Long-term anaerobic digestion of food waste stabilized by trace elements

Waste Management 32 (2012) 1509–1515

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Long-term anaerobic digestion of food waste stabilized by trace elements Lei Zhang a, Deokjin Jahng b,⇑ a

Key Laboratory of Industrial Ecology and Environmental Engineering, School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, PR China b Department of Environmental Engineering and Biotechnology, Myongji University, San 38-2, Namdong, Cheoin-Gu, Yongin, Gyeonggi-Do 449-728, Republic of Korea

a r t i c l e

i n f o

Article history: Received 14 August 2011 Accepted 17 March 2012 Available online 24 April 2012 Keywords: Anaerobic digestion Cobalt Food waste Iron Trace element

a b s t r a c t The purpose of this study was to examine if long-term anaerobic digestion of food waste in a semicontinuous single-stage reactor could be stabilized by supplementing trace elements. Contrary to the failure of anaerobic digestion of food waste alone, stable anaerobic digestion of food waste was achieved for 368 days by supplementing trace elements. Under the conditions of OLR (organic loading rates) of 2.19–6.64 g VS (volatile solid)/L day and 20–30 days of HRT (hydraulic retention time), a high methane yield (352–450 mL CH4/g VSadded) was obtained, and no significant accumulation of volatile fatty acids was observed. The subsequent investigation on effects of individual trace elements (Co, Fe, Mo and Ni) showed that iron was essential for maintaining stable methane production. These results proved that the food waste used in this study was deficient in trace elements. Ó 2012 Published by Elsevier Ltd.

1. Introduction Food waste is the one of the main types of municipal solid waste (MSW), which is the material resulting from the processing, storage, preparation, cooking, and handling of food and uneaten loss. For instance, the Korean government reported that generation of food waste reached 14,452 tons per day in 2007, which accounted for 28.7% of the MSW (MOE, 2011). The management of food waste is a challenging task due to its special nature of high moisture content and easy decay. Conventional treatment and disposal methods such as landfill, incineration and ocean dumping became less desirable due to stringent environmental regulations and low economic feasibility. Recently, anaerobic digestion of food waste has become an intensive field of research, due to the fact that both waste treatment and energy production can be achieved at the same time. For process configurations, it is generally accepted that the single-stage process is the most frequently used (De Baere, 2000). However, there are rare reports on successful performance of this system treating food waste alone. For example, El-Mashad et al. (2008) found that a digester treating food waste was not stable at the OLR (organic loading rate) of 4.0 g VS(volatile solid)/L day or even at the reduced OLR of 2.0 g VS(volatile solid)/L day, as it was indicated by accumulation of volatile fatty acids, low pH and low biogas production. Climenhaga and Banks (2008) reported that at OLR of 1.45 g VS/L day, an extended HRT (hydraulic retention time) of 180 days was required to achieve a steady state. These ⇑ Corresponding author. Tel.: +82 31 330 6690; fax: +82 31 336 6336. E-mail addresses: [email protected] (L. Zhang), [email protected] (D. Jahng). 0956-053X/$ - see front matter Ó 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.wasman.2012.03.015

low OLRs make anaerobic digestion of food waste economically infeasible. Lee et al. (1999) also reached a similar conclusion that single-stage anaerobic digestion was impractical for easily degradable food waste. Interestingly, many studies have shown that the anaerobic process was more resistant to environmental changes when mixed wastes were used (Creamer et al., 2010; Heo et al., 2003; Kayhanian and Rich, 1995; Romano and Zhang, 2008; Wu et al., 2010). We have also reported that co-digestion of food waste with piggery wastewater was steady and stable for 367 days (Zhang et al., 2011). We have additionally found that this stable and successful operation was achieved because insufficient trace elements in food waste were supplemented by metal-abundant piggery wastewater. In this study, we re-confirmed that addition of trace elements stabilized anaerobic digestion of food waste alone in a single-stage reactor at varying OLRs and HRTs for prolonged operation. Moreover, effects of each trace element were studied, respectively, to answer the question which element was the most effective. 2. Experimental 2.1. Materials and methods The food waste containing mainly rice, vegetables, and meats was obtained from a restaurant on the campus of the Myongji University, Yongin, Korea and pretreated before use as described previously (Zhang et al., 2011). The anaerobic sludge inocula for experiments were drawn from a lab-scale digester treating piggery wastewater. The analytical methods for general environmental

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parameters such as pH, concentrations of TS (total solid), VS, COD (chemical oxygen demand), and ammonia, gas chromatographic substances including volatile fatty acids and methane, and metal elements can be found elsewhere (Zhang et al., 2011).

than animal manures (Zhang et al., 2011). As for the seed sludge, the trace element content was high because it was obtained from an anaerobic digester that had been treating piggery wastewater. 3.2. Comparison of anaerobic digestion of food waste in the presence of different trace elements

2.2. Experimental design Semi-continuous anaerobic digestion was run at 37 °C in Schott Duran bottles as described in our previous study (Zhang et al., 2011). Seven reactors (R1–R7) with a 150-mL working volume each were run in parallel. R1 was operated for 95 days as the control without receiving any trace element. In order to identify the role of an individual trace element, R2, R3, R4 and R5 were fed with trace element solutions of cobalt (Co), molybdenum (Mo), nickel (Ni), and another combined solution containing all of Co, Mo, and Ni, respectively. R1–R5 were operated for 95 days. For R6 feeding of trace element solution was varied during experimental period: Phase 1, trace element solution containing Co, Fe, Mo and Ni was fed until day 95; Phase 2, from day 96, the trace element solution containing Co and Fe was fed; Phase 3, only Fe was fed from day 148; Phase 4, no trace element was fed from day 277 until the cease of cultivation. In order to examine the long-term stability of the process, R7 was fed with the trace element solution containing Co, Fe, Mo and Ni for 368 days of the entire operating period. Except R1, other reactors received 0.25 mL of trace element solutions every 5 days. The designated concentrations of each trace element in the reactor was 2.0 mg/L for Co, 5.0 mg/L for Mo, 10.0 mg/L for Ni, and 100 mg/L for Fe. During the experimental period, the HRT was reduced from 30 to 20 days on day 229 for R6 and R7, and the OLR was gradually increased from 2.19 to 6.64 g VS/L day. 3. Results and discussion 3.1. Characteristics of food waste and seed The food waste used in this study contained 18.1% (w/w) of TS and 17.1% (w/w) of VS. The high ratio of VS/TS (0.94) suggested that most components of food waste were biodegradable. The volatile solid content was mainly comprised of carbohydrate (111.7 g/ L), protein (32.9 g/L), and lipid (23.3 g/L). The C/N ratio (carbon/ nitrogen) was 13.2. The trace element contents in raw food waste and seed sludge were measured and given in Table 1. Among the trace elements determined in this study, the food waste contained significantly lower concentrations of cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), and zinc (Zn) than those of seed sludge. These trace element contents of food waste were comparable with the food wastes used in other studies (Zhang et al., 2007; Zhu et al., 2008), and was significantly lower Table 1 Concentrations of trace elements in food waste and inoculum (lg/L on wet basis). Trace element

Raw food waste

Food waste feeda

Seed sludge

Cadmium (Cd) Chromium (Cr) Cobalt (Co) Copper (Cu) Iron (Fe) Lead (Pb) Manganese (Mn) Molybdenum (Mo) Nickel (Ni) Silver (Ag) Tungsten (W) Zinc (Zn)

53.1 403.1 ndb 7092.7 7360.0 407.3 2229.4 58.5 441.7 ndb ndb 19196.5

21.7 165.0 ndb 2903.4 3012.9 166.7 912.6 23.9 180.8 ndb ndb 7858.2

ndb 203.4 140.1 23511.3 98908.7 132.8 15231.0 322.3 518.3 ndb ndb 249372.3

a Trace element concentration was calculated based on 7.0% of VS content, which was used as the feeding substrate. b Lower than detection limit, and the detection limits were: Ag, Cd, Co >30 lg/L, W >300 lg/L.

Fig. 1 shows the performance of seven reactors (R1–R7) during 95 days of operating period, which were supplied with individual or combined trace elements except R1 (the control receiving no trace element). R1 exhibited an unsustainable anaerobic digestion of food waste in terms of low biogas production, decreased pH and significant accumulation of VFA, which were basically the same as the known fact that the single-stage anaerobic digestion of food waste alone is very difficult (Ma et al., 2011; Xu et al., 2011; Zhang et al., 2011). In contrast to R1, R6 and R7 fed with the identical trace element solution containing Co, Fe Mo and Ni showed a very stable performance during 95 days of operating period in terms of biogas production, pH value and VFA level (Fig. 1). As also summarized in Table 2, under the conditions of a 2.2 g VS/L day OLR and a 30 days HRT, the pH remained stable at approximately pH 7.7 without any pH control, the methane productivity was stable at 1.0 L/L day, and methane yield was as high as 440 mL CH4/g VSadded) with low levels of VFA (about 600 mg/L) and SCOD (about 3500 mg/L). These results confirmed that the addition of trace elements was effective for sustainable anaerobic digestion of food waste alone. R2, R3 and R4 were designed to investigate the effects of Co, Mo, and Ni, respectively. As shown in Fig. 1, Co supplementation (R2) slightly extended the time duration for stable methane production compared to the control (R1). The stimulatory effects of cobalt were also observed in a grass-clover silage-fed biogas process (Jarvis et al., 1997) and semi-continuous anaerobic fermentation of a model substrate for maize silage (Pobeheim et al., 2011). However, methane production of R2 began to sharply decrease after about 80 days, which indicated that Co was not a limiting factor for stable anaerobic digestion of food waste in a prolonged operating period. With regard to Mo and Ni, both R3 (supplied with Mo) and R4 (supplied with Ni) did not show any noticeable difference from the control (R1) in terms of methane productivity, methane content, pH and VFA concentrations. These results might be due to the low requirement for Mo by anaerobic microorganisms. In many cases, the effects of Mo were examined together with other trace elements (Murray and van den Berg, 1981; Wilkie et al., 1986). Pobeheim et al. (2010) reported that the addition of molybdenum (Mo) did not significantly affect methane production in anaerobic digestion of a synthetic model substrate for maize silage. Unlike Mo, many studies showed that Ni was stimulatory for anaerobic digestion of diverse substrates (Hu et al., 2008; Kida et al., 2001; Pobeheim et al., 2011). Methanogenic archaea are known to use several pathways to utilize the various substrates (e.g., methanol, acetate, and H2/CO2), and all pathways converge to the common nickel containing cofactor, methyl-S-CoM (Kida et al., 2001). In addition, carbon monoxide dehydrogenase (CODH), which possesses two nickel-containing metallocenters, is present in both aceticlastic methanogens and acetogenic microorganisms (Hu et al., 2008). Since Ni supplementation did not significantly stabilize anaerobic digestion of food waste in this study, it was thought that Ni concentration in the food waste might be higher than the limiting level. As shown in Table 1, nickel concentration (180.8 lg/L) in the food waste feed was in the range of nickel concentrations found in many full scale biogas plants (155–420 lg/L) (Pobeheim et al., 2010) and higher than the limiting level (60–100 lg/L) reported by Pobeheim et al. (2011). As anticipated from the results of R2, R3 and R4, R5 supplemented with Co, Mo and Ni did not show any better results than R2.

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L. Zhang, D. Jahng / Waste Management 32 (2012) 1509–1515 1.4

CH4 productivity (L/L.day)

A

1.2 1.0 0.8 R1 (Control) R2 (Co) R3 (Mo) R4 (Ni) R5 (Co, Mo, Ni) R6 (Co, Mo, Ni, Fe) R7 (Co, Mo, Ni, Fe)

0.6 0.4 0.2 0.0 70

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CH4 content (%)

60 50 40 R1 (Control) R2 (Co) R3 (Mo) R4 (Ni) R5 (Co, Mo, Ni) R6 (Co, Mo, Ni, Fe) R7 (Co, Mo, Ni, Fe)

30 20 10 0 8.5

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8.0

pH value

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Operating

R1 (Control) R2 (Co) R3 (Mo) R4 (Ni) time (day) R5 (Co, Mo, Ni) R6 (Co, Mo, Ni, Fe) R7 (Co, Mo, Ni, Fe)

5

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Total VFA concentration (mg/L)

18000 16000 R1 (Control) R2 (Co) R3 (Mo) R4 (Ni) R5 (Co, Mo, Ni) R6 (Co, Mo, Ni, Fe) R7 (Co, Mo, Ni, Fe)

14000 12000 10000 8000 6000 4000 2000 0 0

5

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Operating time (day) Fig. 1. Performance profile of single-stage anaerobic digestion of food waste in the presence of individual or combined trace element(s): volumetric methane productivity (A); methane content in biogas (B); pH profile (C); total VFA concentration (D). OLR and HRT were 2.19 g VS/L day and 30 days, respectively. R6 and R7 were fed with the same trace elements until day 95.

Different from R1–R5, R6 and R7 contained not only Co, Mo and Ni but also Fe. R6 and R7 showed stable methane production for 95 days without pH drop and VFA accumulation. Thus, the iron contained in food waste was thought to be insufficient or less bioavailable. As shown in Table 1, Fe concentration in food waste feed

(3.0 mg/L) was found to be lower than that in seed sludge (98.9 mg/L). In the continuous operation, the process performance would be markedly affected without externally added iron because the inoculum will be gradually replaced by the low iron containing food waste.

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L. Zhang, D. Jahng / Waste Management 32 (2012) 1509–1515

Table 2 Summary of performance parameters in long-term anaerobic digestion of food waste. Operating phase

Phase 1

Reactor

R6

Operating period (day) Trace element composition HRT (day) Influent VS content (%) OLR (g VS/L day)

0–95

Process performance CH4 productivity (L/L day) CH4 content (%) CH4 yield (mL/g VSadded) pH value Acetate (mg/L) Propionate (mg/L) iso-Butyrate (mg/L) n-Butyrate (mg/L) iso-Valerate (mg/L) n-Valerate (mg/L) Total VFA (mg/L) SCOD (mg/L) Ammonia-N (mg/L) TS content (%) VS content (%)

Phase 2 R7

R6

Phase 3 R7

96–147

Co, Mo, Ni, Co, Mo, Ni, Co, Fe Fe Fe 30 30 6.6 ± 0.3 7.0 ± 0.1 2.2 ± 0.1

R6

Phase 4 R7

R6

R7

R6

148–228

229–276

277–335

Co, Mo, Ni, Fe Fe 30 6.8 ± 0.2

Co, Mo, Ni, Fe Fe 20 7.2 ± 0.5

Co, Mo, Ni, Non Fe 20 9.0 ± 0.2

2.3 ± 0.1

3.6 ± 0.1

4.5 ± 0.1

2.3 ± 0.1

R7

R7 336–368

Co, Mo, Ni, Co, Mo, Ni, Fe Fe 20 13.3 6.6

1.0 ± 0.1

1.0 ± 0.10

1.1 ± 0.1

1.0 ± 0.1

0.9 ± 0.1

1.0 ± 0.1

1.3 ± 0.2

1.4 ± 0.2

1.0 ± 0.7

1.7 ± 0.1

2.4 ± 0.3

52.7 ± 2.9 443 ± 44

52.7 ± 2.7 439 ± 43

51.0 ± 1.5 450 ± 25

51.2 ± 1.8 441 ± 27

48.0 ± 2.4 409 ± 45

49.9 ± 0.8 446 ± 27

46.6 ± 1.7 359 ± 44

48.8 ± 0.7 390 ± 28

29.8 ± 21.3 209 ± 157

49.2 ± 0.7 384 ± 16

48.4 ± 0.5 352 ± 10

7.7 ± 0.1 584 ± 356 15 ± 20 66 ± 13

7.7 ± 0.1 496 ± 273 19 ± 30 65 ± 12

7.6 ± 0.1 443 ± 138 8±4 64 ± 11

7.6 ± 0.1 863 ± 467 24 ± 41 70 ± 16

7.5 ± 0.1 999 ± 508 1705 ± 1130 76 ± 43

7.6 ± 0.1 376 ± 72 290 ± 341 49 ± 13

7.1 ± 0.1 1974 ± 963 2359 ± 1172 157 ± 76

7.4 ± 0.1 378 ± 89 165 ± 258 51 ± 11

6.3 ± 1.4 5695 ± 3000 3701 ± 2651 220 ± 137

7.5 ± 0.1 399 ± 46 16 ± 6 56 ± 8

7.5 ± 0.1 543 ± 98 15 ± 3 57 ± 7

0.5 ± 2.6 2.6 ± 6.4

0.7 ± 2.3 1.2 ± 3.5

1.3 ± 3.5 1.3 ± 3.4

0.7 ± 2.4 2.4 ± 5.2

7 ± 10 121 ± 109

0.3 ± 1.7 4.8 ± 6.4

38 ± 43 375 ± 202

0 3.3 ± 6.3

157 ± 140 578 ± 375

0 0

0 0

0.3 ± 1.6 669 ± 355 3834 ± 179 –

0 582 ± 275 3171 ± 422 –

0 517 ± 147 3729 ± 187 –

0 960 ± 484 4049 ± 276 –

27 ± 26 2935 ± 1697 6606 ± 1855 1304 ± 85

0 720 ± 351 4223 ± 655 1265 ± 73

86 ± 63 4990 ± 1491 8450 ± 1670 1365 ± 79

0 597 ± 322 4148 ± 722 1286 ± 97

130 ± 111 10,481 ± 5700 19,748 ± 13,740 1766 ± 341

0 471 ± 56 3626 ± 321 1931 ± 264

0 615 ± 104 6481 ± 2189 2758 ± 428

2.3 ± 0.1 1.4 ± 0.1

2.4 ± 0.1 1.4 ± 0.1

2.5 ± 0.2 1.4 ± 0.1

2.6 ± 0.2 1.5 ± 0.1

2.8 ± 0.2 1.7 ± 0.2

2.5 ± 0.1 1.5 ± 0.1

2.8 ± 0.1 1.8 ± 0.1

2.6 ± 0.1 1.7 ± 0.1

3.3 ± 0.9 2.5 ± 1.0

2.5 ± 0.2 1.8 ± 0.1

2.9 ± 0.4 2.2 ± 0.3

Raju et al. (1991) and Preeti Rao and Seenayya (1994) reported that the supplementation of iron (Fe) enhanced anaerobic digestion of mango processing waste and cow dung and poultry litter waste, respectively. The stimulatory effect of iron can be ascribed to two reasons. One might be the essential role of Fe in metabolizing enzymes as other trace elements (Co, Ni and Mo) (Hu et al., 2008; Jarvis et al., 1997; Pobeheim et al., 2011; Wilkie et al., 1986). The other reason might be the detoxification effect of iron on sulfide inhibition as suggested by Gonzalez-Silva et al. (2009) and Preeti Rao and Seenayya (1994). Since food waste is a protein-rich feedstock, anaerobic digestion of food waste would generate a high level of sulfide, which is a universal metabolic inhibitor. Although our results clearly showed the stimulatory effect of iron on anaerobic digestion of food waste, further study to identify the role of iron on this process is needed. 3.3. Confirmation of the effects of cobalt and iron on the anaerobic digestion of food waste In order to confirm the positive effect of iron and cobalt on stable anaerobic digestion of food waste, R6 was incubated until day 335 (Figs. 2 and 3). The operating period of 335 days was divided into four stages by feeding different trace elements: Phase 1 supplemented with Co, Fe, Mo and Ni, Phase 2 with Co and Fe, Phase 3 with Fe only, and Phase 4 without any trace element. In Phase 2 (day 96–147), R6 without Mo and Ni exhibited equally steady performance to R7 containing Co, Fe, Mo and Ni in terms of methane productivity, methane content, VFA level, SCOD and solid content (Table 2, Figs. 2 and 3). On day 148–276 (Phase 3), Co and Fe were replaced by Fe, and the process performance was still stable in terms of methane productivity, methane content and methane yield (Fig. 2). However, compared to Phase 2 and R7, about 8% lower methane productivity and methane yield were observed (Table 2). The decreased biogas production was accompanied by an

increase of organics level in the aqueous phase as shown in Fig. 3 and Table 2. For example, the total VFA concentrations were higher than R7 by 2215 mg/L and 4393 mg/L for HRT 30 days and HRT 20 days, respectively. Similarly, the SCOD level of R6 was also higher than R7. The ammonia level and solid removal were slightly lower than that of R7, indicating that the extent of food waste degradation was negatively affected by stopping cobalt (Co) feeding. On day 277, feeding of Co and Fe into R6 was stopped; the methane productivity gradually decreased, and the methane productivity dropped to zero on day 320. The pH decreased from pH 7.1 to around pH 4.0 with a rapid accumulation of VFA (about 20,000 mg/L). SCOD also accumulated to the level of 45,000 mg/L. The significant increase of solid content and the decrease of ammonia level indicated that the solid degradation, especially protein, was greatly slowed (Fig. 3). This result again confirmed that it was not possible to achieve long-term stable anaerobic digestion of food waste in a single-stage reactor without any trace element addition. Also this suggested that iron played an important role for the sustainable anaerobic digestion of food waste. 3.4. Long-term anaerobic digestion of food waste supplemented with trace elements In order to further evaluate the long-term stability under the conditions of shortened HRT and increased OLR, R7 culture was continued until day 368. During this period, the HRT was shortened from 30 to 20 days on day 229, and the OLR was gradually increased from 2.19 to 6.64 g VS/L day (Fig. 2A). Consequently, the methane productivity increased from 0.96 to 2.41 L/L day (Fig. 2B). The methane content slightly decreased from 52.7% to 48.4%, and the methane yield decreased from 439 to 352 mL CH4/ g VSadded (Fig. 2 and Table 2). Decreases of methane content and yield were thought to be due to the general rule that the high OLR and short HRT may not provide sufficient time for conversion

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L. Zhang, D. Jahng / Waste Management 32 (2012) 1509–1515 Phase 1 (Co, Mo, Ni, Fe)

B

OLR (g VS/L.day)

A

Phase 2 (Co, Fe)

Phase 4 (No trace elements)

Phase 3 (Fe)

8 6 4 2 0

Arrow 1

2.50

CH4 productivity (L/L.day)

2.25 2.00 R6 (Varying trace elements) R7 (Co, Mo, Ni, Fe)

1.75 1.50 1.25 1.00 0.75 0.50 0.25 0.00

C

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60 55

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50 45 40 35 30

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25 20 15 10 5 0

D

650

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CH4 yield (mL CH4 /g VSadded )

550 500 450 400 350 300 250 200

R6 (Varying trace elements) R7 (Co, Mo, Ni, Fe)

150 100 50 0 0

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Operating time (day) Fig. 2. Biogas profile of long-term single-stage anaerobic digestion of food waste under the conditions of different HRTs and OLRs in the presence of various trace elements: process operating conditions (A); volumetric methane productivity (B); methane content in biogas (C); methane yield (D). HRT was changed from 30 to 20 days on the operating day of 229 (Arrow 1 in A). For R6, additions of Co, Mo, Ni, and Fe were extended to day 95 (Phase 1). Since then, Co and Fe were fed in Phase 2, and only Fe was added in Phase 3. In Phase 4, R6 was not fed with any trace element.

of some organic substrates into methane in continuous operation. Nevertheless the profile of aqueous parameters (Fig. 3) clearly showed the long-term stability of R7. During the entire operating period, the pH value was maintained in the narrow range of pH 7.4–7.7 without any pH control (Fig. 3B). In addition, R7 showed a satisfactorily low level of VFA (471–960 mg/L) even under the conditions of a higher OLR (6.64 g VS/L day) and a shorter HRT

(20 days). In general, the process upsets are accompanied by a significant accumulation of VFA, which causes the pH to drop and irreversible process failure in the long run. Fig. 3 shows the profiles of soluble COD, solid content (TS and VS) and ammonia concentration. SCOD was maintained in the level of 4000–6000 mg/L, which fell into the typical range found in the anaerobic digestion of organic solid wastes (Zhang et al., 2011). The ammonia concentration

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L. Zhang, D. Jahng / Waste Management 32 (2012) 1509–1515 Phase 1 (Co, Mo, Ni, Fe) OLR (g VS/L.day)

A

B

Phase 2 (Co, Fe)

Phase 4 (No trace elements)

Phase 3 (Fe)

8 6 4 2 0

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7 R6 (Varying trace elements) R7 (Co, Mo, Ni, Fe)

6

5

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3

C Total VFA concentration (mg/L)

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Soluble COD cocentration (mg/L)

D 45000

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40000 35000

was between 1265 and 3400 mg-N/L, which appeared to be proportional to the VS content and C/N ratio (13.2) of food waste feed. Although the solid content slightly increased in the late stage of Phase 4 due to the increased solid content of the food waste feed, the VS removal was kept relatively constant (75–80%). In short, it was possible to obtain a long-term sustainable anaerobic digestion of food waste by maintaining trace element concentrations in the reactor to be 2.0 mg/L Co, 5.0 mg/L Mo, 10.0 mg/L Ni and 100 mg/L Fe. High methane productivity (2.41 L CH4/L day) and a low level of VFA (471–960 mg/L) were obtained in this study under the conditions of 20 day HRT and 6.64 g VS/L day OLR in a single-stage reactor. Banks et al. (2011) ran the biogas plant with source-sorted food waste at the conditions of OLR of 2.5 g VS/L day and HRT of 80 days, and the obtained methane productivity was 1.0 L CH4/L day with 15,000 mg/L of total VFA accumulation. The methane yield of food waste (352–439 mL CH4/g VSadded) observed in this study was also much higher than those of anaerobic digestion of other organic wastes, such as animal manures (242–300 mL CH4/ g VSadded) (Zhang et al., 2011) and organic fraction of municipal solid waste (273–314 mL CH4/g VSadded) (Dong et al., 2010). This study suggested that supplementation of trace elements could be a simple and easy way to accomplish stable and efficient anaerobic digestion of food waste. Instead of adding metallic compounds directly into the anaerobic digester, trace elements can be supplemented to food waste by mixing with trace element rich wastes such as animal manure and sewage sludge. The economic feasibility will strongly depend on the cost of trace element sources, especially for the iron which was found to be needed at a relatively high concentration.

R6 (Varying trace elements) R7 (Co, Mo, Ni, Fe)

30000 25000

4. Conclusions

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Solid content (%)

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6 5 4

It was found that the cafeteria food waste used in this study was deficient in trace elements, which caused the failure of single-stage anaerobic digestion. With supplemented trace metals, methane productivity and pH were maintained almost constantly, and the concentrations of volatile fatty acids in the reactor remained low. Among examined trace elements, Fe was identified as the most effective for stabilizing anaerobic digestion of food waste. These results offered a simple and easy way to successfully treat food waste via anaerobic digestion in a single-stage reactor.

3

Acknowledgement

2 1

F

4000

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3500

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R6 (Varying trace elements) R7 (Co, Mo, Ni, Fe)

3000 2500 2000 1500

This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0022968) and the 2011 research fund of Myongji University. We also thank for the supports from Key Laboratory of Industrial Ecology and Environmental Engineering (MOE) (KLIEEE-11-07), Fundamental Research Funds for the Central Universities (DUT11RC(3)67) and Special Fund of Environmental Protection Research for Public Welfare of China (201109035).

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100 125 150 175 200 225 250 275 300 325 350 375 400

Operating time (day)

Fig. 3. Performance parameters in aqueous phase in long-term single-stage anaerobic digestion of food waste under the different conditions of trace element supplementation, HRTs and OLRs: process operating conditions (A); pH profile (B); total VFA concentration (C); soluble COD level (D); solid content (E); ammonia– nitrogen concentration (F). Operating conditions are listed in Fig. 2.

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