Semi-continuous anaerobic co-digestion of thickened waste activated sludge and fat, oil and grease

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<ul><li><p>of</p><p>Ohio</p><p>Micronutrients</p><p>wasmesVSaaineofducfaile</p><p>gnizedofferine stabienergy9). Seweconda</p><p>et al., 2009) have been studied for anaerobic co-digestion withsewage sludge. Among these substrates, lipid-rich waste is consid-ered to be most attractive due to its high methane potential. The-oretically, the methane potential of lipids is 1014 L/kg volatile solid(VS) (Buswell and Neave, 1930), which is much higher than of car-bohydrates (e.g., 370 L/kg VS for glucose) (Kim et al., 2004) andproteins (740 L/kg TS) (Zupancic and Jemec, 2010). The synergistic</p><p>1981). Also, operational problems, such as clogging, scum forma-tion, and sludge otation, could be caused by adsorption of the li-pid layer around the sludge or biomass surface (Hwu et al., 1998;Kim et al., 2004; Rinzema et al., 1994). The inhibition caused byLCFAs was reported to be reversible when microorganisms wererecommenced to degrade LCFAs after a lag phase (Pereira et al.,2005). Due to the adsorption of LCFAs to microbial surfaces, theinhibition could be caused by the limitation of nutrient transportto cells (Pereira et al., 2005). Co-digestion of FOG and sewagesludge (Davidsson et al., 2008), manure (Lansing et al., 2010) or</p><p> Corresponding author. Tel.: +1 330 263 3855; fax: +1 330 263 3670.</p><p>Waste Management 31 (2011) 17521758</p><p>Contents lists availab</p><p>an</p><p>elsE-mail address: li.851@osu.edu (Y. Li).vated sludge, WAS), of which WAS is more difcult to digest(Rulkens, 2008). Anaerobic co-digestion of sewage sludge andother organic wastes could enhance biogas production and organicmatter degradation due to benets such as diluted inhibitory com-pounds and a more balanced carbon to nitrogen ratio (Luostarinenet al., 2009; Mata-Alvarez et al., 2000).</p><p>The organic fraction of municipal solid waste (MSW) (Derbalet al., 2009); confectionary (Latte-Trouquq and Forster, 2000);food waste, such as fruit and vegetable waste (Habiba et al.,2009; Heo et al., 2003); lipid-rich waste such as grease trap sludge(Luostarinen et al., 2009); and fat, oil, and grease (FOG) (Kabouris</p><p>dewatered FOG to sewage sludge.Lipids in waste mainly consist of neutral fats and long-chain</p><p>fatty acids (LCFAs). Neutral fats can be readily hydrolyzed intoLCFAs and glycerol by lipase secreted by acidogenic bacteria duringanaerobic digestion (Cirne et al., 2007). LCFAs are further convertedto acetate and H2 via b-oxidation by syntrophicacetogenic bacteriathen in turn converted to methane by H2-utilizing methanogensand acetoclastic methanogens (Cirne et al., 2007; Palatsi et al.,2010). LCFAs can be inhibitory to several essential reactions, e.g.,degradation of LCFAs and methanogenesis, due to their toxicityto both syntrophicacetogens and methanogens (Hanaki et al.,1. Introduction</p><p>Anaerobic digestion has been recoogy for treatment of sewage sludge,and economic benets, such as sludgreduction, nutrient recycling, andet al., 2005; Luostarinen et al., 200composed of primary sludge and s0956-053X/$ - see front matter 2011 Elsevier Ltd.doi:10.1016/j.wasman.2011.03.025as an efcient technol-g many environmentallization, sludge volumeproduction (Bougrierage sludge is typicallyry sludge (waste acti-</p><p>effects of anaerobic co-digestion of sewage sludge and lipid-richwastes have been reported by several researchers. An increase inmethane yield of 927% was observed when grease trap sludge(1030% of total VSadded) was co-digested with sewage sludge(Davidsson et al., 2008). With the addition of grease trap sludge (upto 46% of total VS) to sewage sludge, methane yield was increasedby 66% (Luostarinen et al., 2009). Kabouris et al. (2009) also reporteda 2.95 times increase in methane yield with the addition ofLipidsWaster activated sludge</p><p>about 50% of a healthy reactor with the same organic loading rate. 2011 Elsevier Ltd. All rights reserved.Semi-continuous anaerobic co-digestionand fat, oil and grease</p><p>Caixia Wan, Quancheng Zhou, Guiming Fu, Yebo Li Department of Food, Agricultural, and Biological Engineering, The Ohio State University/Wooster, OH 44691-4096, USA</p><p>a r t i c l e i n f o</p><p>Article history:Received 8 November 2010Accepted 19 March 2011Available online 4 May 2011</p><p>Keywords:Anaerobicco-Digestion</p><p>a b s t r a c t</p><p>Co-digestion of thickenedsemi-continuously understeady state was 598 L/kg137% higher than that obtat a CH4 and CO2 contentnot improve the biogas proVS), the digester initially</p><p>Waste M</p><p>journal homepage: www.All rights reserved.thickened waste activated sludge</p><p>Agricultural Research and Development Center, 1680 Madison Ave.,</p><p>te activated sludge (TWAS) and fat, oil and grease (FOG) was conductedophilic conditions. The results showed that daily methane yield at thedded when TWAS and FOG (64% of total VS) were co-digested, which wasd from digestion of TWAS alone. The biogas composition was stabilized66.8% and 29.5%, respectively. Micronutrients added to co-digestion didtion and digestion stabilization. With a higher addition of FOG (74% of totald but was slowly self-recovered; however, the methane yield was only</p><p>le at ScienceDirect</p><p>agement</p><p>evier .com/ locate/wasman</p></li><li><p>easily degradable substrates (e.g., glucose, cysteine) (Kuang et al.,2006) and the addition of adsorbents (Angelidaki et al., 1990) havebeen used to overcome the inhibition of LCFAs. Change of feedingpatterns, addition of adsorbents, and dilution with active inoculumfor increasing the microbial/LCFA ratio are effective methods to re-cover failed digesters (Palatsi et al., 2009).</p><p>In addition to the aforementioned methods for inhibition re-moval or digestion recovery, sufcient availability of micronutri-ents, e.g. Co, Fe, Mo, Ni and Se, is important to the stability ofanaerobic digestion as micronutrients are essential for the growthand metabolism of anaerobes (Ilangovan and Noyola, 1993;Kayhanian and Rich, 1995). A substrate decient in micronutrientsmay result in incomplete, unstable biodegradation of the organicmatter or in digestion failure. To improve digestion performance,this type of substrate can be supplemented with synthetic nutri-ents or mixed with a micronutrient-rich substrate. In anaerobicdigestion of lipids, process stability is a big challenge due to inhi-bition of LCFAs, as previously mentioned; thus, addition of micro-nutrients can potentially stabilize the lipid digestion. There are noreports of the effect of micronutrients on anaerobic co-digestion ofsewage sludge with lipid-rich waste.</p><p>In this study, co-digestion of raw FOG (un-dewatered) andthickened WAS (TWAS) was conducted to determine the effecton biogas production. The effect of synthetic micronutrient addi-tion on biogas production and stability of digestion was also eval-uated. Different strategies for recovery of a failing digester were</p><p>(WWTP) in Columbus, Ohio, where waste air-activated sludgewas removed from the nal clarier and concentrated to approxi-</p><p>FOG receiving facility in Columbus, Ohio. Both substrates werestored in plastic buckets, sealed, and kept at 4 C prior to use.The inoculum was efuent from a 25-gallon digester fed withTWAS under mesophilic conditions. The characteristics of sub-strates and inoculum are shown in Table 1.</p><p>2.2. Semi-continuous digestion</p><p>Digestion was carried out in duplicate using 4-L glass reactorswith constant mixing at 1000 rpm by a magnetic stirrer. All reac-tors were placed in a walk-in incubator that was controlled at37 C. The digester was initially lled with 2 L of seed sludge andthen ushed with N2 to create an anaerobic environment. The seedsludge was acclimated until no signicant amount of biogas wasproduced. Then, feeding was conducted once a day at organic load-ing rates (OLR) as shown in Table 2. Prior to each discharge andafter feeding, the sludge in the digester was completely mixed bya motor driven mixer for 30 s. Biogas was collected daily using aTedlar bag for volume determination and composition analysis.</p><p>2.3. Analytic methods</p><p>Gas composition was measured by a GC (HP6890, Agilent Tech-nologies, Wilmington, DE) equipped with a 1.8 m 3.2 mm stain-less-steel Porapak Q (180100 mesh) column and a thermal</p><p>was</p><p>FOG Inoculum SCS</p><p>C. Wan et al. /Waste Management 31 (2011) 17521758 1753mately 6% total solids using centrifuges. FOG was collected from a</p><p>Table 1Characteristics of substrates and inoculum.</p><p>Characteristics Unit Organic</p><p>TWAS</p><p>TS (%) % 5.9VS (% of TS) % of TS 83.7C/N ratio 6.7pH 7.5VFA mg/L1 as HAc 8263.5Alkalinity mg/L1 as CaCO3 3373.9</p><p>Macronutrientsa (wet base)C % 2.6N % 0.4Na ppm 65.9Mg ppm 291.0P ppm 1231.0K ppm 321.6Ca ppm 1432.0Mn ppm 54.3Fe ppm 628.0</p><p>Micronutrientsb (wet base)Ni ppb 5.68E+03Cu ppb 1.78E+04Zn ppb 5.45E+04Se ppb 736.00Mo ppb 0.00Co ppb 0.00also investigated.</p><p>2. Materials and methods</p><p>2.1. Substrates and inoculum</p><p>TWAS was obtained from a wastewater treatment plant* Synthetic chemical solution.a Detection limit 0.2200 ppm.b Detection limit 0.2200 ppb.559.80 2153.00 4.19E+06579.10 7723.00 3.76E+062498.00 3.03E+04 1.25E+06231.90 385.90 335.800.00 15.9E+03 1.20E+060.00 0.00 3.52E+063.2 3.093.9 69.522.1 4.24.2 7.8 2.0</p><p>857.4 1943.6 5777.4</p><p>2.2 1.3 0.1 0.3 </p><p>220.0 85.2 564.913.3 255.7 28.330.1 1293.0 0.061.3 382.3 80.2</p><p>320.4 783.9 52.60.0 12.3 9357.0</p><p>29.6 285.3 0.0conductivity detector (TCD). Helium was used as a carrier gas ata ow rate of 5.2 ml/min. The temperatures of the injector anddetector were 150 and 200 C, respectively. The column was main-tained at 40 C for 2 min, then ramped to 200 C at 15 C/min andheld at this temperature for 10 min. The daily biogas productionwas measured using a wet drum gas meter (TG 5, CalibratedInstruments Company, NY). Methane production was calculatedby multiplying the biogas volume by the corresponding methanecontent in the produced biogas.</p><p>TS, VS, pH and alkalinity were measured according to the Stan-dard Methods for the Examination of Water and Wastewater</p><p>te</p><p>*</p></li><li><p>cans, Mt Laurel, NJ). Total volatile fatty acids (TVFAs) were mea-</p><p>FOG(L) + M, respectively (Table 3). Correspondingly, the averagedaily methane yield based on volatile solids added during the stea-</p><p>Fig. 1. Biogas production (a), and its CH4 content (b) and CO2 content (c).</p><p>gemsured by a modied two-part titration method using a titrator(DL22, Metterler Toledo, Columbus, OH). A 5-ml sample was di-luted with 50 ml DI water and then titrated with a standard HClsolution (1.0 N). The volumes of HCl consumed at pH values of5.0 and 4.4 were recorded. The following equations were usedfor TVFAs and alkalinity calculations according to Nordmann(1977): TVFAs (mg/L as HAc) = ((ml of acid consumed betweenpH 5 and pH 4.4) 6.640.15) 500; alkalinity (mg/L as Ca-CO3) = (ml of acid consumed from start to pH 5) 1000. Thus, alka-linity in this study specically referred to alkalinity at pH 5. TheVFA/TIC was dened as a ratio between TVFAs and alkalinity,which was based on the FOS/TACmethod developed by Hach LangeLaboratory in Germany (Lossie and Ptz, 2008) and used as an indi-cator of the process stability of anaerobic digestion.</p><p>Macro-nutrients (except C and N) and micronutrients were ana-lyzed by Inductively Coupled Plasma/Mass Spectrometry (ICP/MS)(Aglient 7500, Agilent Technologies, Wilmington, DE). Argon wasused as a carrier gas with pressure kept at 100 2.8 psi. The ex-haust airow was 5 m3/min. Calibration standards ranging from0.2 to 200 ppb for micronutrients and 2010,000 ppb for macronu-trients were used. The samples were prepared by digestion using amicrowave oven with programmable power (CEMMDS 2100, Mat-thews, NC). Exactly 0.5 g sample was mixed with 10 ml nitric acidin a TEFL microwave vessel and digested at 190 C for 10 min. Thedigested samples were diluted to 1:1000 for analysis.</p><p>2.4. Statistical analysis</p><p>Analysis of variance (ANOVA) tests were performed using SASsoftware (Version 8.1, SAS Institute Inc., Cary, NC, USA). Signi-cance of the difference between responses were compared at a5% level of probability.</p><p>3. Results(APHA, 2005). Total carbon and nitrogen were determined with anelemental analyzer (Elementar Vario Max CNS, Elementar Ameri-</p><p>Table 2Operational parameters of reactors.</p><p>Parameters TWAS TWAS +FOG(L)</p><p>TWAS +FOG(L)+M</p><p>TWAS +FOG(H)+M</p><p>Working volume (L) 2 2 2 2HRT(d) 15 15 15 10Micronutrients (ll) 7 7TS loading (g/L/d) 3.93 2.61 3.73 3.73WAS loading (% of total OLR) 100 36 36 25FOG loading (% of total OLR) 64 64 75Total OLR (g VS/L/d) 2.34 2.34 2.34 3.40</p><p>1754 C. Wan et al. /Waste Mana3.1. Biogas production</p><p>As shown in Fig. 1a, biogas production of TWAS with a lowerFOG loading (TWAS + FOG(L)) increased gradually until day 3 andthen decreased in the following few days. Thereafter, biogas pro-duction gradually increased until it became stable on around day18. In contrast, addition of micronutrients (TWAS + FOG(L) + M) re-sulted in a lower daily biogas production before day 12. Digestionof TWAS alone led to relatively lower biogas production comparedto co-digestion with FOG at the lower loading rate. The biogascomposition was stabilized after day 12 with CH4 and CO2 contentmaintained at approximately 6568% and 2830%, respectively, forall tests except those with high FOG loading (Fig. 1b and c). At thesteady state, the daily biogas production was maintained at aver-age values of 1849, 4174, 4258 ml/d for TWAS, FOG(L) andent 31 (2011) 17521758dy state was 252.4, 598.4 and 614.1 L/kg VSadded/d for TWAS,FOG(L), and FOG(L) + M, respectively (Table 3). Co-digestion ofTWAS with FOG(L) resulted in a 137% increase in methane yieldover TWAS alone. Although a small increase (about 3%) in biogasproduction and methane yield was observed with the addition ofmicronutrients to co-digestion of TWAS and FOG(L), this increasewas not signicant (P &gt; 0.05). Co-digestion of TWAS and FOG(H)failed even with micronutrient addition. The biogas production be-gan decreasing after day 3 until it ceased completely. In this diges-ter, CO2 was the dominant biogas and maintained at 5054% afterday 7 while CH4 decreased gradually from the beginning. N2 ac-counted for the vast majority of the remaining biogas, and it keptdecreasing untill day 7 and then gradually increased (data notshown). The addition of micronutrients did not appear to improvethe stability of the digestion.</p></li><li><p>bilization as it converts organic materials in the sludge to biogas.</p><p>esteas th</p><p>gem3.2. VS reduction</p><p>In comparison with the 3.5% VS content in the feed, the VScontents in efuents withdrawn from the steady-state digesterwith TWAS alone and co-digestion of TWAS + FOG(L) were 2.1%and 1.5%, respectively (Table 3). About 57% of the VS were de-graded in the digester with TWAS + FOG(L), which was 17% higherthan VS degraded in the reactor with TWAS alone. Addition ofmicronutrients did not improve the VS destruction. Co-digestionof TWAS and FOG(H) with micronutrients resulted in the lowestVS destruction (29.4%).</p><p>3.3. Variances of VFA, pH, and alkalinity</p><p>The variations in VFA, pH, and alkalinity during the digestionprocess are indicated in Fig. 2. The pH and alkalinity of TWAS alonewere relatively stable, with average values around 8.0 and8863 mg/L, respectively (Table 3). VFA levels increased in the rst6 day...</p></li></ul>

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