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 14


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