A METHODOLOGY FOR DETERMINING BEST AREAS

TO EXPAND WASTEWATER COLLECTION SYSTEMS IN

THE U.S.-MEXICO BORDER REGION

PROJECT NUMBER W98-2

DONALD F. HAYES, UNIVERSITY OF UTAH

MAMUNUR RASHID, UNIVERSITY OF UTAH

SANDY RHEA, UNIVERSITY OF UTAH

BERNARDO P. FLORES-BAEZ, UNIVERSIDAD AUTONOMA DE BAJA

CALIFORNIA

LORENA RIOS, UNIVERSIDAD AUTONOMA DE BAJA CALIFORNIA

INTRODUCTION

The discharge of untreated wastewater generated from residences, commercial,institutional, recreational, and other facilities causes environmental and public healthproblems. Wastewaters (WW) contain suspended solids, organics, nutrients,pathogenic microorganisms, and toxic compounds. If untreated wastewater is allowedto accumulate, decomposition of organics produce malodorous gases, nutrientsstimulate aquatic plant growth, and the pathogenic microorganisms have the potentialto transmit communicable diseases like typhoid and paratyphoid fever, dysentery,diarrhea, and cholera, etc. Due to the infectious nature of these organisms, they areresponsible for thousands of deaths each year in areas with poor sanitation, especiallyin the tropics. In addition, some heavy metals and bio-refractory organics found inwastewater are toxic and carcinogenic to human life. If discharged into rivers, lakesand other bodies of water without treatment, wastewater can be a major source ofwater pollution. If discharged on land, it can cause the soil and groundwatercontamination (Moeller 1998; Metcalf and Eddy 1991).

Due to the adverse effects of wastewater on the environment, the immediate removaland subsequent treatment of wastewater at its source is now mandated by numerousfederal and state laws in the United States. The Clean Water Act (CWA) establishednational goals and objectives “to restore and maintain the physical, chemical, andbiological integrity of the Nation’s waters.”

Two methods of wastewater collection and treatment are commonly used:• Onsite disposal and treatment, usually by pit privies and septic systems• Sanitary sewerage collection system and centralized treatment facilities

Wastewater from individual dwellings and other community facilities in unseweredareas can be managed by onsite treatment and disposal systems. Onsite systemsinclude pit privies, septic tanks, cesspools, imhoff tanks, onsite containment, and

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holding tanks. A pit privy is a hole (usually one meter in diameter and 2 meters indepth) in the ground with a small closed shelter and a toilet built above it. Since verydeep soil and a great separation from groundwater are needed to use pit privies, thecontinued use of seepage pits should be discouraged because of the potential they willcontaminate the underlying groundwater (Metcalf and Eddy 1991).

A septic tank is usually constructed of concrete, with an inlet for sewage to enter andan outlet for liquid effluent. Thus, septic tanks are subsurface sewage disposalsystems. When effluent from septic tanks move through soil for a sufficient period oftime, bacteria and viruses are likely to be removed by straining, adsorption, and die-offs. However, if the absorption fields are too close to a high water table, adequatesewage attenuation may not occur and microorganisms may enter the groundwater(Tuthill 1999).

From previous experience, researchers indicate that onsite systems can be a majorsource of groundwater pollution, especially in densely populated areas (Spencer1958; Rizaiza and Hamidur 1998; Tuthill 1999). In some rural locations the density ofresidential development has increased to the point that continued use of individualonsite systems is no longer feasible and construction of wastewater collection system(WCS) and a centralized treatment system is required.

The U.S.-Mexican border is experiencing a high population growth and a substantialeconomic expansion. The population in the border region grew from approximately 4million to 10 million between 1980 to 1996, and currently has a population of 12million and will likely double by the year 2025. The population and trade growth andpoverty place a significant stress on water and energy sources and could have seriousenvironmental consequences for the health and quality of life of border residents(Ganster 2000).

Water pollution resulting from untreated sewage is a major environmental and publichealth problem in the U.S.-Mexican border region. In the colonias along the RioGrande, a major regional environmental problem is created by the lack of wastewatercollection and treatment. Untreated wastewater discharge results in fecalcontamination of surface water (marine pollution) and groundwater resources,degrades water quality, and severely restricts potential uses of the limited waterresources available along the border. A recent study by the National WaterCommission or Comision Nacional de Aqua (CNA) showed that only 44% of thetotal wastewater in the four major populous cities in Baja California (Tijuana,Mexicali, Ensenada, and Tecate) is treated. Due to inadequate wastewater treatment,waterborne intestinal diseases like cholera have spread. However, colonias are not thelone problem areas; although metropolitan areas on the Mexican side of the border(Tijuana, Mexicali, Ciudad Juarez, and others) have wastewater collection andtreatment systems, they service just over 50 percent of the population. Existing sewersystems in urban areas are old and require rehabilitation or replacement.

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Although it is evident that the solution to the environmental and public healthproblems is to construct a wastewater collection system for the entire region, this isnot possible due to limited economic funding. It would be unusual for a municipalbudget to be adequate to construct a collection system in all un-sewered areas at thesame time. So, the decision-makers must carefully analyze costs and benefitsassociated with every alternative. Thus, the selection of an area or areas for collectionsystem expansion should be determined on the basis of estimated costs and benefitsof each possibility before the design and construction of the system. The cost includesall expenses accrued to produce the desired output of a project, and the benefits arethe goods or services, environmental improvements, public health improvements, etc.The cost and benefits from each area are compared and a ranking system can beestablished to prioritize areas that would benefit most by being serviced by acollection system.

TerminologyThe words “wastewater collection system” and “sewerage” as well as “wastewater”and “sewage” will be used interchangeably in this report. The term “wastewatersystem” will indicate a wastewater collection, transportation, treatment, and disposalsystem. The acronym “WCS” for wastewater collection system and “WTS” forwastewater treatment system also will be used.

RESEARCH OBJECTIVES

The objective of this research is to develop a methodology to identify and prioritizethe areas that need to be considered for wastewater collection system expansions.Ideally, the methodology can be used in developing areas like U.S.-Mexican borderregion. The project will look specifically at wastewater collection system expansionalternatives in Ensenada, Baja California. Ensenada is one of the four most populouscities in Baja California, and wastewater management decisions need to be made onthe future expansion of the existing collection system there. The existing sewersystems in the urban areas of Ensenada are old and require rehabilitation and/orreplacement. Some areas of Ensenada are either serviced, partially serviced or notserviced at all by the collection system. There are some partially developed areas thatare only serviced by septic tanks or pit privies. In addition, high strength industrial(i.e., fishing industry) wastewater is discharged into the aquatic environment withoutany treatment.

The methodology uses mathematical optimization (Dynamic Programming) to selectthe “best” areas of Ensenada in which to expand wastewater collection. Themethodology considers existing environmental and public health conditions of the un-sewered areas, the cost of constructing wastewater collection systems, the reductionof environmental and public health impacts once a collection system is in place, andavailable funds. The methodology is implemented in a Visual Basic computerprogram that will assess the necessary parameters and establish an optimal plan forconstructing a wastewater collection system.

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Once developed, it is anticipated that municipal and local government officials willuse the methodology to identify collection system projects competing for limitedfunds and ensure those funds are being used to minimize the environmental andpublic health impacts, thus maximizing benefits. The methodology will not replace orcompete with existing decision-making bodies and processes, rather, it willcompliment these processes by providing a sound basis for comparing the technicalaspects of complex project alternatives. Economic and political considerations willnot be included in the methodology since these are considered by local officials andfunding agencies. Specific objectives of this research are to:

• Develop an algorithm to assist in identifying the “best” areas for wastewatercollection system expansion

• Develop a methodology for assessing the environmental and public healthbenefits of wastewater collection system projects

• Apply the computer program and the above methodology to the Ensenadawastewater system and rank prospective expansion alternatives on the basis ofenvironmental benefits and system costs

• Implement dynamic programming (DP) in the form of a Visual Basic computerprogram that can be used to evaluate expansion alternatives for wastewatercollection systems

RESEARCH METHODOLOGY/APPROACHES

Literature ReviewHistorically, areas selected for sewage collection were often chosen on the basis ofthe political and economic strength of the recipient communities rather than whetherthe schemes are likely to be feasible, cost-effective, or needed. This basis of selectionprobably does not maximize the greater good, and therefore a more objectivemethodology for site selection is necessary (Reed 1996).

There are limited papers that discuss methodologies for objectively prioritizingcommunities competing for sewerage. However, there have been a number ofpublications that describe the application of systems analysis techniques for sewerpipe layouts, wastewater collection system design, wastewater planning, and solidwaste management. Several studies have also been done on decision support systems(DSS) for water resources management. Since the objective of this research is todevelop a methodology to make decisions for the prioritization of sewerage, DSS ispertinent to this research. There have also been a number of studies on cost of sewersystem expansion. This literature review summarizes the state of the science or art ineach of these areas.

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Matrix Approach for Prioritization of SewerageIn the only paper specifically on the subject of this research, Reed (1996) presented amechanism called the “matrix approach” for objectively selecting which of a group ofcommunities should be the first to receive sewerage. A community’s prioritization forsewerage addition or expansion is based on the area’s population density, sanitationsystem failures, pollution intensity, costs, and other aspects of environmental impacts.In this approach, each community is awarded a score for each of the criteria, and thatscore is multiplied by the weight assigned to that criterion. The weighted scores arethen totaled and the communities are ranked according to their scores. The criticalfactors affecting the prioritization of areas for sewerage are many and will vary bylocation and over time. Reed (1996) suggests that planners should subtract or addcriterion to suit local conditions.

Sewerage plans are designed to meet the needs of a community for many years andmust be capable of handling the maximum sewage flow, which usually comes at theend of the design life. Selection of criteria should relate to the conditions expected atthe end of the design life, rather than those currently experienced. If, however, thereare current conditions relevant to the prioritization process, then they should beincluded. For instance, Reed (1996) offers that a current condition of heavy surfacewater pollution is relevant to the prioritization process.

• The first criterion Reed (1996) uses is the projected total population. Collectionplans expected to serve large populations at the end of the design life are consideredto have a higher priority than do those with a lower projected population.

• The second criterion of population density is also important since higher thepopulation density, the greater the health hazards from poor sanitation and the lowerthe unit cost of sewerage. Also, as housing density increases and plot sizes decrease,the chances of on-site sanitation systems failing increase.

• The third criterion, failure of existing on-site sanitation systems, is one of the mostcommon reasons for needing sewerage.

• The fourth criterion is the presence of industrial pollution. This is a significantfactor when deciding to provide sewerage due to the fact that industrial waste is muchmore concentrated than domestic sewage and it can produce serious pollution ofwater sources.

• The cost of a collection plan is the fifth criteria. A comparison of total capital costwill identify which schemes are the cheapest and which are within the budgetavailable. Operational costs must also be considered. Most sewerage systems operateunder gravity and therefore the operational cost is proportional to the size of thenetwork. Schemes including pumping will have higher operational costs and thiscould be taken into account in the selection process.

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• The sixth criterion is the impact of tourism. Tourism is a major source ofemployment and foreign exchange for many countries. Non-standard sanitationpractices, bad odors, and visible sewage may have a negative impact on tourism.

• The seventh criteria, environmental impact, can be both positive or negative. Thereduction of odor and visible pollution (positive impact) may be counterbalanced byincreased pollution concentrations (negative impact) from waste treatment in thereceiving waters.

• The affordability of a collection system is the eighth criteria. A community’s abilityto pay for a service is not the same as a willingness to pay for the service. Acommunity may be willing to pay the collection service if it is perceived to beimportant. Communities having little desire for sewerage will be unwilling to pay apercentage of their income for the service.

• The ninth criterion is economy of scale. If a number of communities are close toeach other or an existing sewer network, there may be some economy of scale if asingle treatment plant or sewer collection can be constructed to serve all thecommunities.

• The institutional capacity is the tenth criterion to be determined. An institution’sability to cope with the demands of a new sewerage plan will greatly affect the plan’slong-term success, both technically and financially. Communities having institutionswith the capabilities to handle new responsibilities efficiently should be favored in aselection process.

• The eleventh and final criterion is the health benefits to the community. One of themain reasons to consider sewerage is to improve standards of health. The existinglevels of health depend on current sanitation and hygiene practices. Possibleimprovements due to sewerage are difficult to assess because of the number ofvariables involved. In most situations, it may be assumed that similar schemesproduce similar health benefits.

After determining the appropriate criteria that reflect the conditions, the weightcarried by each criterion should be assigned. The weight indicates the importance of acriterion in the selection process. It is necessary to weigh the scores so that importantcriterion has a larger impact on the final result than minor ones. Reed (1996) used aweight of 1, 2, or 3 for each criterion. Once a weight has been determined for eachcriterion, a scale of 1 to 10 is used to score each community. The score indicates theimpact of a criterion on a community. A score from 1 to 10, with 10 being the mostsevere, is assigned for each criterion. Multiplying the score by the weight results in aweighted score for the criterion. A total score is calculated by adding up the weightedscore from each criterion. The communities are then ranked according to their totalscore. The higher total score of a community indicates higher priority for establishinga sewer collection system in that community. The example matrix used in Reed’s(1996) research is summarized in Table 1.

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Gawad and Butter (1995) also developed a stepped approach to prioritize clustertowns and villages that are not served by sewerage systems. The optimum size of acluster, from a financial point of view, is estimated by considering aspects such asconstruction and operation costs of the facilities, existing infrastructure, thegeography, environmental impact, alternative treatment technologies, and phasing ofimplementation. Variables are sizes of clusters, distances from a facility, andtreatment technologies. Once the clusters are determined, each cluster is tested onenvironmental impact, and a priority ranking is made of the villages with regard tothe need for sewerage. The priority ranking considers five parameters: number ofhabitants, service level of drinking water, the health condition inside the town due tothe absence of sewerage, the socio-economic development, and the regionalimportance of locality. The priority list serves as a guide in the selection of newsewerage projects.

The criteria and parameters mentioned in the above two papers can be useful inestimating benefits in this research.

Optimization AlgorithmsMathematical optimization is a powerful technique for solving many engineeringproblems. The optimization algorithm searches for the best decision set to satisfy aquantifiable objective. Linear programming (LP) consists of linear functions that areeasily solved. The applications of LP algorithms are limited, however, because mostphysical systems are inherently non-linear. For a non-linear system, non-linearprogramming (NLP) can be used. A drawback of NLP algorithms is that they aretypically able to find only local maxima or local minima.

Dynamic programming (DP) can be used to solve both a linear and non-linear systemto obtain a global solution (global optimal) since DP is adaptable to both linear andnonlinear functions. DP is useful for sequential or multi-stage problems that maycontain many interrelated decision variables. However, sometimes DP is difficult toimplement because it suffers from the “curse of dimensionality.” In DP, the numberof decision variables need to be kept as small as possible, preferably less than two, tokeep the computational time to a reasonable limit (Hiller and Liebman 1986;Klemetson and Grenney 1976).

Wastewater System OptimizationWastewater system planners have applied systems analysis and optimizationtechniques for decades. Examples of prior applications include:

• Optimization of the hydraulic design of water and wastewater collection systemnetworks

• Regional planning of wastewater collection and treatment systems• Management of wastewater and sludge disposal

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Meritt and Bogan (1973), Mays and Yen (1975), and Argman et al. (1973) have usedlinear and dynamic programming to minimize the cost of a wastewater collectionsystem network during the hydraulic design phase. Spencer (1958) demonstrates thatan economy of scale can be obtained by the regional planning of a wastewater systemsince the unit cost of a larger system is smaller than that of a smaller system.Regional planning involves the selection of the combination of a collection systemand a treatment plant that best meets the desired objectives at the lowest cost (Spencer1958).

Lauria (1976) developed a “critical distance” parameter (as a function of wastewaterflow, length and diameter of sewers, elevation of waste source, etc.) that is usefulwhen deciding whether to build a regional wastewater treatment facility or to build atrunk sewer that will convey sewerage to a distant facility. Leighton and Shoemaker(1984) used a mixed-integer programming model to identify the most attractiveregionalization plans for the expansion of sewerage facilities. The result suggests thatan economy of scale can be obtained by building a regional wastewater treatmentplant, and that cost savings can be realized by using the excess capacity of existinginterceptor links. Joeres et al. (1974) developed a mixed-integer programming modelto select an optimal regional wastewater treatment plan that considers the trade-offbetween the economics of scale inherent in wastewater treatment plants and the addedpipe network collection costs. This model selects a treatment plant and interceptorssuch that the entire demand for wastewater service is satisfied at the lowest overallcost to the region, without violating any of the environmental impact limits.

Deninger and Su (1973) applied a linear programming formulation using Murty’s(1968) ranking extreme point approach to derive an optimal solution to a wastewater-planning problem that involved a number of communities and/or industries in ageographical area. The following questions were considered: where should thetreatment plant be built, how many plants should be built, at what time shouldconstruction occur, and which intercepting sewers are necessary to connect themunicipalities and industries to these plants such that the total cost of wastewatercollection and treatment is minimum. McConagha and Coverse (1973) developed aheuristic algorithm that determines the best combination of treatment facilities andconnecting trunk sewers. This methodology was applied to a wastewater systemcontaining seven communities along a river and its tributary. These seven sites couldbe connected by trunk sewer and could each have a treatment plant built in thecommunity. The optimal solution that was derived was to build only one treatmentplant at one community (community 6) and pipe all the waste to that plant. Panital(1978) developed a capacity expansion model and initial deficit model for estimatingthe optimal design period for an intercepting sewer governed by interest rates, relativeinflation, and increased cost of construction due to development of the service area.

Markland et al. (1977) developed a large scale mixed integer programming model forthe regional planning of a land application system to dispose of sewage effluent fromsecondary treatment plants serving a large metropolitan area. This model can be used

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to identify which disposal sites should serve which treatment plants, when initialconstruction should be completed, and when capacity expansion should occur.

Bhagwan and Polprasert (1982) developed a multi-objective programming method toanalyze wastewater and sludge management systems. The objectives considered werethe minimization of: costs for piping and treatment of waste at potential treatmentplant sites and costs for transportation of sludge to disposal sites; water qualityimpacts due to the discharge of wastewater into a receiving water body; and land useimpacts due to the land application of sludge.

Harvey and O’Flaherty (1972) developed a mixed integer-programming modelformulated as a “transportation network” for analysis of solid waste transportationand disposal alternatives. The objective of this model was to identify which sanitarylandfill site (or sites) and transfer stations, among many, should be seriouslyconsidered in terms of minimizing the present worth and total cost of solid wastetransportation and disposal. Naraynaswamy and Kennedy (1994) developed aheuristic algorithm to locate solid-waste disposal sites. The objective was tomaximize the service to an existing population while minimizing the distance fromthe site. These two objectives can be achieved by minimizing the sum of weightedpopulation miles (population times distance).

Ford (1984) developed a dynamic programming methodology to minimize the totaldiscounted cost of a dredged material disposal system. The cost includes removal,transportation, and addition of dredged material from the source to the disposal sites.The formulation represents material sources and available disposal sites as nodes ofthe network and represents transportation links and carry-over storages as arcs.

Conclusions From Literature ReviewThe objective of this research is to select the “best” service areas among many thatare competing for sewerage. The selection of best areas requires making “yes” or“no” decisions. This selection involves allocating a different amount of resourcesamong different areas that will minimize the impacts and maximize the benefit withinthe available funds. So, the problem in this research can be modeled as a multistageproblem consisting of many un-sewered areas in which many inter-related decisionsare to be made. This multistage problem can be broken down into a series of inter-related single-stage (single area) problems using dynamic programming (DP)approach. The findings from this literature research can be summarized as follows:

• The dynamic programming approach used for optimization in this research isadaptable to both linear and nonlinear systems and guarantees a global optimumsolution.

• The computational procedure in a DP approach provides an optimal solution anda sub-optimal solution set for the whole system. This approach can also provideoptimal and sub-optimal solution sets at a given cost.

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• The mixed integer programming model proposed by Markland et al. (1977) is aviable option for this research. However, this approach uses non-linearprogramming, which does not guarantee a global solution.

• The “matrix approach” proposed by Reed (1996) is based on decision-makers’qualitative judgment and is not an optimization method. But, this approach canbe used to estimate environmental and public health impacts from sewerageprojects, and thus will be integrated in this research.

Developing The Dynamic Programming ApproachThe Dynamic Programming (DP) approach breaks down a large, complex probleminto a series of smaller sub-problems and then combines the solutions of the smallerproblems to obtain the solution for the entire model composition (Mays and Tung1992). Since the problem in this research has multiple stages (16 stages for 16unserviced areas) where only two (sewer or not) decisions have to be made indifferent stages, DP seems the best approach. Due to two decision variables, thecomputations in the problem will increase exponentially (instead of linearly) with theincrease in number of sub-problems. Thus, the “curse of dimensionality” is not a bigproblem for a smaller number of stages in this research.

Wastewater SystemA wastewater system is comprised of three components: source, collection, andtreatment. The wastewater source is the flow received from residential, commercial,and industrial facilities. Collection and treatment components include the sewernetwork and treatment plant. Optimization of wastewater systems involve identifyingsystem constraints, including the limitation of funding to provide sewerage in all un-sewered areas. Sewer network and treatment plant have hydraulic constraints,including include the sewer pipe capacity to carry the projected flow and massloadings (product of wastewater flow and strength). Hydraulic constraint of thetreatment plant is the capacity to meet the projected flow and mass loadings carriedby the sewer network. Figure 1 illustrates a wastewater system with its variouscomponents.

The wastewater system in Figure 1 consists of seven un-sewered areas (Q1, Q2, Q3,Q4, Q13, Q14, Q15) and eight sewered (Q5, Q6, Q7, Q8 Q9, Q10 Q11, Q12) areas. As seenfrom the figure, treatment plant T1 receives wastewater from junction 1 (MH1)through collector 1 (Col1) and from junction 2 (MH2) through collector 2 (Col2). Thesewered area 5 (Q5) also directly sends its wastewater to treatment plant T1. Similarly,the treatment plant T2 receives wastewater from junction 3 (MH3) through collector 3(Col3). The sewered areas 9, 10, 11, 12 (Q9, Q10, Q11, and Q12) directly send theirwastewater to the treatment plant T2. This system has eight existing connections (l)and eight possible connections between wastewater sources and sinks (treatmentplants and/or manholes).

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Problem FormulationFor the wastewater system in this research, different unsewered areas are representedas stages (i = 1, 2, 3, …, n) where decisions are to be made. The system is arepresentation of all the areas. State variables describe the condition of the system.For this model, the condition of the system is the remaining funds and capacity(wastewater treatment capacity) available to collect sewerage from unsewered areas.The decision variables represent the control or ability to change the system. At eachstage a decision is made to either collect sewer or to not collect sewer from that area.The state and decision variables are represented as xi and ui for the wastewatercollection plan model. The decision variable ui is either “yes” or “no” (1 or 0), toeither collect or to not collect. The objective function is to minimize the impacts andmaximize the benefits of the whole system. The benefits represent the reduction ofenvironmental and public health impact if sewerage is provided in all areas. As moreareas are chosen to be sewered, the environmental and public health condition of thesystem improves, and consequently impacts are reduced. On the other hand, seweringmore areas deplete the remaining funds and capacity of treatment more quickly,affecting the state of the system.

The available funds (F), and capacity of treatment plants (T) for the expansion andconstruction of the collection system are the constraints of the model. A collectionplan should be chosen that provides the minimum impacts or the maximum benefitsfor the available funds and capacity. A visual representation of the wastewatercollection system model is given in Figure 2.

The DP formulation for the wastewater collection system is given below:

Minimize Â=

n

i 1

f(xi ui) ------------------------------------------------------------------ (1)

Subject to: xi = xi+1 + uiCi -------------------------------------------------------------------- (2)Qi = Qi+1 + uiQi ------------------------------------------------------------------- (3)Bi(ui) = Ei(1-uiREi) + Hi(1-uiRHi) + Oi(1-uiROi) + Bi+1(ui+1) --------------- (4)

Qi

i nT

j

j mi j

=

=Â £

=

=Â

1 1-------------------------------------------------------------------- (5)

Q Si

i nM

j

j mi i j

=

=Â £

=

=Â

1 1--------------------------------------------------------------------------(6)

(6) xn+1 = 0 ------------------------------------------------------------------------ (7)ui ≥ 0 ------------------------------------------------------------------------------- (8)

Figure 2 presents a graphical explanation of the above equation.ui < 1 ------------------------------------------------------------------------------- (9)D ui =1----------------------------------------------------------------------------- (10)x1 < F ------------------------------------------------------------------------------ (11)

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where:xi = cumulative cost for stages i ($)ui = decision to collect sewerage for stage iBi(ui) = benefits accrued from stage iQi = wastewater flow transported from source (stage) i (volume/time)Mj = mass-loading capacity of wastewater treatment plant j (mass/time)Tj = hydraulic capacity of wastewater treatment plant j (volume/time)Ei = environmental impacts for stage iHi = public health impacts for stage iOi = other impacts for stage iREi = reduction in Ei due to collection for stage i (%)RHi = reduction in Hi due to collection for stage i (%)ROi = reduction in Oi due to collection for stage i (%)Ci = estimated cost for stage i ($)x1 = total cost for collection plan of system ($)F = available funds for collection plan ($)f(xi ui) = value of criterion as given by recursive equation for a particular state-

decision combination (xi ui) in stage i

The aforementioned DP algorithm can be solved as a backward dynamic programusing a Microsoft Excel spreadsheet. This simply means that the algorithm solution(i.e., wastewater collection plan matrix) was determined by marching through eachstage, beginning with the last stage and continuing backward to the final stage (i.e.,stage 1), and accumulating every possible combination for system cost, treatmentplant’s hydraulic and mass loading capacity, and system benefits. For determining awastewater collection plan matrix, it is assumed that all funds, capacity and massloadings are exhausted after the last stage. A wastewater collection plan matrixincludes all optimal and sub optimal solutions for every combination of areas to besewered. At the final stage, a matrix of total collection costs, treatment plant’shydraulic and mass loading capacity and corresponding total benefits can be obtainedfrom Excel. Theoretically, the optimal solution with minimum impacts or maximumbenefits is to collect wastewater from every area. This also results in the highest totalcost, and the depletion of available hydraulic and mass loading capacity of thetreatment plants. The other possible combinations represent the sub-optimal solutions.When the available funding constraints and hydraulic capacity constraints of thetreatment plants are known, a trace back can be conducted to determine the optimalwastewater collection plan. This plan would represent the areas that would be mostbeneficial to sewer with the resources available.

Costs Of Sewerage ProjectsThis research assumes that the costs for sewerage projects are known and thus noeffort will be made to estimate costs. But a synopsis of previous research to estimatecosts is provided below.

The essential elements for estimating the cost of a sewerage system are length,diameter and unit costs of sewer, and the number and capacity of required pump

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stations. The largest single component of the construction cost of a sewerage networkis the cost of supplying and installing the different pipe sections, which make up thenetwork. A study based on average national figures estimates that 85 percent of thecost of gravity sewer systems is devoted to excavation, pipe supplies, and installation.The remaining 15 percent covers the cost of manholes (Baffa 1955). These estimatesare based on the most common method of construction, namely that of laying out apipe in pre-excavated open-cut trenches. While a number of other technologies arepossible (such as tunneling, elevated pipes, pipes in embankments, etc.), they areusually considered to be solutions to special situations where the cheaper open-cutmethod is not feasible.

The cost estimation of a collection system project requires information on land useforecast, a plan of sewer subsystems for forecasted development, and the descriptionof topography (i.e., slope, surface and rock elevation). Land use forecast includespopulation forecast, which can be predicted from previous data. Although a collectionsystem lasts longer, for cost estimation purposes, the life of a sewer system isgenerally considered to be 50 years (considering no salvage values) from theinstallation date. Pipe sizes and their attendant costs are determined on the basis ofmaximum demand for the entire design period.

Once a system layout of pipes and routing of flows have been determined, thedesigner must decide on a combination of slopes, diameters, and depths for each pipe.Dajani, Gemmel, and Morlok (1972) present an empirical analysis of the cost ofconstructing a sewer line and developed an overall optimization model for the system.Baffa (1955) developed a sewer construction cost model on the basis of existingconstruction data. The general form of the model is:

Cs = a + bDn2 + cXe

2-------------------------------------------------------------------- (12)

where:Cs = cost of the system ($/feet)Dn = nominal diameter of the sewer pipe (feet)Xe = average excavation depth (feet)a, b, c = regression coefficients (determined from existing construction data)

However, the design and construction costs of a collection system are site specificand difficult to estimate accurately without design details. Generally, the total costs ofdesign, construction, operation, and maintenance can be achieved from municipal andlocal authorities or from extrapolation of previous data. Heuristic methods have beendeveloped based on so-called “ruled of thumb” and experience that can provide “ballpoint” estimates of costs and those are presented below.

Sewer Pipe and Manhole CostsThe cost of a sewer collection system has three components: design and constructioncost, or first cost; operation and maintenance cost; and discounting of future capitalinvestment beyond the first cost. Construction cost, or first cost, can be divided into

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three categories: pipe-in-place cost, including manhole cost; pumping station (liftstation) cost; and lateral cost. A pumping station is generally used if the elevation ofan area or district to be serviced is too low to be drained by gravity to existing orproposed trunk sewers. So, where the topography is favorable, a simple gravitysystem can be built and no pumping costs are incurred. Pipe-in-place cost is thelargest cost component of a sewer system. Pipe in place cost includes the cost of earthexcavation and backfill, under-drains, handling water, manholes, sheeting, furnishingand installing pipe, and concrete cradles. In simplest terms, pipe costs are a functionof the depth and size of pipe and geology of its environment. In the absence of moreprecise cost estimates, the following empirical relationships can be used as suggestedby Klegerman (1964):

For h £19 inch, and d £ 36 inch, Cl = 17.0 + 1.6(h-8) + 1.2(d-12)---------------- (13)

For h >19 inch, and d < 36 inch, Cl = 70 + 2.5(h-24) + 2.1(d-24)---------------- (14)

For d > 36 inch, Cl = 138.0 + 5.9(h-11) + 2.5(d-72)-------------------------------- (15)

k = 0.095l -------------------------------------------------------------------------------- (16)

where:Cl = cost of sewer-line ($/ft)h = mean invert depth (inch)d = diameter of pipe (inch)k = pipe maintenance cost ($/ft)l = the length of pipe (ft)

Meredith (1972) suggested the following empirical equations for pipe and manholeinstallation cost and total manhole cost of a sewer system.

For d < 3 feet, and h < 10 feet, cp = 10.98 d + 0.8 h -5.9--------------------------- (17)

For d = 3 feet and h = 10 feet, cp = 5.94d + 1.166h + 0.504hd - 9.64 ------------ (18)

For d > 3 feet, cp = 30.0d + 4.0h -105.9---------------------------------------------- (19)

cm = 250 + hm2 -------------------------------------------------------------------------- (20)

where:cp = pipe installation cost ($/ ft)d = the pipe diameter (ft)h = mean pipe invert depth below the ground surface (ft)cm = total cost of manhole ($)hm = depth of manhole (ft)

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Pumping Station CostPumping station cost is a function of pump capacity and the vertical distance throughwhich the sewage is to be lifted, as well as pipe characteristics. Required pumpcapacity can be determined from peak flow and head, and from the vertical distancethrough the sewage is to be lifted. Klegerman (1964) showed that pump station cost ismuch more sensitive to peak flow than to head and thus can be estimated as:

For qp ≥1.55 cfs, cP = 97,000qp - 43,700qp2------------------------------------------ (21)

For 1.55 cfs < qp £ 40 cfs, cP = 29,000qp - 363qp2 -------------------------------- (22)

For qp > 40 cfs, cP = 14,500qp -------------------------------------------------------- (23)

where:cP = cost of equipment, structure, and land ($)qp = peak flow (ft3/sec)

The annual maintenance and operating cost of a pumping station is a function of flowand head. Pumping costs are generally expressed in terms of head, flow, power-rateand pumping efficiency. Klegerman (1964) estimated the annual power costs as:

e = 23.9hqa ------------------------------------------------------------------------------ (24)

where,e = annual power cost ($)h = the head (f)qa = the average daily flow over the life of the pump (ft3/sec)

Wastewater Collection System BenefitsThe evaluation of benefits is crucial to this research. Benefits accrued from a WCSproject can be categorized as environmental benefits and public health benefits, andother tangible and intangible benefits. Environmental benefits are the reduction ofraw sewage discharges, leach-fields, and septic fields, and public health benefits arethe reduction of diseases and immature deaths. The tangible and intangible benefitsfrom a WCS expansion include: Increased employment and economic growth in anarea, increased recreational activities in waterways, and healthier citizens who areconfident that they have adequate wastewater treatment and cleaner waters (Nemerow1978).

The benefits of environmental improvement from pollution abatement are obtained inthree distinct stages as: reduction of pollutant discharge improves the environmentaland public health quality, the improvement in environmental quality increases thetypes and levels of human use of the environment, and the increases in human uses ofthe environment affect utility or welfare. Tangible benefits can be quantified, whereasintangible benefits are more difficult to measure. From a practical point of view,

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environmental and public health benefits from sewerage projects are normallymeasured in non-monetary units.

Conceptual Framework Of Benefit EstimationThe basic concept of “benefit” has taken on a range of definitions in the technicalliterature and other disciplines of education. For the purposes of this report, the notionof non-monetary “benefit” of a sewerage project is related to the difference inexpected impact (output of the sewerage project) between the current condition(without the project) and the expected condition with the project. The expectedimpact is the reduction of potential risk of environmental pollution from untreatedwaste (wastewater content), reduction of surface and groundwater pollution fromexisting onsite systems, and reduction of diseases and morbidity of human and otherbeings.

Due to this reduction of pollution, the environmental and public health and othertangible and intangible improvements can be drawn from investment in sewerageprojects in un-sewered areas. In the absence of collection and treatment, wastewater isdischarged to the environment causing degradation, while collection and treatment ofwastewater will protect the environment as shown in Figure 3.

As seen in Figure 3, environmental benefits from a WCS project are the reduction ofraw sewage discharges, leach-fields, and septic fields. This reduction will improvewater quality, aquatic life, and ecology. Public health benefits include the reductionof exposure to contamination, which will reduce diseases and morbidity. As a resultof these impact reductions, the quality of life will be improved through theimprovement of health and livelihood systems and the reduction of the vulnerabilityof environmental conditions.

The numerical value of benefit that can be accrued from the provision of sewerage isa function of environmental, public health, and other impacts that would occur if thearea is not provided sewerage, and reductions of these impacts if the area is providedsewerage. The following empirical equation demonstrates that:

Bi = Ei(1-uiREi) + Hi(1-uiRHi) + Oi(1-uiROi) ---------------------------------------- (25)

The variables in the above equation are defined in the previous section. So, estimatingbenefit involves quantifying impacts (environmental impact, E; public health impact,H; and other impact O), and reduction variables (RE, RH, and RO), and identifyingdecision variable (u). It is important to note here that theoretically if an area isprovided sewerage, 100 percent reductions should occur. However, reductions arealways less than 100 percent (RE, RH, RH < 1) since some of impacts are reduced bythe existing on-site systems serving the area.

According to Equation 25, if an area is sewered (u = 1), the benefit values decreasecompared to an unsewered area (u = 0), provided that all other variables in theequation are the same for both conditions. For the sake of discussions, assume that E

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= 20, H = 15, O = 10, RE = 0.85, RH = 0.75, and RO = 0.65. When an area is notsewered, that is for u = 0, B = E + H + O = 45 impact units. When an area is sewered,that is for u = 1, B = 20.75 impact units. The numerical value B equal to 45 indicatesthat the impact of 45 units would occur on this area if this area is not providedsewerage, and B equal to 20.75 units (for u = 1) indicates that even though this area isprovided sewerage, all the impacts could not be reduced and 20.75 units of impactsare still remaining since some of the impacts are reduced by existing onsite systemsresulting a reduction values less than one (1). So, the reductions of impact are 24.25(i.e., 45-20.25 = 24.25) impact units. Therefore, a smaller value of B (for u = 1)indicates larger reductions of impacts. From this discussion, it can be concluded thatB indicates “remaining impact” and minimizing B implies maximizing the reductionof impacts or maximizing benefit. Therefore B is synonymous to “maximization ofbenefit,”

Empirical Method For Quantification Of ImpactThe proposed empirical method for quantifying impact calculates a numerical indexor score (in non-monetary unit), indicating how the welfare of the society (or thequality of life) would be affected if particular alternatives (sewer one area vs. other)are not implemented. This method is subjective and not an absolute answer toestimate impact. The general philosophy of this method is expressed in the followingsaying:

“It is the mark of an instructed mind to rest satisfied with thedegree of precision which the nature of the subject permits, andnot to seek an exactness where only an approximation of the truthis possible.” -Aristotle

Quantifying impact may involve three activities, such as a general description of thebaseline environmental setting where the proposed sewerage will be provided; thecharacteristic of the environment (i.e., environmental variables or factors thatdescribe the baseline environmental setting of the un-serviced area and upon whichimpacts may occur); and impact predication and assessment.

Previous Studies On Impact AssessmentMcAllister (1980) developed a method known as the Environmental EvaluationSystem (EES) for assessing environmental and certain social impacts of water-relatedprojects including intangibles. In this method, the composite score of environmentalimpact is calculated in three steps. First, impact is estimated by forecasting parameterlevels with and without the project. Then each parameter is weighted by a numberindicating a parameter’s relative importance. The second step is to convert theseparameter estimates to commensurate units. In the third step, each parametermeasurement is transformed by a value function into a measurement on an“environmental quality scale” ranging from 0 to 1, where 0 represents “extremely badquality” and 1 represents “very good quality.” The predetermined value functions canreflect a linear or a nonlinear relationship. Once the environmental quality units have

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been determined with and without the project, the composite score for theenvironmental and social impacts is calculated as follows (McAllister 1980):

ESI = Â=

n

i 1

[(V1)i – (V2)i]wi------------------------------------------------------------- (26)

where:ESI = composite score of environmental and social impacts(V1)i = value of parameter i with the project, environmental quality units(V2)i = value of parameter i without the project, environmental quality unitswi = the value weight assigned to parameter i, parameter importance unitn = the number of total parameters

Gawad and Butters (1995) and Reed (1996) suggested some specific non-monetaryaspects (factor) that can be considered for impact assessment of sewerage projects.They discuss projected population, population density, existing human healthconditions, drinking water condition, and other environmental conditions. Reed(1996) discussed a weighted scoring matrix to prioritize the unsewered communities.A scoring scale of 1 to 10 is used to assign a score to the non-monetary benefits. Eachscore is multiplied by a weight factor, indicating the importance of that criterion. Aweighted factor of 1, 2, or 3 is assigned, with 3 being the most importance factor.Reed’s weighted matrix approach is a practicable procedure to evaluate the non-monetary aspects of impacts and thus will be integrated in this research.

Nero et al. (2001) developed and used decision models to screen potential land areasfor wastewater system facility siting and evaluate wastewater treatment managementalternatives in Monroe County, Florida. The approach in the models incorporatedtechnical input (i.e., cost, performance estimates, and net environmental impact) withpolicy input (i.e., importance of achieving different performance goals), both ofwhich were merged with the values and concerns expressed by the key decision-makers and stakeholders (the public). The policy level objectives used to evaluate thealternatives were to maximize environmental benefits, implementability andreliability, and to minimize costs and secondary impacts. A total of 19 criteria weredeveloped to evaluate each individual alternative.

Each criterion used was assigned a weight indicating its importance . The higher theweight, the more important that criteria in the evaluation. The weight (also calledpolicy weights) expresses priorities and values that reflect the policy views of thecommunity and stakeholders. These policy weights drive the direction of the decisionprocess and its conclusions.

In the wastewater management alternatives decision model, the stakeholders rankedmaximizing environmental benefit the highest, while minimizing the costs receivedthe second highest ranking. In the facility siting decision model, stakeholders rankedmaximizing public acceptance the highest, while minimizing costs received thesecond highest ranking. This model was used to develop a Wastewater Master Plan,which included extensive evaluations of existing systems in the Florida Keys and

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applicable technologies that would fulfill the objective of the Monroe County (Neroet al. 2001).

Environmental ImpactThe major environmental impact that is likely to occur in the absence of sewerage inan area is the degradation of surface and groundwater quality. These degradations areproportional to the sum of the pollution potential (also called numerical pollutionindex) and can be quantified using the following empirical equation.

E = QrQw + SrSw + XrXw + NrNw + frfw+ wrww + YrYw + xrxw + lrlw + SWrSWw +OwOr

+ STrSTw + dwdr + DrDw+ ZrZw+ KrKw+ brbw + GWrGWw)/NE -------------- (27)

where:w = weighted value assigned to a score for each factorr = rating scoresQr = score for wastewater flow rateSr = score for wastewater (domestic or industrial) strength (i.e., BOD)Xr = score for proximity of wastewater source to the surface water body (i.e.,

lakes, ponds, and reservoirs, etc.)Nr = score for average nitrate concentration (from wastewater or onsite system)fr = score for phosphate concentration (from wastewater or onsite system)wr = score for concentration of fecal coliform bacteria (i.e., E. Coli) in wastewaterYr = score for total area/volume of water bodyxr = score for proximity of onsite system (i.e., privy) to the flowing streamlr = score for intensity of color of water body due to wastewater dischargeSWr = score for the current condition of surface water quality (i.e., already

polluted, somewhat polluted, not polluted at all, etc.)Or = score for current percentage of priviesSTr = score for current percentage of septic tankdr = score for visible sewage draining above ground (current condition)Dr = score for average depth of groundwaterZr = score for number of private wells currently supplying drinking waterKr = score for hydraulic conductivity of groundwater aquiferbr = score for evidence and/or probability of soil contaminationGWr = score for the current condition of ground water quality (i.e., already

polluted, somewhat polluted, not polluted at all, etc.)NE = number of factors used for assessing environmental public health impact

Guidelines For Assigning RatingsThe estimation of impact requires the assignment of rating scores and importanceweights for the selected factors. The ratings for every factor for a given number ofareas differ, and every area can be assigned a relative rating for a given factor. Forinstance, if there are 20 different areas with 20 different population densities, then thearea that has the highest density can be assigned a rating of 10. A linear relationship

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can be established between the rating and density for the different areas as shown inFigure 4.

According to Figure 4, the rating for an area with a maximum density of 100 is 10,while rating for area with a density of 60 is 6. So, a rating curve can be constructedfor any factor if the minimum and maximum values of that factor are known. Therating for a factor that lies between the minimum and the maximum (i.e., rating of 5for medium-dense areas) can be determined using this rating curve or linearinterpolation. The same approach can be used to construct a rating curve (assuminglinear or nonlinear) for any of the other factors for a given number of areas. Reed(1996) uses a rating score of 1 to 10 for the prioritization of sewerage projects. Canter(1997) also presented a rating methodology for several factors that contribute togroundwater pollution by nitrate.

The higher sum values of impact represent greater potential for pollution or greaterimpact on environmental and public health quality if an area is not providedsewerage. In addition to indicating the vulnerability of an area to pollution, thesevalues (also called policy values) are likely to reflect the policy views and concerns ofthe community and stakeholders (i.e., country officials, environmental activists, taxpayers, and decision makers) and drive direction of the decision process and itsconclusions. Several generic steps are associated with the quantification of impact,such as factor identification; assignment of rating score and importance weights forevery factor; and establishing a guideline or method for factor evaluation, assigning arating score and importance weight. Then the impact is calculated by multiplyingeach factor importance weight by its rating score, summing the total, and thendividing the total by the number of factors.

Guidelines for Assigning Importance WeightsA relative importance weight must be assigned to each factor to reflect its importancewith respect to the others when forecasting the pollution potential. The assignment ofan importance weight is based on professional judgment and may vary depending onwhat is contributing to pollution. For instance, the nitrate concentration and depth ofgroundwater may carry more weight than that of BOD concentration when forecastingground water pollution potential. It is important to mention that two or more factorscan be assigned the same importance weight.

Each of the weighted values can be changed to meet a particular situation. Reed(1996) assigned a weighted value of 3 for the most important criterion, while acriterion that does not pertain to a particular situation is assigned a weighted value ofzero.

Public Health ImpactsThe public health impacts that will incur if a sewerage project is not implemented arethe degradation of public health quality due to the infectious diseases caused byhuman excreta and other wastes. Public health quality can be reduced by three modesof exposure – drinking, swimming, and bathing. Surface and/or drinking water can be

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contaminated by wastewater discharge. The overall public health impact can bequantified according to the following equation:

H =(PrPw + rrrw + CHrCHw+ araw + PWrPWw + DWrDWw+ RWrRWw+ OwOr + dwdr

+ drdw+ prpw + mrmw)/NH --------------------------------------------------------------- (28)

where:P = score for current/projected total populationr = score for current density of populationCH = score for current density of childrena = score for percent of the population with low income (less than the average

income).PW = score for potable water source contaminationDW = score for drinking water source contaminationRW= score for extent of surface water used for recreational purposesd = score for the percent of the population with high income (more than the

average income).p = score for length of sullage drain environmentm= score for intensity of odor (usually measured by olfactometer) from

wastewater gases or directly from human feces.NH = number of factors used for assessing public health impact

Other ImpactsThe other impacts that may occur if a sewerage project is not implemented are theabsence or reduction of existing tourist and/or recreational activities, expected sewerrevenue, and the decrease of other qualities of life, to name a few. This impact can beestimated (but not limited to) using the following factors as given below:

O = (VrVw + RIrRIw+ SDrSDw + Rr Rr + erew + drdw)/ NO--------------------------- (29)

where:V = score for forgone tourism and/or other recreational activities in the areaRI= score for the regional importance of the areaSD = score for the existing or expected socio-economic development in the areaR = score for expected sewer revenue from the aread = score for the percent of the population with high income (more than the

average income) in the areae = score for existence (or more likely existence) of other public utilities such as

electricity, road, gas and water supply in the area

The following tangible/intangible factors are usually considered important for theprioritization of sewerage and can be useful in brainstorming assigning importanceweights and ratings of areas competing for sewerage:

• Public acceptance of the area to receive sewerage (if sewerage is not provided,people will be upset)

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• Implementability or technical feasibility of the area to receive sewerage (i.e.,ease of management and siting)

• Sustainability of the area to be developed (no development and growth willoccur if sewerage is not provided)

• Negative impacts (i.e., noise, dust, pipe-working, etc.) of sewerage constructionon the area

• Private property and land use (residential or nonresidential) restrictions• Physical constraints to construct sewerage (i.e., existence of river, lake, highway

or any other major construction in the area or distance from the treatment plant)

Theoretical Basis Of FactorsThe list of factors considered for impact assessment is long and will vary from placeto place and over time, depending on the prevailing conditions (i.e., environmentaland public health conditions and others) when the analysis is being done. Thetheoretical basis of using the aforementioned factors to impact assessment is givenbelow. The assignment of importance weight for each factor is site specific. However,a general guideline for assigning weight and a synopsis of pertinent literature is alsodiscussed below.

Projected Total Population And Density (Pw and rw)Sewerage projects expected to serve large populations (Pw) at the end of design life isthe first factor to consider since increasing population leads to increased developmentand increased industrial activity. Sewerage projects serving larger populations tend tohave a lower per capita cost, produce greater social and environmental gains,maximize the number of people having access to improved sanitation, and improvethe local environment through reductions in odor and inconvenience from opensullage drains (Reed 1996).

A high population density (rw) is both an environmental and public health concernsince the amount and characteristics of wastewater (BOD, COD, nutrient andmicroorganism concentration ,etc.) and other waste generation outlets have a directcorrelation to the density of people in a community. Wastewater from the high-density area shows high values for turbidity, suspended solids, oxidizable organicmatter, BOD, ammoniacal nitrogen, and fecal coliforms (Oluwande, Sridhar, andOkubadejo 1978). In a densely populated area, close proximity to other peopleincreases the chances of transmitting illness. The total number of pathogenicorganisms discharged from a household depends on whether an individual is ill and isshedding pathogens. If one or more members of a family are ill and sheddingpathogens, the number of measured organisms can increase by several orders ofmagnitude (Crites and Tchobanoglous 1998).

Maynard (1969) reported the relationship between population density and septic tankperformances in an area with impervious soil conditions, poor surface drainage, andan extremely high water table. Results indicate that due to high population density,the urban-sized lots were incapable of absorbing all the sewage effluent produced byan average household. Consequently, almost all of the septic systems were

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malfunctioning, especially during periods of high precipitation. The effluent fromthese systems flowed directly into the ditches causing an extreme public healthhazard. The citizens in this area found that their property values reduced from asmuch as $4,000 to practically nothing because of the unsanitary conditions (Maynard1969).

A high population density (rw) can be also an environmental concern since it cancontribute to surface water and groundwater pollution. Steynberg (1995) reported thathigh-density population (with the provision of limited sanitation services) is one ofthe parameters that contribute increased microbial numbers to the surface water bodyin developing areas. Concentrated land-use activities near populated areas also havethe potential to affect water quality (Stednick, Gilbert, and Lee 1998). Tuthill (1998)reported that as lot size increases with the increase of population density, septicsystems might cause increased levels of groundwater well contamination throughleaching.

Children Density (CHw )The high density of children (CHw) in a community is a public health concern sincechildren are more susceptible to illnesses and waterborne diseases than adults presentin poor sanitation areas. If open drains are created from sewage disposals, childrenare more likely to have either occasional or regular contact with the drains’ contentand may have parasite infection. Study shows that these drains usually containwastewater varying between those of fresh and septic domestic sewage (biochemicaloxygen demand, or BOD, varying between 35 to 450 mg/L) carrying many types ofpathogenic organisms (Oluwande, Sridhar and Okubadejo 1978).

For scoring purposes, the area with the largest population (number and density), andchildren density will receive a score of 10 while an area with the lowest populationand children density will receive a score of 1for the corresponding factors. The scorefor other communities could be found by a linear interpolation between 1 and 10.

Poverty Of Population (aw)The principal purpose of urban sanitation is to reduce poverty and improve publichealth by providing necessary infrastructure (i.e., wastewater treatment plants,municipal disposal projects and etc.) in low-income communities. The health profileof these communities is poor since poor people generally are more susceptible toillnesses and waterborne diseases than wealthier people (Reed 1996). Researchindicates that, water-borne diseases such as typhoid, cholera, paratyphoid fever,bacillary dysentery etc. are more prevalent in areas with poor sanitary conditions(http://www.edugreen.teri.res.in/explore/water/health.htm). Poverty of population isalso an environmental concern due to the fact that a well-established link existsbetween poverty and environmental degradation (Ganster 2000).

For scoring purposes, the highest income communities could be assigned a score of 1while the lowest income communities could be assigned a score of 10. The score forother communities could be found by a linear interpolation between 1 and 10.

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Wastewater Flow Rate And Strength (Qw, Sw)Larger volume of wastewater with high strength can also provide monetary benefitfor the wastewater treatment plant. For example, larger QiSi indicates a larger amountof methane production in the digester for sludge digestion, which is useful in powergeneration and can be counted for monetary benefit. Volume of methane productionis expressed as:

Vmethane = 5.62[(So-S)(Q) -1.42Px] ---------------------------------------------------- (30)

where:Vmethane = volume of methane produced at standard conditions; 32oF and 1 atm

(ft3/day)So = ultimate BOD in influent (mg/L)S = ultimate BOD in effluent (mg/L)Px= net mass of bacterial cell tissue produced, (lb/day) in the activated sludge

process (Metcalf and Eddy, 1991)

Visible Sewage Draining Above Ground (dw)Visible sewage, typically drained from the kitchen, may include raw or gray waste(also called sullage). Gray waters usually do not contain wastewater with toilet paperand feces, but may contain high concentrations of grease, soaps, detergents, andorganics that may cause soil clogging. Visible sewage in an immediate surroundingmay generate mal odorous gas and pose a potential threat to public health, especiallyin developing countries where water supply and proper sanitation is inadequate. Italso has the potential to contaminate different environmental media like soil,groundwater, surface water, and vegetation (Laak 1986).

In addition to gray waste, the absence of a wastewater collection system causes rawwastewater from houses with no septic tanks or latrines to run freely through thestreets, creating drains. These runoffs are sporadic, and primarily occur during therainy season. In addition to the nuisance they represent, these runoffs pose a publichealth risk, as they are a vehicle for the transmission of diseases, either by directcontact or through vectors.

Drainage on the ground, whether gray waste or sewage, can also encourage mosquitobreeding with the attendant risk of Bancroftian filariasis (Mara 1996). Oluwande,Sridhar, and Okubadejo (1978) reported that these drains could be the foci ofinfection in developing countries. Children may be exposed to drainage materialswhen playing in the street and contract parasite infections.

For scoring purposes, an area with visible sewage draining above ground will receivea score of 10 while an area with no visible sewage will receive a score of zero (0).

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Current Condition Of Surface Water Quality (SWw)Surface water (i.e., rivers, streams, ponds, lakes, wetland, reservoirs, oceans or bays,etc.) quality degradation is both an environmental and public health concern. Theintroductions of wastewater to surface water greatly increase the total coliform leveland nutrient loading into the body of water. The total coliform level might causewaterborne diseases while the nutrient loading might stimulate the excess plantgrowth (Metcalf and Eddy 1991; Chapra 1997). Eutrophication alters the appearance,taste, and odor of surface water. If surface water is used as a source of drinking water,eutrophication may increase the cost and difficulty of the drinking water purificationprocess in some cases. Eutrophication also directly affects oxygen and carbon dioxidelevels of water, in turn threatening the survival of aquatic life, and making waterunsafe for top-trophic levels such as birds and livestock (Pierzynski 1994) and otherhuman-centered uses. Many types of fish and bottom-dwelling animals cannotsurvive when dissolved oxygen levels drop below 2ppm to 5ppm. When this occurs,eutrophication kills large numbers of aquatic organisms, which leads to majordisruptions in the food chain.

Bell, et al. (1989) studied the impacts of runoff and sewage discharges from touristresorts on coral reef communities. The authors reported that that sewage discharges,primarily nutrients nitrogen and phosphorus, result in increased algal growth inmarine freshwater, which leads to the destruction of the coral reef ecosystem. It isimportant to mention here that eutrophication from the discharge of sewage isintensified in the vicinity of the untreated sewage discharge, where, occasionally, lowoxygen values occur (Theodorou 1992).

More recently, Chen and Weng (2000) investigated the effect of wastewater dischargefrom industry, agricultural land and urban areas on the water and shallow sedimentsof Grand Canal in Hangzhou, China. The authors found that canal flow transportedthe contaminants from source areas downstream and caused water pollution. Becauseof this pollution, the canal water, which was suitable for drinking, contained lowoxygen values and was not able to support any aquatic animal life.

For assigning rating scores, a low score may indicate that only a small number ofsurface water bodies are present in the area, water bodies are not polluted, waterbodies are at a distance from unsewered houses, and/or surface water is not used as asource of drinking water. A high score indicates that unsewered houses border thebody of water, there are evidences of increased level of coliform and/or eutrophicconditions, or surface water is used as a source of drinking water.

Current Condition Of Ground Water Quality (GWw)Groundwater is the most important and ecologically safe source of potable watersupply that supplies one-quarter of all the fresh water used by agriculture and asdrinking water in the United States (Zektser 2000; Environmental Quality 1980).Because more than 90% of the rural population and 50% of the total population inNorth America gets its domestic water supply from groundwater, groundwaterpollution is both an environmental and public health concern. (Zoller 1994;

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Environmental Quality 1980). In the U.S.-Mexican border region, groundwaterpollution is also an important environmental and public health concern sincegroundwater deposits are limited and have been overused in many areas (Ganster2000).

Groundwater is normally assumed to be free of pathogenic microorganisms, however,if the soil is permeable or the groundwater table is shallow, adequate filtering ofbacteria may not occur before the wastewater and discharge from onsite system(especially septic tanks) enters the aquifer. For these reasons, discharge of municipalwastewater and septic tanks are significant sources of groundwater pollution(Environmental Quality 1980). Many waterborne disease outbreaks have been causedby contaminated groundwater (Tuthill 1998). Research indicates that one-third of allwater-borne disease outbreaks in the United States from 1971 to 1976 are traced tothe consumption of water from untreated groundwater sources. In the least-developednations, area-wide releases of human sewage, especially near rural groundwater wellsand in burgeoning urban areas, cause the most serious damage, with an estimated onebillion people suffering from waterborne diseases at any one time (Duda 1993).

The parameters necessary for assessing potential for groundwater contamination arethe capacity of the natural soil to attenuate the system effluent; the depth of thegroundwater table; the depth to the impervious layer; drainage, surface topography,and the hydraulic conductivity of the aquifer (Zektser 2000; Canter 1997). Theseparameters should be considered when assigning a score for groundwater quality toan area.

An area that is partially sewered and has a deep groundwater table may be subject to alow potential for contamination, whereas an area that is not sewered at all, has ashallow groundwater table, and has privies and wells is subject to a high potential forpollution. The following table (Table 2) shows a guideline that can be used forassigning a rating score varying between 1 to 10 for the current condition ofgroundwater to estimate environmental impact.

Table 3 can be used for assigning a rating score varying from 1 to 10 for differentuses of groundwater to determine public health impact.

Average Depth Of Groundwater (Dw)Depth of groundwater determines the depth of subsurface material (vadose zone)through which contaminants must travel before reaching the aquifer and the amountof time during which there is contaminant contact with the surrounding media. So,contaminant levels in groundwater are dependent on the processes (i.e., transport andbiochemical processes) occurring in the vadose zone. Canter (1997) used a ratingscore varying from 1 to 10 for the different depths of groundwater to determinenitrate pollution index as given in Table 4.

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Soil Properties (bw)The contamination of soil and groundwater is dependent on the properties of soil. Thesoil (vadose zone) properties important for contaminant transport includes pH,texture, carbon and water-content, permeability, bulk density, and porosity, etc. SoilpH may affect the mobility of some contaminants, while organic content control theabsorption of nonionic compounds (Wilson, et al. 1995). Soils with a high carbonconcentration had minimal leaching of nitrate compared to soils with lower carbonconcentrations. The soil texture refers to the relative percentage of sand, silt, and clay.Canter (1997) used a rating score varying between 1 to 10 for different soil texture todetermine the nitrate pollution index as given in Table 5.

The destruction of natural soil filter will cause groundwater pollution. The failure ofonsite sewage systems is a direct source of soil contamination, since both privies andseptic tanks are built underground and they discharge sewage to the soil if notemptied periodically. If untreated sewage containing toxic organics and pathogenicmicroorganisms are discharged to the ground surface, it may leach to the unsaturatedvadose zone of soil and may move through porous media and penetrate thegroundwater aquifer. The contaminant movement in the porous media is dependenton physical characteristics (i.e., pore size) of the media. Microorganisms that have asmaller diameter than the pore size will pass through the vadose zone, while thelarger diameter microorganism will be strained out. Pathogenic microbes that maymove through the vadose zone ranges in size from 0.020 mm to 12 mm. The presenceof the pathogenic microorganisms in groundwater is responsible for many waterborneoutbreaks causing many thousands of deaths every year (Wilson, et al. 1995).

Presence And Performance Of Onsite System (STw and Ow)Approximately 19.5 million housing units in the United States use onsite disposalsystems. Septic tanks are the most widely used method, while privies are the secondoldest method (McClelland 1980). Approximately one-third of all housing units andabout 25% of all new homes use septic tanks for their sewage treatment in the U.S.(Zoller 1994). However, due to poor locations, poor designs, poor construction andmaintenance practices, and unsuitability of soils for a “natural” purification of theeffluents, septic tanks fail and pollute, or have the potential to pollute, groundwater(Canter and Knox 1985; Zoller 1994). Through leaching of onsite systems, the totalamount of sewage and wastewater discharged to the subsurface in the U.S. is morethan 1 trillion gallons each year. The amount of discharge from septic tank systems iscommonly estimated as 280 liter/cap-day. Based on reported efficiencies of soilabsorption systems, Novotny (1993) confirms the following typical concentrationsentering the groundwater: BOD of 28 mg/L to 84 mg/L; COD of 57 mg/L to 142mg/L; ammonia nitrogen of 10 mg/L to 78 mg/L; and total phosphorus of 6 mg/L to 9mg/L. The other ground water constituents of concern include bacteria, virus, nitrates,synthetic organics, and toxic metals (Liptak and Liu 2000).

If the density of septic tank systems in a locality is greater than the natural ability ofthe subsurface environment to receive and purify system effluents prior to theirmovement into groundwater, than the continuing use of septic tank systems can be a

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major concern to its surrounding environment, especially in high density areas (Reed1996). According to Novotny (1993) septic tanks are the most likely contributor toground water contamination in areas where there is a high density of homes withseptic tanks, the soil layer over permeable bedrock is thin, the soil is extremelypermeable such as gravel, and/or the water table is shallow (i.e. one meter or lessbelow the surface).

Septic discharges often contaminate private wells. The problem can further bemagnified by the fact that in many areas—especially rural communities—theprevalence of septic tanks is paralleled by reliance on private wells for drinking water(Environmental Quality 1980).

Since both the septic tank and privies are onsite systems, the percentage of septictanks (STw), and privies (Ow) are both important criteria. However, the percentage ofseptic tanks criterion can be assigned a higher importance weight than the percentageof privies, since septic tanks discharge the highest total volume of wastewater directlyto the groundwater and are the most frequently recorded sources of contamination ofgroundwater and surface flow (Novotny 1993). Reed (1996) used the “likelihood offailure” approach for assigning rating scores for on-site systems, as summarized inTable 6.

Condition Of Drinking Water (DWw) and Recreational Water (RWw) QualityDrinking water source contamination (DWw) by microorganisms and nitrates is apublic health concern because of its consumption by humans. Studies have found that30% of the mortality and 50% of the morbidity in Delhi, India, was attributed towaterborne infectious diseases (i.e., diarrhea). According to a survey of eightdeveloping countries by the World Health Organization (WHO), Sharma (1988)reported that as much as 90% of children’s deaths could be avoided with safe water(and sanitation). More recently, in 1993, half of Milwaukee’s 800,000 people becamesick and more than 100 people died because of the contamination of drinking waterby the protozoan parasite Cryptosporidium. In 1999, the National Research Councilin the U.S. estimated that this incident now would cost at least $25 billion.

A water source used for recreational purposes (RWw) is also important because of itsdirect contact (i.e., contact through swimming, fishing, boating, etc.) with humans.Untreated sewage and other waste are often discharged into the water bodies,including lakes, coastal areas, and rivers, endangering their use for recreationalpurposes.

Tourism Activities (Vw)Tourism activities (Vw) are a major source of employment and foreign exchange inmany communities. The impact of unsightly polluted drains, bad odors, and “below-standard” sanitary fixtures on tourists can far exceed the potential health hazards.Studies indicate that foreign tourists are more susceptible to pollution due to lack ofimmunity and suffer from gastro-intestinal diseases more than others (Kocasoy 1995).This is why tourism activities are an important factor to consider for the provision of

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sewerage in a community where tourism is, or could be, an important activity.Assigning an importance score for communities that have tourism activities aredifficult, however. Numbers of tourists, length of visible and/or invisible polluteddrains, or pollution of beaches and bathing waters are all possible criteria to consider(Reed 1996). Reed (1996) used the following table (Table 7) for assigning a ratingscore to determine tourist impact for the provision of sewerage.

The most important factors used in assessing environmental and public health impactsinclude the extent of potential service areas (and thus the size of un-serviced areas),wastewater flow and strength generated from the service area, total population, andthe population density of the service area. A synopsis of their estimation is providedbelow. Using existing data, field study, and experimental analysis, other factors canbe approximated.

Extent of Service AreaDetermination of the extent of the service area is important since without the size ofthe area flowrate population density, population growth, and other parameters cannotbe estimated. Typically a sewage collection system is extended or provided in areaswith existing on-site disposal systems and in newly developed residential,commercial, or industrial areas. In this research, an area that competes for sewerage isdefined as “potential service area.” The parameters that are important whendetermining the desirability and extent of the potential service area are:

• Future land use pattern and population size and distribution• Proximity of a service area to an existing sewer system or treatment plant• The conditions of existing on-site systems, sewer collection and/or treatment

systems• The feasibility of using gravity systems• The economic, political, and administrative processes (Shapiro, et al. 1976)

Howe (1971) used a service area of 160 acres in sewerage planning, and Shapiro andRogers (1978) used service area sizes ranging from 0.048 square miles to 3.25 squaremiles in their study. The service areas can also be distinguished based on land usepatterns.

The extent of service areas currently served with on-site systems can be determinedbased on the unit cost of on-site systems and the unit cost of a wastewater collectionsystems. The unit costs of collection systems increase with increasing service areasbecause of area diseconomies and decreasing densities. The unit cost of an on-sitesystem is generally assumed to be constant throughout the service area. The minimumcost is obtained where the marginal cost of collection system expansion is equal to thecosts of onsite systems. The extent of the service area at this minimum cost is theoptimum service area (Shapiro et al. 1976).

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Wastewater Flow And StrengthThe components that make up the wastewater flow from a community are presentedin the following equation:

Qi = (Qd)i + (QI)i + (QI/I)i -------------------------------------------------------------- (31)

where:Qi = the average wastewater flow for design life from area i (m3/day)Qd , QI, andQI/I = wastewater generated by domestic houses, industrial activities, and

contributed from infiltration/inflow from area i respectively (m3/day)

It is frequently assumed that the average rate of domestic wastewater flow generatedfrom a community (including inflow/infiltration) is equal to the average rate of waterconsumption. Generally, about 60% to 80% of water supplied to an area becomeswastewater. Application of appropriate percentages to records from metered wateruse generally can be used to obtain a reasonable estimate of wastewater flow rates,excluding infiltration/inflow. In some cases, however, excessive infiltration, roofwater, and water used by industries that is obtained from privately owned watersupplies make the quantity of wastewater larger than the water consumption from thepublic supply (Metcalf and Eddy 1995). In this research, it was assumed thatwastewater generation per person from unsewered areas in Ensenada equals averagewater consumption per person. So, if the projected population for sewer design life(usually 25 years to 50 years) is known, expected wastewater production at the end ofdesign life can be estimated.

Zukovs and Adams (1980) proposed a model for wastewater generation as a functionof population. Typical industrial wastewater flows are 1,000 to 1,500 gal/acre-day forlight industrial developments, and 1,500 to 3,000 gal/acre-day for medium industrialdevelopments (Tchobanoglous, et al. 1992; Metcalf and Eddy 1991; Gupta 1989).Groundwater infiltration is estimated as being between 100 to 10,000 gallon per dayper inch of diameter per mile (0.01 to 1.0 m3/day/mm/km) of sewer (Tchobanoglous,et. al. 1992; Metcalf and Eddy 1991; and Gupta 1989).

The biochemical oxygen demand (BOD), chemical oxygen demand (COD), and totalorganic carbon (TOC) are the commonly used methods of measuring the organicstrength in wastewater. Among these, the five-day BOD is the most widely usedparameter, which can be estimated using existing methods.

BOD of untreated domestic wastewaters with a strong, medium, and weak strengthare 400, 220, and 110 mg/L respectively. Typical values for the ratios of BOD/TOCand BOD/COD for untreated wastewater ranges 1.20 to 1.50 and 0.50 to 0.65respectively. Typically, the per capita BOD contribution to municipal wastewatervaries from 0.16 to0.26 depending on the type of wastewater (Tchobanoglous et al.1992). This BOD value can be used to compute the organic loading to a treatmentplant for a given population.

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If a treatment plant receives wastewater of different strength from a number of areas,the following equation can be used to estimate the organic strength received by thetreatment plant.

------------------------------------------------------------------------------ (32)

where:Sj = organic strength of wastewater that reaches to the treatment plant j (kg/m3)Si = organic strength of wastewater from area i (kg/m3)Qi = wastewater flow rate generated by area i (m3/day)

Population Growth EstimationA wastewater collection system is usually designed for 25 years to 50 years, andtreatment plants are usually designed for the same. There are four broad categories toforecast population growth for the sewer design year, including: graphical extension,mathematical curve fitting, ratio and correlation, and component methods. Graphicalextension methods include arithmetic growth, and declining growth method. Gupta(1989) presents an excellent discussion of all these techniques. The mathematicalcurve-fitting approach known as “Mathematical Logistic Curve” method is popularbecause it is relatively easy to apply. According to this approach, population at anytime from an assumed area is:

PP

aet

sat

bt=+1

---------------------------------------------------------------------------- (33)

where:Pt = population at any time tPsat = saturation populationa, b = empirical constants

However, due to the lack of data for saturation population and other data for theunserviced areas, Equation 32 can’t be used to forecast population in Ensenada.Instead, the geometric growth method is used to forecast population, since therequired data (i.e., current population and the constant growth rate) for this methodare known. The current growth rate (percent of current population) for all areas inEnsenada is 4%. The current population of different areas in Ensenada will beprovided in the following sections. According to this approach, rate of growthpopulation at any time from an assumed area is:

-------------------------------------------------------------------------- (34)

where:Pt = population at any time tPo = present population (at t = 0)t = period of projection (for sewer design life of 25 years to 50 years)

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Reduction In Environmental Public Health And Other ImpactsThe variables for reduction in environmental and human health impacts (RE, RH, andRO) once an area is serviced with a sewer collection system are difficult to assess.The reduction is related to the actual percentage of area that is already connected tothe new collection system and the number of households that choose to make theconnection to the system. If the use of on-site collection systems continues, then thereduction rate is minimal. If an area is developed with paved roads and has limitedseptic tanks and privies, then reduction may be almost 100%. The reduction variableshould be given great consideration due to its significant affect on the determinationof a wastewater collection plan. However, the reduction of environmental and publichealth impacts can be calculated using the following equation:

RE, RH, RO = [(Seweredi) + (Connectedi)+ (1-Septici) + (1- Priviyi) +(1- Servicedi)+ (1- Othersi)]/6------------------------------------------------------------------------- (35)

where:RE = reduction in environmental impacts (%)RH = reduction in human health impacts (%)RO = reduction in other health impacts (%)Sewered = percentage of the area likely to be sewered (%)Connected = percentage of households likely to be connected to sewer lines (%)Septic = percentage of septic tanks still in use (%)Privy = percentage of privies still in use (%)Serviced = percentage of area already sewered, (%)Other = percentage of other types of onsite systems still in use (%)

SUMMARY

The methodology discussed above can be used to estimate the projected benefit froma sewerage project. This approach includes only the general factors and parameters onwhich a sewerage project may have impact if provided. The decision makers may addor subtract any factors that may be pertinent to a particular project. Once the cost isidentified and the benefit is estimated, the DP algorithm could be used to identify thebest area, or areas, for the provision of sewerage. An overview of this methodology ispresented in Figure 5.

RESEARCH FINDINGS

This section discusses the wastewater system in Ensenada, Mexico. Ensenada is thethird largest city in Baja California with a population of approximately 370,000 and agrowth rate of 4%. Children from the ages of 0 to14 years, make up 34% of thepopulation, while 62% are 15 years to 64 years (Central Intelligence Agency 2000).Tourism, fishing, agriculture, and industrial activities are the most important factorsin the economical development of the city. An established wastewater collectionsystem is crucial for the continued growth of tourism and the fish processing industryof Ensenada. Ensenada is currently experiencing new development growth, which isnot paralleled with new sewage collection to handle the increased demands.

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Existing Wastewater Collection and Treatment System in EnsenadaThe city of Ensenada is currently faced with crucial wastewater managementdecisions. Collection of wastewater is not provided through the entire city. Someareas have complete sewer systems, while other areas are partially developed and/orare only serviced by septic tanks or outhouses. A total of 26 areas have beenidentified as needing some kind of wastewater collection system. These areas mayconsist of more than one colonia.

Currently, Ensenada has three wastewater treatment plants: El Gallo, El Narajo, andEl Sauzal. El Gallo is located in Colonia Carlos Pacheco, near the coastal bay. ElNarajo is located at the southeast of El Gallo, near Colonia La Esperanza and ColoniaJose Maria Morelos y Pavon. El Sauzal is in northern Ensenada near ColoniaPedregal Playitas. The city also has a plan to construct another treatment plant it willcall Pantenoes. Figure 6 shows the existing wastewater treatment facilities inEnsenada.

The El Gallo treatment plant was constructed in 1972 with a capacity of 250liters/second and was further extended to treat a wastewater flow rate of 350liters/second. Currently, this plant receives wastewater with a flow rate of 400liters/second to 450 liters/second and a BOD of 450 mg/L, even though it wasdesigned to treat a BOD of 350 mg/L. Due to this excess BOD loading, the plantshows a strong deficit in its treatment, and consequently the effluent that isdischarged needs further treatment.

Wastewater from fishing industries in the Colonia Ejido Chapultepec are carried tothe El Gallo treatment plant for pretreatment and then this wastewater is carried to theEL Naranjo treatment plant for further treatment. After treatment from El Naranjoplant, the effluent is discharged to the ocean with a BOD of about 100 mg/L and aflow rate of 350 l/sec. The city of Ensenada permits discharge of 75-100 mg/L ofBOD.

El Naranjo, the newest treatment plant, consists of preliminary, primary, and secondarytreatment systems; its total design and construction cost is $11 million. This plant iscurrently operating below design capacity, leaving excess capacity for additionalwastewater treatment for future collection system expansion.

Due to tourist activities, Ensenada is growing rapidly and generating additionalwastewater that requires treatment. In addition, the existing fishing industries generatehigh-strength wastewater that is discharged directly to the bay without any treatment. Totreat the additional wastewater from the developing communities and industrial areas, anadditional capacity of 500 liters/sec will be added to El Naranjo plant in near future,resulting in a new total capacity of 1,000 liters/sec. The other design criteria for this plantare outlined in Table 8.

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El Sauzal treatment system consists of a pretreatment process, biological process, sludgeextraction and thickening process, and a disinfections process. The current capacity ofthis plant is 60 liters/sec, but it receives only 45 liters/sec. It is projected that El Sauzalwill expand its capacity by 22 liters/sec, totaling a flow capacity of 82 liters/sec. Thetreatment efficiency of this plant is about 95% and thus, effluent that returns to the bay isof excellent quality and does not need any further treatment.

Due to the available wastewater treatment capacity in both El Sauzal and El Naranjo, theexpansion of a sewerage collection system is possible. The additional wastewater fromdeveloping communities and industries can be collected and carried to either of thesetreatment plants for adequate treatment.

Collectors In Sewerage SystemsThe state public service commission for Ensenada, Comision Estatal De ServiciosPublicos De Ensenada (CESPE), has allocated resources and initiated plans for theimprovement of potable water and wastewater management for the fiscal year of 1996-2003. This improvement plan includes the provision of drinking water sources in someareas, construction and/or rehabilitation of existing collectors, and providing sewerage insome un-serviced areas. Table 9 shows the approximate costs for construction ofcollectors.

Unsewered AreasThere are 26 areas competing for wastewater collection system in the city of Ensenada(Figure 7). The demographics, size, and population densities of the areas varyconsiderably. The physical characteristics range from flat topography to steep hillsides.All of the unsewered areas have unpaved roads with the exception of Colonia California,Moderna Oeste, and Fraccionamiento Chapultepec. Approximately half of the areas areentirely unpaved and the other half have a combination of paved and unpaved roads.Some areas have drains created from raw wastewater discharge. The average family sizein these areas is 4.5 persons per family, average per capita water use is 250 liters/day andbiological oxygen demand strength is 0.07 lb/person/day. Wastewater managementtechniques in these areas vary. Primarily, septic tanks and outhouses service most all theunsewered areas. These areas are provided electricity (approximately 90%) and gas(almost 100%). The unsewered areas can be divided into developed and developing(under residential and/or commercial development) areas as listed in Table 10.

The following information was obtained from Ensenada officials:

Both areas 1 and 2 are developed urban residential areas with flat topography. Area 1consists of Colonia Fraccionamiento Playa Hermosa, Fraccionamiento Acapulco, andRancho Bonito with an approximate size of 0.51 square kilometers and a populationdensity of 1,226 persons/km2. Approximately 50% of Fraccionamiento Acapulco andFraccionamiento Playa Hermosa have drainage for removal of excess water, but there isnone in Rancho Bonito. There are no paved roads in these colonias in area 1.

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Area 2, identified as Fraccionamiento Loma Linda, is next to area 1 and approximately0.32 square kilometers with a population density of 4,929-persons/km2. Approximately50% of this area has drainage and paved roads.

Septic tanks are used in areas 1 and 2 (approximately 50% of the areas) for wastewatermanagement. There is a mix of tourist and commercial activities among the residentialhouses in these areas. Since areas 1 and 2 are located along the beach of Bahia de TodosSantos, there is a high potential for bay water contamination if these areas are notsewered. El Gallo wastewater treatment plant is located north of these areas. An 18-inchsewer line runs along the road Calle Acapulco in Fraccionamiento Acapulco and connectsto a direct sewer line to El Gallo. Along the east boundary, a 15-inch sewer line runsdown the road Avenida Reforma.

Area 3 is Colonia Abelardo Rodriguez. This is a flat area in downtown that consists ofauto repair shops and residential homes. The size of this area is approximately 0.052square kilometers and it has a population density of 3,852-persons/km2. Approximately50% of this area is serviced by septic tanks, the remainder is sewered. Approximately,50% of this area has paved roads. This area also has drainage for surface water removal.An 8-inch sewer line runs along the south boundary of the area following the road CalleDelante to El Gallo treatment plant.

Area 4, Fraccionamiento California, is 0.132 square kilometers in size. Approximatelyhalf of this area is serviced by septic tanks and the remainder is sewered. This area is adeveloped urban area with little slope, visible drainage, and paved roads, and it isbordered by sewer lines. On the southwest corner of the area, a connection to a 10-inchsewer line is available that runs along the road Calle Esmeralda to El Gallo treatmentplant.

Area 5, identified as Colonia La Esperanza, is a popular urban area with little slope andno visible drainage on the ground. It is approximately 0.0435 square kilometers in sizewith a population density of 4,595-persons/km2. This area has paved roads (50%) and isvery close to the wastewater treatment plant El Naranjo. It is estimated that 50% of thearea is serviced by septic tanks and 25% by privies.

Area 6, Colonia Jose Maria Morelos y Pavon, is developed yet features hills with steepslopes. The size of this area is 0.67 square kilometers and it has a high population densityof 4,827-persons/km2. Approximately 50% of this area is serviced by septic tanks and theremaining 50% by privies. There are no paved roads or visible drainage in this area. Thesewered area, Colonia Granjas El Gallo, borders the west. Both areas 5 and 6 are locatednorth of El Naranjo wastewater treatment plant.

Area 7 consists of several colonias, including Fraccionamiento Mar, Colonia 17 de AbrilColonia Aguajito, Colonia Libertad, Colonia Emiliano Zapata, Ampliacion Aguajito,Rancho Bonito, and Francisco Villa. This area is the third largest unsewered area with anapproximate size of 1.57 square kilometers and a population density of 2,031-persons/km2. Colonias Emiliano Zapata and Franciso Villa is an urban area developed

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over hills, while the rest of the colonias have a slight slope. Approximately, 50% septictanks and 50% privies service the entire area. Since a groundwater well in Colonia 17 deAbril Colonia Aguajito supplies drinking water to part of the population, there is apossibility of well water contamination through groundwater pollution from on-sitesystems. The sewage drains into the streets attracting mosquitoes, flies, and cockroachesand causing environmental and public health concerns. Welts and granulations arebreaking out on children’s faces, which is a sign of the poor health conditions in this area.There is also a drainage pathway that runs through the middle of this area.Fraccionamiento Marquez de Leon is sewered on the west side of this area.

Area 8, Colonia Industrial, is approximately 0.20 square kilometers, and has a populationdensity of 6,929-persons/km2. This area is serviced entirely by septic tanks. The ColoniaFracc. Mariano Marquez on the west side of this area is sewered.

Area 9 is Colonia Las Margaritas and Lazaro Cardenas II. It is 0.146 square kilometers insize with a high population density of 5,207-persons/km2. Both septic tanks andouthouses service half of the area, and the remaining half is sewered. There are no pavedroads in this area. Visible drainage is present.

Area 10 consists of Lazaro Cardenas I, and 2 de Septiembre, with a size of 0.149 squarekilometers and a population density of 6,264-persons/km2. A 12-inch sewer line runs nearthe northwest side of this area. Both areas 9 and 10 are urban areas developed over small-sloped hills. The majority of the development of area 10 is on a plateau.

Area 11 is rapidly developing and is the largest area (2.38 square kilometers andpopulation density of 638-persons/km2). It consists of Ejido Ruiz Cortinez and Cumbresde la Presa. Approximately, 75% of the area is serviced by septic tanks while outhousesservice the remaining 25%. This area is located along the Emilio Zamora Reservoir nearthe dam with a slight slope toward the reservoir. Due to the existing onsite system and theslope, potable water withdrawn from dam outflow could be contaminated from thesewage discharge. The soil in the developing areas is sandy, which could easily beeroded. Groundwater contamination could also be a concern due to the existence of onsitesystems and the presence of the city landfill in this area.

Area 12, consisting of the colonias of Relotificacion Lomitas and Lomitas Ensenada, isconnected to area 11 along Geranios Street. This area is 0.53 square kilometers with apopulation density of 6,394-persons/km2. This area is developed over the hills, withapproximately 75% of the area serviced by septic tanks and the remaining 25% byprivies. Drainage is visible above ground in this area. An 8-inch sewer line runs along theroad Calle Prof. Lic. David H. Sokolow on the south and along Calle Alameda on thewest border.

Area 13, identified as Colonia Popular 1989 is connected to area 11 along Ave. de LasAnimas. This area has a size of 1.75 square kilometers and a population density of 2,202-persons/km2. Approximately 50% of this area is being serviced by septic tanks andprivies service the remaining 50%. Visible drainage above ground is common is this area.

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Area 14 consists of Las Rosa, Bronce, 28 de Agosto, Pro-Hagar, and 3 de Octubre. Thesize of this area is 0.31 square kilometers and it has a population density of 5,155-persons/km2. This area is serviced entirely by septic tanks. However, a 16-inch sewer linethat runs from east to west through this area is eventually connected to El Gallo treatmentplant. This area seems to have the most developed sewer line system among allunsewered areas.

Area 15 consists of II-III-IV Fovisste, Fraccionamiento Piedras Negras, andFraccionamiento Mexico and is connected to area 14. The size of this area is 0.31 squarekilometers with a population density of 3,872-persons/km2. It is estimated that 75% ofthis area is serviced by septic tanks while the remaining 25% is sewered. A 30-inch sewerline runs along the road Avenida Reforma on the east border. Areas 14 and 15 should besewered at the same time, due to their proximity and existing interconnected system.

Area 16, identified as Los Laureles, has a size of 0.03 square kilometers with the highestpopulation density of 21,113-persons/km2. This is a flat area in downtown that consists ofa strip mall and apartment buildings. The large population density is attributed to theapartment buildings. Approximately half of this area is serviced by septic tanks while theremaining is sewered. A 30-inch sewer line runs along the road Av. De Los Olivos on theeast border, while an 8-inch sewer line runs along Avenida Reforma on the west side.

Area 17 consists of Balcones de la Presa and Rogelio Appel, it has a size of 0.65 squarekilometers and a population density of 612-persons/km2. This developing urban area islocated within the hills along the shoreline of the La Presa Emilio Zamora reservoir.Visible drainage that could potentially contaminate the reservoir as well as thesurrounding soils exists in this area. Approximately 50% septic tanks and 25% priviesservice this area.

Area 18, identified as Granjas Chapingo, has a size of 0.07 square kilometers with apopulation density of 2,898-persons/km2. Approximately 50% septic tanks and 50%privies service this area. However, a developed sewer systems exists along the southborder of this area.

Area 19 consists of La Joyita, Fraccionamiento Las Penitas, and Carlos Salinas de Gortari(not shown on the map). The size of this area is 0.30 square kilometers with a populationdensity of 2,496-persons/km2. Approximately 50% septic tanks and 25% privies servicethis area.

Area 20 is identified as Colonia Loma Linda. This area is 0.74 square kilometers in sizewith a population density of 2,346-persons/ km2. Approximately, 50% of this area isbeing serviced by septic tanks, and privies service the remainder.

Area 21 is identified as Sexto Ayuntamiento. It is 0.75 square kilometers and has apopulation density of 1,256-persons/km2. Septic tanks and privies service this area.

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Area 22 is Colonia Bellavista, with a size of 0.58 square kilometers and a populationdensity of 2,164-persons/km2.

Areas 19, 20, 21, and 22 are developing urban areas located in the hills in the vicinity ofEl Sauzal Wastewater Treatment Plant. Visible drainage and trash is common in all thesefour areas.

Area 23, identified as Moderna Oeste, is a developed area. The majority of this areaappears to be serviced by a collection system. This area is 0.15 square kilometers and hasa population density of 1,353-persons/km2.

Area 24 consists of Coronita, Pedregal Playitas, and Quintas Papagayo and is locatednear El Sauzal Treatment Plant, and the university UABC (Universidad Autonoma BajaCalifornia). This area is 0.83 square kilometers and it has a population density of 765-persons/km2. The Colonia Pedregal Playitas is located in the hills, which have a mediumslope, while the Colonia Coronita is located near the beach of Bahia de Todos Santos andattracts tourists. Approximately, 75% septic tanks and 25% outhouses service this area.

Area 25, Fraccionamiento Chapultepec, is 0.40 square kilometers in size and has apopulation density of 1,736-persons/km2. This area is serviced primarily by septic tanks.Areas 23, 24 and 25 are located near El Sauzal treatment plant. There are several luxuryresidences in this area with high-income residents.

Area 26 consists of Ampliacion Gomez Morin and the colonia Gomez Morin and islocated four kilometers east of downtown. This area is 0.69 square kilometers and has apopulation density of 366-persons/km2 (smallest density). Approximately 50% of thisarea is serviced by septic tanks and privies service the remaining 50%. There is notreatment plant nearby to receive this area’s sewage. Although this is a low-incomecommunity, due to its distance from the treatment plant, this area might result in a higherconstruction cost and consequently might be at the bottom of the list to receive sewerage.

Table 11 summarizes the characteristics and cost to provide sewerage in each area andTable 12 summarizes an estimate of wastewater generation for the sewer design life inEnsenada.

Water Supply SourcesThe people of Ensenada are supplied water from La Presa Emilio Zamora reservoir aftertreatment for household use. Since the unsewered areas Fraccionmiento Mar and ColoniaEsperanza Y Amplacion surround this reservoir, these areas may pollute the reservoirwater by wastewater discharge from residential houses or from leaking septic tanks andouthouses. However, people are prohibited from using the reservoir for recreationalpurposes, which eliminates a potential pollution pathway.

Due to the potential for human health concerns, the limited water sources in Ensenadamust be preserved, and the water quality of the reservoir must be maintained. Collectingsewerage from the bordering communities may help eliminate fecal contamination to the

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reservoir, which in return eliminates potential waterborne diseases in the city’s potablewater source.

The ocean bay Bahia de Todos Santos is the primary discharge point for all treatmentplant effluent discharge, runoff streams, and rivers in Ensenada. The physical nature ofthe bay segregates the bay waters from the Pacific Ocean. This limits the dilution thatwould occur otherwise. High BOD and coliform levels may result due to the discharge ofcontaminated water. The bay is used for tourism and recreational activities, and thatdirect exposure of humans to the bay waters poses a health concern.

Summary From Wastewater System In EnsenadaThe developed and developing areas of Ensenada need collection systems for wastewatertreatment and disposal. The current amount of wastewater (which is not currentlycollected for treatment) generated from unserviced areas is approximately 88 liters/sec.This amount (flow rate) was calculated (excluding inflow/infiltration) assuming anaverage water consumption of 250 liters/cap/day. That water consumed becomeswastewater and will reach the sewer once a collection system is in place. In some cases,however, excessive infiltration, roof water, and water used by industries obtained fromprivately owned water supplies make the quantity of wastewater larger than the waterconsumption from the public supply (Metcalf and Eddy 1995).

Among existing three treatment plants in Ensenada, El Naranjo and El Sauzal haveexcess capacity. Currently, El Sauzal has an excess capacity of 37 liters/sec and ElNaranjo is operating below its design capacity. As well, due to rapid industrial growth ofthe city an additional 500 liters/sec of capacity will be added to El Naranjo treatmentplant in the near future. However, El Gallo treatment plant has a deficit of 100 liters/secand will end its operation soon. Once El Gallo ends it operation, El Naranjo treatmentplant will be able to remove the deficit. So, the current total excess capacity of thetreatment plants in Ensenada is approximately 87 liters/sec and thus will be able to acceptadditional wastewater from the unsewered areas for treatment and disposal.

Applications of DP approach to the Ensenada wastewater systemThe aforementioned methodology (DP approach and impact assessment) will be appliedto Ensenada wastewater system, to obtain the best collection system plan. In order to usethe DP approach, benefits need to be estimated for each area. The conceptual frameworkfor benefit estimation is described in the previous sections. The following section shows amechanism to estimate environmental and public health impacts that may occur in eachunsewered area if sewerage is not provided in Ensenada.

Estimation of impacts of unsewered areasTo estimate the impacts of unsewered areas, the conceptual framework discussedpreviously is used here. Each area is awarded a score for each environmental and publichealth criterion and that score is multiplied by the weight assigned to that criterion. Thisframework is very similar to the “weighted matrix approach” developed by Reed (1996).However, due to the availability of data, the previously described framework is simplifiedto fit the conditions in Ensenada. In this estimation, a scale of 1 to 10 (with 10 being the

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most crucial and an assigned weight of 1, 2, or 3 (with 3 being most important, 2 beingmediocre, and 1 being least important) are used. The criteria determined to represent theenvironmental impacts are a direct reflection of the existing environmental conditions ofthe area and indicates the potential environmental pollution. The criteria used for thepublic health impact assessment also reflects the status of existing public conditions andthe contaminants’ human exposure potential . The same criteria may be represented inboth environmental and public health impacts carrying different weight. Table 13displays a guide to assign scores for different environmental and public health criterionand Table 14 summarizes the corresponding weight for different factors applicable to theEnsenada system.

The environmental impact (Ei) and public health impact (Hi) for an area is an average ofeach of the weighted criterions. A representative equation for each impact applicable tothe conditions in Ensenada is given below:

Ei = (drdw + SWrSWw+ GWrGWw + STrSTw +OrOw + brbw)/NE -------------------- (36)

Hi = (rrrw+ CHrCHw+ araw + PWrPWw + DWrDWw+ RWrRWw ------------------ (37) + OwOr + dwdr)/NH

The definition and theoretical basis for the above factors were discussed above. Thejustification for assigning importance weight and rating score for each criterion forEnsenada is discussed below.

The first environmental criterion – whether there is visible sewage draining above ground(dw) – was given the highest weight of 3, due to its high potential to contaminate variousenvironmental media such as soil, groundwater, surface water, and vegetation. Drainageon the ground, whether gray waste or sewage, is unhealthy when humans – especiallychildren – come into contact with it. Due to this potential for exposure to humans, thiscriterion was assigned a weight value of 2 in the public health impact equation. Forscoring purposes, a value of 10 is assigned if there is evidence of visible sewage drainingabove ground, and 0 is assigned if there is no evidence of visible sewage draining.

Potential contamination of a surface water (SWw) source is an environmental concern.The introduction of wastewater to a body of water greatly threatens the water quality byincreasing the total coliform level and nutrient loading, which causes eutrophication(Metcalf and Eddy 1991; Chapra 1997). Eutrophication directly affects the oxygen andcarbon dioxide levels, which threatens the survival of aquatic life. Due to this severeenvironmental concern, the highest weight, 3, was assigned to this criterion inenvironmental assessment. For scoring purposes, a low potential (low score) forcontamination may indicate that only a small number of surface water bodies are presentin the area and/or are at a distance from the unsewered houses. A high potential indicatesthat unsewered houses border the body of water. Evidence of contamination may includehigh levels of coliforms or eutrophic conditions.

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The potential for contamination of groundwater (GWw) is both an environmental andpublic health concern. Onsite sewage systems can be a major source of groundwaterpollution, especially in densely populated areas. For the conditions in Ensenada, thegroundwater criterion was only used for assessing the environmental impacts due to thefact that groundwater is not used or consumed by the people, limiting the public healthconcern. A weighted factor of 1 was used for an environmental impact assessmentbecause of the limited amount of data obtained on groundwater characteristics of theEnsenada region.

The potential for soil contamination (bw) is an environmental concern. The failure ofonsite sewage systems is a direct source of soil contamination. Both privies and septictanks are built underground and they discharge sewage to the soil if not emptiedperiodically. Although a high percentage of onsite systems are currently used inEnsenada, the failure rate of these systems is not known. For this reason, the soilcontamination criterion was assigned a weighted value of 0 for the conditions inEnsenada. Due to the limited data gathered, it is assumed that the potential forcontamination to the soil is equal throughout all the areas and can be neglected.

The first public health criterion of population density (rw) was given the highest weightedvalue of 3 due to its strong correlation with public heath quality. The amount ofwastewater and other waste generation have a direct correlation to the density of peoplein a community. Generally, the higher the population density, the greater the healthhazard from poor sanitation and higher the probability of on-site sanitation systemfailures (Reed 1996). The projected total population (Pw) criterion was not used in thepublic health assessment due to the existence of a constant growth rate (a growth rate of4) in all unsewered areas in Ensenada.

The children density (CHw) in a community is an important public health factor sincechildren are more susceptible to illnesses and waterborne diseases than adults that arepresent in poor sanitation areas. Since both the percentage of children and size of an areamay vary from one community to another, it is likely that children density will bedifferent among all areas. Due to its high importance for public health, children densitywas assigned a weighted value of 2.

The poverty or percent of poor (aw) in a community is important since poor peoplegenerally are more susceptible to illnesses and waterborne diseases than wealthy peoplein poor sanitation areas. As well, a well-established correlation exists between povertyand environmental degradation. However, due to the lack of quantitative data on povertyin Ensenada, a weighted value of 0 was assigned to this criterion.

The potential for contamination of potable water (PWw) is dependent on the source. Apotable water source may be a reservoir, river, stream, lake, or groundwater. Potablewater contamination is important because it is used directly by humans. The only potablewater source in Ensenada is La Presa Emilio Zamora reservoir. Due to the highimportance of this reservoir as a potable water source, the highest weighted value, 3, wasassigned in assessing its public health impact.

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The potential for contamination of both the drinking water (DWw) and surface water usedfor recreation(RWw) are dependent upon the water source. Drinking water sourcecontamination is important because of its consumption by humans. A water source usedfor recreational purposes is also important because of its direct contact (i.e., contactthrough swimming, fishing, boating, etc.) with humans. The people of Ensenada usecommercially available bottled water as their drinking water, and as a result, the weightedvalue for “drinking water source contamination” criterion was assigned a value of 0. Theonly water body allowed for recreational use is the Bahia Todos Santos, the PacificOcean bay. All, streams, rivers runoff from ditches, and reclaimed water from wastewatertreatment plants are eventually discharged into the bay. So, it is likely that all the areasmay eventually discharge contaminated water to the ocean bay. Thus, contamination ofocean water is a concern, but individual impact from each unsewered area on the baycannot be assessed accurately. As a result, a weighted value of 0 was assigned to thiscriterion.

The failure of septic tanks (STw) is a big concern to their surrounding environments sinceseptic tanks are frequently sources of groundwater pollution. The percentage of privies(Ow) servicing an area is both an environmental and human health concern. A privy thathas failed and is leaking sewage may pollute the soil and groundwater. However, thepercentage of septic tanks (STw) criterion should be assigned higher importance weightthan that of percentage of privies (Ow), since septic tanks usually discharge the highesttotal volume of wastewater directly to the groundwater. So, a weighted value of 2 wasgiven to represent the importance of the “percentage of septic tank” criterion, and aweighted value of 1 was assigned to “percentage of privies” in environmental impactassessment. For the human health impacts, “percentage of privies” also received aweighted value of 1, while the “percentage of septic tank” criterion was ignored in publichealth impact assessment.

Tables 15 and 16 summarize the assigned scores for each environmental and publichealth criterion and the resulting impacts respectively for each area.

Reduction In Environmental And Public Health ImpactsThe conceptual framework for estimation of reduction impacts (environmental, publichealth, and others) is described above. For this research, it is assumed that if sewerage isprovided in Ensenada, the expected reductions of environmental and human healthimpacts are equal. Due to the lack of necessary data, the reduction values for all theunsewered areas in Ensenada were estimated (using available data and judgment) ratherthan calculated. It is important to mention here that due to the lack of cost data, only 16areas among 26 are used to find the collection plan. The estimated reduction values forthese 16unsewerad areas are given in Table 17.

Example Of Impact Scoring (Area 17)The scoring approach discussed in the previous section is used here to assign ratingscores for environmental and public health impacts for unsewered area 17. Area 17 is adeveloping urban area located in the hills along the shoreline of the La Presa Emilio

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Zamora reservoir. This area consists of Colonias Balcones de la Presa and Rogelio Appel.It is 0.65 square kilometers and has a population density of 612-persons/km2. Visibledrainage exists in this area that could potentially contaminate the reservoir as well as thesurrounding soils. The area is serviced by 50% septic tanks and 25% privies. Eachcriterion will be assessed and then the score of the environmental impact will becalculated, followed by the public health impact.

Five criteria are used to estimate environmental impacts that could occur if sewerage isnot provided in area 17. These factors are: visibility of sewerage draining above ground(dr), potential for surface water quality pollution (SWr), potential of groundwaterpollution (GWr), percentage of septic tank use (STr), and percentage of privy use. Theassigned weights for environmental impact assessments for these factors are: (a) dw = 3,(b) SWw = 3, (c) GWw = 1, (d) STw = 2, and (e) Ow = 1 (according to Table 13).

The first criterion asks “is there visible sewerage draining above ground?” This indicatesthat a score of 10 should be assigned to dr for area 17 according to Table 13.

The second criterion scores the potential for contamination of surface water. Area 17borders La Presa Emilio Zamora reservoir, indicating the potential for contamination tothe reservoir water. Now the severity must be assessed. Due to the proximity of the areato the reservoir, and the high percentage of septic tanks and privies, and the evidence ofsewage draining on the ground, there is a high possibility of contamination of reservoirwater. However, since no water quality testing was conducted on the reservoir water todetect the evidence or extent of contamination, the highest score of 10 cannot beassigned. Instead, a score of 9 for SWr was assigned for area 17.

The third and fourth criteria are indications of the percentage of privies (Or) andpercentage of septic tanks (STr) being used in the area. Approximately, 25% of area 17uses privies and 50% uses septic tanks for wastewater management. This correlates to ascore of 4 for Or and 6 for STr (according to Table 13).

The fifth criterion is the potential for contaminating the groundwater (GWr). This areahas a high percentage of septic tanks (50%) and privies (25%), and these onsite systemshave high potential to contaminate groundwater through permeation. In addition, the soilin this area is primarily sand, which has a high permeability, as shown in Figure 9. Thisarea also has many unpaved roads as shown in Figure 10.

With sewerage draining above ground and into the streets, the presence of unpaved roadsallows the sewage to seep into the soils and may contaminate both the soil and eventuallythe groundwater. However, since no groundwater quality testing was conducted forevidence and/or extent of contamination, the highest score of 10 cannot be assigned.Instead, a score of 5 for GWr was assigned for area 17 assuming some potential ofgroundwater pollution. The calculation for the environmental impact of Ensenada is asfollows:

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Ei = (drdw+ SWrSWw+GWrGWw + STrSTw + OrOw)/NE

E17 = [3(10) + 3(9) + 1(5) + 2(6) + 1(4)]/5 = 15.2.

So, the environmental impact for Area 17 is 15.2.

Five criteria were used to estimate the public health impact that could be reduced ifsewerage is provided in area 17. They were: population density (rr), children density(CHr), potential for contamination of potable water source (PWr), visibility of seweragedraining above ground (dr), and percentage of septic tank use (Or). The assigned weightfor public health impact assessment for these factors according to Table 14 are: rw = 3,CHw = 2, PWw = 3, dw = 2, Ow = 1.

The first and second criteria is population density and children density respectively. Thepopulation density for Area 17 is 612 persons/km2 and children density is 208 child/km2,which correlates to a score of 2 for both rr and CHr (according to Table 13).

The third criterion is the potential for contamination of the potable water source. Asmentioned before, the only potable water source in Ensenada is La Presa Emilio Zamorareservoir. Surface water contamination potential has already been assigned a score of 9(SWr = 9) in environmental impact assessment. For public health assessment, the samescore will be used for PWr, assuming the reservoir water has a similar effect on area 17 inenvironmental and public health quality.

The fourth and fifth criteria are the presence of visible sewerage above ground and thepercentage of privies used to service the area. Both of these criteria have previously beenassigned scores. As mentioned before, the score does not change whether it’s used forcalculating the environmental or public health impact, only the weight may change. So,the score for dr is10 and for Or is 4. The calculation for the existing public health impactsis as follows:

Hi = (rw rr + CHwCHr + PWwPWr + OwOr + dwdr)/NH

H17 = [3(2) + 3(9) + 2(2) + 2(10) + 1(4)]/5 = 12.2.

So, the public health impact for area 17 is 12.2.

Solution ApproachA few different approaches can be used to analyze the results applying theaforementioned DP algorithm. In the first approach, a resulting matrix is calculated,assuming funds and capacity to treat wastewater in treatment plants are available toprovide sewerage in all sixteen areas. This solution represents the optimal solution. In thesecond approach, solutions are obtained for different costs. This approach enables theuser to choose an optimal and sub-optimal collection plan for the given ranges of cost. Inthe third approach, the funding constraint and capacity constraint are applied from thebeginning of the solution. This results in a wastewater collection plan for the indicated

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total cost and capacity. An effort was also made to rank the areas once they are selectedfor the provision of sewerage.

To obtain a solution for Ensenada wastewater system, the DP algorithm was solved as abackward dynamic programming using a Microsoft Excel spreadsheet. The DP algorithmwas also converted into a Visual Basic (VB) program. However, only 13 areas among 16were used for the Visual Basic program due to the size constraint of the worksheets,which were used for the solution output. Research is in progress to convert the algorithminto a VB program for all sixteen areas.

Optimal Solution (First Approach)A desirable optimal wastewater collection plan was determined assuming funds andcapacity are available to provide sewerage in all 16 areas. In this approach, an availablefunding of $80.7 million and a capacity of 522 l/sec were assumed. Results show that theoptimal collection plan for the available funds is to expand sewerage to all the areas for aminimum impact of 84.16 units. Table 18 shows the costs to provide sewerage,corresponding required capacity (wastewater flow), and accrued benefit for all 16 areasof Ensenada.

Figure 11 shows the relationship between the costs of sewer collection and correspondingbenefits. The figure shows that the benefits increase faster as more funds are allocated forwastewater collection. But as cost increases, the curve begins to flatten out at the end,where the slope of the curve gradually lessens, indicating that allocating more money forcollection system expansion does not drastically increase the benefits.

As more wastewater is collected, more funds will be required, resulting more benefits asseen in Figure 12.

Optimal/Sub-Optimal Solution (Second Approach)This approach enables the user to obtain optimal and sub-optimal solutions for a similarfund range. Table 19 shows the matrix of optimal and sub-optimal solutions for threedifferent funds.

Provision Of Sewerage At Different Funding Constraint (Third Approach)This approach enables the user to obtain a matrix of wastewater collection plans for anyavailable funds and capacity of treatment plants. Once the matrix is obtained and theareas are selected for the provision of sewerage, they can be internally ranked on thebasis of a benefit-over-cost (B/C) ratio for prioritization. Tables 20, 21, and 22 show thewastewater collection plan matrix for available funds and capacity, and the ranking of theareas, respectively.

Sensitivity AnalysisA sensitivity analysis was conducted to examine which of the model parameters (i.e.,environmental impact, public health impact, and impact reduction) used in quantifyingbenefits are the most important in determining wastewater collection plan for Ensenada.These three parameters were altered individually for one of the sixteen areas while

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remaining parameters were held constant. The resulting system benefits were comparedfor each parameter change.

Figure 13 shows the change of system benefits as environmental and public healthimpacts are changed for area 1. According to Figure 13, the environmental and publichealth impacts have similar influence on the system benefits, as these curves haveidentical slopes for both provisions of sewerage. The same results can be obtained for anyareas.

Figure 14 shows the change of system benefits as impacts reduction values are changedfor area 1. The figures shows the system benefit curve has constant slope when area 1 isnot provided sewerage. So, when an area is not provided sewerage, change of impactreduction value has no effect on system benefit, since no impact would be removed.However, when area 1 is provided sewerage, the system benefit curve changes sharplywith a negative slope (i.e., slope = 8.16). So, it can be concluded that when an area is notprovided sewerage, change of impact reduction value has a significant effect on systembenefit. This phenomenon is true for any area.

As seen in Figures 13 and 14, the environmental and public health impacts have identicalinfluences on the system benefits, whereas the impact reduction values (i.e., RE, RH)have the largest influences on benefit of the system. So, the task of assigning values forimpact reduction must be examined thoroughly due to its considerable influence onresulting benefits.

Sensitivity FactorsAnother effort was made to conduct the sensitivity analysis to examine which factors(i.e., potable water condition, children and population density, visible sewage drainageetc.,) used to estimate environmental and public health impact are the most important inquantifying overall benefit. Five factors with different importance weights (varying from0 to 3) were used in estimating environmental impact. These importance weights werevaried for all factors for all 16 areas, while remaining parameters were held constant. Theresulting system benefits were compared for importance weight change for all the factors,as shown in Figure 15.

According to Figure 15, the system benefit curve has the largest negative slope (slope =7.0) when weights for the factor “visible sewage drainage” are varied, The curve has thesmallest negative slope (slope = 1.7) for the factor “surface water condition.” So, it canbe concluded that the visible sewage drainage factor is the most sensitive, as it providesthe largest effect on system benefits among all environmental factors. So, great careshould be taken while assigning weight for this factor. Conversely, the sensitivity of“surface water condition” indicates that this factor has a less significant effect on thesystem benefit and thus would have little effect on the provision of sewerage inEnsenada. The sensitivity of other factors can be determined using their slope, as shownin Figure 16. This sensitivity analysis was performed assuming all areas are providedsewerage.

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In public health impact assessment, the same procedure was used. These importanceweights of all public health factors were varied for all sixteen areas, while remainingparameters were held constant. The resulting system benefit curves are shown in Figure16.

Figure 16 shows that the system benefit curve has the largest negative slope (slope = 7.0),for “visible sewage drainage” and has the smallest negative slope (slope = 0.4) for“potable water condition” factors when weights are varied. Visible sewage drainage wasalso used as an environmental impact and proved to be the most sensitive. So, it can beconcluded that that the “visible sewage drainage” factor is the most sensitive and has thelargest influence on system benefits and in the provision of sewerage for Ensenada.Therefore, great care should be taken in assigning weight to this factor. Conversely, thesensitivity of “potable water condition” indicates that this factor has a less significanteffect on the system benefit and thus would have little effect on the provision of seweragein Ensenada.

Provision of SewerageIn this research, it was assumed that an unsewered area is either sewered completely ornot sewered at all. An effort was made to investigate the change of system benefits assewerage percentages vary from 0 to 1 as shown in Figure 17.

According to Figure 17, if none of the 16 areas are sewered, the total impact(environmental and public health) that would occur in Ensenada is about 300 units. But,if all 16 areas are provided sewerage by allocating $80.6 million to collect a total of 522l/sec of wastewater, this impact will be reduced to 84.16 units. It is important to note thatthe benefit would reduce linearly for different provisions as is seen in the above figure.So, from an economics points of view, Ensenada’s willingness to pay is approximately$380,000 [or, (80.6/(299.2-86.4) = 0.375] for per unit reduction of impact, andapproximately $150,000 [80.6/522 = 0.154] for removal of 1 liter/sec of wastewater.

CONCLUSIONS

The DP algorithm and non-monetary impact assessment methodology used in thisresearch has the ability to determine an optimal collection plan for any number ofcompeting areas. These methodology posses the ability to determine a resulting matrixsummarizing the optimal and sub-optimal collection plans, and can also find an optimalcollection plan for a given funding constraint. This gives flexibility to the decision-maker.

A decision for the provision of sewerage on the basis of net benefit (i.e., monetary costminus monetary benefit) is not practical because estimation of benefit in monetary termsis not feasible. The non-monetary scoring approach, or so-called “matrix approach,” wasused to estimate benefit as a surrogate of impact that will be reduced, which will occuronce an area is provided sewerage. The findings from impact assessment in this researchcan be summarized as follows:

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• Impact or benefit assessment is complicated, subjective, and not perfected. Thevariables and parameters necessary for their estimation are site-specific.

• The information needed to assess the environmental and public health impacts andimpact reductions requires data collection, surveying, or similar methods, and mayrequire visual inspection of the competing areas. So, the process of obtaining informationmay become time- and resource-intensive. Once data is collected and methods are used,the accuracy of the optimal solution may become dependent on the reliability of theinformation gathered.

• Consistency must be maintained in the scoring procedure to estimate the environmentaland human health impacts. Thus, the procedure for assigning scores may be limited toonly one scorer. This methodology will work for any situation as long as the scoring isconsistent across all the areas. However, the absolute magnitude of the scores isunimportant, only the relative scores are important.

• If more than one scorer is used, an initial ranking of the total impacts should becomparable among different scorers.

So, the methodology presented in this research is a powerful tool if fueled with all thenecessary information.

The project made several determinations about the wastewater system in Ensenada:

• The flourishing tourism, fish processing, and other industrial activities essential to theeconomic development of Ensenada may be threatened by current environmental andhuman health concerns existing in some unsewered areas. Currently, partial collectionsystem and/or septic tanks or privies are managing the wastewater from these areas.

• Due to the excess wastewater treatment capacity in both El Sauzal and El Naranjo, theexpansion of sewerage to bring additional wastewater from developing communities andindustries for adequate treatment is possible.

• Funds allocated for expanding the wastewater collection system to the unsewered areasdoes not cover the price tag. Currently, there are 26 areas competing for the availablefunds. The parameters of the environmental impact, public health impact, impactreductions, and cost for wastewater collection must be examined before establishing aplan to expand the collection system to the competing areas.

• Drinking water source pollution is not a problem in Ensenada since most of people inthe unsewered areas buy commercially available bottled water for drinking purposes.

• Since there is no significant surface water body in Ensenada, except the reservoir LaPresa Emillio Zamona, surface water pollution is not a significant issue. However,Ensenada officials studied the microorganism concentration of the water in the bay“Bahia Todos Santos” and found that microorganism content is in excess of the water

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quality standards. It is likely that excess microorganism concentration has beencontributed from the discharge of the waste from the fishing industries and the effluentdischarge from the wastewater treatment plant.

• Two of the unsewered areas, Fraccionmiento Mar and Colonia Esperanza Y Amplacionborder the only potable water source of Ensenada, La Presa Emilio Zamora reservoir.because of these areas’ geographic locations, , the potential for contamination of thereservoir is a serious concern.

The specific impacts that are expected to occur from the expansion of thewastewater system in Ensenada include:

• The existing mal-functioning septic tanks, privies, and other onsite systems will bereplaced by sanitary systems to collect and convey all household sewer andwastewater to treatment plants El Naranjo and El Sauzal.

• The condition of raw sewage spilling out onto the surface and the exposure of theresidents and visitors to vectors and disease will be eliminated. This will beneficiallyimpact the health of the residents, and will reduce the potential threat to the healthof visitors from the U.S.

• Overflows and spillage of household sewage into the streets will no longer occur.Consequently, soil contamination will be reduced and water quality in reservoir LaPresa Emilio Zamona and the bay Bahia Todos Santos will improve.

• Threats of groundwater pollution due to surface runoff of sewage and onsitesystems will be reduced or eliminated.

•The spread of wastewater-related disease to the U.S. by Mexican nationals fromEnsenada traveling toward the U.S. border will be reduced.

• Unpleasant odors from fishing industry waste currently impacting residents ofEnsenada and U.S. travelers to Ensenada will be eliminated.

• In addition to providing wastewater service to the existing population, thewastewater system will also allow and encourage moderate community growth andeconomic opportunity.

Implementation of the project will not otherwise negatively affect the naturalenvironment. The consequences of implementing the project will be, on balance,positive, far outweighing any temporary negative effects.

RECOMMENDATIONS FOR FURTHER RESEARCH

The primary goal of the research was to prove that mathematical optimization could beused to evaluate sewage system expansion options for a complex variety of physical and

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environmental conditions. This goal was successfully accomplished. First-orderalgorithms to estimate the environmental impacts associated with various conditions weredeveloped to facilitate the research. Now that the approach is proven to work, thesealgorithms should be improved so they are more robust and reliable. A detailed study isneeded to develop sound theoretical bases for these algorithms and gather sufficient datato verify their applicability under a wide range of conditions.

The computer program developed in this project facilitated the computations withrudimentary functionality. There are no provisions for interactive data entry and much ofthe site-specific data required is hard-coded. If the approach is to be applied to areas otherthan Ensenada and potentially used by others, its user interface must be improvedsubstantially. Additionally, one of the initial goals of the research not realized was theintegration of GIS data to form the basis for data entry. It quickly became clear that theeffort required to complete the primary thrust of the research would not have beenpossible if this had been pursued. However, integrating GIS into an interactive graphicaluser interface would be an appropriate next step.

Several assumptions were made for the development of this methodology. It wasassumed that wastewater treatment plants that will receive sewerage from newly seweredareas already exist or, in the absence of existing plants, planners knew the location,timing of construction, and capacity of these treatment plants. A logical extension of thisresearch will be to develop a methodology along with the current methodology to identifybest areas for construction of wastewater treatment plants.

RESEARCH BENEFITS

This research developed a fundamental approach for comparative analysis of expansionoptions for sewage collection systems in highly populated, underdeveloped areas. Itdeveloped first-order methodologies for determining environmental and public healthbenefits under a wide range of conditions. A software program implementing thisapproach was initiated. Although the program is not developed yet sufficiently fordistribution or use by others, it forms the basis for future development of such software.The study focused specifically on Ensenada and the results provide quantitativejustification for sewage system expansion plans there. The results will assist Ensenada indetermining their priorities for expansion. The results provide a reliable estimate of thereal benefits of sewage collection system expansion and will help city officials preparefunding requests for future expansion projects.

If the expansion is implemented, it will have untold impacts on human andenvironmental health in these areas. Most importantly, it will ensure children nolonger have access to sewers, where many will play if they have the chance, and willeliminate human exposure to the pathogens that thrive there. As well, it will lessenthe need for septic tanks and privies, both of which carry a high potential forleakage, and thus contamination of soil, groundwater and surface water bodies.

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ACKNOWLEDGEMENTS

The authors gratefully acknowledge the cooperation of CESPE throughout the research.They provided much of the data used and were enthusiastic supporters of the effort fromthe beginning. The professionalism they brought to their work was impressive andencouraging.

REFERENCES

Argman, Y., U. Shamir, and E. Spivak. 1973. “Design of Optimal Sewerage Systems.”Journal of the Environmental Engineering Division Proceedings of the American Societyof Civil Engineers, 99 (EE5): 703-716.

Bhagwan, R. and C. Polprasert. 1982. “Integrated Wastewater Management.” Journal ofHydraulics 109(3), 619-630.

Baffa, J. 1955. “Lateral Sewer Construction Costs.” Public Works LXXXVI.

Bell, F., F. Greenfield, D. Hawker, and D. Connell. 1989. “Impact of waste discharges ofcoral reef regions.” Water Science and Technology 21(1), Pp. 121-130.

Budnick, F., R. Mojena, and T. Vollman. 1977. Principles of Operations Research forManagement. Richard D. Irwin, Inc., Burr Ridge, Ill.

Canter, L. 1997. Nitrates in Groundwater. Lewis Publishers, New York.

Canter, L., and R. Knoxx. 1985. Septic Tank Effects On Groundwater Quality. LewisPublishers, New York.

Central Intelligence Agency. 2000. The World Factbook 2000.http://www.umsl.edu/services/govdocs/wofact2000.

Chapra, C. 1997. “Surface Water Quality Modeling.” The McGraw-Hill Companies Inc.,New York.

Chen, X., and H. Weng. 2000. “Impact of Polluted Canal Water on Adjacent Soil andGroundwater Systems.” Environmental Geology, 39(8), Pp 945-950, Springer-VerlagGmbH & Company KG, Berlin, Germany.

Council on Environmental Quality. 1980. The Eleventh Annual Report of the Council onEnvironmental Quality.

Crities, R., and G. Techobanoglous. 1998. Small and Decentralized WastewaterManagement Systems. McGraw-Hill, Boston.

52

Dajani, J., R. Gemmel, and E. Morlok. 1972. “Optimum Design of Urban WastewaterCollection Networks,” Journal of the Sanitary Engineering Division Proceedings of theAmerican Society of Civil Engineers, 98(SA6), 853-865.

Deninger, R., and S. Shaiw. 1973. “Modeling Regional Wastewater Treatment Systems.”Water Research. Pergamon Press, 7, 633-647.

Ford, D. 1984. “Dredged Material Disposal Management Model.” Journal of the WaterResources Planning and Management 110(1).

Ganster, P. 2000. The U.S.-Mexican Border Environment: A Road Map to a Sustainable2020. SCERP Monograph Series, No. 1. San Diego State University Press, San Diego,Calif.

Gupta, R. 1989. Hydrology and Hydraulic System. Prospect Heights, Illinois.

Gawad, H., and J. Butter. 1995. “Clustering of Towns and Villages for CentralizedWastewater Treatment.” Water Science and Technology 32(11), 85-95.

Harvey, D., and T. O’Flaherty. 1973. “An Analysis of Solid Waste Transportation andDisposal Alternatives.” Canadian Journal of Operational Research and InformationProcessing 11(3), 187-200.

Hiller, F., and G. Lieberman. 1990. Introduction to Operation Research. 5th Edition,McGraw Hill Book Company, New York.

Howe, S. 1971. Planning Sewerage Service for New Towns. Ph.D Dissertation,University of Wisconsin.

Joeres, E., J. Dressler, C. Cho, and C. Falkan. 1974. “Planning methodology for theDesign of Regional Waste Water Treatment Systems.” Water Resources Research 10(4),643-649.

Klegerman, H. 1964. “Fairfax Country Integrated Sewerage Program.” Sanitary SewerService in the Accotink and Pohick Creek Watersheds, Preliminary Design, CostEstimates, Financial Requirements, A Feasibility Report Prepared for Fairfax Country,Virginia. New York: Alexander Potter Associates.

Klemetson, S., and W. Grenney. 1975. Development of a Dynamic Programming Modelfor the Regionalization and Staging of Wastewater Treatment Plants. Utah WaterResearch Laboratory.

Kocasoy, G. 1995. “Waterborne Disease Incidences in the Mediterranean Region as aFunction of Microbial Pollution.” Water Science and Technology 32 (9-10), 257-266.

53

Laak, R. 1986. “Wastewater Engineering Design for Unsewered Areas,” 2nd Ed.Technomic Pub. Co., Lancaster, Pa.

Lauria, D. 1976. “Desk Calculator Model for Wastewater Planning.” Journal of theEnvironmental Engineering Division Proceedings of the American Society of CivilEngineers, 105(EE1), 113-120.

Leighton, J., and C. Shoemaker. 1884. “An Integer Programming Analysis of theRegionalization of Large Wastewater Treatment and Collection Treatment.” WaterResources Research 20(6), 671-681.

Liptak, B., and D. Liu. 2000. Groundwater and Surface Water Pollution. Pp. 48. LewisPublishers, New York.

Mara, D. 1996. Low-Cost Urban Sanitation. John Wiley & Sons, New York.

Markland, R., D. Smith, and J. Becker. 1977. “Computerized Regional Planning for LandDisposal of Wastewater.” Urban Systems 2, 1-14.

Maynard, J. 1969. “Septic Tank Performance Related to Ground Conditions andPopulation Density.” Canadian Journal of Public Health 60, 173-175.

Mays, L., and Y. Tung. 1992. “Hydrosystem Engineering and Management.” McGraw-Hill Inc., New York.

Mays, L., and B. Yen. 1975. “Optimal Cost Design of Branched Sewer Systems.” WaterResources Research 11(1), 37-47.

McAllister, D. 1980. Evaluation in Environmental Planning: Assessing Environmental,Social, Economic, and Political Trade-offs. MIT Press, Cambridge, Mass.

McCongha, D., and A. Converse. 1973. “Design and Cost Allocation Algorithm forWaste Treatment Systems.” Journal of Water Pollution Control Federation 45(12), 2550-2566.

McClelland, N. 1980. Individual Onsite Wastewater Systems. Proceedings of the SixthNational Conference 1979, Ann Arbor Science.

Metcalf and Eddy, Inc. 1991. Wastewater Engineering: Treatment, Disposal, Reuse, 3rded., McGraw-Hill, New York.

Merrit, L., and B. Bogan. 1973. “Computer-Based Optimal Design of Sewer Systems.”Journal of the Environmental Engineering Division American Society of Civil Engineers,99(EE1), 35-53.

54

Meredith, D. 1972. “Dynamic Programming with Case Study on Planning and Design ofUrban Water Facilities.” Treatise on Urban Water Systems, Colorado State University,Fort Collins, Colorado.

Moeller, D. W. 1998. Environmental Health. Revised ed., Harvard University Press,Cambridge, Mass.

Murty, G. 1968. “Solving the Fixed Charge Problem by Ranking the Extreme Points.”Operations Research 16, 268-279.

Naraynaswamy, V., and W. Kennedy. 1994. “Heuristic Algorithm to Locate Solid-WasteDisposal Site.” Journal of the Urban Planning and Development 120(1), 14-21.

Nemerow, N. 1978. “Industrial Water Pollution, Origins, Characteristics, and Treatment.”Addison-Wesley Publishing Company, Boston, Mass.

Nero, W., K. Williams, and G. Garrett. 2001. “Using Decision Models for Sitting andWastewater Management Alternatives.” Decision support Systems for Water ResourceManagement, AWRA/UCOWR Summer Specialty Conference. Pp.19-23.

Novotny, V., and H. Olem. 1993. Water Quality: Prevention, Identification, andManagement of Diffuse Pollution. Van Nostrand Reinhold, New York.

Panital, S. 1978. “Revise Your Optimal Design Periods.” Water and Wastes Engineering15(10), 45-57.

Pierzynski, G., J. Sims, and G. Vance. 1994. Soils and Environmental Quality. CRCPress, Boca Raton, Fla.

Reed, R. 1996. “Selecting Communities for Sewerage.” Low-Cost Sewerage, DunhamMara, Ed. John Wiley & Sons, New York.

Rizaiza, O., and R. Hammadur. 1999. “Cost Comparison between Cesspools and SanitarySewerage Systems in Saudi Urban Areas.” Journal of American Water ResourcesAssociation 34(4), 855-863.

Sharma, B. 1988. “Pollution Control.” Chemical Age of India 39(8), 483-490.

Shapiro, M., P. Rogers, and R. Tabors. 1976. Land Use and the Pipe. Lexington Books,D. C. Health and Company, Lexington, Mass.

Shapiro, M., and P. Rogers. 1978. “Analyzing Impact of Land Use on Sewer ServiceArea Planning.” Water Resources Bulletin 14(6).

Spencer, C. 1958. “Metropolitan Planning for Sewers on a Country Basis.” Public Works89(8), 83-85.

55

Steynberg, C., N. Venter, E. Wet, G. Plessis, D. Holhs, N. Rodda, and R. Kfir. 1995“Management of Microbial Water Quality: New Perspectives for Developing Areas.”Water Science and Technology 32(5-6), 183-191.

Stednick, D., M. Gilbert, and D. Lee. 1998 “Development of a National Water QualityMonitoring Programme for Belize.” International Association of Hydrological Sciences253, 379-386.

Tata Energy Research Institute. “Health impacts of water pollution.”(http://edugreen.teri.res.in/explore/water/health.htm)

Oluwande, A., C. Sridhar, and O. Okubadejo. 1978. “Health Hazards of Open Drains inDeveloping Countries.” Progress in Water Technology, Sci and Med in the Prev of TropWater-Relat Dis 11(1), 121-130.

Tchobanoglous, G., R. Linsley, J. Franzini, and D. Freyberg. 1992. Water-ResourceEngineering 4th Edition, 660-742.

Theodorou, J. 1992. “Ecological Consequences of Untreated Wastewater Discharge inthe Saronikos Gulf, Greece.” Water Science and Technology 25(9), Pp 115-124.

Tuthill, A., D. Meikle, and M. Alvanja. 1998. “Coliform Bacteria and NitrateContamination of Wells in Major Soils of Frederick, Maryland.” Environmental Health6(8), 16-21.

Zektser, I. 2000. Groundwater and the Environment, Applications for the GlobalCommunity, Lewis Publishers, New York.

Zoller, U. 1994. Groundwater Contamination And Control. Marcel Decker, Inc., NewYork.

Zukovs, G., and B. Adams. 1980. “Capacity Expansion Planning Model for WastewaterTreatment and I/I Control.” Water Resources Bulletin American Resources Association,Vol. 16, No. 4, 601-607.