Waste Management 31 (2011) 1752–1758

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Waste Management

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

Caixia Wan, Quancheng Zhou, Guiming Fu, Yebo Li ⇑Department of Food, Agricultural, and Biological Engineering, The Ohio State University/Ohio Agricultural Research and Development Center, 1680 Madison Ave.,Wooster, OH 44691-4096, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 November 2010Accepted 19 March 2011Available online 4 May 2011

Keywords:Anaerobicco-DigestionLipidsWaster activated sludgeMicronutrients

0956-053X/$ - see front matter � 2011 Elsevier Ltd.doi:10.1016/j.wasman.2011.03.025

⇑ Corresponding author. Tel.: +1 330 263 3855; faxE-mail address: li.851@osu.edu (Y. Li).

Co-digestion of thickened waste activated sludge (TWAS) and fat, oil and grease (FOG) was conductedsemi-continuously under mesophilic conditions. The results showed that daily methane yield at thesteady state was 598 L/kg VSadded when TWAS and FOG (64% of total VS) were co-digested, which was137% higher than that obtained from digestion of TWAS alone. The biogas composition was stabilizedat a CH4 and CO2 content of 66.8% and 29.5%, respectively. Micronutrients added to co-digestion didnot improve the biogas production and digestion stabilization. With a higher addition of FOG (74% of totalVS), the digester initially failed but was slowly self-recovered; however, the methane yield was onlyabout 50% of a healthy reactor with the same organic loading rate.

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1. Introduction

Anaerobic digestion has been recognized as an efficient technol-ogy for treatment of sewage sludge, offering many environmentaland economic benefits, such as sludge stabilization, sludge volumereduction, nutrient recycling, and energy production (Bougrieret al., 2005; Luostarinen et al., 2009). Sewage sludge is typicallycomposed of primary sludge and secondary sludge (waste acti-vated sludge, WAS), of which WAS is more difficult to digest(Rulkens, 2008). Anaerobic co-digestion of sewage sludge andother organic wastes could enhance biogas production and organicmatter degradation due to benefits such as diluted inhibitory com-pounds and a more balanced carbon to nitrogen ratio (Luostarinenet al., 2009; Mata-Alvarez et al., 2000).

The organic fraction of municipal solid waste (MSW) (Derbalet al., 2009); confectionary (Lafitte-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) (Kabouriset 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

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effects of anaerobic co-digestion of sewage sludge and lipid-richwastes have been reported by several researchers. An increase inmethane yield of 9–27% was observed when grease trap sludge(10–30% 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 ofdewatered FOG to sewage sludge.

Lipids in waste mainly consist of neutral fats and long-chainfatty 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.,1981). Also, operational problems, such as clogging, scum forma-tion, and sludge flotation, 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

C. Wan et al. / Waste Management 31 (2011) 1752–1758 1753

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).

In addition to the aforementioned methods for inhibition re-moval or digestion recovery, sufficient 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 deficient 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.

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 werealso investigated.

2. Materials and methods

2.1. Substrates and inoculum

TWAS was obtained from a wastewater treatment plant(WWTP) in Columbus, Ohio, where waste air-activated sludgewas removed from the final clarifier and concentrated to approxi-mately 6% total solids using centrifuges. FOG was collected from a

Table 1Characteristics of substrates and inoculum.

Characteristics Unit Organic was

TWAS

TS (%) % 5.9VS (% of TS) % of TS 83.7C/N ratio 6.7pH 7.5VFA mg/L�1 as HAc 8263.5Alkalinity mg/L�1 as CaCO3 3373.9

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

Micronutrientsb (wet base)Ni ppb 5.68E+03Cu ppb 1.78E+04Zn ppb 5.45E+04Se ppb 736.00Mo ppb 0.00Co ppb 0.00

* Synthetic chemical solution.a Detection limit 0.2–200 ppm.b Detection limit 0.2–200 ppb.

FOG receiving facility in Columbus, Ohio. Both substrates werestored in plastic buckets, sealed, and kept at 4 �C prior to use.The inoculum was effluent from a 25-gallon digester fed withTWAS under mesophilic conditions. The characteristics of sub-strates and inoculum are shown in Table 1.

2.2. Semi-continuous digestion

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 filled with 2 L of seed sludge andthen flushed with N2 to create an anaerobic environment. The seedsludge was acclimated until no significant 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.

2.3. Analytic methods

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 (180–100 mesh) column and a thermalconductivity detector (TCD). Helium was used as a carrier gas ata flow 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.

TS, VS, pH and alkalinity were measured according to the Stan-dard Methods for the Examination of Water and Wastewater

te

FOG Inoculum SCS*

3.2 3.093.9 69.522.1 4.2

4.2 7.8 2.0857.4 1943.6

– 5777.4

2.2 1.3 –0.1 0.3 –

220.0 85.2 564.913.3 255.7 28.330.1 1293.0 0.061.3 382.3 80.2

320.4 783.9 52.60.0 12.3 9357.0

29.6 285.3 0.0

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+06

Table 2Operational parameters of reactors.

Parameters TWAS TWAS +FOG(L)

TWAS +FOG(L)+M

TWAS +FOG(H)+M

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

Fig. 1. Biogas production (a), and its CH content (b) and CO content (c).

1754 C. Wan et al. / Waste Management 31 (2011) 1752–1758

(APHA, 2005). Total carbon and nitrogen were determined with anelemental analyzer (Elementar Vario Max CNS, Elementar Ameri-cans, Mt Laurel, NJ). Total volatile fatty acids (TVFAs) were mea-sured by a modified 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) � 6.64–0.15) � 500; alkalinity (mg/L as Ca-CO3) = (ml of acid consumed from start to pH 5) � 1000. Thus, alka-linity in this study specifically referred to alkalinity at pH 5. TheVFA/TIC was defined as a ratio between TVFAs and alkalinity,which was based on the FOS/TAC method developed by Hach LangeLaboratory in Germany (Lossie and Pütz, 2008) and used as an indi-cator of the process stability of anaerobic digestion.

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 airflow was 5 m3/min. Calibration standards ranging from0.2 to 200 ppb for micronutrients and 20–10,000 ppb for macronu-trients were used. The samples were prepared by digestion using amicrowave oven with programmable power (CEM MDS 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.

2.4. Statistical analysis

Analysis of variance (ANOVA) tests were performed using SASsoftware (Version 8.1, SAS Institute Inc., Cary, NC, USA). Signifi-cance of the difference between responses were compared at a5% level of probability.

4 2

3. Results

3.1. Biogas production

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 65–68% and 28–30%, 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) and

FOG(L) + M, respectively (Table 3). Correspondingly, the averagedaily methane yield based on volatile solids added during the stea-dy 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 significant (P > 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 50–54% 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.

Table 3Reactor performance and effluent characteristics.a.

Parameters TWAS TWAS + FOG(L) TWAS + FOG(L) + M TWAS + FOG(H) + M Recovered

Biogas productionBiogas (ml/d) 1849.0 ± 91.3 4173.6 ± 269.3 4257.6 ± 439.1 40 ± 0.00 972.9 ± 57.7CH4 (ml/d) 1201.1 ± 54.8 2800.6 ± 247.1 2873.7 ± 317.7 – 512.3 ± 38.2CH4 yield (L/kg VSadded/d) 252.4 ± 16.6 598.4 ± 5.3 614.1 ± 67.9 – 109.5 ± 8.2CH4 (%) 65.1 ± 1.9 66.8 ± 2.0 67.5 ± 1.0 21.3 ± 0.68 52.7 ± 1.3CO2 (%) 28.1 ± 1.2 29.5 ± 0.5 29.6 ± 0.8 51.9 ± 3.24 36.1 ± 1.3

EffluentTS (%) 2.8 ± 0.0 2.0 ± 0.0 2.1 ± 0.1 2.9 ± 0.1 NDVS (%) 2.1 ± 0.1 1.5 ± 0.1 1.6 ± 0.1 2.4 ± 0.1 NDTVFA (mg/L as HAc) 2317.3 ± 233.2 2094.4 ± 265.6 2558.0 ± 150.0 8393.8 ± 500.0 NDpH 8.0 ± 0.0 7.7 ± 0.1 7.7 ± 0.0 5.6 ± 0.1 NDAlkalinity (mg/L as CaCO3) 8862.7 ± 141.2 5540.4 ± 205.7 5374.6 ± 98.9 974.4 ± 128.5 NDTVFA/TIC 0.24 ± 0.03 0.40 ± 0.06 0.47 ± 0.07 7.74 ± 0.95 NDVS destruction (%) 40.0 57.1 54.3 29.4 ND

a For TWAS + FOG (H) + M, data were reported as the average of two replicated digesters on day 17 (n = 2); for recovered digester, data were reported as the average of onedigester at last 6 days (n = 6);for the other tests, steady-state values were reported as the average of two digesters at the last 6 days (n = 12).

C. Wan et al. / Waste Management 31 (2011) 1752–1758 1755

3.2. VS reduction

In comparison with the 3.5% VS content in the feed, the VScontents in effluents 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%).

3.3. Variances of VFA, pH, and alkalinity

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 first6 days for the co-digestion of TWAS + FOG(L), reached a peak valueof 8000 mg/L on day 6, and then gradually decreased to around2000 mg/L on day 15 (Fig. 2a). The pH change was dependent onVFA levels and maintained at an average value of 7.7 (Fig. 2b).Alkalinity was also closely related to VFA levels, which decreasedin the first few days and then increased steadily until stabilizedat 5540 mg/L after day 12 (Fig. 2c). Co-digestion of TWAS + FOG(L)with addition of micronutrients followed very similar patternsas co-digestion of TWAS + FOG(L) without micronutrients forVFAs, pH, and alkalinity (Fig. 2). For the co-digestion of TWAS +FOG(H)+M, increase in VFAs accompanied by a decline in pH wasobserved and a VFA of 8394 mg/L and pH of 5.6 were reached atthe end of the digestion (day 15). Correspondingly, bufferingcapacity of this digester was completely disrupted as alkalinitydropped to 1000 mg/L at the end of the digestion (Fig. 2c). Forthe three functional digesters, VFA/TIC was stabilized at 0.24–0.47 (Fig. 3). However, VFA/TIC of the upset digester with TWAS +FOG(H)+M increased to 7.7 at the end of digestion.

3.4. Recovery of the upset digester

Recovery of the upset digester was evaluated using two meth-ods: (1) addition of NaOH and (2) self-recovery without additionof NaOH. Addition of NaOH (0.2%, w/v) on day 16 did not restorebiogas production although the pH increased from 5.60 to 7.72.In contrast, without the addition of NaOH, the digester slowlyself-recovered and 2 weeks after the initial cessation of biogas pro-duction, the pH increased to 6.3. Biogas production from the self-

recovered digester reached a total volume of about 1600 ml over3 days without feeding of additional substrate. After approxi-mately 40 days of self-recovery, semi-continuous feeding of TWASwas initiated. The biogas production reached 973 ml/d when thedigestion was stabilized (Fig. 4), but it was about half the biogasproduction of the undisrupted digester with the same TWAS load-ing rate (Table 3). This result indicates that a part of the microor-ganism’s activity was permanently damaged due to theoverproduction of VFAs.

4. Discussion

Anaerobic digestion is an important technology for TWAS sta-bilization as it converts organic materials in the sludge to biogas.The typical methane potential of sewage sludge is approximately300–400 L/kg VSadded (Davidsson et al., 2008; Einola et al., 2001).As listed in Table 4, methane yields of sewage sludge obtainedfrom several semi-continuous anaerobic digestion studies were250–280 L/kg VSadded, regardless of experimental scale. Thedestruction of VS is important to anaerobic digestion and it is a di-rect indicator of the metabolic activity of the microorganism com-munity. VS destruction of TWAS by anaerobic digestion is typicallyas low as 30–45% (Davidsson et al., 2008; Ghosh, 1991). This studyhad similar results with about 40% VS removed during digestion.Application of a low OLR (i.e., <2 g VS/L/d) has been shown to im-prove VS destruction, with the potential to be increased to morethan 50% (Habiba et al., 2009; Luostarinen et al., 2009).

Co-digestion of sewage sludge and FOG increased methaneyield by 137% compared to digestion of sewage sludge alone. Themethane yield of 598.4 L/kg VSadded obtained in this study was sub-stantially higher than that obtained from other co-digestion stud-ies. In a study by Kabouris et al. (2009), a methane yield of 512 L/kgVSadded was obtained from thermophilic co-digestion of sewagesludge and dewatered FOG while 449 L/kg VSadded was obtainedfrom mesophilic digestion (Table 4). The higher methane yield inthis study was probably due to a higher FOG addition (64% of feedVS), which had a high methane potential compared to sewagesludge. The batch laboratory test also indicated that enhancementof the methane potential was well correlated to the grease sludgefraction of total VS (Davidsson et al., 2008). On the other hand, pro-cess stability could be negatively affected with the higher FOGaddition due to potential LCFA inhibition. Luostarinen et al.(2009) showed higher addition of grease trap waste (55–71% offeed VS) resulted in a decreased methane yield and increased VS,COD and total VFAs in the effluent. Similarly, as shown in Fig. 1,the co-digestion of sewage sludge and FOG at a much higher level

Fig. 2. Variance of VFA (a), pH (b) and alkalinity (c) during anaerobic digestion.

Fig. 3. Profile of the ratio of VFA/TIC throughout the digestion.

Fig. 4. Biogas production of the inhibited digester.

1756 C. Wan et al. / Waste Management 31 (2011) 1752–1758

(75% of feed VS) led to digestion failure due to acidification of thedigester. A relatively short hydraulic retention time (10 d) couldalso contribute to digestion failure, probably due to washout ofmicroorganisms during removal of effluent.

During co-digestion of TWAS and FOG(L), an increase in biogasproduction was observed initially due to the readily available or-ganic matter in the co-substrates. Instead of gradually increasedbiogas production, a lag phase in biogas production was observedbetween days 3 and 8 (Fig. 1a), most likely due to predominant aci-dogenic activity. The biogas composition also reflected correspond-ing microbial metabolism. During the lag phase, VFAs increasedrapidly and reached up to 8000 mg/L on day 6 and then decreased(Fig. 2a). The CO2 production correspondingly increased fromapproximately 18% to 42% and then declined gradually (Fig. 1c).The further conversion of VFAs and CO2 to methane brought theCO2 content down to 30% and VFA concentration down to2000 mg/L in the digester during the stable operational periodand the resulting methane content was maintained at about 67%.The pH of co-digestion was well maintained between 7.6 and 8.0due to the high buffering capacity of TWAS. However, the highloading of FOG (75%) in the feed disrupted the buffering capacityof the digester, resulting in an acidic environment in this digester(Figs. 3 and 4). The high content of CO2 (up to 52%) in this digesterappeared to have induced an even further drop of pH (Lansinget al., 2010) (Fig. 1b), and it was observed that the pH was muchlower than that observed from co-digestion of TWAS and FOG(L)before day 9 (Fig. 2a and b).

The ratio between VFAs and buffering capacity (VFA/TIC) is usedas an indicator of the process stability. According to Riau et al.(2010), a VFA/TIC ratio higher than 0.5 indicates there is not a goodbalance in the microbial population between acidogenic and meth-anogenic bacteria. Based on empirical values provided by Lossieand Pütz, 2008, when feeding at a proper rate, the range of VFA/TIC is generally between 0.2 and 0.6. In this study, VFA/TIC re-mained at 0.24 when TWAS was the single substrate for digestion.The ratio of VFA/TIC of digestion of TWAS + FOG(L) increased grad-ually, reaching 1.87 on day 6, indicating acidification of the diges-ter. Thereafter, with the consumption of VFAs by methanogenicbacteria, the ratio went down and then leveled off. The VFA/TICwas between 0.4–0.5 during the stable operational period. A rela-tively lower OLR may be preferred to obtain more consistent biogasproduction. Not surprisingly, VFA/TIC of digestion of TWAS +FOG(H)+M kept increasing, reaching as high as 7.6 at the end ofdigestion. This digester failed for continuous biogas production.By monitoring variance of VFA/TIC, an appropriate action such asdiscontinuing feeding after day 6 should be taken to avoid furtheracidification of this digester.

Table 4Comparison of results from studies of co-digestion of sewage sludge and lipid-rich organic waste.a

Study Digested sludge types CH4 yield (L/kg VSadded/d) VS reduction (%) HRT (d) OLR (kg VS/m3/d) Reactor size (L)

Luostarinen et al. (2009) SS 278 52 16 1.56–2.09 595% SS + 5%GS 374 59 16 1.67–2.23 554% SS + 46%GS 463 67 16 3.46 545% SS + 55%GS 318 72 16 3.99 5

Davidsson et al. (2008) SS 271 45 13 2.5 3590% SS + 10%GS 295–308 54–55 13 2.5 3570% SS + 30%GS 344 58 13 2.4 35GS – – – 0–2.3 35

Kabouris et al. (2009) 40%PS + 60%TWAS 159 25 12 2.45 421% PS + 31%TWAS + 48%dFOG 449 45 12 4.35 4

Heo et al. (2003) 90%WAS + 10%FW 192–202 34–39 10–20 1.19–2.37 3.570%WAS + 30%FW 228–235 38–44 10–20 2.51–4.39 3.550%WAS + 50%FW 339–366 54–56 10–20 2.93–5.96 3.5

Present study WAS 252 40 15 2.34 436%WAS + 64%FOG 598 57 15 2.34 425% WAS + 75% FOGb – 29.4 10 3.4 4

SS: sewage sludge; GS: grease trap sludge; TWAS: thickened waste activated sludge; dFOG: dewatered fat, oil, and grease; and FW: food waste.a Semi-continuous anaerobic digestion under mesophilic condition.b Co-digestion with addition of micro-nutrients.

C. Wan et al. / Waste Management 31 (2011) 1752–1758 1757

Macro- and micro-nutrients (e.g., Co, Fe, Mo, Ni, and Se) are re-quired for microbial growth and metabolism, each having specificfunctions in the digestion process (Ilangovan and Noyola, 1993;Kayhanian and Rich, 1995). As presented in Table 1, the TWASwas rich in macro- and most micro-nutrients while, in comparison,the concentration of macro- and micro-nutrients in the FOG wasrelatively low. Co-digestion of TWAS and FOG, therefore, ensuredsufficient micronutrients required for anaerobes. The syntheticchemical solution (SCS) contained a high amount of micronutrientsexcept selenium. The addition of these micronutrients to TWASand FOG also stabilized the digestion process, but did not signifi-cantly stimulate anaerobic digestion of TWAS and FOG. The biogasproduction during the stable period was close to that obtainedfrom digestion of TWAS and FOG without SCS addition. It wasnoted that biogas production at the initial stage was lower thanthat without the addition of nutrients, indicating potential inhibi-tion, possibly due to toxicity of accumulated nutrients. As shown inTable 1, significant amounts of key macro- or micro-nutrients werealready in the TWAS and met the reported stimulatory ranges(Kayhanian and Rich, 1995). Therefore, the addition of chemicalsmight result in excess nutrients. Kayhanian and Rich (1995) com-pared results of co-digestion of manure with the biodegradablefraction of municipal solid waste (BOF/MSW) with and withoutmacro- and micro-nutrient addition and showed that addition ofnutrients did not enhance biogas production. Considering the po-tential complexity of the chemical solution and potential costsassociated with it, addition of micronutrients to a digester maynot be practical for commercial application. Instead, organicwastes, such as manure or sewage sludge, that are sufficient innutrients could be added to a substrate that is deficient in nutri-ents, improving the stability of the digestion and stimulating bio-gas production.

FOG is a lipid-rich organic waste and an attractive substrate fordigestion because of its high methane potential. The digester upsetcould have resulted from overloading of FOG. Thus, the recoverystrategies for an inhibited digester are useful for the operation ofcommercial digesters. In this study, although NaOH addition re-stored pH to 7.72, the digestion process did not recover, whichindicated that NaOH did not increase buffering capacity of the di-gester. On the other hand, high sodium concentration might havefurther inhibited activity of acetogens and methanogens (Hanakiet al., 1981). Self-recovery of the digester was slow with a smallamount of biogas production, and approximately 40 days were

required for full recovery. This result was in agreement with theself-recovery observed by Palatsi et al. (2009). Feeding TWASsemi-continuously to a self-recovered digester resulted in a rela-tively stable digestion (Fig. 4). However, biogas production waslower than that obtained from normal digestion of TWAS when asimilar OLR was applied, indicating that some microbial activitymight be permanently damaged due to the toxicity of LCFAs andoverproduced VFAs. Self-recovery by stopping feeding when inhi-bition/imbalance of the process occurs is a widely accepted strat-egy for digestion recovery. However, the long recovery time andpotential instability reduce the economic feasibility of thispractice.

5. Conclusions

Co-digestion of raw FOG (up to 64% of VS) and TWAS enhancedmethane production remarkably, with about a 137% increase overdigestion with TWAS alone at the stable operational period. Theaddition of micronutrients did not significantly stimulate co-diges-tion of TWAS and FOG, nor improve the stability of the digestionprocess. The characteristics of substrates showed that TWAS alsocontained high levels of macro- and micro-nutrients, which sug-gested that co-digestion of FOG with TWAS has advantages overexternal nutrient supplementation. For the inhibited reactor, itwas successfully self-recovered after a long recovery time.

Acknowledgements

This work was supported by funding from Ohio Agricultural Re-search and Development Center Seeds Program and quasar energygroup. The authors wish to thank Mrs. Mary Wicks (Departmentof Food, Agricultural and Biological Engineering, OSU) for readingthrough the manuscript and providing useful suggestions.

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