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Sewage sludge drying by energy recovery from OFMSW composting:Preliminary feasibility evaluation

Elena Cristina Rada a, Marco Ragazzi a, Stefano Villotti a, Vincenzo Torretta b,⇑a University of Trento, Department of Civil, Environmental and Mechanical Engineering, via Mesiano 77, I-38123 Trento, Italyb Insubria University of Varese, Department of Biotechnologies and Life Sciences, Via G.B. Vico 46, I-21100 Varese, Italy

a r t i c l e i n f o

Article history:Received 10 September 2013Accepted 19 February 2014Available online 18 March 2014

Keywords:CompostingDrying of sewage sludgeEnergy recoveryGreenhouseHeat recovery

a b s t r a c t

In this paper an original energy recovery method from composting is analyzed. The integrated systemexploits the heat available from the aerobic biochemical process in order to support the drying of sewagesludge, using a specific solar greenhouse. The aim is to tackle the problem of organic waste treatment,with specific regard to food waste. This is done by optimizing the energy consumption of the aerobic pro-cess of composting, using the heat produced to solve a second important waste management problemsuch as the sewage waste treatment. Energy and mass balances are presented in a preliminary feasibilitystudy. Referring to a composting plant with a capacity of 15,000 t/y of food waste, the estimation of thepower from recovered heat for the entire plant resulted about 42 kW. The results demonstrated that theenergy recoverable can cover part of the heat necessary for the treatment of sludge generated by the pop-ulation served by the composting plant (in terms of food waste and green waste collection). The additionof a renewable source such as solar energy could cover the residual energy demand. The approach is pre-sented in detail in order for it to be replicated in other case studies or at full scale applications.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

EU waste policy is based on the concept of a ‘‘waste hierarchy’’,which specifies preferred options for managing waste. The best isthe prevention of waste generation, while the worst is the disposalof non-valorizable materials in landfills (De Feo et al., 2013). Withregard to the prevention of waste generation, much progress hasbeen made especially in the packaging industry, by changing therange of products on the market and even trying to change thehabits and behaviour of the general population (Valerio, 2010; Tor-retta, 2013; Ciuta et al., 2012). Recycling and energy recovery aresome of the intermediate options (Fricke et al., 2011). Regardingrecycling, there have been numerous studies and applicationsaimed at recovering reusable materials by minimizing the environ-mental load. Therefore EU member states and others (Torrettaet al., 2012; Vaccari et al., 2012), have focused on the collectionof waste that does not belong to traditional circuits of recycling,for example WEEE (Torretta et al., 2013a), vegetable oils (Torreset al., 2013) and especially the organic fraction of waste – withparticular reference to less developed nations. The techniques forenergy recovery as well as the technical areas of research and

development have been very different, but always trying to maxi-mize the energy efficiency of waste subjected to the treatmentprocess and to minimize the economic and environmental implica-tions of the treatments themselves. Of course the most significantdevelopments have focused on the various technologies for recov-ery of energy from waste to be burned (Shao et al., 2010; Radaet al., 2013a; Torretta et al., 2013b; Lombardi et al., 2013; Consonniet al., 2011; Mancini et al., 2010) or anaerobic treatment for theproduction of methane (Rada et al., 2013b; Callegari et al., 2013;Martinez et al., 2013; Morris et al., 2013).

In this frame composting is a process by which material isrecovered. The aim is to achieve a sufficiently stable state, thusallowing easy storage and possible use as a humified matrix. Com-posting has a lower impact on the climate than burning or burying(Morris et al., 2013; Burnley et al., 2011; Iakovou et al., 2010).

Organic waste, and in particular food waste, is one of the mostimportant fractions of waste, thus the crucial role that compostingcan play in a well-functioning integrated waste management isobvious. During this aerobic process, variations in the organic ma-trix produce a considerable amount of heat; part of which can beremoved and reused in other applications. This feature can beexploited for heat recovery, thus obtaining process optimization.

For instance, the conventional energy exploitation of food wasteis based on different options depending on its availability in mixedwaste (residual municipal solid waste, RMSW) or as a source

http://dx.doi.org/10.1016/j.wasman.2014.02.0130956-053X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +39 0332218782; fax: +39 0332218779.E-mail address: vincenzo.torretta@uninsubria.it (V. Torretta).

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separated stream (selective collection). The following four casescan thus be considered:

� Sanitary landfilling (Rada et al., 2010): taking into account apotentially generable amount of biogas from food waste (Curryand Pillay, 2012) and a rate of recovery (Beylot et al., 2013), theenergy available results around 1.5 MJ/kg of food waste.� Anaerobic digestion (Andreottola et al., 2012): the amount of

biogas generated can be fully exploited apart from an amountsent to a torch when the biogas engine is under maintenance;thus the energy available from food waste is higher than theprevious case and is concentrated in a shorter period thanksto the optimization of the process. However, the residual vola-tile solids after conversion into biogas may be lower in landfills,thus the biogas available cannot be the same of the case of ananaerobic reactor.� Direct thermo-chemical treatments (Bernstad and la Cour,

2012) and indirect thermo-chemical treatments (Rada et al.,2009): the amount of energy available is related to the lowerheating value (LHV) of food waste; considering an LHV equalto 1.7–6.3 MJ/kg of food waste (Bernstad and la Cour, 2012), thisvalue represents the energy available for recovery.

Each pathway is related to different energy recovery efficien-cies, depending on the suitability of either combustible gas orhot off-gas.

A different perspective concerns the aerobic processes. Indeed,the fourth case (indirect thermo-chemical treatments) is relatedto bio-drying as a pre-treatment for energy explotation (Radaet al., 2009). In this case food waste is the engine of the process(Ragazzi et al., 2011) and its consumption is related to the evapo-ration of water in order to concentrate the initial LHV of RMSWinto a lower amount of mass.

Also in bio-stabilization, before landfilling, the role of foodwaste is to guarantee an adequate exothermy, but without energyrecovery.

The role of food waste in composting is twofold: to support exo-thermy in order to accelerate the bio-chemical reactions and guar-antee an adequate mass flow of the final product. Heat in theexhaust air is not exploited in conventional composting.

As LCA application can demonstrate, composting guaranteesenvironmentally favourable results compared with the mass incin-eration of food waste. However, aerobic composting is less pre-ferred compared to anaerobic digestion due mainly to CO2 andNH3 emissions, as well as, low amount of bio-compost produced(Khoo et al., 2010).

In this paper an original energy recovery option from compost-ing is analyzed.

The extraction of heat from composting has already been stud-ied in the past, but the originality of this preliminary feasibilitystudy lies in the application of the recovered heat. The temperaturereached by the water inside the heat exchanger prevents it frombeing re-used in district heating plants, where operating tempera-tures reach 60–80 �C. An application for lower temperatures istherefore needed. A team from the University of Edinburgh thusconducted a study to assess the feasibility of using heat recoveredfrom compost to heat the living spaces of a farm next to the samecomposting facility (Irvine et al., 2010). The University of Kentuckyproposed a project in which students had to first estimate the heatremoved from the composting of leaves from a local companynearby and then design a greenhouse used to grow plants, to en-able cultivation in the winter months (Schmaltz, 2008). A furtherapplication of the heat extracted from compost can be found in acomposting plant in Canada at GMSC (Greater Moncton SewageCommission) Biosolids Composting Facility. In the above men-tioned company, composting is done outside, where the piles are

placed on cement platforms and then covered with special fabricsto limit heat dispersion. During the winter, when the climate isparticularly harsh, layers of ice formed at the edges of the plat-forms, thus preventing the smooth running of operations. A heatexchanger located in the floor below the compost heaps was con-structed to heat the frozen areas (Allain, 2007).

The heat removed from composting can be used in differentways. An interesting solution would be to exploit it in a green-house for solar drying of sewage sludge. Disposing of sludge ishowever another major problem, since sewage sludge storage inlandfills involves:

� Hygienic problems due to the presence of pathogens.� Practical problems, because it occupies a considerable volume

of space, due to the fact that it must be mixed with a largeamount of inert material which serves as a structuring agent.� Problems of an environmental nature, because it can generate

biogas, and thus possible emissions of methane.

The cost of sludge treatment is an important part of the operat-ing funds of local governments. One option is drying, which re-duces the sludge volume before it is further treated (Smollen,1990). Solar drying in greenhouses, in particular, is a low mainte-nance system, where the demand for thermal energy is entirelycovered by the sun (Ekechukwu and Norton, 1999).

The main idea of this work is the need to pursue a sustainablealternative that can meet the need to dispose of organic wasteand sewage sludge. In our feasibility evaluation we used a bio-cellcomposting system, fed with food waste and green waste, whilethe solar drying plant was a greenhouse with transparent cover.The heat was recovered using a heat exchanger positioned in theupper part of the bio-cell. The evaluation of the solar drying ofsludge was carried out by setting the balance of mass and energyand considering different operational scenarios with the additionof a bulky agent. In fact, the characterization of food waste and lo-cally available bulking agents are a prerequisite for optimizingcompost recipes. The bulky agents (hay, straw, pine wood shav-ings, cardboard, left over cattle feed and wheat residue pellets)with the highest moisture adsorption capacity were found to bewheat residue pellets, followed by chopped straw. Wheat residuepellets, chopped hay and leftover cattle feed all present a balancedC/N ratio (Adhikari et al., 2008).

Our study was designed to find the best way to manage the or-ganic fraction of the waste and sludge from wastewater treatment.We believe that our solution is sustainable, both from a technicalpoint of view as well as in economic and environmental terms:the heat produced by the process of aerobic composting of the or-ganic fraction of the waste is exploited in the solar drying of sludgefrom wastewater treatment. It is thus a solution that could be veryeffective in full-scale applications in actual plants.

2. Materials and methods

2.1. Plant description

The bio-cell technology was chosen for studying the feasibilityof the composting plant, which consists of a closed constructionin special concrete, equipped with an aerated floor. According tothis configuration, pipes embedded in the concrete casting enablethe insufflation of air, which is necessary for the composting pro-cess, enabled by pipes embedded in the concrete casting. Eachbio-cell is supposed to be served by a fan located at the rear ofthe tunnel: the suction port of the fan is connected to a pipelineequipped with two dampers. The process air can therefore comefrom the inside of the tunnel (through a recirculation air damper)

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or from the pipe conveying the fresh air coming from outside thebio-cell (via a damper). The air which has not been re-circulatedis ducted through a further opening located on the bio-cell cover,and finally reaches the scrubbers of the bio-filter. This openingtoo has a damper in order to ensure a vacuum inside the bio-celland to avoid any unpleasant odours reaching the surrounding envi-ronment. Each cell has a manual gate for hermetic closure and awater spray for humidification. The original element of the bio-cellis the heat exchanger positioned in the upper part of the oxidationchamber.

In our feasibility evaluation, we considered a theoretical plantwith a yearly capacity of 15,000 tonnes. With an average of100 kg ab�1 year�1 for the collection of organic waste in northernItaly, each facility would be designed to serve 150,000 inhabitants.The capacity of the plant and the per capita collection of organicwaste come from the planned choices and expected efficienciesin a region in the North of Italy (PAT, 2006). Considering an averageresidence time of 21 days for each bio-cell, 6 bio-cells are needed,with a capacity of 164 tonnes per single bio-cell.

The solar drying plant considered for this feasibility studyis a greenhouse with a transparent cover of honeycomb polycar-bonate. It is divided into two compartments, both with the samedimensions:

� Length: 30 m.� Width: 12 m.� Height: 4 m.

To reduce transmission losses in the greenhouse, the long sideof the greenhouse was matched with the overall width of the com-posting plant, which consists of six bio-cells, each of 5 m in width.This side of the greenhouse faces north because the contribution ofradiation is zero. In this way the plant is optimal, also avoiding anyloss of heat from the heat exchanger. By extracting heat from thebio-cells where the composting takes place (Fig. 1), the compostheats the floor of one of the two lines of the plant for drying. Asthe calculated thermal power would not be sufficient to coverthe entire surface of the greenhouse, under-floor heating is sup-posed to be installed in only one of the two compartments. Becauseof the low drying temperatures in the rooms, between 25 �C and45 �C, the risk of technical problems is limited. The ventilation ofthe solar greenhouse can be essentially provided by fans arrangedvertically (used for the expulsion of exhaust gases) and horizon-tally (used to create air in a turbulent motion on the sludge andto dry it).

2.2. Theoretical approach

The substrate to be composted consists of food waste and greenwaste. The characteristics with regard to the content of H2O, solidfraction S and volatile solids content was calculated on an averagemass percentage:

H2Osubstrate ¼H2O �%massð Þfood þ H2O �%massð Þgreen

ð%massÞfood þ ð%massÞgreenð1Þ

Ssubstrate ¼S �%massð Þfood þ S �%massð Þgreen

ð%massÞfood þ ð%massÞgreenð2Þ

ASHsubstrate ¼ASH �%massð Þfood þ ASH �%massð Þgreen

ð%massÞfood þ ð%massÞgreenð3Þ

As a first approximation, the solid content of green waste wasconsidered neutral during the first 21 days of the process. In thiscontext, it is useful to consider that the behaviour of ligno-cellulosicmaterials can be approximated to that of inert solids, at least withregard to the first three weeks of the trial. However for longer peri-ods, there is a greater difference and the maturity of the compostfrom wood material has a great impact on the composition of humicacids (De Guardia et al., 2010; Fukushima et al., 2009). The potentialrole of an additional bulky agent can easily be described using thesame approach, with a limited effect on the results (Adhikariet al., 2008; Chang and Chen, 2010). In fact the addition of the bulkyagent with a moisture level comparable to the final compost andbiologically non-biologically reactive solid in the first two to threeweeks of the process does not modify the results (Adhikari et al.,2008). The fraction of green waste that is not lignocellulosic playsa secondary role in the balance of heat generated by the biochemi-cal reactions in the first 21 day; that depends on three aspects:

– The lignocellulosic content of green waste can be assumedlower than 50% of the total (Francou et al., 2008).

– The biological reactivity of this fraction is significantly lowerthan the one of food waste (Francou et al., 2008).

– The green waste stream is about one order of magnitude lowerthan the one of food waste in the considered region (PAT, 2006).

Concerning the sewage sludge the problem of the treatmentand disposal of this sludge constitutes 30–40% of the cost ofdesigning and building a plant and 40–50% of its operating costs(Wei et al., 2003). It is thus clear how important the correct man-agement of sludge is. Thus we analyzed how to dry the sludge inorder to reduce the volume and facilitate storage; thus alsodecreasing the cost of treatment. The production of 20% dehy-drated sludge is in the range of 180–340 g ab�1 day�1.

The percentages by mass of the chemical components of the in-put substrate can be assessed from a basic analysis of the substrateitself. Carbon C, hydrogen H, oxygen O and nitrogen N are the keyelements involved in the oxidation reaction. The organic substancecan be approximated by the formula CaHbOcNd where a, b, c, d arecoefficients derived from the percentage by weight of the individ-ual elements divided by their molecular weight. This formula ap-plies to the organic fraction of the substrate and does not includeash, which is why the weight percentages of individual itemsshould be corrected to take into account the total weight of theash. The oxidation of organic matter (and therefore only the biode-gradable volatile solid BVS) can be described by the chemicalreactions (Haug, 1993) that allow the calculation of oxygen con-sumption and CO2, H2O, NH3 generation.

It is often difficult to estimate the reaction heat for organicwaste from tabulated values of enthalpy. Usually the compositionof this waste is unknown. A reasonably accurate empirical formulafor organic compounds is to assume that 1 g of COD of the organicwaste releases approximately 3.3 kcal (Angelidaki et al., 2000). Theheat released per unit of COD is relatively constant because theCOD is a measure of transferred electrons. During aerobic oxida-tion, 4 mol of electrons must be transferred for each mole of oxy-gen required by the substrate.Fig. 1. Scheme of the integrated plants.

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The COD is thus proportional to the number of electronstransferred during aerobic oxidation. Combustion heat for electrontransfer is fairly constant and is approximately 26.05 kcal perelectron equivalent; since oxygen requires four electrons, thecombustion heat is 104:20 kcal mol O�1

2 . This is equivalent to3.260 kcal gCOD�1 (Haug, 1993).

The mass balance, according to the laws of thermodynamics, isdivided into three phases of matter: solid, gas and water. The solidfraction can be divided into ash and volatile solids fraction. The ashis inert and for calculation purposes it is regarded as conservative,and the volatile solids fraction can be further divided into non-bio-degradable volatile solids (NBVSs) and biodegradable ones (BVSs).BVSs are responsible for the decrease in weight of the solids, duringexhaust gases are formed.As a first approximation, the followinghypothesis is proposed: the heat released during oxidation ismostly removed from the hot and wet gases leaving the system;heat will also heat the solids output and in part will be dispersed.It is assumed, therefore, that the heat released by the organicdecomposition (HORG) heats the input air (HDGASO for dry air,and HSWVI for water vapor), provides the latent heat of vaporiza-tion HLAT and warms the water vapor evaporated at a temperatureoutput (HSWVO).

These quantities are calculated as follows (with some obvioussymbols, such as m = mass):

� Heat released from the bio-oxidation of organic matter:HORG = kgBVS QBVS.� Heat to warm up the dry air at Tout: HDGASO = m cp,air DT.� Heat to heat the water vapor to Tout: HSWVI = m win cp,wat vap

DT.� Latent vaporization heat: HLAT = m wnet Qlat.� Heat to warm up the water vapor evaporated (wnet) to Tout:

HSWVO = m wnet cp,wat vap DT.

Considering only these terms and neglecting other minor con-tributions, a solution can be found:

HORG ¼ HDGASOþHSWVIþHLATþHSWVO ð4Þ

The specific humidity w can be calculated using the followingthermodynamic formula:

w ¼ 18:01528:96

P:V:P:AIR � P:V:

ð5Þ

where P.V. is the vapor pressure and P.AIR is the air pressure. Thevapor pressure can be calculated through the relative humidity:P.V. = R.H. P.V.S., where P.V.S. is the saturation vapor pressure. Thelatter is a function of the temperature and can be calculated usingAntoine’s equation:

log PVS ¼ 5400� 1838:7T � 31:7370

½atm� ð6Þ

where T is the temperature of the air.The moisture removed from the air flow will then be the

difference:

wnet ¼ wout �win ð7Þ

where win is the specific humidity of the incoming air, and wout isthe specific humidity of the air leaving (the outlet air is supposedsaturated, so R.H.out = 1).

The value found for the air flow required m is a first approxima-tion, to have an order of magnitude; which will then be manipu-lated to meet the energy balance.

The aeration rate QAIR is the input of the model, chosen fromthe amount of air (already calculated) required to remove heatfrom the reaction and maintain the process at a defined tempera-ture. To calculate the saturation vapor pressure, Antoine’s equation

is used again, with the temperature T of the air. The vapor pressureof water P.V. is obtained by multiplying the relative humidity bythe saturation pressure. Using the equation of perfect gases, themass flow of dry air DAIRI and water vapor WATVI are obtained.

The mass flow of dry gas output DGASO is determined by theamount of gas input to which the gases resulting from the decom-position of the organic biodegradable substrate are added (calcu-lated with the stoichiometric reaction). The pressure of watervapor PVSO is obtained again using Antoine’s equation, but usingthe temperature of the exit gas. In the first approximation, it canbe assumed that the exit gas is saturated and, thus, that the vaporpressure can be considered equal to that of saturation.

Bio-organic oxidation involves the production of water WATP.This quantity was thus calculated from the stoichiometric reaction:

WATP ¼ H2O produced � BVS ð8Þ

With relatively dry substrates (as with the organic fraction)water needs to be added. The compost produced must have an im-posed moisture content, usually a value of 40% (Haug, 1993).Knowing the solid content in the final compost, the amount ofwater can be calculated back by the water mass balance.

As regards the calculation of the sensitive heat, the formula for aprocess at constant pressure is adopted:

Q ¼ mcpDt ð9Þ

The contributions of the input energy are:

� The sensitive heat to heat the substrate.� The sensitive heat to heat the water.� The sensitive heat to heat the dry air.� The sensitive heat to heat the water vapor associated with air.� The heat released by the oxidation of organic matter.

The contributions of output energy are calculated in a similarway, with the difference that the sensitive heat of solids, water,gas, and water vapor is related to temperature output. In additionto these terms, the latent heat of vaporization should also be takeninto consideration, as it causes some of the water in the substrateto evaporate.

The type of heat exchanger chosen for the preliminary feasibil-ity study was a stainless steel tube located in the upper part of thebio-cell. Thus the exhaust gases at a temperature of 55 �C exchangeheat by convection and conduction with the water inside the tubes.The tubes are located in the upper part of the bio-cell to avoid mes-ophilic temperatures inside the compost pile; the heat exchanger istherefore located to ensure that the compost produced meetshygiene standards (Klejment and Rosinski, 2008). In fact, the im-pact that cooling would have on the composting process is notknown.

Despite the growing importance of the solar drying of sludge,there are currently no models that predict the drying rate. The Uni-versity of Hohenheim, in collaboration with the Thermo-SystemCompany, has developed a predictive model based on a series ofdata collected from existing plants (Seginer and Bux, 2006). Theoptimal control of the drying process requires a good predictivemodel for the drying rate in a range of likely conditions. The meth-od used is the water vapor balance, which consists in measuringthe relative humidity of the air inlet and outlet w and multiplyingthe difference wout–win by the air density q and the volumetricflow rate Qv. The result is an estimation of the evaporation rate:

E ¼ q wout �winð Þ � Qv ¼ q � Dw � Qv ð10Þ

In general the control variables (such as ventilation, mixing theair and mud) and the environmental variables (weather), deter-mine the drying speed. Of the various predictive models, the mul-tiplicative model was chosen:

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E ¼ qQv1:962� 10�11 � ðR0 þ 1100Þ2:322 � ðT0 þ 13:0Þ1:292

� ðQvÞ�0:577 � ðQ m þ 0:0001Þ0:013 � ðrþ 0:26Þ�0:353 ð11Þ

where R0 is expressed in W/m2, T0 in �C; Qv and Qm are in m3 m�2

h�1 and r in kgsolids/kgsludge; q is equal to 1.2 kgair m�3 and E is inmm h�1, that is equal to kgwater m�2 h�1.

The equation highlights that r has a negative exponent, whichmeans that, with the increase in solid content, the evaporation rategradually decreases. By analyzing the function it becomes clearthat both the solar radiation and temperature have significantlyinfluenced the evaporation rate.

To estimate the potential of the plant it is assumed that thepower of the solar greenhouse is discontinuous. Thus the sludgeis distributed on the floor with a thickness of 40 cm and coversthe entire surface. The dried mud is extracted when it reaches a so-lid content equal to the desired value. At the end of the cycle, it isrecharged again and the procedure is repeated.

In order to assess the potential of the plant, it is useful to deter-mine the heat required to evaporate the water from the mud. It isthus is necessary to determine the water content in the mud at thebeginning. First, the content in the dry mud and the water contentof the complement:

drycontent ¼ 100� Uinitial

100�wetweightsludge ð12Þ

waterinitial ¼Uinitial

100�wetweightsludge ð13Þ

The dry content is considered as conservative; it is then possibleto calculate the amount of water by imposing a certain degree ofmoisture in the final mud Ufinal:

waterfinal ¼Ufinal

100� Ufinal� drycontent ð14Þ

The total amount of water to be removed is obtained by thedifference:

water to remove ¼ waterfinal �waterinitial ð15Þ

Once the drying temperature and the corresponding latentvaporization heat have been established, the energy required tovaporize the water can be determined easily:

Q water to remove ¼ Q latentðwater to removeÞ ð16Þ

The sensitive heat required for heating the sludge must also beadded to this energy, evaluated with a specific heat equal to that ofwater (cp = 4.186 kJ kg�1�C�1). The following formula:

Q sludge ¼ m � cp � DT ð17Þ

was used, where m is the mass of mud and DT is the difference be-tween the drying temperature and the temperature of the mud. Theheat required to evaporate a kilogramme of water is calculated byadding the two contributions and dividing by the amount of waterto be evaporated:

Q needed ¼ ðQwater þ Q sludgeÞ=water to remove ð18Þ

3. Results and discussion

From an elementary analysis of the organic and green fraction(Consonni and Viganò, 2011), the average composition of the inputsubstrate can be calculated (Table 1). Values reported in Table 1 re-fer to Italy.

With regard to the stoichiometry of the oxidation reaction ofthe organic substance, the values in Table 2 are obtained fromthe following expression:

C3:983H7:686O2:629N0:168 þ 4:464 O2

¼ 3:983 CO2 þ 3:591 H2Oþ 0:168 NH3 ð19Þ

The calorific value of the organic fraction is calculated, giventhat from the stoichiometric analysis made earlier, the COD is1:426 kgO2 kg�1

BVS:

QBVS ¼ 1:426kgO2

kgBVS� 3260

kcalkgCOD

¼ 4649:5kcal

kgBVSð20Þ

The mass balance (Fig. 2) was calculated based on a tempera-ture of 20 �C and a relative humidity 0.75. During the process thereis a reduction of about 47% in substance weight. The water vaporincreases of one order of magnitude as effect of the biological pro-cess. Ash is not affected from the process, as not involved in thebiological reactions.

The energy balance was calculated under the same climaticconditions. The monthly change is rather low because the condi-tions inside the bio-cell are fairly constant. Analyzing the input en-ergy components (Fig. 3), it is clear that almost the whole fractionis made up of the bio-oxidation of organic matter (HORG). Only asmall part is contained in the sensitive heat of dry air (HDAIRI)and water (HWI) input to the system. For the output energy(Fig. 4), about 75% is latent vaporization heat (HLWVO). The latteris responsible for the dehydration of organic matter and its conse-quent reduction in weight. Just over 20% is the sensitive heat of thehot gases leaving the system. The remaining 5% is divided into neg-ligible parts.

A value of 40 W m�2 K�1 was used for the overall heat transfercoefficient U. The method e-NTU set the size of the heat exchanger(Cengel, 2005). The exchange area is calculated considering a tubediameter of 2.54 cm and a length of 60 m. This value is obtainedassuming that the heat exchanger completes two circuits withinthe bio-cell. The calculated value (7.082 W) is related to a singlebio-cell where the temperature of the hot fluid (exit gas) is 55 �C,with a fluid flow rate of 3090 m3 h�1 (corresponding to1.03015 kg s�1), with a specific heat of 1.004 kJ (kg �C)�1 and a heatcapacity rate of 1034.3 W (�C)�1. This condition of the exit gasdetermines an output temperature of the cold fluid (water in theheat exchanger) of 13 �C. Considering the water flow rate of3 L min�1 (corresponding to 0.05 kg s�1), a specific heat of4.1868 kJ (kg �C)�1 and a heat capacity rate of 209.3 W (�C)�1,and applying the calculation to an exchanger surface area of9.58 m2 (NTU = UA/Cmin = 1.830) and a heat capacity ratio (Cmin/Cmax) of 0.202, the estimation of the power from recovered heatfor the entire plant is thus equal to 42,493 W.

Table 1Mean composition of the input substrate (S = solid fraction).

% mass H2O S ASH (wet wt)

Food waste 72.78 59.00 41.00 6.15Green waste 27.22 75.00 25.00 0.38Substrate 100.00 0.634 0.366 0.046

Table 2Production of reaction gases. Acronyms: each WT-FACTOR refers to the weight ratioof consumption or generation of a gas (O2, CO2, H2O, NH3); DGASFACTOR refers to theconversion of BVS into dry gas.

O2 consumed WTO2FACTOR 1.426 kgO2=kgBVS

CO2 produced WTCO2FACTOR 1.758 kgCO2=kgBVS

H2O produced WTH2OFACTOR 0.641 kgH2 O=kgBVS

NH3 produced WTNH3FACTOR 0.027 kgNH3=kgBVS

Change in dry gases DGASFACTOR 0.359 kgGAS=kgBVS

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In a study by the Ecole des Mines de Paris the benefits obtainedby coupling greenhouse solar sludge drying with two heat pumpswere analyzed: the first for heating the floor and the second forheating the air (Slim et al., 2008). In testing the different combina-tions, the floor temperature was in the range of a minimum of42 �C and maximum 68 �C. It is therefore clear that the tempera-ture of 48 �C (used in the case study) is compatible with the se-lected application; although it is the only source of additional heat.

A comparison of a conventional solar greenhouse (heated onlyfrom solar energy) and a solar greenhouse with additional heatwas carried out. Firstly it is assumed that the water content inthe final sludge reaches 40%. As a reference, the configuration of

the plant under standard conditions is taken into consideration:Qm ¼ 150 m3 h�1 m�2

floor and Qv ¼ 100 m3 h�1 m�2floor.

If we consider the size of the greenhouse of 720 m2 and a thick-ness of the sludge of 40 cm (with a density comparable to that ofwater) we can calculate the amount of water that must evaporateto bring the dry content from 20% to 60%:

� Amount of sludge treated cycle: 288 tonnes.� Dry content (assumed conservative): 57.6 tonnes.� Initial water content: 230.4 tonnes.� Final water content: 38.4 tonnes.� Amount of water to evaporate per cycle: 192 tonnes.

Fig. 2. Mass balance diagram (kg/cycle).

Fig. 3. Input energy overview.

Fig. 4. Output energy overview.

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In Fig. 5 the X-axis shows the days of the year, while the Y-axisthe amount of evaporated water accumulated. As the value used todetermine the end of one cycle and the beginning of the next one isthe amount of water to be removed and its multiples; it is thuspossible to determine the capacity of plants.

In the case of the conventional plant, the annual plant potentialis 1440 tonnes, which means 2 tonnes m�2 year�1, with a reduc-tion in volume that is around 66%. In the case with additional heat,six cycles can be completed; so the plant potential increases to avalue of around 2.4 tonnes m�2 year�1.

A comparison with similar real plants was also made. The sys-tem in Murnau, located in Bavaria, is also equipped with a heatedfloor, but the heat recovered comes from the wastewater treat-ment plant and has a potential of 2.4 t m�2 year�1 (Wendewolf,2012). The same result was obtained by a system with heat recov-ery in northern Italy, despite the fact that the weather conditionsare more favourable than Bavaria. This can be justified by the useof waste heat at a low temperature (about 48 �C) recovered fromthe composting process. From this analysis we can conclude thatthere is a good consistency between the potential of the systemunder study and real plants with similar characteristics.

Given the potential described above, our system can treat15,000 t year�1 of organic fraction (for a total of about 150,000people) and dry 1700 t year�1 of mud (for a total of approximately20,000 residents). In the considered preliminary feasibility study itis possible to make a comparison, in terms of energy recovery, be-tween the proposed solution and other solutions for food wastemanagement:

� Sanitary landfilling: when no pre-treatment is present, consid-ering the amount of biogas that can be collected, the grossenergy available can be assessed at around 20 � 106 MJ year�1

(assuming a collection of 60 m3CH4=t of organic fraction and a

percentage of methane as 60% in the biogas).� Thermochemical plant: considering an LHV of 1.7–6.3 MJ/kg of

food waste, the gross energy available for recovery is 25–94 � 106 MJ year�1

� Biostabilization before landfilling: food waste can have a con-siderable exothermy, but without energy recovery (this optionis compatible with the proposed solution of energy recovery).� Anaerobic digestion: the energy available from the organic frac-

tion is similar to the one of the sanitary landfill scenario.

As expected, compared to the above mentioned options, thehere proposed solution has an energy balance less favourable: in-deed the fact that the recovered heat is ‘‘residual’’ energy limitsthe order of magnitude of the recovery.

Calculations could be regenerated also with the addition of abulky agent, as the data presented here relate to a theoretical case

where local green waste can act exactly like the bulky agentneeded.

4. Conclusions

Our system was designed to solve two problems in parallel:recycling the wet fraction and drying sewage sludge. Merging thetwo plants into a single larger one optimizes the two individualprocesses. In fact the drying greenhouse has a better yield in theseconditions than in a separated configuration: by constructing thecomposting plant adjacent to the greenhouse there is a decreasein dispersion loss from the greenhouse and a reduction in heatlosses from the heat exchanger.

A low increase in the initial cost of the heat exchanger is justi-fied by a reduction in operating costs for the entire plant. In thelong run, the solution appears to be effective, both technicallyand financially. Also from an environmental point of view, theplant has been designed to have the best renewable energy re-sources available. The demand for heat energy is totally coveredby solar energy and by the recovery of heat from the compostingprocess.

The plant could be improved by installing solar panels thatwould increase the temperature of the carrier fluid of the heat ex-changer, or to heat line two of the greenhouse drying. However, thecomplexity of the plant would increase.

Our study was designed to find the best way to manage the or-ganic fraction of the waste and sludge from wastewater treatment.We believe that our solution is sustainable, both from a technicalpoint of view as well as in economic and environmental terms:the heat produced by the process of aerobic composting of the or-ganic fraction of the waste is exploited in the solar drying of sludgefrom wastewater treatment. It is thus a solution that could be veryeffective in full-scale applications in actual plants.

Considering that our work was a preliminary feasibility study, apilot- or full-scale experimentation is absolutely needed in order tohave a confirmation about the positive and/or negative aspects ofthe designed system.

Thus future work will involve cross-checking these results witha pilot scale plant that reproduces the features of the system. Theexperimental data obtained would thus serve as feedback for theimplementation of a program to simulate the behaviour of the en-tire plant, also with different amounts of bulky agent in input. Thiswould allow for a more accurate estimation, based on data aver-aged over shorter time intervals, instead of monthly mean dailydata.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.wasman.2014.02.013.

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