Anaerobic digestion of solid slaughterhouse waste (SHW) at laboratory scale: Influence of co-digestion with the organic fraction of municipal solid waste (OFMSW)

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  • Biochemical Engineering Journal 40 (2008) 99106

    Anaerobic digestion of solid slaughterhouseithe (a O, Unitmen193 Aember

    Abstract

    Mesophil on whave been ev ntinwith a hydra 1.70for co-digestion, was not successful. The second set-up was initiated with an HRT of 50 days and an OLR of 0.9 kg VS m3 day1 for digestionand 1.85 kg VS m3 day1 for co-digestion. Under these conditions, once the sludge had been acclimated to a medium with a high fat and ammoniacontent, it was possible to decrease the HRT while progressively increasing the OLR to the values used in the first set-up until an HRT of 25 daysand OLRs of 1.70 and 3.70 kg VS m3 day1, for digestion and co-digestion, respectively (the same conditions of the digesters failures previously).These digesundetected ofound to be 2007 Else

    Keywords: A

    1. Introdu

    Anaeroba widely uwaste andsource thro

    Slaughthuman con(skins, fatswastewaterproducts arorigin not iare not fit fas foodstufurated fat c

    CorresponE-mail ad

    1369-703X/$doi:10.1016/jters showed a highly stable performance, volatile fatty acids (VFAs) were not detected and long chain fatty acids (LCFAs) werer only trace levels were measured in the analyzed effluent. Fat removal reached values of up to 83%. Anaerobic digestion was thus

    a suitable technology for efficiently treating lipid and protein waste.vier B.V. All rights reserved.

    naerobic processes; Biogas; Co-digestion; Process inhibition; Solid slaughterhouse waste; Waste treatment

    ction

    ic digestion of organic matter has been reported assed technology in the efficient treatment of organicthe simultaneous production of a renewable energyugh the use of biogas [13].erhouses generate meat and products marketed forsumption, pollutant solid waste and other by-products, and bones), as well as substantial volumes ofas a result of cleaning operations. Animal by-

    e all bodies or parts of animals and products of animalntended for human consumption, either because theyor human consumption or there is no market for themf [4]. Consumer demand for meats with a low unsat-ontent in order to reduce cholesterol has led to the

    ding author. Tel.: +34 987 291841; fax: +34 987 291839.dress: amoran@unileon.es (A. Moran).

    expansion of the poultry industry and hence to an increase in lipidand protein waste. This waste causes important environmentalproblems as a result of organic pollution and microbial loads,and the increasing problems of its removal must be addressed asa result of legislative constraints and the cost of treatment anddisposal.

    Slaughterhouse waste is an ideal substrate for anaerobicdigestion and elimination of more than 90% chemical oxygendemand (COD) can be attained [5]. Lipids represent an impor-tant fraction of the organic charge in slaughterhouse waste [6].They consist mainly of triglycerides and long chain fatty acids(LCFAs). Triglycerides may be hydrolyzed to LCFA and glyc-erol. Accumulation of LCFA may inhibit anaerobic digestion,because they are toxic for acetogens and methanogens, the twomain groups involved in LCFA degradation [79]. Another inhi-bition mechanism is the result of the adsorption of the surfaceactive acids onto the cell wall [10,11], thus affecting the pro-cesses of transportation and protection. The unionized form ofLCFA adsorbs initially to the microbial cell surface and is then

    see front matter 2007 Elsevier B.V. All rights reserved..bej.2007.11.019scale: Influence of co-digestion wof municipal solid wast

    Mara Jose Cuetos a, Xiomar Gomez a, Marta Institute of Natural Resources (IRENA), Avda. de Portugal 41

    b Centre for Environment and Marine Studies (CESAM), DeparCampus Universitario de Santiago, 3810-

    Received 26 September 2006; received in revised form 21 Nov

    ic anaerobic digestion of slaughterhouse waste (SHW) and its co-digestialuated. These processes were carried out in a laboratory plant semi-coulic retention time (HRT) of 25 days and organic loading rate (OLR) ofwaste (SHW) at laboratorythe organic fraction

    OFMSW)tero b, Antonio Moran a,versity of Leon, 24071 Leon, Spaint of Chemistry, University of Aveiro,veiro, Portugal2007; accepted 25 November 2007

    ith the organic fraction of municipal solid waste (OFMSW)uously operated and two set-ups were run. The first set-up,kg VS m3 day1 for digestion, and 3.70 kg VS m3 day1

  • 100 M.J. Cuetos et al. / Biochemical Engineering Journal 40 (2008) 99106

    taken up into the cell [9]. LCFA are beta-oxidized to acetate,carbon dioxide and hydrogen in anaerobic digestion [12] whichare then coprocess is rLCFA andlowed by r

    Inhibitioorganic cocentrationsnitrogen-riare rich inof non-digto slaughteto ammonimechanismdirectly inhbic free amare rapidlypH conditi

    In studimixtures isthus improvnitrogen, wlems. Thecontent incphase digeimprovemetion of soliand the con

    It shouldis another tislation wifraction [21ers presenthigh methaboth typeslems assoccompounds

    The aimanization ((SHW) anorganic fraratory scaleoperationalcess were

    were comption of muin lipids.

    2. Materia

    2.1. Exper

    The inofrom the wThe slaughpoultry sla

    with the contents of the stomach and intestines were collectedfrom eviscerated poultry. They were ground and frozen at

    unsentcle se cotionsoredo feeed toeparfee

    tageere 4terhoon r

    perdig

    rriedingrs ht-upnt oy digandas a

    set-the fiperativelrt-ue froculuT o

    the dt in aof S

    bicerateer Btly,

    g wahowneach

    operdystat

    on fois ctrati

    naly

    Roufolldig

    alkanverted to biogas. What normally occurs during theapid hydrolysis/acidogenesis, with accumulation ofvolatile fatty acids (VFAs), removal of LCFA, fol-

    emoval of VFA and methane production [7,13].n of the anaerobic digestion of waste with a high

    ntent is usually also caused by high ammonia con-[1,14], produced in the degradation of proteins from

    ch slaughterhouse waste [15]. Poultry by-productsnitrogen because they may contain high proportionsester material, especially if the birds are fed priorring [16]. Two different mechanisms are attributeda inhibition of methanogens. According to the first, activities of methane synthesizing enzymes areibited by free ammonia. In the second, hydropho-monia molecules diffuse passively into the cell andconverted to ammonium owing to the intracellular

    ons [17].es carried out on this type of waste, co-digestion ofemployed to stabilize the feed to the reactor [18,19],ing the C/N ratio and decreasing the concentration ofhich in certain cases may produce inhibition prob-use of a co-substrate with a low nitrogen and lipidreases the production of biogas. Alternatively, two-stion systems may be employed [15,20], with whichnts are achieved in process efficiency, in the reduc-ds, the removal of chemical oxygen demand (COD)version of biogas.not be forgotten that municipal solid waste (MSW)

    ype of residue that is affronting more restrictive leg-th respect to landfill disposal of the biodegradable,22]. Co-digestion of the mixtures of waste with oth-

    ing a lower nitrogen and lipid content may result inne yields due to the fact that the characteristics ofof waste are complementary, thus reducing prob-

    iated with the accumulation of intermediate volatileand high ammonia concentrations [23].of the present study was to carry out the biometh-

    anaerobic digestion) of both slaughterhouse wasted mixtures of solid slaughterhouse waste with thection of municipal solid waste (OFMSW) at labo-in mesophilic semi-continuously fed digesters. Theconditions needed to achieve a stable digestion pro-

    determined. At the same time, the results obtainedared, assessing the contribution of the organic frac-nicipal solid waste to the slaughterhouse waste rich

    ls and methods

    imental set-up

    culum used for starting up the digesters was obtainedastewater treatment plant of the city of Leon (Spain).terhouse waste was collected at the Huevos Leonughterhouse in the same city. The entrails together

    18 Ca reprea partimixturproporthen st

    Twanalyzwas prdesiredpercenfeed wslaughdigestiand VS

    Thewas ca

    a workdigesteTwo sediffereidentifSHW,or 2) wsecond

    Inwere o

    respecThe staing ratthe inotial HRdue tocontenstudiesanaero

    A2 opDigestsequenloadineters s

    Forouslyof steaSteadyvariatiand thconcen

    2.2. A

    2.2.1.The

    ing the(VS),til subsequent use. Given the difficulty of obtainingative sample of OFMSW, a simulated OFMSW withize of less than 3 mm was used as substrate. Thisntained fruit and vegetable waste and followed thereported in previous studies [22,24]. This waste wasat 4 C until used.ds were prepared for the study being periodicallyalways ensure the same solids content. A SHW feed

    ed by diluting the sample with distilled water to thed load; the SHW:H2O ratio being 1:5 in weight. Thes of total solids (TS) and volatile solids (VS) of this.7 and 4.3%, respectively. A combined mixture withuse waste was prepared with an SHW:OFMSW co-

    atio of 1:5 in weight. The mix as prepared had a TScentages of 10 and 9.3%, respectively.estion of SHW and its co-digestion with OFMSWout in two completely mixed stirred digesters, with

    volume of 3 L and thermostatized at 34 1 C. Bothad a side inlet via which the systems were fed daily.s were implemented in order to run experiments underperational conditions. In the nomenclature used toestion systems letter A refers to the digester treating

    letter B to the one treating the mixture. A number (1dded according to conditions imposed in the first orup.rst set-up of two reactors Digesters A1 and B1

    ted at an OLR of 1.7 and 3.7 kg VS feed m3 day1,y, both at a hydraulic retention time (HRT) of 25 days.p of these digesters was performed applying this feed-m day 1 of experimentation to reactors loaded withm previously described. In the second set-up the ini-f the digesters was 50 days. This HRT was selectedifficulty of treating waste with a high lipid and proteinccordance with other successful anaerobic digestionHW, where the HRT are not as low as in the case of

    digestion of other organic wastes [25,26]. Digesterd at a loading of 0.90 kg VS feed m3 day1, and2 at a loading of 1.85 kg VS feed m3 day1. Sub-the HRT was successively reduced and the organics gradually increased in accordance with the param-

    in Table 1.experimental condition, the reactors were continu-

    ated for two consecutive HRTs to ensure a situationstate before changing their operational conditions.e was defined as the situation where the coefficient ofr daily gas production was less than 10% [27,28]

    oincides with a stable effluent volatile fatty acidson.

    sis

    tine analysesowing parameters were monitored periodically dur-estion process: pH, total solids (TS), volatile solidslinity, chemical oxygen demand (COD), ammonia,

  • M.J. Cuetos et al. / Biochemical Engineering Journal 40 (2008) 99106 101

    Table 1Digester operational parameters in the second reactor set-up carried out

    Digester Description Days HRT (days) Loading rate (kg VS feed m3 day1)A2 50 0.90A2 36 1.16A2 25 1.70B2 50 1.85B2 36 2.56B2 25 3.70

    yield and cof volatile ftwice a wea week, and

    The anaalkalinity aMethods [2

    The conEqs. (1) an

    FA =1 +

    pKa = 0.0

    where FA igen, pKa isT is the tem

    The cheHanna Insthomogenizat 150 C f

    Daily gawith liquid

    BiogasGC gas chity detecto80/100 Meumn 13to separate(N2), hydrhelium andof 50 C. Sapparatus.

    Volatilesame gastor (FID)(30 m 0.2gas was heand 250 C150 C fordetection lwas calibraSupelco (fopreviouslyfiltrated thr

    Other analyses performedoriginal waste and digestates were characterized.

    radability analysis was carried out, total nitrogen concen-s were determined by the Kjeldahl method [30], organicwas analyzed according to the Walkey-Black method

    nd orconts su

    tio.totahodhl-N

    chrfattyed awithat

    TheL ch

    0.2oven

    en iininaturlibrance

    forwere

    (C1.maiay

    nt pr

    eristic

    ersSHW digestion 0100SHW digestion 101175SHW digestion 176225SHW + OFMSW co-digestion 0100SHW + OFMSW co-digestion 101175SHW + OFMSW co-digestion 176225

    omposition of the biogas produced and concentrationatty acids (VFAs). All these variables were measuredek, except for ammonia, which was monitored once

    gas production, which was daily measured.lyses of pH, total and volatile solids (TS and VS),nd ammonia were carried out according to Standard9].centration of free ammonia was calculated based ond (2) [1,17]:

    TAN10(pKapH)

    (1)

    9018 + 2729.92T + 273.15 (2)

    s the free ammonia, TAN is the total ammonia nitro-the dissociation constant for the ammonium ion andperature in C.

    mical oxygen demand (COD) was determined using aruments Series C99 multi-parameter photometer. Theed sample was digested in the presence of dichromateor 2 h in a Hanna C9800 reactor.s production was measured using a reversible devicedisplacement with a wet-tip counter.

    composition was analyzed using a Varian CP-3800romatograph equipped with a thermal conductiv-

    r (TCD). A 4 m long column packed with Hayesepsh followed by a 1 m long Molecular Sieve col-80/100 Mesh (1.0 m 1/8 in. 2.0 m) were usedmethane (CH4), carbon dioxide (CO2), nitrogen

    ogen (H2) and oxygen (O2). The carrier gas wasthe columns operated at 331 kPa at a temperature

    tandard 234 from Supelco was used to calibrate the

    fatty acids (VFAs) were determined on thechromatograph, using a flame ionization detec-

    equipped with a Nukol capillary column5 mm 0.25m) from Supelco. The carrier

    2.2.2.The

    Biodegtrationmatter[30], amatterbon waC/N ra

    Thelet metKjelda[31].

    Gaschainextractmixed30 minfilters.tem X(30 m initialand thmaintatemperwas ca

    with colimitlyzedstearicSigma

    TheAt it mdiffere

    Table 2Charact

    Parametlium. Injector and detector temperatures were 220, respectively. The oven temperature was set at3 min and thereafter increased to 180 C. The

    imit for VFA analysis was 5.0 mg L1. The systemted with a mixture of standard volatile acids fromr the analysis of fatty acids C2C7). Samples werecentrifuged (10 min, 3500 g) and the supernatantough 0.45m cellulose filters.

    Total organicOrganic matteTotal nitrogenC/N ratioTotal fat (%)Total proteinTS (%)VS (%)NA: not analyganic carbon was determined considering an organicent/organic carbon ratio of 1.7241. The organic car-bsequently divided by the total nitrogen to obtain the

    l lipid content was determined using a Standard Soxh-[29], and the protein content was calculated from thecontent using a conversion factor of 6.25 (for meat)

    omatography was used for the analysis of the longacids (LCFAs). Samples for LCFA analysis were

    s described by Fernandez et al. [32]. Samples weren-heptane, the solution was then centrifuged for

    3500 g and filtrated through 0.45m cellulosesample was injected into a Perkin-Elmer AutoSys-

    romatograph equipped with an HP Innowax column5 mm 0.25m). The carrier gas was helium. Thetemperature of 120 C was maintained for 1 min,

    ncreased to 250 C, with a ramp of 8 C min1,g this temperature for 7 min. Injector and detectores were 250 and 275 C, respectively. The systemted using a mixture of LCFA from individual acids

    ntrations in the range of 0100 mg L1. The detectionLCFA analysis was 5.0 mg L1. The acids ana-lauric (C12:0), myristic (C14:0), palmitic (C16:0),8:0), oleic (C18:1) and linoleic (C18:2), all from

    n properties of the original waste are given in Table 2.be seen the used SHW and OFMSW wastes haveoperties. While SHW has lower organic carbon (%),

    s of the original waste and the inoculum used

    OFMSW SHW Inoculumcarbon (%) 42.7 0.7 23.1 0.4 23.6 0.7r (%) 73.4 0.9 39.6 0.9 31.3 0.7(%) 1.3 0.4 6.2 0.3 4.2 0.4

    32.1 1.2 3.7 0.4 5.5 0.40.7 0.4 40.5 0.5 NA

    (%) 8.3 0.7 38.9 0.9 NA6.4 0.2 28.3 0.3 4.1 0.46.0 0.2 26.0 0.5 2.9 0.4

    zed.

  • 102 M.J. Cuetos et al. / Biochemical Engineering Journal 40 (2008) 99106

    organic matter (%) and C/N than OFMSW, its total nitrogen(%), fat (%), protein (%) and both TS (%) and VS (%) are muchhigher.

    3. Results and discussion

    Anaerobic digestion of SHW and anaerobic co-digestion ofthe mixtures of SHW with the OFMSW were carried out atlaboratory scale at 34 1 C. Two set-ups were operated underdifferent conditions to study the effect of the variation in HRTand organic loading, key parameters in the digestion of SHW[9,33].

    3.1. First reactor set-up

    Gas production commenced immediately after loading thedigesters, but after a few days of functioning (around day 10 ofthe study in Digester A1, and day 15 in Digester B1), the volumeof biogas decreased progressively (Fig. 1). This decrease wasfollowed by a drop in methane production from values of 65%to values of below 45% in both reactors. From this point on, areduction in pH was observed decreasing from values of 7 to 7.5to below 6.5.

    Fig. 1 shows that there is a significant decrease in daily gasproduction in correlation with an increase in VFA concentra-tion in the effluents. Acetic acid was the most abundant VFAin Digester A1, mainly causing inhibition of acetate consumingmethanogesured in thethe total Lalmost douboth cases,

    stituted over 50% of the analyzed LCFA, followed by stearicand linoleic acids, the lowest concentrations corresponding topalmitic, lauric and myristic acids (data not shown).

    The data obtained greatly exceeded the concentrationsreported as toxic by other authors [3437].

    Therefore, the low methane production rates may beattributed to the accumulation of VFA, intermediary compoundsin the metabolic pathway of methane fermentation, which causemicrobial stress if present at high concentrations, thus result-ing in a decrease of pH and leading to failure of the digesters[38,39]. VFA, along with LCFA, are potential inhibitors of theformation of methane in reactors, stopping the overall process.In view of the results obtained, and after 30 days of function-ing, it was decided to dismantle the failed reactors, without anyattempt to recover them.

    VS removal remained around 70% in Digester A1 and 65%in Digester B1. However, during the period of time in which thereactors were working the VS concentrations measured in thereactors (Fig. 1) decreased from 15.0 to 6.0 g L1 in DigesterA1 and from 13.0 to 5.5 g L1 in Digester B1. Despite mechan-ical stirring of the systems, it was not possible to prevent theformation of foam on the top of the reactors, thus demonstratingthe high tendency of slaughterhouse waste to a non-uniformdistribution in the digesters. This agrees with the results ofother authors, who state that lipids have a tendency to formfloating aggregates and foam that may cause problems of strati-fication [13,40]. The value of measured solids in the dismantled

    rs, athe

    nd 3nal

    Fig. 1. Daily concreactor set-upns. Furthermore, the concentrations of LCFA mea-effluents after dismantling the digesters were high;

    CFA content of Digester 1 was 4300 mg L1 andble for the co-digestion reactor (Digester B1). Inoleic was the most abundant acid, which alone con-

    digesterest of20.0 ators, ficase.

    biogas and VS concentration in Digester A1 (A) and Digester B1 (B), and VFA.fter homogenization of the floating layer with thecontent of the reactor, increased significantly to

    0.9 g L1, respectively. Despite failure of the reac-fat removal reached values of 50 and 65% in each

    entrations in Digester A1 (C) and Digester B1 (D) during the first

  • M.J. Cuetos et al. / Biochemical Engineering Journal 40 (2008) 99106 103

    In both digesters, the final free ammonia concentration asconsequence of protein mineralization was 10.6 mg L1. Thisvalue is quite lower than those considered as inhibitory in stud-ies published to date [41], in which free ammonia concentrationof 150200 mg L1 mark the limit of being inhibitory. More-over, other authors [5,42] have reported that concentrations ofammonium of up to 58 g L1 may be tolerated by microorgan-isms if the pH is sufficiently high. These findings indicate thatthe VFA and LCFA accumulation, and not the presence of Nin the system, was the fundamental cause of inhibition of thedigestion process in the present study.

    3.2. Second reactor set-up

    The short HRT selected for the start-up of the systems resultedin overloading and subsequent failure of the digestion process.A new approach was chosen in order to shorten the HRT, start-ing off from a greater value and progressively decreasing theHRT while increasing the organic loading. This methodologywas an attempt to adapt anaerobic microorganisms to the newconditions by pre-exposing them to non-inhibitory concentra-tions.

    The main results corresponding to the steady-state periods,for different HRT and loading, are presented in Table 3.

    To avoid problems of system instability, a new set of reac-tors was assembled reducing the organic loading and employingan HRT of 50 days. The reactors were kept under the opera-tional conditions reported in Section 2 during two consecutivehydraulic retention times. Under these conditions, operation of

    the reactors was highly stable, a fact confirmed by the presenceof stable pH, alkalinity and COD profiles (Fig. 2).

    Levels of VFA of below 2.5 and 1.0 g L1, respectively, weredetected at the commencement of digestion of the SHW andco-digestion with the OFMSW (Fig. 2). A short time after start-up, and without having completed one operational hydraulicretention time, no values of VFA were detected in either of thetwo reactors (Fig. 3).

    The average values of total ammonia during the steady statein the digesters with an HRT of 50 days (Table 3) were belowthe toxic limits reported in the literature [41,42]. However, therelease of ammonia from the proteins provoked an increase inalkalinity concentration of the system, leading to an averagevalue for the period higher than 5000 mg L1, the recommendedvalue for standard rate sewage sludge digesters [43]. Due tothe high protein concentrations in the waste, pH values in thereactors were expected to be slightly higher than those reportedas suitable for the development of methanogenic microorgan-isms with another type organic substrate; pH values remainedbetween 7.5 and 8.0. Therefore, as a result of the increase in pH,the free ammonia concentration is high, but not inhibitory forthe development of microorganisms.

    Only trace concentrations (

  • 104 M.J. Cuetos et al. / Biochemical Engineering Journal 40 (2008) 99106

    Fig. 2. pH, Csecond reacto

    As the pwere reducThe OLR wfed with SHthe reactorshows that,increase inwere charaammonia asteady statesystems wetions. Altho

    compared to the results obtained from the previous period (alka-linity, ammonia and COD), the systems were able to reach and

    in sty oftions1

    ,

    er twf th, the1 an

    VS.in th

    sysmaintastabilicentra8 mg L

    Afttion oHenceup (A3.70 kgtively)

    Asing theOD, alkalinity and ammonium in Digesters A2 and B2 during ther set-up.

    erformance of these reactors was stable, their HRTsed to 36 days, while gradually increasing in the OLR.as set at 1.16 kg VS feed m3 day1 for the reactorW (Digester A2) and 2.56 kg VS feed m3 day1 forfed with SHW and OFMSW (Digester B2). Fig. 3at the beginning of this new period consisting of anorganic loading and a decrease in HRT, the systemscterized by increases in the values of alkalinity ands well as a decrease in COD values until reaching the. Once a period equivalent to an HRT had elapsed, there capable of adjusting to the new established condi-ugh higher values for the parameters were observed

    imposed coVFA concein the effluoleic acidtration of le

    On decrwas observthe percentfat removalreactors. Inremained bmuch loweof slaughteinhibition o

    Daily biMethan

    feed in thebetween 0SHW andanaerobic d1.0 m3 kgbetween 0ods studiein the literVS feed forelativelytion of thethe high th[4648].

    The higportion ofto 66 and 6well as theof the implslaughterho

    Anaerobtization ofmedium [2the new cois agreeme[10,13,32],microorganfeasible.table profiles until the end of the period. The highthe reactors was confirmed by the lack of VFA con-(Fig. 2) and traces of LCFA concentrations, of below

    detected in the effluents.o HRTs under these conditions, a further reduc-

    e HRT to a value of 25 days was carried out.same loading at which the reactors in the first set-

    d B1) had been inhibited was repeated (1.70 andfeed m3 day1 for Digesters A2 and B2, respec-

    e previous case, with the change of HRT and load-tem needed a period of time to adapt to the newlynditions until reaching steady profiles. Once more,ntrations were imperceptible. LCFA concentrations

    ents at the end of the period were very low (Table 3),being the most abundant, with a maximum concen-ss than 25 mg L1.easing HRT and increasing the organic loading, ited an increase in the volume of gas produced and inage of methane in the analyzed biogas, and a high, as a consequence of lipid and protein content of theall circumstances, the pH value of the systems alwaysetween 7 and 8. VFA and LCFA concentrations werer than the concentrations reported in other studiesrhouse waste, in which their accumulation cause thef the anaerobic process [9,40].ogas yield increased in line with increased loading.e yield ranged between 0.6 and 0.7 m3 kg1 VS

    anaerobic digestion of slaughterhouse waste, and.4 and 0.5 m3 kg1 VS feed with the mixture ofOFMSW. Specific gas production (SGP) in theigestion of SHW likewise ranged between 0.8 and

    1 VS feed and in the co-digestion with OFMSW.7 and 0.8 m3 kg1 VS feed for the different peri-d. These values are higher than those reportedature, which are not higher than 0.60.7 m3 kg1r co-digestion of other substrates [22,44,45]. Thishigh SGP is a result of the efficient degrada-

    slaughterhouse waste and as a consequence ofeoretical methane potential of lipids and proteins

    h values of methane yield achieved and the high pro-methane in the biogas produced in the process (up5% in digestion and co-digestion, respectively), as

    lack of volatile intermediaries during the steady stateemented treatments, indicate that fat removal in theuse waste was carried out efficiently.ic digestion was carried out thanks to the acclima-

    the bacterial consortium to an ammonia-rich6,49], the sludge was progressively acclimated tonditions, high fats and LCFA concentrations. Thisnt with the research carried out by other authorsand prove that the adaptation of methanogenic

    isms to different LCFA concentrations is

  • M.J. Cuetos et al. / Biochemical Engineering Journal 40 (2008) 99106 105

    Fig. 3. Daily FA cosecond reacto

    4. Conclu

    Anaerobseem a com

    the waste. Hout in semi

    Anaeroborganic frasuccessfulorganic loaThese systewere not aload. This(LCFA andever, it wasand co-digesively decrthe organi1.70 and 3the adaptabammonia-rtures to non

    Total fafor the co-d

    The addtributed toco-digestintors. The bof the SHWat 25 daysalleviates thmencemen

    concentrati

    re pc loa

    wled

    s reso. 20tillated b

    ncebiogas and VS concentration in Digester A2 (A) and Digester B2 (B), and Vr set-up.

    sions

    ic digestion of slaughterhouse waste (SHW) mayplex task due to the high lipid and protein content ofowever, the methanization was successfully carried

    -continuously fed digesters at 34 C.ic digestion of SHW and its co-digestion with thection of municipal solid waste (OFMSW) was notin initial assays working with an HRT of 25 days andding of 1.70 and 3.70 kg VS m3 day1, respectively.ms were working without a period of adaptation and

    are mo

    organi

    Ackno

    Thience n

    of Cassuppor

    Refere

    ble to overcome the disturbance of the initial shockled to the accumulation of volatile intermediariesVFA) and failure of the digestion process. How-possible to carry out anaerobic digestion of SHW

    stion of mixtures of SHW with OFMSW by progres-easing the HRT from 50 to 25 days while increasingc loading from 0.9 and 1.85 kg VS m3 day1 to.70 kg VS m3 day1, respectively. In consequence,ility of anaerobic microorganisms to a fat-and free

    ich medium was observed by pre-exposing the cul--inhibitory concentrations.

    t removal was 61% for the SHW digestion and 83%igestion of the mixture of SHW with OFMSW.ition of OFMSW to the co-digestion system con-a significant increase in the daily biogas yield wheng with SHW along with an increase of VS in the reac-iogas yield of the co-digestion systems doubled that

    digestion system (i.e. 8.6 L day1 cf. 4.3 L day1of HRT). The presence of a co-substrate slightlye concentration of volatile intermediaries at the com-

    t of the treatment. However, total and free ammoniaons are higher during co-digestion, differences which

    [1] K.H. Hamanure:

    [2] F.J. Calldigestionmanure,

    [3] L. de Baeof-the-arvol. 1, 20

    [4] RegulatiOfficial J

    [5] H. Siegriwaste wiammonia

    [6] L. Massefat degraProcess

    [7] K. Hanalong-cha23 (1981

    [8] C.-S. Hwfatty aci34 (56)

    [9] E. Salmitry slaugWater Rencentrations in Digester A2 (C) and Digester B2 (D) during the

    ronounced the lower the HRT and the higher theding as a result of the contribution of the OFMSW.

    gements

    earch was made possible through the projects: refer-05/48 supported by Technological Agrarian Institutey Leon and reference no. ENE 2005-08881-C02-01y the Ministry of Education and FEDER funds.

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    .

    Anaerobic digestion of solid slaughterhouse waste (SHW) at laboratory scale: Influence of co-digestion with the organic fraction of municipal solid waste (OFMSW)IntroductionMaterials and methodsExperimental set-upAnalysisRoutine analysesOther analyses performed

    Results and discussionFirst reactor set-upSecond reactor set-up

    ConclusionsAcknowledgementsReferences

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