Feasibility of anaerobic co-digestion of pig waste and paper sludge

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    " Semi-continuous digester shows higher performance for co-digestion than pig waste.

    a b s t r a c t

    and paper waste, which represent the two largest fractions ofwaste biomass generated in the USA; waste biomass amounts aresummarized in Table 1.

    Pigwaste (PW) represents a signicant fraction of animalwastes,the largest waste stream in Table 1. Likewise, paper sludge (PS),which is mainly residues from various stages of paper mill opera-tion, is the largest pulp and paper waste. While containing a large


    5 g N/L or above (Braun et al., 1981). Overcoming inhibition has ahigh payback, because 3040% of the total COD in PW is solubleand immediately bioavailable (Jindal et al., 2006), a value muchhigher than in stabilized biomass, such as waste activated sludge(Jindal et al., 2006). In addition, the high N content of pig wastegives it a high alkalinity, because the organic N is hydrolyzed toNH3, a moderately strong base (pKb = 3.7, Snoeyink and Jenkins,1980): 1 mol of bicarbonate alkalinity is released for every moleof ammonia released, and this corresponds to 3.6 mg as CaCO3per mg N. Alkalinity is essential for stable pH control.

    Corresponding author. Tel.: +1 480 727 0849; fax: +1 480 727 0889.

    Bioresource Technology 124 (2012) 163168

    Contents lists available at


    elsE-mail address: prathap@asu.edu (P. Parameswaran).1. Introduction

    The carbon in organic wastes has high-energy electrons that canbe transformed into useful forms of energy for society. Anaerobicdigestion, a mature technology for capturing these electrons asmethane gas (CH4), is widely used worldwide. In the USA, forexample, over 1500 anaerobic digesters are currently in operation:approximately 135 treating livestock/agricultural wastes, 850 formunicipal solid waste removal, and 544 in wastewater treatmentplants (Alternative and Advanced Fuels Biogas, 2009). Whilethe current application of anaerobic digestion is signicant, muchmore anaerobic digestion is possible for animal waste and pulp

    potential for energy recovery, both of these large waste streamspose unique challenges when subjected to anaerobic digestion.

    Due to the high protein content in the diet of young pigs, PWhas a very high organic-nitrogen (N) content that is converted tototal ammonia during hydrolysis and fermentation. Inhibition ofmethanogenesis due to high concentrations of total ammonia is awell-established fact (Van Velsen, 1979; Cheung et al., 2002; Sossaet al., 2004; Sawayama et al., 2004; Kayhanian, 1993; PoggiVeraldo et al., 1997; Koster and Lettinga, 1984; Hansen et al.,1998). The major inhibition is caused by unionized ammonia(NH3) at a concentration of 150 mg N/L or higher, but the ammo-nium ion NH also exhibits toxicity at very high concentrations,a r t i c l e i n f o

    Article history:Received 28 May 2012Received in revised form 24 July 2012Accepted 26 July 2012Available online 15 August 2012

    Keywords:MethanogenesisPig wastePaper sludgeCo-digestionHydrolysis0960-8524/$ - see front matter 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.biortech.2012.07.116Pigwaste (PW) andpaper sludge (PS) possess complementary properties that canbe combined for successfulanaerobic digestion. Biochemicalmethanepotential (BMP) tests revealed that a PW:PS 3:1 (v/v) ratio had thehighest normalized CH4COD removal (54%), while PS had the lowest value (11%) and PW had 44%. BatchBMP tests revealed a signicant decrease in lag times for methane production in the order of PW:PS 1:3(14 days) < PW:PS 1:1 (17 days) < PW:PS 3:1 (20 days) < PW (23 days). Hydrolysis constants (khyd) werehigher for all PW:PS combinations than for either of the individual waste streams: 0.004 d1 (PS) < 0.02 d1

    (PW) < 0.024 d1 (PW:PS 3:1) < 0.03 d1 (PW:PS 1:1) < 0.05 d1 (PW:PS 1:3). Semi-continuous reactors per-forming co-digestion of PW and PS at a 2:1 ratio showed 1.5 times highermethane production than baselinePW-only reactors, conrming the BMP results.

    2012 Elsevier Ltd. All rights reserved.Feasibility of anaerobic co-digestion of p

    Prathap Parameswaran , Bruce E. RittmannSwette Center for Environmental Biotechnology, The Biodesign Institute at Arizona State

    h i g h l i g h t s

    " Benets of co-digestion of pig waste and paper sludge demonstrated by" Hydrolysis constants for co-digestion 220 times higher than baseline w


    journal homepage: www.ll rights reserved.waste and paper sludge

    versity, P.O. Box 875701, Tempe, AZ 852875701, USA

    P assays.es.

    SciVerse ScienceDirect


    evier .com/locate /bior tech

  • d ar

    Biomass waste category Amount of waste produced/year Waste quantitove

    Milbrandt (2005) and Perlack et al. (2005)

    lls wal w

    esouPS is generated in large quantities during several stages of apaper mill operation, such as chipping, paper machine rejects,and packaging (Mahmood and Elliott, 2006). The organic matterin these waste streams is lignocellulosic and has a very low N con-tent, poor buffering capacity, and low soluble COD; hence, it hasproven difcult to sustain a diverse anaerobic microbial commu-nity for good methanogenesis with paper sludge (Banks andHumphreys, 1998). PS from paper mill rejects is also known tohave a signicant fraction of inorganics making anaerobic diges-tion difcult.

    A potential solution for successful anaerobic digestion of bothlarge, but challenging waste streams is to mix pig and paper-sludgewastes, since they have complementary properties for methano-genesis. Mixing the two streams dilutes the high N content of thePW, which lowers inhibition, and adds readily biodegradableCOD and alkalinity from the PW to help establish a stable anaero-bic methanogenic community that can efciently degrade the par-ticulate matter present in the PS.

    Previous efforts to co-digest PW with other organic wastestreams have had success: e.g., with municipal solid wastes(Campos et al., 1999), food and vegetable wastes (Alvarez andLidn, 2007), wastewater from olive-oil bleaching and ltering(Ahring et al., 1992), grass silage (Xie et al., 2012), wasted sardineoil (Ferreira et al., 2012) and crude glycerol (Astals et al., 2011).Zhan et al (2012) already demonstrated the benets of successfulco-digestion of grass silage and pig manure at 1:1 ratio. Anaerobicdigestion of pig manure with pure cellulose as a co-substrate wasresearched by Van Assche et al. (1983), who demonstrated threetimes higher gas production than with pig waste alone. However,no studies have evaluated co-digestion of the combination of PSand PW, which are the two largest biomass waste sources in theUSA and should have especially complementary characteristics.

    The rst goal of this study was to test whether or notco-digestion of PW and PS provided advantages for methanogene-sis. This goal was achieved using batch biochemical methane po-tential (BMP) tests for various ratios of PW:PS compared againstPW or PS by itself. The second goal was to interpret the data fromthe batch BMP tests and using a rst-order model to estimate the

    (million US dry tons) energy rec

    Animal wastes 335 3.6 a 0.07b

    Pulp and paper 149 142Food processing 113 6Municipal wastewater 7 1.3Total 604 153

    a Amount of biomass estimated to be recovered as energy for heating from landb The amount of biomass recovered as biogas in anaerobic digesters treating animTable 1Summary of various waste-biomass-to-energy sources is the USA. The values indicatealready utilized for energy recovery.

    164 P. Parameswaran, B.E. Rittmann / Biorhydrolysis constant (khyd). khyd values were used to explain theimproved efciency for the co-digestion over baseline PW or PSanaerobic digestion and to t the experimental data to a wellestablished empirical model, namely Gompertz equation. The thirdgoal was to conrm the benets during long-term anaerobic diges-tion with a workable ratio of PW to PS. The semi-continuous reac-tor operation along with a control semi-continuous reactortreating PW alone helped to achieve the last goal.

    2. Methods

    2.1. Biochemical methane potential (BMP) tests

    BMP tests (Owen et al., 1979; Angelidaki et al., 2009; Salerno etal., 2009) were performed to compare CH4 production from PWand PS alone and with different volume ratios of co-digestion mix-tures. PW slurry was obtained from Hormel foods, Snowake, AZand PS from Abitibi pulp and paper mill, Snowake, AZ. Threeratios of PW:PS by volume were evaluated: 1:3, 1:1, and 3:1. Aninoculum control was additionally prepared that consisted of theanaerobic inoculum with added trace minerals. The anaerobicinoculum was obtained from the anaerobic digesters at Mesa(AZ) Northwest Wastewater Reclamation Plant (MNWWRP), whichis fed with mixed primary + waste activated sludge that is centri-fuged at about 2000 rpm for 10 min.

    For all BMP tests, 70 mL of sample and 30 mL of inoculum wereadded to 160-mL serum bottles. The tests were performed in dupli-cate and the average values are presented here. To make theco-digestion mixtures, appropriate volumes were added to makeup 70 mL: e.g., 17.5 mL of PW and 52.5 mL PS added to 30 mL ofanaerobic inoculum for the 1:3 PW:PS condition. Before sealingthe serum bottles with rubber stoppers and aluminum caps, theserum bottles were purged with 100% N2 gas to create anaerobicconditions. During the BMP test, the serum bottles were shakenat 180 rpm and incubated at 37 1 C.

    Gas production was periodically measured in the headspaceusing a frictionless glass syringe (Perfektum, NY) after allowingthe syringe to equilibrate with atmospheric pressure. The CH4composition was analyzed by gas chromatography (GC-2010,Shimadzu, Japan) equipped with a thermal conductivity detector(TCD) and a packed column (Shincarbon ST 100/120, 2 m, Restek,Bellefonte, PA). The GC-TCD was operated at a 145 C iso-thermalcondition (inlet 120 C; detector 150 C; current 45 mA) duringgas-composition analysis. Heliumwas used as carrier gas. Standardcurves were prepared using certied CH4, CO2, and H2 mixed gas(40%:30%:30%, Matheson Tri-Gas, Twinsburg, Ohio).

    Chemical Oxygen Demand (COD) total and soluble (after l-tration through a 0.45-lm membrane lter) for the initial andnal samples of the BMP tests were also measured to establishCOD mass balances. COD, total nitrogen (TN), and ammonia nitro-gen (NH3-N) were measured using a HACH COD kit (concentrationrange 101500 mg/L) and measuring the absorbance at 620, 410,and 655 nm, respectively, using a spectrophotometer. Total sus-

    Water Environment Federation (2002)

    ith livestock wastes.astes.e the total amount of waste generated by each source and the total quantity that is

    y currently withry (million US dry tons)


    Manure & By-product Utilization Program (2006)McKeever (2004) and Perlack et al. (2005)

    rce Technology 124 (2012) 163168pended solids (TSS) and volatile suspended solids (VSS) were mea-sured per Standard Methods (APHA, 1998). Volatile organic acidswere analyzed using a Shimadzu HPLC equipped with an AMINEXHPX-87H column at 50 C with 2.5 mM H2SO4 at a ow rate of0.6 mL/min.

    2.2. Semi-continuous anaerobic reactors pig wastes and co-digestion

    A1-L semi-continuous methanogenic bioreactor treating PWalone was operated along with a 3-L semi-continuous methano-genic bioreactor performing anaerobic co-digestion of PW and PSat a 2:1 volume ratio. The reactor was semi-continuous withrespect to feeding/wasting material, since the reactors were oper-ated with hydraulic retention time (HRT) = solids retention time(SRT) = 35 days. This SRT was achieved by removing a dened

  • volume of liquid (29 mL for the 1-L pig-waste reactor and 86 mLfor the 3-L co-digestion reactor), at which time we also added freshfeed of the same type.

    Anaerobic digested sludge from the MNWWRP was used asinoculum at a 1:1 volume ratio with the wastes. pH controllerscoupled with in situ pH meters were used to maintain a pH rangeof 6.87.6. A water jacket was also used with a temperature con-troller to maintain 35 2 C. Gas production was quantied in acumulative fashion using a wet-tip gas meter (Rebel Point wetgas meter company, TN). This equipment is sensitive enough todetect ow rates as low as 100 mL/day, but still able to measureup to 500 mL/min.

    co-digestion ratios reported higher CH4 production and normalized

    and Table 3, the following insights were gained:

    (i) Co-digestion of PW and PS had a strong mutual benet onthe COD conversion efciency of either waste stream. While thiseffect was most remarkable by comparison with PS by itself (11%versus any of three ratios of PW:PS shown in Table 2), co-digestionwith a lower volume ratio of PS (i.e., 1:1 and 3:1 PW:PS) increasedthe COD conversion efciency of PW as well.

    (ii) Co-digestion signicantly decreased the lag time for achiev-ing rapid methane production. 1:1 PW:PS reported the shortest lagtime of 14 days (Fig. 1a), while PW alone reported the longest lagtime of 23 days. PS never entered rapid phase of methane produc-tion. All three ratios of PW and PS tested in the study had shorterlag times than either waste stream tested individually.(iii) Higher SCOD in the initial feed mix correlated with higher

    COD conversion to CH4 at the end of the test. This is supportedmost strongly by the PS control (lowest input SCOD), whichreported the lowest COD conversion efciency to CH4 (11%).(iv) Lower SCOD and NH4-N during start-up of the co-digestion

    studies correlated with the shortest lag times for methanogenesis(Table 3). The effect of NH4 was probably two-fold: increasingthe lag time for CH4 production and reducing overall methanogenicactivity. In further support of the NH3 effect, the saturation regionof the methane-production curve for the PW control, which hadthe highest total and soluble COD input, was lower than or equalto other co-digestion mixtures in Fig. 1a.

    3.3. Quantication of hydrolysis rate constants (khyd) and hydrolysisefciencies with BMP data

    00 20 40 60 80 100

    Duration of batch BMP tests (days)

    Fig. 1. (a) Methane production from co-digestion experiments compared withcontrol experiments with PW or PS. The contribution from the inoculum controlwas too small to be plotted in the gure. (b) Normalized CH4-COD to the respectivefeed COD values for the BMP tests with different volumetric ratios of PW to PS.

    P. Parameswaran, B.E. Rittmann / BioresouTable 2Summary of feed characteristics for pig waste (PW) and paper sludge (PS) andestimated values for a 1:1 co-digestion mixture.

    Paper sludge Pig waste 1:1 Mixture forco-digestion

    Parameter (units) Value Value ValueTotal solids (g/L) 56 36 46Total COD (g/L) 68 53 60Soluble COD (g/L) 4.2 20 12SCOD/TCOD 0.06 0.39 0.20Total nitrogen (mg/L) 180 1350 760Soluble nitrogen (mg N/L) 16 860 440TCOD conversion efciency than either PW or PS control. This dem-onstrates benets of co-digestion. Until about 60 days, the 1:1 PW/PS mixture had the greatest volumetric methane production. How-ever, CH4-COD removal normalized to the feed COD showed thatthe 1:1 and 3:1 ratios of PW:PS were similar at 60 days. At theend of 80 days, the normalized COD conversion efciency washighest for the 3:1 ratio of PW and PS, even though PW led allthe sam...


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