Final Manuscript published 2009 in Irrig Drainage Syst (2010) 24: DOI 10.1007/s10795-009-9083-9

Bad for the environment, good for the farmer?

Urban sanitation and nutrient flows

Marco Erni & Pay Drechsel & Hans-Peter Bader &

Ruth Scheidegger & Christian Zurbruegg & Rolf Kipfer

Abstract

Due to poor urban sanitation farmers in and around most cities in developing countries

face highly polluted surface water. While the sanitation challenge has obvious

implications for environmental pollution and food safety it can also provide ‘free’ nutrients

for irrigating farmers. To understand the related dimensions, a box-flow model was used to

identify the most important water and nutrient flows for the Ghanaian city of Kumasi, a

rapidly growing African city with significant irrigation in its direct vicinity. The analysis

focused on nitrogen and phosphorus and was supplemented by a farm based nutrient

balance assessment. Results show that the city constitutes a vast nutrient sink that releases

considerable nutrients loads in its passing streams, contributing to the eutrophication of

downstream waters. However, farmers have for various practical reasons little means and

motivation in using this resource of nutrients. This might change under increasing fertilizer

prices as the nutrient load will continue to increase by 40% till 2015 assuming a widening

gap between population growth and investments in water supply on one side and

investments in sanitation on the other. However, even a strong investment into flushing

toilets would not reduce environmental pollution due to the dominance of on-site sanitation

systems, but instead strongly increase water competition. Key options to reduce the nutrient

load would be via optimized waste collection and investment in dry or low-flush toilets.

The latter seems also appropriate for the city to meet the water and sanitation Millennium

Development Goals (MDGs) without increasing water shortages in toilet connected

households.

Keywords

Urban water balance . Nitrogen . Phosphorus . Water pollution . Wastewater irrigation .

Scenario-analysis . Material flow analysis . Modeling . Kumasi

M. Erni (*) : H.-P. Bader : R. Scheidegger : C. Zurbruegg : R. Kipfer

Eawag, Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, P.O. Box 611,

8600 Duebendorf, Switzerland e-mail: erni.marco@gmail.com

P. Drechsel

International Water Management Institute (IWMI), P.O. Box 2075, Colombo, Sri Lanka

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Introduction

Water scarcity and water pollution have become major issues for many peri-urban areas in

developing countries and will become even more acute if the trend of rapid population

growth and urbanization continues. This is strongly affecting smallholders engaged in

informal irrigation taking advantage of market proximity. A recent survey showed that in

and around four of five cities in the developing world, farmers have little alternative than to

use highly contaminated water (Raschid-Sally and Jayakody 2008). This also applies to

Kumasi (Obuobie et al. 2006). Disregarding the related microbiological challenges in this

paper, the nutrient content of the wastewater is often praised as benefit (Scott et al. 2000;

Qadir et al. 2007), while considered by others a major environmental problem (Gray and

Becker 2002; Færge et al. 2005). Studies addressing related challenge remained either

limited to water quality analysis at the farm level (Cornish et al. 1999; McGregor et al.

2002; Keraita 2002), while those addressing the root cause, i.e. urban sanitation, focused

mostly on source control and ecological sanitation (Larsen and Guyer 1996; Otterpohl et al.

1999; Langergraber and Müllegger 2005; Huang et al. 2006; Jeppson and Hellstrom 2002;

Montangero et al. 2007).

This paper tries to link both parts for the case of Kumasi, Ghana, by quantifying water

and nutrient flows from their origin to the farm or environment in general. In order to

identify weaknesses and prioritize improvement measures in urban and agricultural water

and nutrient management, a model based on material flow analysis (MFA) was used to

show the order of magnitude of all major water and nutrient flows in order to gain a better

picture of the city’s water budget. For reasons of modeling and data availability, nitrogen

and phosphorus were chosen as indicators of pollution which can also be beneficial for

agriculture. Apart from understanding possible impacts on irrigated agriculture downstream

of the city, and ways to manage the urban nutrient output, the paper aims to answer how the

wastewater streams will change in the coming years, considering population growth and

possible investments in different sanitation systems.

As the modeling is data intensive, the authors draw from a number of earlier studies on

Kumasi which contributed to this work: Cornish et al. (1999), CEDAR (2002), Obuobie et

al. (2006) and Keraita (2002) have investigated water flows and quality to assess the value

and risks of (peri-)urban irrigated agriculture as well as human health and to gain a more

holistic view of natural resources management at watershed-level. An overview of carbon,

nitrogen and phosphorus flows related mainly to food and timber in and out of Kumasi to

assess the sense of co-composting organic waste has been given by Leitzinger (2000). Apart

from Kindness (1999), who assessed the supply and demand for soil amelioration, a

number of studies have been conducted by IWMI and SANDEC (Eawag Department of

Water and Sanitation in Developing Countries) into the use of nutrients from solid and

liquid wastes. The Ghana Statistical Service (GSS 2002, 2005), the Waste Management

Department of Kumasi (Mensah 2006) and the Ghana Water Company Limited (GWCL

2006) have produced sanitation and household statistics used in this article.

Materials and methods

Location

The study was carried out in and around the city of Kumasi which lies about 150 km north-

west of Accra, Ghana’s capital. Kumasi is the capital of the Ashanti Region and the

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commercial capital of central Ghana. Its population has grown fourfold since 1970. The last

population census in 2000 counted 1.17 million inhabitants with an annual population

growth of 5.2% since 1984. Based on a rate of 5% we assumed about 1.57 million

inhabitants in 2006; the year used for all data. The urbanized area of Kumasi is estimated to

be around 205 km2 (Erni 2007). It was defined as the built-up area within the administrative

boundaries of Kumasi and its immediate surroundings shown on 2005 satellite images and

verified on the ground.

Kumasi’s elevation above sea level is between 230–290 m, which is reflected in a range

of undulating hills with slopes of generally less than 5%. The climate of the area is humid,

semi-equatorial with mean annual rainfall of 1350 mm (MSD 2006). Minimum and

maximum monthly average temperatures are around 21°C and 30°C respectively, with only

little variability throughout the year. Two smaller streams are passing parts of the city, the

Owabi and Wiwi, while the Subin stream originates in the city. Due to the hilly landscape,

the streams run through inland valleys unsuitable for construction but of high value for

urban vegetable production. The streams are draining into the Oda river at the southern

boundary of the city (see Fig. 1).

Along all streams in and around the city, farmers are cultivating crops. Cornish and

Lawrence (2001) estimated about 12000 ha under at least seasonal vegetable irrigation

which is twice the area under formal irrigation in the whole country. Also in the city, about

41 ha are irrigated (Obuobie et al. 2006). All farming is done by smallholders who sell their

products on the urban markets. It can be described as informal irrigation (Drechsel et al.

2006) as water fetching and distribution is usually manual or supported by small pumps, i.e.

irrigation is not organized in formal schemes and without governmental support. In Kumasi,

Fig. 1 River network and urbanized area of Kumasi

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vegetable farming is done all-year-round, while in the peri-urban area it is mostly a dry-

season activity when prices are high, while maize and cassava are more competitive in the

rainy season. Farm sizes are increasing from urban to peri-urban areas with usually less

than 0.05–0.1 ha in the cities compared to 1.5 ha in peri-urban areas. Vegetable farming is

done on raised beds with an average area of 3–8 m2. Due to high labor requirements (land

preparation, weeding, watering etc) farmers with bigger land areas have to hire labor or rent

a water pump. In peri-urban areas more family labor is used. Poultry manure is all-over the

preferred and cheapest nutrient source, but also fertilizer is used, especially on cabbage

(Obuobie et al. 2006).

Methods

Methods consisted mainly of water and nutrient modeling which was supplemented by

information from surveys on nutrient management in smallholder irrigated farms around the

city. The main method used to derive urban water and nutrient flows is a mathematically

extended material flow analysis called MMFA. It describes, quantifies and models the

material flows of the system considered (Baccini and Bader 1996; Eawag 2007). Detailed

Information and results on a MMFA as conducted according to the system in Table 2 can be

found in Erni et al. (2010). Recent studies using material flow analyses in urban water

management and related problems can be found on the corresponding website of www.

eawag.ch. The SIMBOX software (Bader and Scheidegger 1995) is used for this MFA. For

farm nutrient balances a simplified NUTMON model was used (see below).

System analysis of water and nutrient flows in Kumasi Kumasi’s system boundaries are

delineated by its urbanized area as shown on satellite images of 2005. All major water

flows in, within and out of this area are considered in this study in order to identify the

relevant flows and allow the problems to be prioritized. The total nitrogen (N) and total

phosphorus (P) that are transported by water in the system area were considered. Water and

nutrient flows are given in million cubic meters per year [Mm3/yr] and tons per year [t/yr]

respectively. The time period forming the basis of this study is the year 2006. The model

outputs show annual average water flows and nutrient loads. The system considered is

shown in Fig. 2.

The boxes represent the balance volumes, or spatial units in which the balance of the in-

and outflows is calculated. The balance volumes considered in the model are Kumasi’s

two drinking water reservoirs, its soils & aquifers, its surface water network, larger

industrial & service sector entities, domestic households, landfills, and the cities’ fecal

sludge and sewage treatment plants. The boxes are connected through three different

types of flows: input flows, internal flows and output flows (Fig. 2). The inputs rainfall,

groundwater and piped water are the water sources for the households as well as industry

and service. Water leaves the system as output, either as discharge to surface water, or

soils & aquifer or as evaporation. All relevant water-related nutrient flows within the

system boundaries were considered. Details of equations and parameters used to model

the water and nutrient flows and the use of Monte-Carlo simulation are described in Erni

(2007). Available data from the literature, reports and expert opinions was used to

calibrate the parameters. each parameter the uncertainty and trustworthiness of the data

was estimated.

Modeled scenarios Scenarios were modeled to quantify future urban water needs and

wastewater generation and to identify the impact of changing sanitation practices on water

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Fig. 2 System analysis of Kumasi’s water and nutrient flows

and nutrient pathway, and availability for farming. They are compared with the current

situation (CS, Table 1).

S1: The implications of city growth (scenario 1, Table 1) are assessed by assuming a

2015 population of 2.23 million (4% annual growth from 1.57 million in 2006 to

2015). Most variables such as water use of industry and service as well as solid waste

production are assumed to increase proportionally, but this is not true for household

Table 1 Summary of water use and nutrient flows for different scenarios

CS S1 S2 S3

Assumptions

Drinking water reservoirs

Mm3/yr

27.9

44.5

27.9

27.9

Total available piped water Mm3/yr 20.9 33.4 20.9 20.9

Piped water per capitaa l/cap/d 49 59 49 49

Total extracted household GW Mm3/yr 4.3 8.4 4.3 4.3

Results N input to surface water t/yr 2490 3480 2280 1640

P input to surface water t/yr 770 1090 770 490

N input to soil and aquifers t/yr 2410 3350 2860 2140

P input to soil and aquifers t/yr 580 810 660 480

S1 Scenario 1 (2015 according to current plans), CS Current state, S2 Scenario 2 (50% water closet coverage,

up from 25%; population growth not included), S3 Scenario 3 (integrated waste reduction concept;

population growth not included) a Includes water from industry and service, otherwise figures would be 20% lower

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water use (increase as planned by authorities) and the system area (increasing housing

density). In the view of the city’s expansion beyond the pipe distribution system, an

increase of the fraction of groundwater users from 25% to 30% is assumed.

S2: The impact of changing sanitation practices is visualized by assuming a higher

fraction of water toilets (50%, compared to 25% at present) that is expected as living

standards rise. City growth is not included in this scenario. It is further assumed that a

more reliable water supply, fewer malfunctioning toilets and more regular flushes will

lead to 25 l/cap/d being used instead of 15 l/cap/d (current conditions). This is still

significantly lower than typical values for developed countries (40–50 l/cap/d; EEA

2003; Vickers 2001

S3 The N and P reduction potential due to improved waste management assumes

an integrated concept based on three pillars: a) improved solid waste collection, b)

more efficient wastewater treatment plants, and c) reduced nutrient outputs from

domestic and industrial sources through a shift from phosphorus containing washing

detergents to P-free detergents, halving the organic wastes washed down the drainage

system, and source treatment of all industrial effluents reducing nutrient concentrations

to the level of the guidelines of the Environmental Protection Agency of Ghana

(10 mg N/l, 2 mg P/l).

System analysis on farm and farmers’ responses Along all streams leaving the city, farmers

are abstracting water for small-scale irrigation. The urban nutrient flow modeling was

therefore accompanied by farm-based studies looking at the plot level at the nutrient

balance. The approach applies a simplified nutmon model (www.nutmon.org) considering

major nutrient in- and outputs at the field level including harvest, fertilization, irrigation,

crop residues, erosion and leaching (Smaling and Fresco 1993).The farm assessment links

with the urban flow model through the nutrient concentrations in the irrigation water

derived from the streams and the amounts farmers use for irrigation. Farm analysis was

accompanied by interviews about farmers’ perceptions of water quality in view of crop

nutrient supply and how far this might be considered or affecting their crop nutrient

management practices. Interviews (n=75) were carried out among both groups of urban and

peri-urban farmers to see if there are differences with increasing wastewater dilution. For

details see Keraita (2002).

Results and discussion

Urban water and sanitation situation

Water supply About 75% of Kumasi’s population use piped water as their drinking water

source. Most of the remaining population uses mostly wells and boreholes (GWCL 2006).

Rainwater harvesting and the use of surface water are marginal. The piped water originates

from two surface water reservoirs (Owabi and Barakese) in the north-east of Kumasi. As a

result of rapid city growth and the limited capacities, the increase in drinking water

production as well as the expansion and improvement of the distribution network have been

very slow in the past, resulting in unreliable water supplies in terms of quantity and a low

pipe pressure at the periphery. Thus, despite abundant rainfall in the region, the drinking

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water supply in Kumasi via piped water at households, from neighbors or a standpipe,

equals a household consumption of only 43 l/cap/d Including figures for water consumption

of industry and service as well, the average daily per capita water use is 49 l/d (see Table 1),

which still remains below the minimum standard of 50 l/cap/d as proposed by Gleick

(1996). In other words, water supply in 2006 fell short by 350,000 people if 50 l is the

target. For comparison, typical western European water consumption for toilet flushing

alone is in this range while total domestic use is between 115 l/cap/d in Belgium and 260 l/

cap/d in Spain (EEA 2003).

Liquid waste Most people in Kumasi use on-site sanitation systems: 26% use flushing toilets

(WCs) connected to septic tanks, 24% pit latrines, 5% bucket latrines and 37% use public

toilets connected to septic tanks. Less than 4% are connected to a sewage system with a

central treatment plant (TP). The remaining residents do not use any particular toilet but ‘open

defecation’ (GSS 2002; Mensah 2006 and own estimations). These percentages do not show

that a much larger percentage of the population in Kumasi practice open urination, a

culturally accepted habit that helps to avoid searching for public toilets, which are usually

unhygienic and demand usage fees. Septic tanks used in Kumasi are designed to allow most

of the water and urine to leave the tanks while the solid fraction is retained. This overflow as

well as seepage from leaky tanks and pits flows into surfaces waters or enters the soil and

aquifers, while the remaining contents of the toilets are brought to the local fecal sludge

treatment plant. Household greywater coming from kitchens, laundries, baths, cleaning,

watering etc., infiltrates or reaches the mostly open drainage system that consists largely of

stormwater gutters. Given the undulated topography of the city, most grey- and stormwater

drain easily into depressions where streams allow the water to leave the city untreated. The

gutter and stream network constitutes in this way an open drainage cum sewage systems.

Solid waste Despite continuous efforts of the municipal Waste Management Department to

improve the municipal waste collection, up to 200t/d, or 80% of solid wastes remain

uncollected (Mensah 2006). They are dumped into depressions, at roadsides or in gutters

from where they are partly washed down the drainage system with stormwater contributing

to the anthropogenic pollution load. Unlike in more developed countries or cities with

harbours, there are no major industries in Kumasi apart from the sawmills, a larger abattoir,

a soft-drink producer and two breweries that consume less than 1 Mm3/yr (Simon et al.

2001) in total. Efforts are being made by the Environmental Protection Agency of Ghana to

make local industries clean their wastewater, which was already in some cases successful.

Large waste generators are the three main markets that produce about 100t/d of (mostly

organic) waste that are partly collected, partly swept into open gutters. Most other actors

from industry and service neither have high water consumption nor a high impact on

nutrient in- and outputs. The breweries are an exception.

Nutrient flows The city as a consumption center is discharging huge amounts of waste and

nutrients into its system as only parts get collected or absorbed without environmental

leakage. Compared to the water quality upstream of Kumasi, the N and P concentrations

downstream of the city are approximately 14 and 6 times higher for nitrogen and

phosphorous, respectively. With over 3’000 ± 350 tN/yr and 500± 125 tP/yr, which

corresponds approximately with half of the annually generated human excreta in Kumasi,

the largest N and P input into water bodies derives from domestic sources i.e. failing

sanitation. The data correspond well with previous estimates from Leitzinger (2000), if

adjusted to the year 2006.

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% o

f fa

rmers

% o

f fa

rmers

Implications at the farm level

The nutrient load of wastewater polluted irrigation water is often considered as a benefit

of wastewater irrigation (Qadir et al. 2007). This is in particular the case where undiluted

wastewater is available. The amount of nutrients in 1’000 mm irrigation of wastewater

can vary considerably: 160–620 kg N, 40–240 kg P, and 20–690 kg K (Qadir et al. 2007).

In and around Kumasi, the wastewater abstracted by farmers is however in general diluted

by run-off and the natural stream water. As the streams are small, the nutrient loads

applied with the water are still noteworthy and in the range of (upstream—downstream)

10–150 kg N, 3–5 kg P, and 42–66 kg K, based on an average irrigation rate of 1000 mm

(range 640–1’600 mm/yr).Although several farmers responded that they actually

preferred using river water because it has ‘more and better food’ for the crops, the

majority are using it as reliable and free water source, not as nutrient source (Obuobie et

al. 2006). Figure 3 shows that more than 50–60% of the farmers in urban and peri-urban

Kumasi are aware that their irrigation water contains nutrients, but only a small

percentage showed a high level of awareness and indicated regular consideration of this

while managing crop nutrient needs.

The low consideration of the nutrient value does not surprise. Without options for

water, soil or crop analysis i.e. without knowledge about the nutrient content of the

different inputs, farmers’ nutrient management depends on observation that they make

on the crops and general ‘experience’. However, farmers use the water first of all to

irrigate highly perishable crops, thus irrigation frequency depends on crop water needs

and not nutrient needs. All this limits farmers’ options for nutrient management via

irrigation water; a task even difficult to handle under controlled research conditions

(Janssen et al. 2005).Farmers address crop nutrient needs via other means. Poultry

manure is the preferred fertilizer for leafy (exotic) vegetables with short growing periods

of 1–3 months. However, its quality can vary significantly depending on its age and

source (broilers vs. layers) (Drechsel et al. 2000). Poultry manure is applied over the year

at a rate of about 20–50t/ha on cabbage and about 50–100 t/ha on lettuce and spring

onions. In terms of N and P, the fertilizer input from wastewater accounts for only about

10% of what is applied via poultry manure; rates differs between sites and crops from 770

Awareness that water used has nutrients

70

60 Urban

Do you consider that when applying water?

60

50 Peri-urban

50 40

40

30 30

20 20

10 10

Urban

Peri-urban

0 0 Not aware Average High

Fig. 3 Farmers’ responses on the nutrient level of their irrigation water (n=25 in urban and n=48 in peri-

urban Kumasi (Keraita 2002)

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to 1’650 kgN/ha and 180 to 390 kgP/ha (Table 2). In addition, NPK is applied at rates of

75–180 kg, but only on selected crops, like cabbage. Application rates follow mostly

farmers’ own experience.

The high nutrient input is justified under consideration of the number of growing

periods over the year, i.e. high frequency of harvests significantly contributing to nutrient

exports from the farm. The losses are even larger through nutrient leaching especially

where soils are sandy and regularly irrigated, while erosion is no significant factor.

Nutrient balance assessments point at farmers’ constant fight against negative N and K

balances while probably unknown to them phosphates continuously accumulate in the

topsoil. As long as nutrient are supplied at very low costs (especially via poultry manure)

farmers do not see the need for a more complex nutrient management which is likely to

only result in higher direct or indirect costs. Indeed, the nutrient replacement costs are

only in the range of 10 USD per ha and year if poultry manure is used; while they would

be more than 10 times higher if there would be only mineral fertilizer as replacement

(Drechsel et al. 2004). Given the high profit margin from the production of irrigated

exotic vegetables, poultry manure is no cost factor (Danso et al. 2006). In summary, the

nutrient supply via irrigation is in the given situation of limited relevance for market-

oriented vegetable farmers. On the other hand, there are increasing signs of water

eutrophication (McGregor et al. 2002).

The question is if and how this situation might change in the future under different

scenarios of investments in water supply and sanitation. This was analyzed in three

scenarios (S1-S3):

S1) Impact of growing population and water demand and wastewater generation

Given the expected population growth, and under consideration of current plans for

expansion in the water and sanitation sector, nutrient inputs to receiving waters, especially

to the Oda river will increase by around 40% till 2015. This increase will be supported by

the planned expansion of water supply without corresponding measures for collection and

treatment. Water delivery per capita can however only be increased if all possible water

supply expansions (reservoirs and boreholes) are implemented. In detail:

• If the annual piped water supply is increased by 60% from 28 Mm3 to 45 Mm3 as

officially planned (Sam and Asare 2007), piped water users would only have 20%

more water (59±4 l/cap/d including water from industry and service, see Table 1,

scenario S1) than currently given the concurrent population growth. If efforts to

upgrade water supply fail, per capita water availability in Kumasi would decrease

to around 30±3 l/cap/d in 2015, which is as little as 2–3 toilet flushes. In all

scenarios, the amount of water per capita would not reach the limit for making

conventional sewer operation feasible, which is around 70 l/cap/day (GTZ 2007).

Table 2 Nutrient application (kg/ha/yr) through fertilizer, manure and polluted irrigation water applied at

1000 mm per year (Drechsel et al. 2004, updated)

Cabbage/Lettuce/Spring onions

Nutrient Source NPK (15-15-15) Manure Stream water

N

P2O5

K2O

75–180

75–180

75–180

770–1650

420–900

350–750

10–150

7–11

50–80

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• The piped water supply could also be increased without new sources by reducing

physical losses in the distribution system, which are at least 25% and probably

more feasible by extending groundwater use. Assuming a conservative groundwa-

ter recharge rate of 10%, a maximum of 30 Mm3/yr can be sustainably extracted in

Kumasi, which is six times the present consumption.

• Typical for most African countries, there is also in Ghana and Kumasi a gap

between investments in drinking water versus sanitation which continues to

increases the challenge to achieve the related Millennium Development Goal

(MDGs) simultaneously. The bias towards supporting drinking water supply

will result in an increase in the amounts of untreated wastewater entering the

environment. With unchanged sanitation coverage, the nutrient input to soils

and groundwater as well as to surface water will increase by 40% each for

both N and P. Over 95% of the increasing nutrient load will enter surface

water (990 tN/yr and 320 tP/yr) and would end in the Oda River (Table 1,

scenario S1).

• Most likely percental nutrient inputs will increase more than water supply and

therefore increasing the nutrient concentration in water.

The question is how this situation could change if the sanitation sector would get

significant attention and investment? The following two scenarios are addressing this

question.

S2) The impact of changing sanitation practices

Assuming at the current state, i.e. simplified without change in population, a significant

step towards the MDG in the number of water closets (WC) and higher amounts of toilet-

flush, water use for sanitation would also more than double from 3.15±0.81 Mm3 to 7.69±

1.92 Mm3 (Table 1; scenario S2). This water demand would compete with other demands

as it accounts for over 30% of all the available drinking water. If spatially concentrated on a

certain area, this would result in severe water shortages in other parts of the city. Thus

achieving the MDGs through flushing toilets would require significant interventions in

water supply far beyond current plans.

Because most flushing toilets are connected to septic tanks from where water infiltrates

into the ground, more flushing toilets would decrease the amount of nutrients directly

entering surface water via gutters by 9% N and 4% P. On the other hand, more flushing

toilets will increase of the amount of nutrients to groundwater by 19% N and 14% P. The

result is a in a net increase of nutrients released to the environment of 240t N and 50t P. It

becomes thus obvious that investments into a higher fraction of dry sanitation systems

would have a positive impact not only on the water demand but also on the reduction of

nutrients released into the water cycle, as has also been shown e.g. by Montangero and

Belevi (2007) for Vietnam.

The final question we tried to answer was how far the nutrient load could be reduced if

not via dry toilets but through other measures for waste reduction?

S3) Impact of improved waste management

To reduce the nutrient output into the environment, optimized practices for solid and liquid

wastes, such as improved existing treatment plant efficiency, improved solid waste

collection as well as a shift to phosphorus-free washing detergents and industrial source

treatment were assumed. The data show that the maximum reduction in total N and P flows

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into the environment compared to current conditions would be 61% for N (1120 t) and 48%

for P (380 t), respectively (Table 1), composed as follows:

• Improvement of wastewater and fecal sludge treatment plants complying with

Ghana’s EPA standards (see above) could help to save 300 tN and 10 tP, but would

require relatively sophisticated and expensive treatment. This appears unrealistic in

the given time slide.

• Nutrient reduction in the magnitude of at least 420 tN and 250 tP could be achieved

through improved solid waste collection (100% collection rate) which appears

feasible as several Ghanaian cities introduced recently three-wheelers to serve also

less accessible areas, and would result in multiple development objectives, such as

healthier living conditions.

• Nutrient loads in household greywater could be reduced by 210 tN and 110 tP if

less organic wastes are flushed down the drains and more importantly phosphate-

free washing detergents are used.

• Another 190 tN and 10 tP could be saved by source treatment in the industrial and

service sector.

Conclusions

Cities in developing countries are vast nutrient sinks where waste from food and other

goods contribute to environmental pollution at large scale (Drechsel and Kunze 2001). The

resulting nutrient load in streams passing the cities can be significant to the advantage of

farmers in need of nutrients but contributing to the eutrophication of downstream waters on

the hand. This study shows that the stream water in Kumasi transports 2490 t N and 770 t P

per year. However, it is for various reasons very difficult for farmers to consider and

consciously manage this ‘free’ supply, especially where farmers actually irrigate for the

sake of water and not nutrients. It is noteworthy to add that the ‘water focus’ is not only

typical for drier climates but also humid areas, like Kumasi. This concerns not only the dry

season but any dry spell of 2 days or more as under the given climate exotic vegetables like

lettuce need very regular watering.

But there are also other reasons why free nutrient supply does not get much attention. One

is that the cost of poultry manure is especially around Kumasi very low; poultry farms

consider the manure more as a waste than a resource. Thus farmers are not under pressure to

change common irrigation practices to optimize farm nutrient management. Future scenarios

of urban development imply that wherever investments in urban water supply outpace those

in sanitation, like in the case of Kumasi, the amount of untreated wastewater will increase and

farmers and the environment are likely to face significant higher loads of ‘free’ nutrients than

today. Depending on the scenario, nutrient concentrations in surface water would increase

(S1) oder decrease (S3). Given the continuous rise in fertilizer prices, this might offer a

welcome alternative wherever farmers have the means to assess and steer nutrient in- and

outputs. As this is less likely, there is a high risk of increased eutrophication. Interestingly,

even a strong investment into flushing toilets would not reduce environmental pollution, but

at the same time strongly increase water competition. The most realistic option to reduce the

expected increase in nutrients entering the urban environment would be via optimized waste

collection. An actual decrease of the nutrient load could be achieved via dry toilets.

The modeling results are also interesting for municipal decision makers. Ghana made

significant progress in the provision of improved drinking water between 1990 and 2006. If

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the current rate of progress is sustained, the country is said to achieve its target for the

MDG for drinking water supply by 2015 (WSMP Ghana 2009). However, the here

presented data show that these effort is likely to fail if anyone start investing in flushing

toilets. With the continuing scarcity of drinking water, Kumasi, probably like many other

African cities is advised to achieve the sanitation MDG better through investments in water-

saving toilet systems.

Acknowledgements

This article and the MSc. thesis on which it is based were supported and financed by Eawag, IWMI, the Swiss Federal Institute of Technology Zurich (ETH) and the Swiss National Center of Competence in

Research North-South (NCCR North-South). Special thanks go to Bernard Keraita for the farm interview

data, and Daan Van Rooijen and Richard Kuffor from IWMI for their valuable discussions and inputs. We also thank the various partners in Kumasi that contributed to the system analysis and data collection,

particularly the Department of Civil Engineering of KNUST, the Kumasi Metropolitan Assembly and the

Ghana Water Company Limited.

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