Dry-thermophilic anaerobic digestion of simulated organic fraction of Municipal Solid Waste: Process modeling

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    Keywords:Anaerobic digestionOFMSWKinetics


    istics and the microorganisms involved in the process and, hence, it should be determined specically in

    hasrs grea

    Hashimoto, 1980).3. An un-structured segregated model (Hill, 1983).4. Structured kinetic models for dynamic simulation of the anaer-

    obic degradation, as IWA anaerobic digestion models, are basedon a complex matrix with kinetic constants for different organicmatter degradation steps (Bagley and Brodkorb, 1999; Batstoneet al., 2002; Siegrist et al., 2002).

    ic matter removal takes place. In the present study, an analysisof evolution of the consumed organic loading rate (OLRC),removal efciency (measured as DOC and VS removal percent-ages) and biogas production have been realized when the SRTis decreased.

    The effect of solid retention time (SRT) on organic matter con-centration in the efuent and its relation with the substrate con-centration in the inuent (both measured as COD or VS) can beused to determine parameters of adequate kinetic models, as theproposed by Romero Garca (1991), for the prediction of the pro-cess behavior.

    * Corresponding author. Tel.: +34 956016379.

    Bioresource Technology 102 (2011) 606611

    Contents lists availab


    elsE-mail address: alberto.fdezguelfo@uca.es (L.A. Fdez.-Gelfo).effective degradation of organic matter to produce biogas with ahigh methanehydrogen rate, but also contributes to save fossilenergy (Mata-lvarez and Llabrs-Luengo, 1996).

    There are three main tendencies in modeling of anaerobic pro-cesses to predict the reactor behavior (Garcia-Ocha et al., 1999):

    1. Based model on kinetic equations such as Monod (1949) andContois (1959) or Romero et al. (1988, 1990, 1993) and RomeroGarca (1991)).

    2. An un-structured non-segregated model (Chen, 1983; Chen and

    organic matter removal and biogas productivity that can bereached (Wang et al., 1997; Kiyohara et al., 2000; Halalshehet al., 2001). When the SRT decreases in an anaerobic reactor,the consumed organic matter rate and, therefore, the specicmethane production increase (De la Rubia et al., 2006a). How-ever, this behavior presents an inexion point from which theopposite effect is observed. Depending on the feeding character-istics, fundamentally size and organic content, every system pre-sents an optimum SRT below which the reactor destabilizes andthen an important reduction in the biogas generation and organ-Modeling

    1. Introduction

    The anaerobic digestion of OFMSWin recent years. This technology offe0960-8524/$ - see front matter 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.07.124each case.In this work a series of experiments were carried out to determine the effect of SRT, from 40 to 8 days,

    on the performance of the dry (30% Total Solids) thermophilic (55 C) anaerobic digestion of organic frac-tion of Municipal Solid Wastes (OFMSW) operating at semicontinuous regime of feeding.The experimental results show than 15 days is the optimum SRT (the best between all proved) for this

    process. Besides, data of organic matter concentration and methane production versus SRT have beenused to obtain the kinetic parameters of the kinetic model of Romero Garca (1991): the maximum spe-cic growth rate of the microorganisms (lmax = 0.580 days1) and the fraction of substrate non-biode-gradable (a = 0.268).

    2010 Elsevier Ltd. All rights reserved.

    attracted much interestt potential for fast and

    To calculate the size of the SSTR, the organic loading rate(OLR) and the SRT are the most used parameters in practice.The SRT is a fundamental operational variable of the processwhich determines its performance and the efciency of theReceived in revised form 16 July 2010Accepted 31 July 2010

    age of Dissolved Organic Carbon (% DOC) or Volatile Solids (% VS) removed and the biogas or methaneproduction are closely related with the SRT imposed. Optimum SRT is depending on the waste character-Dry-thermophilic anaerobic digestion ofof Municipal Solid Waste: Process model

    L.A. Fdez.-Gelfo a,*, C. lvarez-Gallego a, D. Sales MaDepartment of Chemical Engineering and Food Technology, Faculty of Science, UniversibDepartment of Environmental Technologies, Faculty of Sea and Environmental Sciences

    a r t i c l e i n f o

    Article history:Received 14 April 2010

    a b s t r a c t

    Solid retention time (SRT)waste treatment processes


    journal homepage: www.ll rights reserved.ulated organic fractiong

    uez b, L.I. Romero Garca a

    f Cdiz, 11510 Puerto Real, Cdiz, Spainiversity of Cdiz, 11510 Puerto Real, Cdiz, Spain

    a very important operational variable in continuous and semicontinuousce the organic matter removal efciency expressed in terms of percent-

    le at ScienceDirect


    evier .com/locate /bior tech

  • eses of linear regression technique are invalidated. Whereas thenon-linear regression does not modify the error distribution, sinceit is not necessary to realize any transformation of the variables.

    Therefore, in this work, the tting of the model to experimentaldata has been performed by means of both linear and non-linearregression.

    1.2. Computer support

    Linear and non-linear regressions have been applied, to t the-oretical equations to experimental data and to determine the ki-netic parameters of the model, using a statistical program(Software Statgraphics 5.0). The non-linear regression is based onthe minimization-square-residues algorithm of Marquardt (1963).

    2. Methods

    2.1. Experimental equipment

    rce Technology 102 (2011) 606611 6071.1. Kinetic model description

    Process modeling, based in kinetic models, permits to predictthe effect of the most important process variables on system per-formance. In this sense, development of adequate models and itsparametrization by means the t of model equations to experi-mental results, obtained in specic assays, is a very important task.

    Besides, industrial plants for anaerobic treatment of organicfraction of Municipal Solid Waste are characterized for a wide var-iation in waste composition feeding to reactors and hence, processmodeling acquires a special signicance to determine the optimumoperational conditions.

    In this work, the substrate consumption model proposed byRomero Garca (1991) has been used. This model has been usedsuccessfully to t experimental results of anaerobic digestion ofdifferent organic wastes: wine vinasses (Prez et al., 2001a,b;Romero Garca, 1991), sludges from WWTP (De la Rubia et al.,2006b) and OFMSW (Fernndez et al., 2010).

    The model assumes that substrate consumption rate is given by

    rs lmaxh S S SNB

    h SNB 1

    By including Eq. (1) in the mass balance for a completely mixedreactor and assuming steady state conditions, the model equationused in this work can be obtained

    S SNB S0 SNBlmaxh2


    SNB represents the non-biodegradable substrate concentration(g/L).S0 represents the initial substrate concentration (g/L).S represents the substrate concentration (g/L).h is the maximum substratum concentration that can beinvested in the biomass formation (g/L).lmax represents the specic grown rate of the microorganisms(days1).h represents the SRT (days).

    In Eq. (2), as postulated by the author (Romero Garca, 1991), itwas assume that parameter h is equivalent to the initial sub-strate concentration S0 for continuous and semicontinuousoperation.

    Besides, the above equation has been developed on the basisthan the activity of the overall microbial population can be repre-sented by the parameter lmax, which represents the rate of themicrobial group which is the rate-limiting of the process.

    Eq. (2) can be modied by considering than the quantity of non-biodegradable substrate by the microorganisms involved in theprocess, is proportional to total substrate concentration in thefeeding. Thus, SNB can be expressed as the fraction of non-biode-gradable substrate (a) with respect to initial substrate concentra-tion S0 to obtain Eq. (3).

    S a S0 S01 almaxh3

    Obviously, the above equation could be easily linearized (Eq. (4)):


    a 1 almax



    However, according to Leatherbarrow (1990), the linearization of an

    L.A. Fdez.-Gelfo et al. / Bioresouequation supposes a modication of the tted variables becausethis procedure causes a modication of the random error distribu-tion, associated to the aforementioned values and thus, the hypoth-The reactor used for the semicontinuous process had a totalcapacity of 5 l and a working volume of 4.5 l (Fig. 1).

    In this study, the semicontinuous stirred tank reactor has beenconsidered as ideal completely mixed reactor without microorgan-ism retention. In this case the SRT is the same that the HRT.

    The reactor was jacketed and thermostatically controlled usinga circulating 7L-bath. A ball valve was tted to the lower part of thereactor in order to discharge the contents and in the cover therewere several ports: central hole for agitation system (mixing shaftat 13 rpm), a pH probe, a biogas collector, an opening for the addi-tion of feedstock and two further inputs for the pH-control. A strictpH-control was not necessary. The pH of the system was correctedonly when it was out of range 6.58. To this end, an on/off pH con-troller was used employing 5 N NaOH and 1 N H3PO4 solutions. Theinitial pH of the synthetic OFMSW (OFMSWSYNTH) inoculum mix-ture was 7.2.

    2.2. Sampling and analysis

    For process monitoring and control, the following analyticaldeterminations have been used: Total Solids (TS), Volatile Solids(VS), alkalinity, pH, Dissolved Organic Carbon (DOC) and ammo-nium. All parameters were analyzed once a day and determina-tions were performed according to Standard Methods (APHA,AWWA, WEF, 1995).Fig. 1. Anaerobic 5 l reactor.

  • The volume of gas produced in the reactor was directly mea-sured using a high precision ow gas meter WET DRUM TG 0.1(mbar) Ritter through the Tedlar bag. The gas composition(hydrogen, methane and carbon dioxide) was determined by gaschromatography (SHIMADZU GC-14 B) with a stainless steel col-umn packed with Carbosive SII (diameter of 3.2 mm and lengthof 2 m) and a thermal conductivity detector (TCD). The injectedsample volume was 1 mL and the operational conditions were asfollows: 7 min at 55 C; ramped at 27 C min1 until 150 C; detec-tor temperature: 255 C; injector temperature: 100 C. The carrierwas helium and the ow rate used was 30 ml min1. A standard

    of this work is to study the kinetics of the process in order to char-acterize the maximum specic growth rate of microorganisms.

    w), orange (4.9% w/w), apple (4.9% w/w), bread (3.5% w/w) and pa-per (55.8% w/w). Table 2 shows the physicochemical characteriza-tion of the feeding.

    3. Results and discussion

    3.1. Organic loading rate (OLRC) removed

    The system was operated at SRT from 40 to 8 days. The SRT of40, 35, 30 and 25 days, correspond to the start-up and stabilization

    Alkalinity (g CaCO3/L) 4.29

    608 L.A. Fdez.-Gelfo et al. / Bioresource Technology 102 (2011) 606611That is, the objective is not to obtain kinetic data that can beextrapolated to industrial scale. To this end, synthetic waste wasemployed to avoid the high heterogeneity of the real OFMSW.The real OFMSW shows a very heterogeneous composition thatcauses a great dispersion in the analytical parameters. This disper-sion in the analytical parameters complicates the kinetic studysince it is very difcult to adjust the models. A stable and homoge-neous composition along the time was necessary for that the ki-netic study was exhaustive. About the composition of the waste,the main constituents were potato (6.2% w/w), cabbage (5.3% w/

    Table 1Organic loading rates (daily feeding).

    Stage OLR0 SRT (days) Operation time (days)

    mg DOC/L/d mg VS/L/d

    1 702 4431 40 142 803 5069 35 173 938 5920 30 254 1120 7090 25 505 1404 8862 20 606 1872 11,817 15 45gas (by Carburos Metlicos, S.A; composition: 4.65% H2; 5.33%N2; 69.92% CH4 and 20.10% CO2) was used for the systemcalibration.

    Individual VFA (from C2 to C7, including iC4, iC5 and iC6) levelsweredeterminedbygaschromatography(SHIMADZUGC-17A)witha ame ionization detector and a capillary column lled with Nukol(polyethylene glycol modied by nitro-terephthalic acid). The tem-peratures of the injection port and detector were 200 and 250 C,respectively. Heliumwas the carrier gas at 50 ml min1. In addition,nitrogen gas was used as make up at 30 ml min1 ow rate.

    2.3. Methodology

    In this work, eight experimental stages were used in a decreas-ing sequence of SRT. Test were run at 40, 35, 30, 25, 20, 15, 10 and8 days and the organic loading rates used in each SRT (expressed asmg DOC/L/d and mg VS/L/d) are shown in Table 1.

    It is important to emphasize that OLR tested in this workexceeds limits established by Angelidaki et al. (2006) to reach asuccessful strategy of start-up. Concretely, in stages 7 and 8, theOLR expressed in terms of Volatile Solids (VS) is higher thanthe limit of 15 g VS/L/d established by the aforementioned authors.

    In order to avoid the distortion in process performance causedby the changes in feed composition when real feeding are used, asimulated OFMSW was prepared following the indications ofMartin et al. (1997). This synthetic waste provides all nutritionalrequirements of the main microbial groups involved in anaerobicdigestion. It is very important to emphasize that the main objective7 2808 17,725 10 308 3509 22,156 8 24Ammonium (g NH3N/L) 1.68Total nitrogen (g NH3N/kg) 23.0Total Solids (g/g sample) 0.90Total Volatile Solids (g/g sample) 0.71Suspended Fixed Solids (g/g sample) 0.19period, whereas the SRT of 20, 15, 10 and 8 days describe the fol-lowed sequence to analyze the optimum SRT.

    The removal efciency (based on the DOC and VS) and the OLRCdepends on the OLR0 and the imposed SRT. The relation betweenthese parameters, suggesting the following two aspects:

    1. For SRTs lower than 15 days, the contact time between the bio-mass and the substrate begins to be insufcient and therefore,the organic matter removal efciency decreases notably, espe-cially if it is taken into account the removal efciency in termsof VS.

    2. Besides, SRTs higher than 20 days cause a gradual decrease inthe removal yields. This fact suggests that there are lacks ofsubstrate in the system and, therefore, the growth of microor-ganisms is being limited.

    As a consequence, 15 days can be considered the optimum SRTfor the dry-thermophilic anaerobic digestion of synthetic OFMSWand, in these conditions, the system reaches the maximum organicmatter removal yield, 89% in VS. Table 3 shows the results obtainedfor the different SRT tested in this study.

    As can be seen in Table 3 the methane production, expressed asLCH4 /LREACTOR/day, increases continuously from 40 to 15 dayswhereas a diminishing of methane productivity is detected forSRT lesser than 15 days (10 and 8 days SRT). This behavior is thesame than that observed for organic matter removal efciencyand, hence, it conrms that 15 days is the optimum SRT for theprocess. Data for biogas and methane production, as well as for or-ganic matter degradation, indicate than 10 days SRT could be con-sidered useful since only a slight decreasing with respect to15 days SRT efciency is produced. However, for 8 days SRT a dras-tic decreasing in methane production and total acidity concentra-tion, expressed as mgAcH/L, can be observed (Fig. 2).

    At the lower SRTs (10 days and, especially, 8 days) the destabi-lization of the system takes place and the wash out of the micro-organisms with lower specic growth rate (MethanogenicAcetate-utilizing) occurs. For this reason the methane productiondecreases in these low SRTs. Others research works as Smith andMah (1966) or McCarty (1981) indicate that methane percentagecoming from Methanogenic H2-utilizing population is, approxi-mately, the 30% of total methane production. This fact suggests

    Table 2Features of the simulated OFMSW.

    Analytical parameter Values

    pH 7.78Density (kg/m3) 750DOC (mg/g) 112.3Acidity (mg AcH/L) 1440

  • that, if the best methane production for this simulated waste isaround 40% (Martin et al., 1997), the methane percentage whenMethanogenic H2-utilizing microorganisms are acting, may

    requires the values of biodegradable substrate concentration thatcan be evaluated as DOC or VS. In Table 4, the values of S and S0(expressed as DOC and VS) for the different SRT are shown.

    Table 3Operation values for the different applied SRT.

    SRT (days) OLR0 (g DOC/LR d) OLRc (gD OC/LR d) DOCc (%) OLR0 (g VS/LR d) OLRc (g VS/LR d) VSc (%) Biogas (L/LR d) CH4 (L/LR d) pH

    40 0.702 0.235 33.52 4.431 2.980 67.26 0.638 0.007 8.1935 0.802 0.419 52.19 5.064 3.823 75.48 1.944 0.465 7.8730 0.936 0.547 58.44 5.908 4.830 81.76 1.160 0.499 7.8625 1.123 0.717 63.87 7.090 6.237 87.98 1.396 0.582 7.9720 1.404 0.944 67.26 8.862 7.851 88.59 2.258 0.933 7.8815 1.872 1.228 65.59 11.817 10.516 89.00 3.244 1.149 7.7410 2.808 1.716 61.12 17.725 15.213 85.83 3.783 0.929 7.808 3.509 2.000 56.98 22.156 15.959 72.03 1.615 0.132 7.83

    L.A. Fdez.-Gelfo et al. / Bioresource Technology 102 (2011) 606611 609diminished until 1012%.Similar behavior was described by Linke (2006), who found a

    decrease in the removal percentages, as well as methane and bio-gas yield, when the organic loading rate was increased from 0.8 to3.4 g VS/L/d.

    As can be seen in Table 3, when the SRT correspond to 20 and15 days, the OLRC in terms of VS is very close to the OLR0, showingremoval efciency above 88%. On the contrary, SRTs higher than20 days and lower than 15 days present a reverse effect, more ac-cused for lower SRTs.

    Someauthors have observed an increase in biogas production andVS removal efciencywhen the SRT is decreased (Alatiqi et al., 1998).The same behavior has been observed in thiswork: for SRT among 20and 15 days, the organic matter elimination (expressed as % DOCCand as % VSC) is higher than for SRT higher than 25 days.

    On the other hand, the methane production rate is directly re-lated to DOCC and, therefore, with the OLRC. Hence, when theSRT ranged from 40 to 15 days, the methane production is directlyproportional to the OLRC. In Table 3, the evolution of biogas andmethane production is also shown.

    In summary, the methane and biogas productions present anincreasing trend when the SRT is decreased except for 108 daysstages. In fact, an almost proportional relation between the appliedSRT and the biogas and methane productions was observed. Thesame behavior was described by Snchez et al. (2000). On the otherhand, for high SRT ranged from 40 to 20 days, when the feed organ-ic loading rate (OLR0) is increased, the consumed organic loadingrate and removal efciency are increased. For low SRT, ranged from15 to 8 days, the effect is the opposite.

    3.2. Substrate consumption model of Romero Garca (1991)

    The t of the expressions (Eqs. (3) and (4)) of substrate con-sumption model to determine the kinetic parameters of the modelFig. 2. Evolution of total concentration, exIt is noticeable than SRT from 40 to 25 days were not main-tained for enough time to reach pseudo stationary state conditions.In this sense, it is admitted usually than the minimum period oftime required to assure that the system is near to stability is equiv-alent to three times the SRT used. For hence, only SRT from 20 to8 days have been used in the model tting.

    Linear t for Romero Garca (1991) model is only valid for theDOC data (Table 5). For VS data, intercept is negative and, hence,lack of physical signicance. However, non-linear t is adequatefor both, DOC and VS data. Nevertheless, as can be seen in Table5, the best results were obtained when the DOC data are used(r2 = 0.983; lmax = 0.581 days1; a = 0.268).

    The lmax value obtained from the t of organic matter (DOC)represents the global maximum specic growth rate for the overallof microbial populations involved in the process and consideringthat the system operates in semicontinuous regime it will be rep-resentative of the microbial group rate-limiting.

    In Table 6 the values obtained are compared with the usual val-ues in literature. Kinetic modeling studies of the dry-thermophilicanaerobic digestion of OFMSW (lvarez Gallego, 2005) indicatethat the maximum specic growth rate of the populations involvedin the hydrolytic and acidogenic stages ranged between 0.08 and0.18 days1; for the methanogenic acetoclastic Archaeas lmax ran-ged 0.230.28 days1; whereas for the hydrogen utilizing Archaeasthe values were 0.330.40 days1.

    Equally, this results can be compared with results obtained inthe research group. Thus, kinetic modeling studies of the anaerobicdigestion of wine vinasses in wet conditions indicate that the lmaxof the Acetoclastic Archaeas was 0.3 days1 in mesophilic range(Sales et al., 1989) and 0.6 days1 in thermophilic range, both whenusing the Chen and Hashimoto model (Romero et al., 1988, 1990,1993) as when using the model of Romero Garca (1991). The lastvalue is very similar to the result presented in this paper(0.518 days1).pressed as mgAcH/L, in the efuent.

  • Used values of h, S y S0 for kinetic modeling.

    8 28.075 12.094


    ce TTable 5Kinetic for substrate consumption model of Romero Garca (1991).

    Kinetic models Model tting for DOC

    Model parameters

    Lineal regressionSS0

    a 1 almax


    y = 1.2607x + 0.2679a = 0.268lmax = 0.580 days1

    Non-linear regression S aS0 S01 almaxa = 0.268lmax = 0.581 days1h (days) DOCInuent (g/L) DOCEuent (g/L)

    40 28.075 18.66435 28.075 13.42230 28.075 11.66825 28.075 10.13120 28.075 9.51215 28.075 9.66210 28.075 10.913Table 4

    610 L.A. Fdez.-Gelfo et al. / Bioresour4. Conclusions

    For the SRTs ranged from 20 to 15 days, the higher removal per-centages of DOC and VS are obtained. The DOC removal yield isaround 66% for both periods. In the case of VS, the removal yieldis around 89%. For SRT of 10 and 8 days the removal efciency de-creases with respect to aforementioned. Therefore, 15 days may beconsidered the optimal SRT. Applying the substrate consumptionmodel of Romero Garca (1991), the global maximum specicgrowth rate and the non-biodegradable substrate fraction in thefeeding were estimated with the following values: 0.581 days1

    and 0.268, respectively.


    This work was supported by the Spanish Ministry of Scienceand Innovation (Project CTM2007-62164/TECNO), by the Innova-tion, Science and Enterprise Department of the Andalusian Govern-ment (Project P07-TEP-02472) and by the European RegionalDevelopment Fund (ERDF).


    Alatiqi, A.M., Hamoda, M.F., Dadkhah, A.A., 1998. Kinetic analysis of thermophilicanaerobic digestion of wastewater sludge. Water Air Soil Pollut. 107, 393407.

    Table 6Comparison of the maximum specic growth rates obtained in different studies.

    Population of microorganisms Range of temperature Waste

    Overall Thermophilic Simulated OFMSWHydrolyticacidogenic Thermophilic Industrial OFMSWMethanogenic Acetate-utilizing Thermophilic Industrial OFMSWMethanogenic H2-utilizing Thermophilic Industrial OFMSWMethanogenic Acetate-utilizing Mesophilic Industrial OFMSWMethanogenic H2-utilizing Mesophilic Industrial OFMSWOverall Thermophilic Sludge from WWOverall Mesophilic Organic solid wasMethanogenic Acetate-utilizing Thermophilic Wine-Distillery W

    Acidogenic Thermophilic Sludge from WWMethanogenic Acetate-utilizing Thermophilic Sludge from WWMethanogenic Acetate-utilizing Mesophilic Wine-Distillery WVSInuent (g/L) VSEuent (g/L) cV (LCH4/LR d)

    177.25 58.035 0.007177.25 43.461 0.465177.25 32.336 0.499177.25 21.313 0.582177.25 20.229 0.933177.25 19.506 1.149177.25 21.021 0.929177.25 49.580 0.132

    Model tting for VS

    r2 Model parameters r2

    0.968 y = 1.978x 0.0133a = 0.013lmax = 0.498 days1


    0.983 a = 0.013lmax = 0.512 days1


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    L.A. Fdez.-Gelfo et al. / Bioresource Technology 102 (2011) 606611 611

    Dry-thermophilic anaerobic digestion of simulated organic fraction of Municipal Solid Waste: Process modelingIntroductionKinetic model descriptionComputer support

    MethodsExperimental equipmentSampling and analysisMethodology

    Results and discussionOrganic loading rate (OLRC) removedSubstrate consumption model of Romero Garca (1991)



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