comparison of mesophilic and thermophilic dry anaerobic digestion of ofmsw: kinetic analysis

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Page 1: Comparison of mesophilic and thermophilic dry anaerobic digestion of OFMSW: Kinetic analysis

Chemical Engineering Journal 232 (2013) 59–64

Contents lists available at ScienceDirect

Chemical Engineering Journal

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

Comparison of mesophilic and thermophilic dry anaerobic digestionof OFMSW: Kinetic analysis

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.07.066

⇑ Corresponding author at: Department of Chemistry and Soil Science, Faculty ofScience, University of Navarra, Spain. Tel: + 34 948 425 600 _ 806271

E-mail addresses: [email protected], [email protected](J. Fernández-Rodríguez).

J. Fernández-Rodríguez a,b,⇑, M. Pérez c, L.I. Romero a

a Department of Chemical Engineering and Food Technology, Faculty of Science, University of Cádiz, Spainb Department of Chemistry and Soil Science, Faculty of Science, University of Navarra, Spainc Department of Environmental Technology, Faculty of Sea and Environment Sciences, University of Cádiz, Spain

h i g h l i g h t s

� Kinetic conditions have been compared in dry anaerobic digestion of complex wastes.� The condition evaluated has been the temperature, exactly, mesophilic (35 �C) and thermophilic (55 �C) range.� Important kinetic differences are showed between both processes, even though of complex wastes.� The conclusions may help to increase the efficiency in dry anaerobic process at industrial scale.

a r t i c l e i n f o

Article history:Received 15 March 2013Received in revised form 15 July 2013Accepted 19 July 2013Available online 27 July 2013

Keywords:Anaerobic digestion (AD)OFMSWKinetic Romero’s modelMesophilicThermophilic

a b s t r a c t

Temperature is a significant variable in anaerobic digestion (AD) since it determines the values of themain kinetic parameters for the process and, hence, the rate of the microbiological process. Thus, in thisstudy, batch AD experiments were carried out, at mesophilic (35 �C) and thermophilic (55 �C) conditions,with the aim to compare the kinetic of both processes. Tests were performed in dry conditions (concen-tration of Total Solids (TSs) of 20%) using the Organic Fraction of Municipal Solid Waste (OFMSW) as feed-stock.

Romero model [1] was used to fit the experimental results from both, the organic matter consumptionand the biogas production. This model has been used extensively in the analysis of results of AD processin a wide range of experimental conditions [2–4].

The values of the maximum specific growth rate of microorganisms (lMAX) are 27–60% higher forthermophilic process than for mesophilic ones and, therefore, the same level of organic matter degrada-tion and methane production can be achieved in a shorter operating time, 20 days in thermophilic insteadof 40 days in mesophilic. Moreover, the yield coefficient for methane production (aP/S) and the initialamount of active microorganisms (XV0/YX/S) show values 107% and 129% higher, respectively, in thermo-philic processes.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

AD is a microbiological process capable of degrading the organicmatter into methane and carbon dioxide in the absence of oxygen.

The performance of microbiological processes is closely relatedto the temperature of the system since metabolic activity of micro-organisms is possible only in a certain temperature range and, inaddition, a maximum activity is obtained within this interval forpure species. However, AD is developed by a complex mixed pop-ulation and, hence, several temperature ranges are possible for the

development of the process. The two main ranges of temperaturefor AD are mesophilic (M) and thermophilic (T), whose optimumtemperatures are 35 �C and 55 �C, respectively. These processeshave been widely studied and applied to different wastes [5–9].The heterogeneity of the OFMSW is widespread known and thisvariability in the waste can suppose problems in order to predictthe behaviour of the anaerobic systems. The kinetics analysis couldgive us the key parameters for optimising the anaerobic process.Furthermore, the used FORSU is a non-source sorted organic frac-tion of municipal solid waste from an industrial MBT plant.

In general, the increase in process temperature means a highermicrobiological activity and, hence, the substrate consumption andthe methane generation rates were higher, but obviously it carriesan increased expenditure of energy, also. In short, thermophilicrange shows some advantages like high biogas production,

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60 J. Fernández-Rodríguez et al. / Chemical Engineering Journal 232 (2013) 59–64

removing of pathogens and specific growth rate of microorgan-isms, and this, the speed of the process. Mesophilic range is higherprocess stability and lower operating costs.

It has been observed that higher temperatures in the thermo-philic range reduce the required retention time, which implies,lower retention times are required in digesters operated in thethermophilic range. Moreover the thermophilic bacteria are moresensitive to environmental conditions than those mesophilicbacteria.

The OFMSW generation is increasing in the last years and treat-ing the OFMSW has converted the most attractive strategy. Cur-rently, there are many technologies available which showadvantages and disadvantages. AD is one of the options becauseof less energy consumption and CH4 yield can be obtained fromthe process. The methodology is carried out in the absence ofoxygen and requires less energy input. Additionally, at the end ofthe process, the digested waste can also be utilised as fertilizerfor agricultural uses.

One of the main points for treating the OFMSW is the high con-tent in solids. In this cases, where the solid content is between 20%and 40% TS, the AD is known like dry anaerobic digestion. The mainadvantages for this kind of technology are lower volume for thereactors, less consumption of water and the energy. However, alsoit presents some disadvantages, like slower AD (because of themass transfer); need more robust equipment and more concentra-tion of the toxic compounds. The knowledge of the kinetic in thiskind of wastes is fundamental for the correct performance of theprocess.

In general, kinetics of biological processes can be addressed bymicroorganisms growth models, substrate utilisation models andproduct formation models, which are interrelated through the cor-responding yield coefficients.

One of the most applied kinetic models has been the model ofMonod [10] which states that cell growth is a function of the avail-ability of a limiting substrate. Contois kinetic model [11] is anothertypical model which considers the effect of microorganismsconcentration in the medium on the specific growth rate of micro-organisms. Based on the Contois model, Chen and Hashimoto[12,13] developed two specific equations for substrate consump-tion and methane production, respectively, in CTSR. The mainadvantage of the models based on Contois expression is that, as adifference of Monod based models, consider than substrate concen-tration in the effluent is dependent to the fed concentration. That isan important modification with respect to the Monod model [10], inwhich both parameters are independent. In essence, the abovemodels take into account the organic load involved in the processas a fundamental parameter for the reactor performance and havebeen widely used in the modelling of anaerobic processes [12,13].

Romero’s model [1] has been, also, utilised for modelling ofdifferent microbiological processes and, more specifically, for thekinetic characterisation of the dry AD of OFMSW. Thus, it has beenused to modelling biodegradation of surfactants under differentexperimental conditions [14], the anaerobic degradation of indus-trial wastewaters (wine vinasses and cutting oil wastewaters)[15,16], the biomass immobilization process in anaerobic fluidisedbed and anaerobic filter reactors [17], AD of sewage sludge fromWWTP in CTSR reactors [18] and mesophilic and thermophilicanaerobic degradation of OFMSW [2–4,19,20].

Therefore, in this work, Romero’s model has been used to fit theexperimental results of substrate evolution (DOC) and productgeneration (methane) of the different tests performed and to ob-tain the kinetic parameters of the model, enabling the establish-ment of the pertinent comparisons.

The aim of this paper is to study the effect of operating temper-ature (mesophilic or thermophilic) on the kinetics of the process byfitting the experimental data of dry AD of OFMSW in batch reactors

to the substrate consumption and the methane generation modelsof Romero [1]. The tests were conducted at dry conditions with acontent of 20% TS, since previous studies [2,20] have shown thatprocess efficiency (substrate removal and methane production)operating with 20% TS is greater than for 30% TS. The comparisonof both anaerobic processes, thermophilic and mesophilic, can sup-ply interesting information about the kinetic parameters. The workis focused on choosing the best conditions between both treat-ments with a non-source sorted organic fraction of municipal solidwaste from an industrial MBT plant.

2. Materials and methods

Batch tests were run to study the behaviour of both processes(mesophilic and thermophilic) for the degradation of OFMSW witha content of 20% TS (dry conditions) and the addition of an ade-quate inoculum source (30% in volume-the inoculum rate is basison total volume of the waste).

The analysis were conducted on the basis of the values ofkinetic parameters obtained by fitting experimental results tosubstrate removal and product generation expressions of Romero’smodel. Dissolved organic carbon (DOC) was used as the organicmatter concentration for the substrate removal modelling. More-over, methane was used as the main product of the process forthe product generation modelling.

Linear and non-linear regressions for fit the model expressionsto experimental results were performed using TSATGRAPHICS Plus5.1 software. Non-linear regressions were based on Marquardtalgorithm [25].

In order to characterise the waste and the inoculum, as well asto monitor the effluent of the process, the following parameterswere analysed: pH, dissolved organic carbon (DOC) and Total Sol-ids (TSs). These analyses were carried out according with Standard-ised Methods [26] adapted for waste with high solids content andbased on the previous leaching of the waste in an aqueous med-ium. The biogas volume was quantified in a gas counter and thecomposition using chromatographic methods. The main concen-trations of the components in the biogas were determined with agas chromatograph (Shimadzu Model 2014, Japan) equipped witha thermal conductivity detector (TCD) and a Carbosieve S-II packedcolumn 3 m � 1/8 in. Internal Diameter (Supelco, USA). The injec-tor, detector and oven temperatures were 100 �C, 170 �C, and170 �C, respectively. Helium served as the carrier gas at a pressureof 500 kPa and flow of 30 mL/min. The samples, of the effluent andthe biogas, were taken three times per week.

2.1. Waste and inoculum characterisation

The Organic Fraction of Municipal Solid Waste (OFMSW) usedin this study come from the Mechanical Biological Treatment(MBT) plant ‘‘Las Calandrias’’ located at Jerez de la Frontera (Cádiz,South of Spain). The OFMSW corresponds to the fraction obtainedfrom a 30 mm trommel placed next to the triage units for separa-tion of valuable fractions (metals, plastics, glass, etc.).

Mesophilic digested sludge from the Wastewater TreatmentPlant (WWTP) ‘‘Guadalete‘‘ located at Jerez de la Frontera (Cádiz,South of Spain) was used as inoculum for mesophilic reactors.Moreover, effluent from a thermophilic anaerobic reactor treatingOFMSW at lab-scale, were used as inoculum for thermophilic reac-tors. It showed an Organic Loading Rate (OLR) of 1872 g DOC/(m3

reactor.day) and an organic matter removal of 89% in Volatile Sol-ids (VSs). The production of methane was 1,149 m3/m3

reactor.day.The physical–chemical characteristics of wastes and inocula, as

well as the initial feed mixtures, used in thermophilic and meso-philic tests are shown in Table 1.

Page 3: Comparison of mesophilic and thermophilic dry anaerobic digestion of OFMSW: Kinetic analysis

Table 1Physic-chemical characterisation of the waste and inoculum used in the mesophilic (M) and thermophilic (T) processes.

OFMSW M OFMSW T Inoculum M Inoculum T Mixture 20% M Mixture 20% T

pH 6.51 6.59 7.49 7.84 6.68 6.87Density (kg/L) 0.666 0.638 0.971 0.965 1.035 1.029TS (%) 82.34 81.09 3.64 4.92 16.61 18.32VS (%)* 29.89 26.76 1.85 3.87 10.71 11.66COD (g O2/m3) 1862.54 1698.23 1134.12 794.15 1691.48 1013.37DOC (ppm) 1283.5 1105.56 662.15 593.45 931.1 847.4Total N (g N/L) 27.43 21.79 0.44 0.35 – –Amoniacal N (mgNH3-N/L) 10.93 10.07 12.15 13.98 – –Total Acidity (mgAcH/L) 301.2 306.3 19.24 256.87 – –Alkalinity (mgCa CO3/L) 14.0 17.3 0.64 1.13 9.2 9.9Total P (g P/kg dry weight) 6.07 5.39 – – – –

* SV expressed as% of total sample weight.

J. Fernández-Rodríguez et al. / Chemical Engineering Journal 232 (2013) 59–64 61

2.2. Equipment used

The experimental equipment used in batch system has been de-signed and patented by the research group Biological Treatment ofWastes from the University of Cádiz [27]. The equipment consistsof a 6 reactors battery, submerged in a thermostatic bath, whichmaintain the temperature operation, mesophilic (35 �C) or thermo-philic (55�). This bath has built-in an electrical panel, which allowsindependent operation of each reactor.

Reactors consist in a stainless steel vessel of 2 L of total volumeand 1.7 L of useful volume, with a device for the stirring of reactorsand various ports in top of the reactors to permit the biogas outletand the sampling (Fig. 1).

The biogas produced was collected in Tedlar plastic bags (poly-tetrafluoroethylene). Samples of biogas were taken from Tedlarbags with a 1 mL Dynatech Gastight gas syringe and were analysedfor determination of biogas components.

2.3. Basic description of the Romero kinetic model

Romero’s model [1,21] is based on the hypotheses than micro-biological process can be represented as an autocatalytic reactionas a consequence of the reproduction capacity of microorganisms.

The general equation of the Romero’s model [1] is:

ð�rSÞ ¼ �dSdt

� �¼ lMAX �

ðSt � SnbÞ � ðh� StÞðS0 � SnbÞ

Fig. 1. Batch anaerobic reactors [29].

where ‘‘(�rS)’’ is the substrate consumption rate, ‘‘h’’ is the maxi-mum amount of substrate available in the medium to form biomass(including the substrate necessary for the initial biomass), ‘‘Snb’’ isthe concentration of non-biodegradable substrate by microorgan-isms, ‘‘St’’ is the total substrate concentration (biodegradable andnon-biodegradable), and ‘‘S0’’ is the initial total substrate concentra-tion available in the medium and ‘‘lMAX’’ is the maximum growthrate of microorganisms.

This equation corresponds to a quadratic polynomial with re-spect to the substrate concentration present in the medium, assuggested by the empirical equation of Quiroga and Sales [14,22]for surfactants degradation in the marine environment. Moreover,the equation can be easily related with the classical ‘‘logistic mod-el’’ for the microorganisms growth rate [23,24].

For discontinuous processes, Romero’s expression for substrateconcentration can be obtained by integrating the rate equation, sothe expression for substrate consumption is:

St ¼h � ðS0 � SnbÞ þ Snb � ðh� S0Þ � expðlMAX � tÞðS0 � SnbÞ þ ðh� S0Þ � expðlMAX � tÞ

ð1Þ

The model considers the existence of four parameters with physicalsignificance: S0, SNB, h and lMAX.

From the previous expression and assuming than substrate isstoichiometrically converted into product, the product generationmodel can be obtained:

P ¼ cMAX1� expð�lMAX � tÞ

YXS �S0bXV0

� �expð�lMAX � tÞ þ 1

ð2Þ

where ‘‘P’’ represents the product concentration (methane in thiscase), ‘‘XV0/YXS’’ is the substrate concentration that would be neces-sary to obtain the initial active biomass concentration in the pro-cess, ‘‘S0b’’ is equal ‘‘S0-SNB’’ and represents the initial substratebiodegradable in the medium. ‘‘cMAX’’ is the maximum productivityof the product. This parameter is equal to ‘‘aP/S�Sob’’, i.e. the yieldcoefficient for product generation ‘‘aP/S’’ multiplied by Sob.

Finally, it can be pointed out that Romero’s model [1], consid-ered a special case for systems in which the initial concentrationof microorganisms is much higher than it could be formed duringthe fermentation process. In this case the model is called ‘‘simpli-fied model’’ and the simplified expression that relates the substratewith the incubation time is presented as:

St ¼ S0b � exp � �lMAX � t� �

þ Snb ð3Þ

Equally, the simplified product generation model is:

P ¼ cMAX 1� exp �lMAX�� ��

ð4Þ

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62 J. Fernández-Rodríguez et al. / Chemical Engineering Journal 232 (2013) 59–64

3. Results and discussion

Romero’s model [1] has demonstrated to be suitable for model-ling the experimental data obtained in the dry OFMSW anaerobicdegradation in both range, thermophilic and mesophilic [2–4]. Inprevious studies with OFMSW the value of maximum microorgan-isms growth rate (lMAX) were 0.1–0.2 days�1 in mesophilic and0.3–0.4 days�1 in thermophilic.

In this paper, the general and/or simplified expressions of themodel have been used for the fitting of DOC evolution, as a repre-sentative parameter of substrate concentration, and cumulativeCH4, as a representative of the product generation.

During the first days of the experiments, an increase of organ-ic matter concentration (COD and DOC) was detected resultingfrom the hydrolysis and solubilisation of wastes. Thus, experi-ments have been planned in order to analyse the biomethaniza-tion of OFMSW and, hence, sampling frequency was establishedin order to reach the main proposed plan. Only data posteriorto these days have been used for the regression to modelequations.

3.1. Dissolved Organic Carbon (DOC) removal

The removal of DOC by methanogen microorganisms in meso-philic reactors was observed from the day 10 until the day 40 ofthe test. However, in thermophilic process, DOC removal was fasterand occurred during days 8–22.

General equation of the Romero’s substrate consumption model[1] was used in the modelling of the DOC removal in both reactors.The results obtained from the fit of experimental data of DOC tothe model expression are shown in Table 2 and the goodness offit can be seen graphically in Fig. 2. It can be observed an initialperiod of acclimation to the conditions. Later, the consumption ofsubstrate phase occurs. According to the temperature, the lengthof this phase is different in mesophilic and in thermophilic reac-tors, 25 days approx. and 10 days approx. respectively. Lately nosignificant evolution in the consumption of organic content isdetected.

Table 2Values of the kinetic parameters of the adjustment of DOC inmesophilic (M) and thermophilic (T).

Kinetic parameters M T

lMAX (days�1) 0.192 0.243S0 (g/m3

reactor) 1433.52 962.57

Snb (g/m3reactor) 434.41 523.08

XV0/YX/S (g/m3reactor) 14.93 20.11

R2 0.9523 0.9505

Fig. 2. Fitting the reactors DOC evolution in mesophilic (a) and thermophilic (b)using the Romero’s model [1] substrate consumption.

The maximum microorganisms growth rate (lMAX) and the con-centration of microorganisms initially viable (XV0/YX/S) are higherin the thermophilic system than in the mesophilic one.

It is observed that the concentration of non-biodegradable sub-strate (SNB) is lower in mesophilic (434.41 g/m3

reactor) than in ther-mophilic (523.08 g/m3

reactor) system. The mesophilic processreaches higher stabilization rate of organic matter, because showslower values of volatile fatty acids (VFA) which contributes to theCOD [29].

3.2. Methane production

The methane production in mesophilic reactor is delayed untilday 10, whereas in the thermophilic reactor methane productionpractically takes place from the 2nd or 3rd day. The inoculum usedin thermophilic reactor was adapted quickly to the process as it iscoming from a reactor treating the same substrate. Therefore, thehydrolysis and the organic matter removal were produced very fastgiving a high production of biogas. The beginning of the methano-genic stage in the mesophilic reactor was slower since, in this case,inoculum is not acclimated previously to substrate and, in addition,the rate of the thermophilic process is higher [28]. The accumu-lated volume of methane per m3

reactor is 7.31 m3 in the mesophilicreactor and 9.26 m3 in the thermophilic one.

Table 3 shows the results of the fitting of the methane produc-tion data in mesophilic and thermophilic reactors to the corre-sponding equation of Romero’s model [1]. In Fig. 3 have beenrepresented the experimental data for accumulated methane andthe model predictions. It can be observed the goodness of the fit-ting. The evolution of accumulated biogas is consequence of theDOC evolution. At the first time no production is detected. The per-iod, in which the most quantity of organic matter is consumed, themost volume of methane is produced. The slope also is different inboth cases: higher in the thermophilic reactors, in which a higherproduction of methane takes place in a shorter time. The maximumproduction of methane longs 25 days approx. in mesophilic reac-tors instead of 12 days approx. in thermophilic one. From the day45 and 25 the reactors stop producing biogas in mesophilic andthermophilic respectively. At the end of the methanogenic phase,

Table 3Kinetic parameters of the methane production adjustment.

Kinetic parameters M T

aP/S (m3CH4/g DOC) 0.0072 0.0149lMAX (días�1) 0.256 0.410XV0/YX/S (g/m3

reactor) 1.44 3.30R2 0.9979 0.9961

Fig. 3. Fitting the methane cumulative production in the mesophilic reactors (a)and thermophilic (b) by the general model of product formation, Romero [1].

Page 5: Comparison of mesophilic and thermophilic dry anaerobic digestion of OFMSW: Kinetic analysis

Table 4Comparison of the values obtained for the lMAX (day�1) overall process and onlyacetoclastic archaea activity in different studies and those obtained in the presentresearch.

Thermophilic Mesophilic Waste Bibliography

lMAX (days�1) 0.54 Kitchenwastes

[30]

lMAX metanog.acetoclas.

0.72 Sewagesludge

[31]

lMAX metanog.acetoclás.

0.3 Winery [29]

lMAX metanog.acetoclás.

0.6 Winery [28]

lMAX 0.195 Sewagesludge

[18]

lMAX metanog.acetoclas.

0.23–0.28 OFMSW [19]

lMAX metanog.acetoclas.

0.11–0.19 OFMSW [2]

lMAX 0.166–0.188 OFMSW [3]lMAX metanog.

acetoclás.0.243–0.410 0.192–

0.256OFMSW This study

J. Fernández-Rodríguez et al. / Chemical Engineering Journal 232 (2013) 59–64 63

the mesophilic system managed to accumulate 7 m3/m3reactor

approx. instead of 9 m3/m3reactor approx. in the thermophilic one

[29].The productivity rate (aP/S) and the microorganisms specific

growth rate (lMAX) are higher in thermophilic reactor than in themesophilic one. As was pointed out previously, the evolution ofaccumulated methane production in the mesophilic reactor showsa latency period that corresponds to the adaptation of microorgan-isms to the substrate. This adaptation period was not observed inthermophilic reactor since inoculum was previously adapted toOFMSW. The value of the parameter XV0/YX/S, representing the ini-tial active biomass, is 1.44 in mesophilic reactor whereas in ther-mophilic reactor it is 3.30.

3.3. Analysis of kinetic modelling results

Comparing the regression coefficients of the fittings of DOC re-moval and methane production, it can be observed that methaneproduction get better r2 values. Differences can be related withthe heterogeneity of the organic waste and the difficulties associ-ated to the sampling in high solids content AD. By contrast, thegas samples are homogeneous and data show less dispersion.

Thermophilic dry AD is favoured compared to mesophilic pro-cess, because the specific growth rate of microorganisms (lMAX)and the initial viable microorganisms concentration (XV0/YX/S) arehigher, for both the substrate consumption model and the productgeneration one. The increase of lMAX in thermophilic process ran-ged from 26% (DOC) to 60% (methane) with respect to mesophilicprocess. In this sense, Sales et al. [30] obtained maximum specificgrowth rates of acetoclastic Archaea of 0.3 day�1 in the mesophilicrange and 0.6 day�1 in thermophilic one [28,30], for the wet AD ofwine vinasses. This wastewater presents a high content of solubi-lised organic matter and, thus, the rate limiting stage is methano-genesis rather than hydrolysis.

In addition, in the product generation model it can be observedthat the methane yield (aP/S) in the thermophilic reactor is higher(nearly double) than in mesophilic process.

Table 4 shows values of the maximum specific growth rate ofanaerobic microorganisms from literature, reported for severalauthors [31,32,30,28,18,19,2,3]. Ref. [19] is noteworthy since, inthe same way to this work, is related with the treatment of OFMSWin dry thermophilic AD, but using the UASB technology. The lMAX

value obtained in this work is similar to that obtained in thisstudy.

4. Conclusions

According to the results obtained in this study, a kinetic com-parison between mesophilic and thermophilic AD with high con-tent solids can be done. It has been observed significantdifferences in the value of kinetic parameters for the applicationof the Romero’s models to the process. Exactly, the lMAX, the meth-ane yields (aP/S) and the ratios XV0/YX/S were higher in the thermo-philic compared to mesophilic reactors. The maximum differenceof lMAX has been observe when have been compared the methanegeneration. In this case the values has been 0.256 days�1 in meso-philic process instead of 0.410 days�1, what supposes an incre-ment of 60%. On the other hand, the methane yield (aP/S) and theratio XV0/YX/S have showed an increment of 107% and 129%, respec-tively. So, it can be concluded, that microbial activity is favouredworking at thermophilic range in the dry AD process (20%) ofOFMSW.

Acknowledgments

This study was conducted by the Environmental Technologiesresearch group at the University of Cadiz, a group of excellenceof the ‘‘Plan Andaluz de I + D + i’’ TEP-181. It was funded by theSpanish ‘‘Ministerio de Ciencia e Innovación’’ (Project CTM2010-17654), the ‘‘Consejería de Innovación, Ciencia y Empresa de And-alucía’’ (Project P07-TEP-02 472) and the European Regional Devel-opment Fund (ERDF).

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Glossary and notations

AD: Anaerobic DigestionCTSRs: Continuous Stirred Tank ReactorsDOC: Dissolved Organic Carbon concentration, expressed as g/m3

OFMSW: Organic Fraction of Municipal Solid WasteTFS: Total Fixed Solid concentration, expressed as %TS: Total Solids concentration, expressed as %UASB: Up-flow anaerobic sludge blanket digestionVFA: Volatile Fatty AcidVS: Volatiles Solids concentration, expressed as %MAX: Maximum specific growth rate of the microorganisms, expressed as day�1

h: Maximum biomass concentration in the reactor when all the biodegradablesubstrate had been metabolized by the microorganisms, expressed as gDOC/m3

XV0/YX/S: Initial concentration of viable microorganisms, expressed as gDOC/m3

St0: Initial substrate concentration, expressed as gDOC/m3

SNB: Concentration of substrate non-biodegradable by the microorganisms, ex-pressed as gDOC/m3

P/S: Yield coefficient for methane production, expressed as m3CH4/g DOCremoved