Increasing biogas production by thermal (70 °C) sludge pre-treatment prior to thermophilic anaerobic digestion

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  • Biochemical Engineering Journal 42 (2008) 186192

    Contents lists available at ScienceDirect

    Biochemical Engineering Journal

    journa l homepage: www.e lsev ier .c

    Increasing biogas production by thermal (70 C) sthermo

    Ivet Ferre tb,

    a GIRO Technolb Composting R eria, UCerdanyola delc Institute of Bi la del

    a r t i c l

    Article history:Received 26 MReceived in reAccepted 23 Ju

    Keywords:Anaerobic proBiosolidThermal pre-tThermophilesWaste-water treatmentWaste treatm

    estigaic dig48 anAs) ane wase-treaby ato 3

    Our results suggest that a short period (9h) low temperature pre-treatment should be enough to enhancemethane production through thermophilic anaerobic digestion of sludge.

    1. Introdu

    Anaeromunicipalstabilizatiodewaterinof the provantage otimes (SRTdigesters.large digepsychrophgeneral, mwidely useof the lowcess. Thermorganic maenhances t

    CorrespoE-mail ad

    f.vazquezl@g1 Current ad

    Maritime andGirona 1-3, 0

    1369-703X/$doi:10.1016/jent 2008 Elsevier B.V. All rights reserved.


    bic digestion is a treatment process used in manywastewater treatment plants (MWWTPs) for sludgen. Mass reduction, methane production and improved

    g properties of the treated sludge are the main featurescess. Slow degradation of sewage sludge is a disad-f anaerobic digestion, leading to high solid retentions) of 2030 days in conventional mesophilic (37 C)

    This fact implies signicant space requirements due tosters. Anaerobic digestion may be carried out underilic, mesophilic and thermophilic conditions (55 C). Inesophilic anaerobic digestion of sewage sludge is mored compared to thermophilic digestion, mainly becauseer energy requirements and higher stability of the pro-ophilic digestion, however, is more efcient in terms oftter removal and methane production [1,2]. Moreover, ithedestructionofpathogens,weed seeds and insect eggs;

    nding author. Tel.: +34 93 5814480; fax: +34 93 5812013.dresses: (I. Ferrer), (S. Pons), (F. Vzquez), (X. Font).dress: Environmental Engineering Division, Department of Hydraulic,Environmental Engineering, Technical University of Catalonia, C/Jordi

    8034 Barcelona, Spain.

    thus enabling efuent hygienisation [3], which might be requiredin the short term for land application (3rd Draft EU Working Doc-ument on Sludge [4]). Increased energy requirements may be metby implementing a systemallowing heat recovery from the efuentand cogeneration with biogas [5].

    Hydrolysis is the rate-limiting step of anaerobic digestion ofsemi-solid wastes. In this step both solubilization of particu-late matter and biological decomposition of organic polymersto monomers or dimers take place. Thermal, chemical, bio-logical and mechanical processes, as well as combinations ofthese, have been studied as possible pre-treatments to acceleratesludge hydrolysis. These pre-treatments cause the lysis or dis-integration of sludge cells permitting the release of intracellularmatter that becomes more accessible to anaerobic microorgan-isms. This fact improves the overall digestion process velocityand the degree of sludge degradation, thus reducing anaerobicdigester retention time and increasing methane production rates[6].

    Mechanical sludge disintegration methods are generally basedon the disruption of microbial cell walls by shear stress. Stirredball mills, high pressure homogenisers and mechanical jet smashtechniques have been used for mechanical pre-treatment applica-tion although the most used technique is sludge sonication [610].Microwaves have also been used for cell lysis. However, they havebeen scarcely used for sludge disintegration [1014]. The use ofheat has been widely reported for the disintegration of sludge

    see front matter 2008 Elsevier B.V. All rights reserved..bej.2008.06.020philic anaerobic digestion

    ra,1, Sergio Ponsb, Felcitas Vzquezc, Xavier Fonogical Centre, Rambla Pompeu Fabra 1, 08100 Mollet del Valls, Barcelona, Spainesearch Group, Department of Chemical Engineering, Escola Tcnica Superior dEnginyValls, 08193 Barcelona, Spainotechnology and Biomedicine, Universitat Autnoma de Barcelona, Bellaterra, Cerdanyo

    e i n f o

    arch 2008vised form 16 June 2008ne 2008



    a b s t r a c t

    The objective of this work was to invthe efciency of thermophilic anaerobof sludge pre-treatment time (9, 24,solids (VDSs), volatile fatty acids (VFsemi-continuous process performancdays solid retention time. The 70 C prby almost 10 times after 9h), followed72h). Biogas production increased upom/ locate /be j

    ludge pre-treatment prior to

    niversitat Autnoma de Barcelona, Bellaterra,

    Valls, 08193 Barcelona, Spain

    te the effect of a low temperature pre-treatment (70 C) onestion of primary and secondary waste sludge. Firstly, effectd 72h) was evaluated by the increase in volatile dissolvedd biogas production in thermophilic batch tests. Secondly,studied in a lab-scale reactor (5 L) working at 55 C and 10tment showed an initial solubilization effect (increasing VDSprogressive generation of VFA (from 0 to nearly 5 g L1 after

    0% both in batch tests and in semi-continuous experiments.

  • I. Ferrer et al. / Biochemical Engineering Journal 42 (2008) 186192 187

    [6,10,1518]. A wide range of temperatures has been studied, rang-ing from 60 to 270 C, although the most common pre-treatmenttemperatures are between 60 and 180 C, since temperatures above200 C have been found responsible for refractory compound for-mation [15]. Pre-treatments applied at temperatures below 100 Care considered as low temperature thermal pre-treatments. Suchpre-treatments have been pointed out as effective in increasingbiogas production fromboth primary and secondary sludge [10,19].

    Similarly, two-stage systems coupling a hyperthermophilicdigester (6870 C) and a thermophilic digester (55 C) have beenfound to be more efcient in terms of methane production com-pared to single stage thermophilic digesters treating primary andsecondary sludge [20,21] and cattle manure [22]. In these studies,it is suggested that thermal pre-treatment applied at temperaturesaround 70 C enhances biological activity of some thermophilicbacteria population with optimum activity temperatures in thehigh values of the thermophilic range. Thus, low temperature pre-treatment may be considered as a predigestion step.

    In general, the efciency of pre-treatments has been assessedby the increase of soluble organic matter (i.e. volatile dissolvedsolids (VDSteins). Sombiogas prodBut little woon thermopuous digestfor a low teprimary ananaerobic d

    The objeof thermopprimary anperature (7pre-treatmeacids (VFAsbatch tests;continuousthe hygieni

    2. Materia

    2.1. Sludge

    The mix(Table 1) usater treatmwere collec

    Table 1Composition o


    TS (g L1)VS (g L1)VS/TSTDS (g L1)VDS (g L1)VDS/TDSVDS/VSpHTotal VFA (g L

    Acetate (g L1)Propionate (g Liso-Butyrate (gn-Butyrate (g Liso-Valerate (gn-Valerate (g L

    serves a population of 128,000 equivalent inhabitants. The con-ventional wastewater treatment used in this plant consists ofpreliminary andprimary treatment and secondary treatment in theactivatedsluvated sludgpoint), befoat very highimprove de

    2.2. Low tem

    The loworder to enhwell as enzthermal preand time [2was evaluat(9, 24, 48 an

    Beakersmostatic ba

    d wi(Heirawolati.effecdVFAslu

    d. Sluratioin Eqge sa

    S = (


    as por 9,ests a70 Cogastestset al.inocle 5 LT an

    AS) fests.atmeine bent wh bomanulumttlesby thessutions of mumuhe prnd eues os), soluble chemical oxygen demand or soluble pro-e studies also focus on anaerobic biodegradability anduction, mainly in mesophilic batch assays [8,13,14,16].rk has been done on the effect of sludge pre-treatmenthilic anaerobic digestion [10,19], especially in contin-ers [9,17,23]. To our knowledge, no such work existsmperature pre-treatment of the mixture of thickenedd secondary sludge prior to continuous thermophilicigestion.ctive of thisworkwas then to address the enhancementhilic anaerobic digestion of the mixture of thickenedd secondary sewage sludge, by means of a low tem-0 C) pre-treatment. Firstly by studying the effect ofnt time on organic matter solubilization, volatile fatty) generation and biogas production in thermophilicand secondly by evaluatingprocess efciency in a semi-lab-scale reactor at 55 C and 10 days SRT. The effect onsation of sludge was also studied.

    ls and methods

    sampling and characterization

    ture of thickened primary and secondary sludgeed for thisworkwas obtained fromamunicipalwastew-ent plant (MWWTP) near Barcelona (Spain). Samplested weekly and stored at 4 C until use. This MWWTP

    f the mixture of thickened primary and secondary waste sludge



    1) 0.110.06

    1) 0.05L1) 0.001) 0.00L1) 0.001) 0.00

    coverestirredples of(TSs), vand pH

    TheVDS anthe rawassayein theshown(t) slud


    2.3. An

    Biog70 C fbatch ttion ofand bibatchFerrer

    Thelab-scadays SRand Wbatch ttrol tredetermtreatm

    Eacinoculuof inocThe bolowedSMC Prproducanalysi

    Accfrom t55 C anet valdgeunit. Primarysludge (PS) andsecondarywasteacti-e (WAS) are thickened and mixed (this is the samplingre undergoing mesophilic (38 C) anaerobic digestionSRT (40 days) aimed to reduce the solids content and

    watering in a centrifuge prior to nal disposal.

    perature (70 C) pre-treatment

    temperature pre-treatment was carried out at 70 C inance thermal solubilization of particulate material, asymatic hydrolysis. Bearing in mind that the effect of-treatments depends both on treatment temperature4], in this work the effect of pre-treatment durationed by taking samples at different pre-treatment timesd 72h) in order to study the combined effect.containing 0.5 L of sludge were submersed in a ther-th at 70 C during 9, 24, 48 and 72h. The beakers wereth plastic lm, to avoid water evaporation, and gentlydolph RZR1) to ensure temperature homogeneity. Sam-and pretreated sludge were analysed for total solidsle solids (VSs), total dissolved solids (TDSs), VDSs, VFAs

    t of pre-treatment time was assessed by the increase in, comparing the initial concentration of VDS andVFA in

    dge with those obtained after each pre-treatment timedge solubilization was also evaluated by the increasesoluble to total volatile solids (VDS/VS), calculated as. (1), where the sub-indexes refer to raw (o) and treatedmples:

    VDS/VS)t (VDS/VS)o(VDS/VS)o


    bic batch tests

    roduction of raw and pretreated sludge samples (at24, 48 and 72h) was initially determined by means oft 55 C. Theobjectivewas to study theeffect of thedura-pre-treatment, in terms of anaerobic biodegradability

    production under thermophilic conditions. Anaerobicwere based on Soto et al. [25], adapted according to[26].ulum was thermophilic sludge from the efuent of acontinuous stirred tank reactor (CSTR), operated at 20d 55 C. This digester was fed with sludge mixture (PSrom the same MWWTP as that used for the anaerobicThe substratewaseitherpre-treatedor rawsludge (con-nt). A blank treatment with only inoculum was used toiogas production due to endogenous respiration. Eachas performed in triplicate.

    ttle-reactor (300mL, SIGG) was lled with 100g ofd50gof substrate (theblank treatment onlywith150g) and was subsequently purged with N2 and sealed.

    were incubated at 55 C and biogas production was fol-e pressure increase in the headspace by means of a

    re Switch manometer (1bar, 5% accuracy), until biogasceased. Biogas samples were taken periodically for theethane content by gas chromatography.

    lated volumetric biogas production (mL)was calculatedessure increase in the headspace volume (150mL) atxpressed under normal conditions (20 C, 1 atm). Thef biogas production were obtained by subtracting bio-

  • 188 I. Ferrer et al. / Biochemical Engineering Journal 42 (2008) 186192

    Fig. 1. Schemaage; (3) feed pthermostatic b

    gas producttreatment.

    2.4. Lab-sca

    The effecess performdescribed iconnectedcontrolled.sition Systeextraction pcorrespondsured withcapacitive sperature issubmersedfrom the DA

    Prior to tbeen workidescribed awas maintais the contsludgewere(which corrquently fedwith a total

    Processbiogas prodinuent andand biogasconditionsof biogas ayields (L gVremoval (Tawhere thesludge:

    VSremoval (%

    Total VFA were calculated as the sum of individual VFA analysed(expressed as g L1).


    solidethoy frofromatanss otalwerenic,lso ais we

    andraphyalys(HPdeterationitiaed afor50 anns ofyric,ionsis wal concolu

    splitlinedperated w(99erich2000(CFUe wawertic diagram of the experimental set-up. (1) Reactor; (2) inuent stor-ump; (4) efuent storage; (5) extraction pump; (6) gas meter; (7)ath; (8) temperature sensor; (9) data acquisition system; (10) PC.

    ion of the blank treatment to biogas production of each

    le thermophilic anaerobic digestion

    ct of 70 C pre-treatment on semi-continuous pro-ance was studied in the experimental set-up (Fig. 1),

    n Ferrer et al. [27]. It consists of a jacketed CSTR (5 L)to a thermostatic bath through which temperature isSemi-continuous feeding is automated via aDataAcqui-m (DAS, by STEP S.L.) which activates the feeding anderistaltic pumps twice per day, giving a total volume

    ing to the SRT. The volume of biogas produced is mea-a device designed by Mata-lvarez et al. [28] and aensor (detector) connected to the DAS. Process tem-also monitored on-line by means of a thermal sensorin the liquor and connected to the DAS. Real time dataS is displayed in a PC (software by STEP S.L.).

    2.5. An

    Thedard MdirectlminedSupernsize glations, t(VSSs)propiowere aanalyster.

    VFAmatogVFA ancolumnisationa splitat an iincreastainedwere 2dilutioiso-butcentratanalysthermapackedHe inmaintator temcalibradioxide

    Esch16649:per mLabsencresultsheexperimentswithpretreatedsludge, thedigesterhadng at 55 C for 1 year, fed with the same sludge mixturebove, at decreasing SRT from 30 to 10 days, at which itined under steady-state conditions for 2 months. Thisrol treatment to which experiments with pre-treatedcompared. Keeping the sameow rate of 500mLday1

    esponds to a SRT of 10 days), the digester was subse-with pre-treated sludge (at 70 C, for 9, 24 and 48h),experimental duration of 6 months.performance was followed by on-line measurement ofuction and by periodical analyses (twice per week) ofefuent sludge samples (TS, VS, VFA, pHand alkalinity)

    samples (% CH4). Process efciency under steady-statefor each treatment assayed was evaluated in termsnd methane production rates (L Lreactor1 day1) andSfed1 or L gVSremoved1), as well as organic solids (VS)ble 2). VS removal was calculated according to Eq. (2),sub-indexes refer to the inuent (i) and efuent (e)

    ) = VSi VSeVSi


    3. Results

    3.1. Sludge

    GeneralenedPSand39g L1 (3.ratio of 0.74stabilized morganicmatsolids to totcating thatalmost abseThe only VF

    3.2. Low tem

    The expan increaseble organicfeeding slucal methods

    s content of sludge was determined according to Stan-ds [29] procedure 2540G. TS and VS were determinedm sludge samples, whereas TDS and VDS were deter-the supernatant of samples centrifuged at 7000 rpm.

    ts underwent ltration through 1.2m nominal porebber lters (Albet FVC047, Spain). The...


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