lable at ScienceDirect

Renewable Energy 66 (2014) 461e467

Contents lists avai

Renewable Energy

journal homepage: www.elsevier .com/locate/renene

Electrical energy production from the integrated aerobic-anaerobictreatment of organic waste by ORC

Francesco Di Maria*, Caterina Micale, Alessio SordiDipartimento di Ingengeria, Università di Perugia, Via G Duranti 67, 06125 Perugia, Italy

a r t i c l e i n f o

Article history:Received 27 June 2013Accepted 30 December 2013Available online

Keywords:Aerobic treatmentAnaerobic digestionElectrical energyOrganic Rankine cycleOrganic wasteFigure of merit

* Corresponding author.E-mail address: francesco.dimaria@unipg.it (F. Di M

0960-1481/$ e see front matter � 2014 Elsevier Ltd.http://dx.doi.org/10.1016/j.renene.2013.12.045

a b s t r a c t

The energetic performance of an ORC system fueled by the heat generated from the integrated aerobic/anaerobic treatment of organic waste was analyzed. The temperature and heat content of the exhaust airarising from the aerobic treatment were increased by the combustion of the biogas produced by theanaerobic digestion of a fraction of the same waste. On the basis of the amount of excess air exploited inthe process, for each tonne of organic waste treated, it was possible to produce from 30 to 90 kg ofexhaust air per day with a mean temperature ranging from 330 to 340 K. By processing from 0.5% to 16%of the whole organic waste in an anaerobic digestion section instead of the aerobic one, it was possible toincrease the exhaust air temperature from 340 to 510 K, leading to an increase in the ORC size from about0.05 to about 1 W/tonne/year. The best energetic utilization of the biogas was achieved for ORCcompression ratios from 1.5 to 2 and for maximum air temperatures from 335 to 340 K. In these con-ditions, by using a micro-ORC system (i.e. <15 kW), it was possible to convert about 20% of the energycontent of the biogas into electrical energy.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Anaerobic and aerobic biological treatments are widely exploi-ted in processing organic waste (OW) both for energy productionand for biological reactivity reduction before final recovery and/ordisposal [1e7]. In particular anaerobic digestion (AD) can lead tothe production from about 80 Nm3 up to 210 Nm3 of biogas pertonne of processed OW. The methane concentration usually rangesfrom 50 to 70% v/v [8e14], whereas the other main component isCO2. The corresponding lower heating value (LHV) varies from18,000 kJ/Nm3 to 24,000 kJ/Nm3 and biogas can be exploited as fuelin internal combustion engines for renewable energy production.

The viability of AD is greatly influenced by plant size and by thevariation in the rate and composition of OW during the year[11,12,15]. Aerobic treatments are used to reduce both OW and ADdigestate residual biological reactivity before disposal or for theproduction of organic fertilizer, depending on OW quality [16]. Asextensively demonstrated [17e19], aerobic treatment can lead tolong-term emission reduction in landfills, up to 90%. If OW qualityis compatible with the characteristics of organic fertilizer [20e22],aerobic bioconversion is generally used to convert the OW to sub-stances exploitable for agricultural use. During the aerobic process,

aria).

All rights reserved.

bacteria oxidize the organic matter [23], generating about 17,000e18,000 kJ/kg OM [24] of heat. Due to the high initial concentrationof OM, heat release is particularly high in the first 2e4 weeks,causing an increase in the OWmass and consequently in process airtemperatures. Maximum temperatures achieved in full-scale fa-cilities range from 55 �C to 75 �C, depending mainly on thermalloss, OWmoisture content OM content and process air rate [25,26].In a previous study, Di Maria et al. [26] evaluated the possibility ofrecovering this heat for civil use by heat pumps. Results showedthat the process exhaust air temperature ranged from about 55 �Cto 70 �C and the amount of heat ejected daily ranged from about120 to about 350 kWh/tonne depending mainly on the amount ofOW treated and the process air rate.

Another possible solution for recovering this amount of lowtemperature heat is by using the organic Rankine cycle (ORC) sys-tem. The ORC uses the same components as a conventional steampower plant, but uses an organic fluid to extract low-grade thermalenergy to generate electricity. ORC is commonly used in practicalindustrial applications such as biomass power [27,28] solar power[29] also aimed at water production [30], ocean thermal energyconversion, geothermal power [31,32], and waste heat recoverypower [33]. Bidini et al. [34] analyzed the exploitation of ORC in anintegrated gas turbine-geothermal power plant for recovering low-grade heat ejected from gas turbine exhaust after geothermal fluidheating. Gewald et al. [35] showed that ORC can improve the effi-ciency of landfill gas-fired power plants by about 12%. Desideri and

F. Di Maria et al. / Renewable Energy 66 (2014) 461e467462

Di Maria [36] reported that the exploitation of ORC for recoveringexhaust heat from a humid air turbine system can lead to an overallcycle efficiency increase from 1.6 to 2.2%. Wang et al. [37] analyzedthe effect of different working fluids on ORC efficiency for enginewaste heat recovery. Similarly Hung et al. [38] investigated theeffect of different organic working fluids on ORC efficiency usingheat generated by solar pond and ocean thermal energy. ORC is apromising solution for decentralized, small- (i.e. <100 kW) andmicro- (i.e. <15 kW) scale combined heat and power generation[39e41] and for this reason it is particularly used in biomass-firedplants. Even if its efficiency is low, between 6% and 17%, ORC haslowmaintenance and personnel costs [41]. Dong et al. [42] reportedthat costs are comparable with gasification in the same small- andmicro-scale range. Furthermore, among the small number ofcommercially viable biomass gasification systems, only a few havebeen shown to be economical [39]. On the contrary, several small-scale ORC systems are operating and their viability has been fullyproven [39e41]. Anyway there is a lack of investigation about thepossibility of exploiting ORC for electrical energy production fromthe heat produced during the bioconversion of OW. Considering thelarge amount OW produced yearly in the UE27, about100,000,000 tonnes [42] the efficiency of an ORC using the heatgenerated by integrated aerobic/anaerobic bioconversion of OWwas theoretically investigated. System performances were alsoinvestigated using specific figures of merit.

2. Material and methods

2.1. Aerobic treatment model

One of the most widespread technologies for the first 2e4weeks of OW aerobic treatment is the use of several concrete bio-cells (Fig. 1) operating in parallel and in batch mode. Full-scalebiocells have a mean width and height of about 5 m and a lengthgenerally more than 15 m, depending on the amount of waste to beprocessed. Once loaded, usually by a wheeled loader, the cell is

Fig. 1. Scheme of the integrated aerobic-anaerob

closed by a specially designed door to avoid air leakage. Electricfans supply the air through perforated floor to pass equally throughthe OW heap. Exhaust air, with an increased temperature, is thencollected from the biocell roof before being discharged. The mainprocess parameters such as temperature (T), humidity (H) and airrate are usually monitored and controlled inside the biocells. Oncea large fraction of the initial rapidly biodegradable OM content hasbeen oxidized, causing a drop in heat generation and OW tem-perature, the further aerobic treatment is usually carried out byarranging the material in an open heap. The emptied biocell canthen be loaded with fresh OW for a new cycle. In any case thelargest amount of heat is generated around the 5th to the 20th dayof treatment and can contribute significantly to the heat for feedingthe ORC. OW is composed of several biodegradable substrates suchas fruit waste, meat waste, paper and yard waste, even if traces ofimpurities may also be present. From the results of a study per-formed for an Italian situation [12], the amount of impurities, likeplastics and metals in OW coming from high-quality sourcesegregation was <6% w/w. The other fraction >94% w/w wasbiodegradable materials. The elemental composition of the OM hasbeen investigated by several authors [23,24] who showed that theprincipal elements are C, H, O and N. According to these results, theaverage composition of the OM qA is defined by (Eq. (1)).

C6H10O4 (1)

The stoichiometry of the aerobic process is shown in Eq. (2),where the OM is oxidized generating mainly carbon dioxide, waterand heat.

ðC6H10O4Þx þ 6:5O2 ¼ ðC6H10O4Þx�1 þ 6CO2 þ 5H2O

þ 2578:6 kJ (2)

On the basis of Eq. (2), it is possible to quantify the l coefficientEq. (3) defined as the ratio between the effective ð _meÞ and stoi-chiometric ð _msÞmass air rate (kg/s) supplied during the process. To

ic system with organic Rankine cycle (ORC).

F. Di Maria et al. / Renewable Energy 66 (2014) 461e467 463

reduce the risk of anaerobic or anoxic conditions in full-scale fa-cilities, the value adopted for l is usually >>1.

l ¼ _me_ms

(3)

The reaction kinetics [43] are represented by a first orderequation, Eq. (4), where Ct represents the mass of oxidized OMwithrespect to the total (kg/kg) at time t (day). T (�C) is the processtemperature. Values of other constants in Eq. (4) are according to DiMaria et al. [26].

Ct ¼ 1� e�ð0:00632,1:066ðT�20ÞÞ$t (4)

Thermal losses occurring during the aerobic process involveconvection, internal thermal conduction, and mass transfer phe-nomena and are represented by the following:

1) Heat transfer by biocell walls;2) Heat exchange by the process air;3) Heat absorbed by water evaporation;4) Heat exchanged by spread water (if any);5) Heat absorbed by the OW mass.

The amount of heat generated during the aerobic process Eq. (2)is equal to that exchanged with the external environment. Thesystem temperature can be determined on the basis of this heatbalance. The global heat exchanged during was evaluated usingwell-known heat transmission relationships [26]. As previouslydemonstrated [24,26] more than 80% of this heat comes fromphenomena 2) and 3). The functional unit used in the present studywas 1 tonne of OW processed in the aerobic section (Fig. 1). Fordoing this, the aerobic process was simulated considering a meanaerobic facility size able to treat 20,000 tonnes of OW per year [12].Successively the simulated results were referred to a single tonne ofprocessed OW.

The exhaust air temperature was calculated by assuming atemperature difference with the OW mass of 5 K [26].

2.2. Biogas from AD

The amount of biomethane producible by a full-scale AD facilityis influenced by several aspects involving both AD technology, mainprocess parameters and OW composition. It is usually with respectto the volatile solids (VS) content of the biodegradable substratetreated and expressed as % by weight on dry basis. VS are all thecompounds that can be oxidized after combustion at 550 �C for24 h, including the OM. According to Themelis and Kim [24], theOM concentration was assumed to be 80% of VS, whereas the bio-methane production was assumed to be the average among thevalues reported in Table 1. The biogas generated during AD alsocontains other components such as CO2, H2, N2, NH3 and H2S. The

Table 1Specific biomethane production (Nm3/tonneVS) and methane content (% v/v) fororganic waste (OW).

Nm3/tonneVS CH4 (% v/v) Reference

400 56.0 [8],a

370 e [44],b

366 e [45],b

281 63.4 [14],b

200 60.0 [46],c

305.4 60.0 Assumed in this study

Bold value represnt specific bio-methane production assumed in this studya Full scale.b Lab scale.c Pilot scale.

global concentration of these compounds, with the exception ofCO2, was less than 1% v/v. For this reason and in accordance with[8,14,46], biogas was assumed to be a mixture of only 60% v/v CH4and 40% v/v CO2.

Finally themoisture content (% w/w) and VS concentration (% ondry basis) were assumed to be, respectively, 62.7% and 81%, as re-ported in Ref. [12].

2.3. ORC simulation model

One of the most relevant aspects concerning the ORC powersystem is the selection of the working fluid. Thermodynamic,economic and environmental properties of working fluids can besignificantly different. Several studies have been proposed[27,32,37] giving selection criteria mainly based on first and secondlaw analysis. Due to the complex aspects involving this choice, inthe present study the results proposed by Wang et al. [47] wereadopted. Wang et al. [47] proposed a screening model based on amulti-objective optimization including temperature differences,heat exchange efficiency and heat exchange surfaces. Results pointout that the optimal working fluid for the range of temperaturesused in the present study is R123 (Table 2). Fig. 2 shows a possibletemperature-entropy (Tes) diagram of the ORC (Fig. 1) oncecondenser temperature and evaporation pressure have beenimposed. Themathematical model used for ORC simulation is givenby Eqs. (5)e(8). In all these equations h (kj/kg) is the specificenthalpy of R123, subscripts s represent the isoentropic values,whereas numbers from 1 to 5 represent the points of the cycle.

The power absorbed by the pump (Wp) was evaluated with Eq.(5), assuming a given efficiency (hp), whereas the heat supplied totheworking fluid, including pre-heating and evaporation (Qin), wascalculated using Eq. (6). In both equations _m is the mass rate (kg/s)of the R123.

Wp ¼ _mðh2 � h1Þ ¼ _mðh2s � h1Þhp

ðkWÞ (5)

Qin ¼ _mðh4 � h2ÞðkWÞ (6)

During the expansion, from point 4 to point 5 (Fig. 2), the powergenerated (Wex) depends both on _m and expander efficiency (hex),on evaporator pressure and condenser pressure Eq. (7).

Wex ¼ _mðh4 � h5Þ ¼ _mðh4 � h5sÞhexðkWÞ (7)

The heat ejected at the condenser (Qc) is represented in Eq. (8).

Qc ¼ _mðh5 � h1ÞðkWÞ (8)

Aspects other than hp and hex that can affect system perfor-mances can be pressure drops and thermal loss occurring in ORCpipes and heat exchangers. An accurate evaluation of these

Table 2Main features of the organic Rankine cycle (ORC) and working fluid (R123).

Parameter Value Unit

ORChp 80 %hex 55 %heg 90 %DTpp 10 KTc 298 KTamb 293 KDT9,2, min 8 KR123Molecular mass 152.93 g/molBoiling point 300.97 KCritical pressure 3.662 MPa

Fig. 2. Example of a temperature-entropy (Tes) diagram for the organic Rankine cycle(ORC).

Fig. 3. Effective air rate and temperature vs time (day) for effective to stoichiometricmass air ratios (l) of 5, 7, 9 and 11 per single biocell.

F. Di Maria et al. / Renewable Energy 66 (2014) 461e467464

phenomena requires knowledge of the exact dimensions and fea-tures of all system components, which is not within the scope ofthis work. Assuming that full-scale ORC are also designed forminimizing system losses, according to the analysis performed byWang et al. [47], maximum pressure drops occurring in the heatexchanger were estimated to be about 1000e2000 Pa. Thermal lossoccurring in the evaporator and condenser are mainly influencedby surface area and temperature, ambient temperature and theinsulation layer. Considering the quite low operating temperaturelevels and drop and assuming the presence of an adequate insu-lating layer, these losses were very low and hence disregarded.

Hence according also with Refs. [29,38], only hp and hex wereconsidered.hp and hexwere assumed tobe 80%and55%, respectively[37]. The efficiency of the electrical energy generator (heg) wasassumed to be 90%. The minimum temperature difference betweenthe heating fluid and T3 (DTpp) (Fig. 2) was assumed to be 10 K [47].The temperature at ORC condensed (Tc), considering an ambienttemperature (Tamb) of 293 K, was assumed to be 298 K (Table 2).

Cycle performances were evaluated using the following figuresof merit:

1) ORC net electrical efficiency (hnet) Eq. (9), i.e. the ratio betweenthe electrical power by the ORC and Qin.

2) The heat utilization ratio (HUR) Eq. (10), i.e. the ratio betweenQae þ QAD and Qin. Qae is the heat extractable from the exhaustair arising from the aerobic section by cooling it from T7 (Fig. 1)to Tamb. QAD is the heat supplied by the combustion of the biogasproduced in the AD section. This figure allows the efficiency oftransferring the amount of available heat generated both by theanaerobic and the aerobic section to the ORC to be evaluated.Consequently HUR results always �1 which is Qae þ QAD � Qin.

3) The anaerobic digestion heat utilization factor (ADUF) Eq. (11)expresses the ratio between the electrical power of the ORCand QAD. This figure allows the efficiency in exploiting the heatproduced by biogas combustion to be evaluated.

hnet ¼�Wex �Wp

�$heg

Qin*100ð%Þ (9)

HUR ¼ Qae þ QADQin

(10)

ADUF ¼�Wex �Wp

�$heg

QAD*100ð%Þ (11)

b ¼ p2p1

(12)

Four different l values (5, 7, 9, 11) were assumed for evaluatingthe effect on the aerobic treatment air rate _me and exhaust tem-perature T7 (Fig. 1). For each l, ORC performances were investigatedfor different pressure ratios (b), for the pump outlet (p2) andcondenser (p1) pressures, Eq. (12), and for different T8 > T7. Theincrease in T8 was achieved by the combustion of a given amount ofbiogas produced by diverting a fraction of OW from aerobic treat-ment to the AD section.

3. Results and discussion

For processing about 20,000 tonnes of OW per year, the aerobicsection consists of 8 biocells of about 400 m3 each where the ma-terial stays for 4 weeks. Every 3 days a biocell is emptied and thenloaded againwith fresh material. For each biocell, by Eqs. (2)e(4), itwas possible calculate _ms, _me and T for each l value (Fig. 3). _me

increased until achieving a maximum value around the 4e6th day,after which it decreased constantly till the 28th day. Likewise, OWtemperature increased during the first 8e10 days and thenremained quite constant until the end of the process.

Jokela et al. [48] found temperature levels in the aerobic treat-ment of pulp and paper mill sludge ranging from 325 to 345 K. Yanget al. [49] showed a temperature of about 333 K during compostingof animal fodder. Similar temperatures values were also reportedby Vuorinen and Saharinen [50] during co-treatment of straw andmanure. Higher l caused a higher _me, but lower T.

Combining _me and T evolution for each biocell it was possible tocalculate the mean air rate and T7. Results show that T7 range fromabout 331 K to about 339 K depending on the l value (Fig. 4).Likewise, the mean _me for each tonne of OW ranges from about39 kg/tonne/day to 94 kg/tonne/day.

Under the assumption made, the ORC net electrical efficiencydepends only on b (Fig. 5). For b ranging from 1.5 to 3.5, hnet rangesfrom about 2% to about 6%. For ORC exploiting R123 as workingfluid, Wang et al. [37] reported a thermal efficiency ranging fromabout 9 to 10% with an evaporator and condenser temperature of406 K and 320 K, respectively, and a b of 8. Similar values have beenobtained with the ORC-R123 system using ocean thermal energyconversion and operating at condenser and evaporator tempera-tures of 278 K and 313 K, respectively [38]. At a fixed condensertemperature of 318 K and an evaporation temperature of 353 K, the

Fig. 4. Mean air rate and temperature (T7) for different effective to stoichiometric massair rate ratios (l) for each tonne/day of organic waste.

F. Di Maria et al. / Renewable Energy 66 (2014) 461e467 465

ORC-R123 system achieved thermal efficiencies of about 6e6.5%[33]. In all these cases the compression ratio ranged from about 3 to8. Schuster et al. [41] reported that for commercially availablebiomass-fired systems, electrical efficiency ranged from 9% to 18%.For the same systems with a power output >400 kW Dong et al.[39] showed efficiencies ranging from 16% to 18%. In these cases theheating fluid temperature was usually >400 K.

In the present study the maximum level achievable for b islimited due to the temperature level of the exhaust air arising fromthe aerobic treatment section (Fig. 1). An excessive increase in thecompression ratio could prevent any heat recovery from the

Fig. 5. Anaerobic digestion heat utilization factor (ADUF) and net electrical efficiency (hnet) v(b), 9 (c), 11 (d) and for different T8 values.

aerobic section. This aspect is also evident from the ADUF evolutionfor different b and T8 values (Fig. 5). In fact, for lower T8 values, as bis increased, T3 þ DTpp can be >T8. Hence no heat recovery ispossible and consequently the ADUF curve is interrupted (Fig. 5aed). The maximum ADUF is achieved for l ¼ 5 and T8 ¼ 340 K(Fig. 5a) and is about 22%. For a given b, as T8 rises, ADUF decreasesdue to QAD >> Qae. When T8 > 335e340 K, depending on the l

value, ADUF can also be <hnet. In fact an increase in b causes aconsequent increase in the temperature of the exhaust gasses, T9(Figs. 1 and 2). The higher is T9, the lower is the fraction of availableheat (i.e. Qae þ QAD) transferred to the ORC cycle affecting, over agiven extent, the ADUF value.

The phenomenon is clearly demonstrated by the HUR trend(Fig. 6). For lower values of T8, the higher is b, the higher is HUR. If T8is higher than 360 K, the increase in b has a lower, and in somecases, negligible effect on HUR increase. This is due to the lowerincidence that the value assumed by T9 has on the maximum heattransmittable to the ORC, because of the increase in b.

For T8 < 350 K, HUR rises rapidly as b is increased, achievingvalues up to 60, whereas for higher T8 values, the HUR ranges fromabout 1 to 2, depending on the l value. This means that better heatexploitation is obtained for higher T8 but the ADUF is lower (Fig. 5).For b from 1.5 to 2 and T8 from 335 K to 340 K, the system effi-ciencies in exploitation of the biogas are quite high if comparedwith that of other conventional systems such as internal combus-tion engines [15] of similar size.

On the basis of the assumptions made (Tables 1and 2), eachtonne of OW is able to produce about 93 Nm3 of biomethane. Foreach tonne of OW for a given l, achievement of the imposed T8requires a given amount of OW to be processed in the anaerobicsection. The ratio between the amount of OW processed in theanaerobic and in the aerobic sections (OWR) is greatly influenced

s compression ratio (b) for effective to stoichiometric mass air rate ratios (l) of 5 (a), 7

Fig. 6. Heat utilization ratio (HUR) vs compression ratio (b) for effective to stoichiometric mass air rate ratios (l) of 5 (a), 7 (b), 9 (c), 11 (d) and for different T8 values.

F. Di Maria et al. / Renewable Energy 66 (2014) 461e467466

by T8 (Fig. 7). Going from a T8 of about 330 K to about 520 K, OWRincreases from about 0.05 to 0.16. For a given T8, the higher is thel, the higher is the OWR. Otherwise, the ORC system poweroutput for a facility processing about 20,000 tonnes of OW peryear ranges from about 1 kW to about 25 kW (Fig. 8) (i.e. micro-ORC).

In any case, the most efficient exploitation of the QAD isachievable for lower T8, l and b, even if the power output of the ORCis about 1e1.5 kW.

Fig. 7. Ratio between the amount of organic waste processed in the anaerobic andaerobic sections (OWR) vs T8 for different effective to stoichiometric mass air ratios(l).

4. Conclusions

The heat generated during aerobic treatment of organic waste isan interesting source of energy for renewable energy production.Even with the rather low temperature levels of the aerobic processexhaust air exploitation of the ORC system operating at lowcompression ratios allows electrical energy to be generated. In-crease in the exhaust air temperature using the biogas produced bytreating a given fraction of the organic waste in an anaerobic

Fig. 8. Electrical power output of the organic Rankine cycle vs T8 for different effectiveto stoichiometric mass air ratios (l) and compression ratios (b) for an aerobic facilityprocessing 20,000 tonnes/year of organic waste.

F. Di Maria et al. / Renewable Energy 66 (2014) 461e467 467

digestion section can lead to a significant increase in the poweroutput. These values are within the range covered by micro-ORCsystems. The best utilization efficiency of the energetic content ofthe biogas is achieved for low compression ratios and maximumexhaust air temperatures, similar to that of the aerobic process.Results show that biogas can be exploited with efficiency levelsquite high compared to other conventional systems of similar size.

References

[1] Athanasoulia E, Melidis P, Aivasidis A. Optimization of biogas production fromwaste activated sludge through serial digestion. RenewEnergy2012;47:147e51.

[2] Cavinato C, Bolonzella D, Pavan P, Fatone F, Cecchi F. Mesophilic and ther-mophilic anaerobic co-digestion of waste activated sludge and source sortedbiowaste in pilot and full scale reactors. Renew Energy 2013;55:260e5.

[3] Fricke K, Santen H, Wallmann R. Comparison of selected aerobic and anerobicprocedures for MSW treatment. Waste Manage 2005;25:799e810.

[4] Gomez X, Cuestos MJ, Cara J, Moran A, Garcia AI. Anaerobic co-digestion ofprimary sludge and the fruit vegetable fraction of the municipal solid wastes.Conditions for mixing and evaluation of the organic loading rate. Renew En-ergy 2006;31:2017e24.

[5] Hessami MA, Christensen S, Gani R. Anaerobic digestion of household organicwaste to produce biogas. Renew Energy 1996;9:954e7.

[6] Liu X, Gao X, Wang W, Zheng L, Zhou Y, Sun Y. Pilot-scale co-digestion ofmunicipal biomass waste: focusing on biogas production and GHG reduction.Renew Energy 2012;44:463e8.

[7] Nguyen PHL, Kuruparan P, Visvanathan C. Anaerobic digestion of municipalsolid waste as a treatment prior to landfill. Bioresour Technol 2007;98:380e7.

[8] Bolonzella D, Pavan P, Mace S, Cecchi F. Dry anaerobic digestion of differentlysorted organic municipal solid waste: a full-scale experience. Water SciTechnol 2006;53:23e32.

[9] De Baere L, Mattheeuws B. Anaerobic digestion in Europe: state of the art. In:7th international conference on organic resource in the carbon economyORBIT 2010; June 29 July 3; Heraklion, Crete 2010.

[10] de Laclos HF, Desbois S, Sanit-Joly C. Anaerobic digestion of municipal solidorganic waste: Valorga full-scale plant in Tilburg, The Netherlands. Water SciTechnol 1997;36:457e62.

[11] Di Maria F, Sordi A, Micale C. Optimization of solid state anaerobic digestionby inoculum recirculation: the case of an existing mechanical biologicaltreatment plant. Appl Energy 2012a;97:462e9.

[12] Di Maria F, Sordi A, Micale C. Energy production from mechanical biologicaltreatment and composting plants exploiting solid anaerobic digestion batch:an Italian case study. Energy Convers Manage 2012b;56:112e20.

[13] Lastella G, Testa C, Cornacchia G, Notornicola M, Voltasio F. Anaerobicdigestion of semisolid organic waste. Biogas production and its purification.Energy Convers Manage 2002;43:63e75.

[14] Qiao W, Yan X, Ye J, Sun Y, Wang W, Zhang Z. Evaluation of biogas productionfrom different biomass wastes with/without hydrothermal pretreatment.Renew Energy 2011;36:3313e8.

[15] Walla C, Schneeberder W. The optimal size for biogas plants. Biomass Bio-energy 2008;32:551e7.

[16] Adani F, Tambone F, Gotti A. Biostabilization of municipal solid waste. WasteManage 2004;24:775e83.

[17] Binner E, Zach A. Biological reactivity of residual waste and dependence onthe duration of pretreatment. Waste Manage Res 1999;17:543e54.

[18] De Gioannis G, Muntoni A, Cappai G, Milia S. Landfill gas generation aftermechanical biological treatment of municipal solid waste. Estimation of gasgeneration rate constants. Waste Manage 2009;29:1026e34.

[19] Komilis DP, Ham RK, Stegmann R. The effect of municipal solid waste pre-treatment on landfill behaviour: a literature review. Waste Manage Res1999;17:10e9.

[20] Decreto Legislativo 29 aprile 2010, n�75 (2010) Riordino e revisione delladisciplina in materia di fertilizzanti, a norma dell’art. 13 della legge 7 luglio2009, n�88. Gazzetta Ufficiale 25 May 2010;121(106). Roma, Italy.

[21] Eklind Y, Kirchmann H. Composting and storage of organic household wastewith different litter amendments. I: carbon turnover. Bioresour Technol2000;74:115e24.

[22] Salter RA, Frederickson J. Composting municipal waste in the UK: some les-sons from Europe. Resour Conservation Recycl 2001;32:359e74.

[23] Liwarska-Bizukojc E, Ledakowicz S. Stoichiometry of the aerobic biodegra-dation of the organic fraction of municipal solid waste (MSW). Biodegradation2003;14:51e6.

[24] Themelis NJ, Kim YH. Material and energy balance in a large-scale aerobicbioconversion cell. Waste Manage Res 2002;20:234e42.

[25] Di Maria F. Upgrading of a mechanical biological treatment (MBT) plant with asolid anaerobic digestion batch: a Real case study. Waste Manage Res2012;30(10):1089e94.

[26] Di Maria F, Benavoli M, Zoppitelli M. Thermodynamic analysis of the energyrecovery from the aerobic bioconversion of solid urban waste organic fraction.Waste Manage 2008;28:805e12.

[27] Drescher U, Bruggemann D. Fluid selection for the organic Rankine cycle(ORC) in biomass power and heat plants. Appl Therm Eng 2007;27:223e8.

[28] Rentizelas A, Karellas S, Kakaras E, Tatsiopoulos I. Comparative techno-economics analysis of ORC and gasification for bioenergy application. En-ergy Convers Manage 2009;30:674e81.

[29] Yamamoto T, Furuhata T, Arai N, Mori K. Design and testing of the organicRankine cycle. Energy 2001;26:239e51.

[30] Karellas S, Terzis K, Manolakos D. Investigation of an autonomous hybrid solarthermal ORC-PV RO desalination system. The Chakli island case. Renew En-ergy 2011;36:583e90.

[31] Di Pippo R. Second law assessment of binary plants generating power fromlow-temperature geothermal fluids. Geothermics 2005;33:565e86.

[32] Saleh B, Koglbauer G, Wedland M, Fischer J. Working fluids for low-temperature organic Rankine cycles. Energy 2007;32:1210e21.

[33] Kuo CR, Hsu SW, Chang KH, Wang CC. Analysis of a 50 kW organic Rankinecycle system. Energy 2001;36:5877e85.

[34] Bidini G, Desideri U, Di Maria F. A single flash integrated gas turbine-geothermal power plant with non-condensable gas combustion. Geo-thermics 1999;28:131e50.

[35] Gewald D, Siokos K, Karellas S, Spliethoff H. Waste heat recovery from landfillgas-fired power plant. Renew Sustain Energy Rev 2012;16:1779e89.

[36] Desideri U, Di Maria F. Humid air turbine cycles with water recovery: how todispose heat in an efficient way. In: Proc. 1998 International Gas Turbine &Aeroengine Congress & Exhibition. Sweden: Stockholm; June 2–5, 1998.

[37] Wang EH, Zhang HG, Fan BY, Ouyang MG, Zhao Y, Mu QH. Study of workingfluid selection of organic Rankine cycle (ORC) for engine waste heat recovery.Energy 2011;36:3406e18.

[38] Hung TC, Wang SK, Kuo CH, Pei BS, Tsai KF. A study of organic working fluidson system efficiency of an ORC using low-grade energy sources. Energy2010;35:1403e11.

[39] Dong L, Hao L, Riffat S. Development of small-scale and micro-scale biomass-fuelled CHP systems-a literature review. Appl Therm Eng 2009;29:2119e26.

[40] Chinese D, Managhetti A, Nardin G. Diffused introduction of organic Rankinecycle for biomass-based power generation in an industrial district: a systemanalysis. Int J Energy Res 2004;28:1003e21.

[41] Schuster A, Karellas S, Kakaras E, Splietoff H. Energetic and economic inves-tigation of organic Rankine cycle applications. Appl Therm Eng 2009;29:1809e17.

[42] ISPRA. Rapporto Rifiuti Urbani 2012. 163/2012, Rome, Italy: ISPRA; 2012, ISBN978-88-448-0550-0; 2012.

[43] Keener HM, Elwell DL, Das K, Hansen RC. Specifying design/operating ofcomposting system using pilot scale data. Appl Eng Agric 1998;13:767e72.

[44] Bouallagui H, Touhami Y, Ben Cheikh R, Hamdi M. Bioreactor performance inanaerobic digestion of fruit and vegetable wastes. Process Biochem 2005;40:898e995.

[45] Cho JK, Park SC, Chang HN. Biochemical methane potential and solid-state anaerobic digestion of Korean food waste. Bioresour Technol 1995;52:245e53.

[46] Ten Brummeler E. Dry anaerobic digestion of the organic fraction of municipalsolid waste. PhD thesis. The Netherlands: Wageningen University; 1993.

[47] Wang ZQ, Zhou NJ, Guo J, Wang WY. Fluid selection and parametric optimi-zation of organic Rankine cycle using low temperature waste heat. Energy2012;40:107e15.

[48] Jokela J, Rintala J, Oikari A, Reinikainen O, Mutka M, Nyronen T. Aerobiccomposting and anaerobic digestion of pulp and paper mill sludge. Water SciTechnol 1997;36:181e8.

[49] Yang H,S, Yoon J,S, Kim M,N. Effects of storage of a mature compost on itspotential for biodegradation of plastics. Polym Degrad Stab 2004;84:411e7.

[50] Vuorinen A,H, Saharinen M,H. Evolution of microbiological and chemical pa-rameters during manure and straw co-composting in a drum compostingsystem. Agric Ecosyst Environ 1997;66:19e29.