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The anaerobic co-digestion of food waste and cattle manure
Bioresource Technology 129 (2013) 170–176
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The anaerobic co-digestion of food waste and cattle manure Cunsheng Zhang, Gang Xiao, Liyu Peng, Haijia Su ⇑, Tianwei Tan Beijing Key Laboratory of Bioprocess, Beijing University of Chemical Technology, Beijing 100029, PR China
h i g h l i g h t s " The separate anaerobic digestion of food waste or cattle manure was hardly feasible. " Co-digestion produced significant quantities of biogas, with high methane content. " Addition of cattle manure enhanced the buffer capacity of anaerobic system. " In co-digestion, the C/N ratio contributed to the improving biogas production.
a r t i c l e
i n f o
Article history: Available online 5 November 2012 Keywords: Food waste Cattle manure Methane Co-digestion Buffer capacity
a b s t r a c t This study assessed the anaerobic co-digestion of food waste and cattle manure, in order to identify the key parameters that determine the biogas and methane yield. Results of both batch and semi-continuous tests indicated that the total methane production is enhanced in co-digestion, with an optimum food waste (FM) to cattle manure (CM) ratio of 2. At this ratio, the total methane production in batch tests was enhanced by 41.1%, and the corresponding methane yield was 388 mL/g-VS. In the semi-continuous mode, the total methane production in co-digestion, at the organic loading rate (OLR) of 10 g-VSFW/L/d, increased by 55.2%, corresponding to the methane yield of 317 mL/g-VS. Addition of cattle manure enhanced the buffer capacity (created by NHþ 4 and VFAs), allowing high organic load without pH control. The C/N ratio and the higher biodegradation of lipids might be the main reasons for the biogas production improvement. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Anaerobic digestion has been proven to be an efficient and green technology in disposing of sewage sludge, crop residues, food waste and animal manure (Wan et al., 2011; Li et al., 2009). Advantages are the production of renewable energy in the form of biogas and the possibility to recycle valuable nutrients, concentrated in the digestion residue (Zhang et al., 2012; Angelidaki et al., 2003). Food waste has already been considered as a very attractive feedstock for anaerobic digestion due to its high methane potential (Zhang et al., 2011). Li et al. (2010) reported that the fat content of food waste is about 23%. Under specific operating conditions, lipidrich waste such as fat and oil will significantly contribute to the methane production (Wan et al., 2011). However, long-chain fatty acids (LCFAs) are formed during the degradation of fat and lipids: the 18-C LCFAs (such as oleic and stearic acid) are inhibitory at concentrations exceeding 1.0 g/L (Appels et al., 2008). LCFAs can moreover be toxic to both syntrophic acetogens and methanogens (Hanaki et al., 1981) and limit the transport of nutrients to cells ⇑ Corresponding author. Tel.: +86 10 644452756; fax: +86 10 64414268. E-mail address: [email protected] (H. Su). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.10.138
due to being adsorbed on the microbial surfaces (Pereira et al., 2005). It was therefore difficult if not impossible to treat only FW by anaerobic digestion (Resch et al., 2011; Zhang et al., 2011; Palatsi et al., 2011). Anaerobic digestion was also found to be unstable when the cattle manure is used as mono-substrate due to the low C/N ratio (5–8) (Li et al., 2009). It is therefore important to examine an alternative approach for anaerobic digestion of FW or CM, co-digestion possibly helping to overcome the deficiencies of mono-digestion. This co-digestion is studied in the present research. The carbon to nitrogen (C/N) ratio is one of the important parameters influencing the digestion process (Kumar et al., 2010). Zhu (2007) suggested that anaerobic digestion could be carried out efficiently when the C/N ratio is 15. Kumar et al. (2010) found that a C/N ratio range from 13.9 to 19.6 is acceptable for digestion. Anaerobic co-digestion of different organic materials may enhance the stability of the anaerobic process because of a better carbon to nitrogen (C/N) balance (El-Mashad and Zhang, 2010; Mshandete et al., 2004). Co-digestion may moreover alleviate the inhibitory effect of high ammonia and sulfide concentrations (Hartmann et al., 2003), and exhibit a more stable biogas production
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due to an improved buffer capacity (Nayono et al., 2010). Although the increasing biogas production through anaerobic co-digestion of food waste and cattle manure has been reported, no study has been carried out to address the C/N ratio, the buffer capacity and the removal of LCFAs in co-digestion of food waste and cattle manure. The aim of this study is therefore threefold: (i) assess the performance of anaerobic co-digestion of food waste and cattle manure; (ii) identify the key parameters influencing the increase of biogas and methane yield, including the effect of the C/N ratio; and (iii) examine the extent of the buffer capacity and the removal of LCFAs.
was digested alone at the load of 8 g-VS/L, as the control group. Cattle manure was digested as mono-substrate in digestion R5, R6 and R7 with the amounts of 4.0, 2.7 and 2.0 g-VS/L, respectively. Thus, to determine the performance of co-digestion, the codigestion of R1, R2 and R3 was compared with mono-digestion groups of R4–R5 (group 1), R4–R6 (group 2) and R4–R7 (group 3), respectively. To evaluate the performance of co-digestion, anaerobic co-digestion was further carried out at high organic loads of 24 and 32 g-VSFW/L, with the optimum FW/CM ratio determined from preliminary experimental results. The control group was also designed for comparison of co-digestion and mono-digestion.
Cattle manure was collected at a farm in Zaozhuang, Shandong province. Food waste was provided by the mess of the Beijing University of Chemical Technology. The organic substrates of FW were selected manually and ground into small particles (<3 mm) by a mill (SS3300, Waste King in USA). Both cattle manure and food waste were stored in the freezer before digestion. The properties of the food waste and cattle manure are shown in Table 1, and compared with literature data.
2.2.2. Anaerobic digestion in semi-continuous mode The digester, the effective volume of each digester and the temperature in semi-continuous tests were the same as during the batch tests. The FW/CM ratio in co-digestion was 2, determined as optimum ratio from the preliminary results in batch operation. The experimental conditions in semi-continuous tests are shown in Table 3. The OLR for co-digestion were 8, 10 and 12 g-VSFW/L/d, respectively, along with 4, 5 and 6 g-VSCM/L/d added to each digester. The mono-digestion at the same OLR of food waste/cattle manure was performed to compare its performance with co-digestion. Before each discharge and after feeding, the activated sludge in digesters was fully mixed. The samples of effluent of each digester were taken for pH and VFAs measurements.
2.2. Anaerobic digestion tests
2.3. Biogas and VFAs measurements
2.2.1. Batch tests Batch digestion tests were carried out in 1-L glass digesters at mesophilic temperature (35 ± 1 °C). The effective volume of each digester was 0.8 L. In each digester, anaerobically treated activated sludge was used as inoculum. It had been acclimated for 14 months in a 20-L tank in the laboratory. The TS and VS/TS of inoculum were 3.2% (wet basis) and 46.9%, respectively. The inoculum and substrates were fully mixed before being added to the digesters. Each digester was flushed for 5 min (300 mL/min) with inert gas (N2) to create an anaerobic environment. Food waste, cattle manure, and their mixtures were separately examined, in mono-digestion or co-digestion respectively. The characteristics of the different experiments are shown in Table 2. In co-digestion, the amount of food waste in each digester was kept constant (8 g-VS/L), while varying the amount of cattle manure added. The FW/CM ratios (based on VS) of digestion R1, R2, R3 were designed as 2, 3 and 4, respectively, corresponding to the cattle manure amounts of 4.0, 2.7 and 2.0 g-VS/L. In digestion R4, food waste
Biogas was collected by water displacement method. The biogas volume was calculated daily and was transformed into the volume at STP condition. Biogas samples were examined by gas chromatography (GC-2014C, Shimadzu in Japan) to determine the CH4 and CO2 content. A thermal conductivity detector (TCD) and a stainless steel column of TDX-01 (packed with carbon molecular sieve, 2 m 3 mm) were used in the GC for biogas measurements. The temperatures of injection port, column and TCD were 160 °C, 160 °C and 180 °C, respectively. Argon was used as carrier gas with a pressure of 0.3 MPa and a flow rate of 25 mL/min. The VFAs in the effluent were also measured by GC, using a hydrogen flame ionization detector (FID) and a capillary column (DB-WAX, 30 m 0.32 mm, Agilent). The temperatures of the injection port and FID were both 250 °C. The temperature programming was: 95 °C during 2 min; increase to 210 °C at a rate of 30 °C/min; keep at 210 °C for 2 min; increase to 230 °C at a rate of 30 °C/min; and maintain at 230 °C during 3.5 min. Nitrogen was used as carrier gas at a pressure of 0.4 MPa. The flow rates of
2. Methods 2.1. Collection of substrates and inoculums
Table 1 Properties of food waste and cattle manure (mean value ± standard deviation). Parameter
pH TS (wt.%) VS (wt.%) VS/TS ratio Carbon, C (%, d.b.) Nitrogen, N (%, d.b.) C/N ratio Fat (%, d.b.) Na+ (%, d.b.) K+ (%, d.b.) Mg2+ (%, d.b.) Ca2+ (%, d.b.) Fe3+ (ppm) Mn2+ (ppm) Zn2+ (ppm)
Food waste
Cattle manure
Zhang et al. (2007)
This study
Li et al. (2009)
This study
– 30.9 ± 0.1 26.4 ± 0.1 0.85 46.78 ± 1.15 3.16 ± 0.22 14.8 – – 0.9 ± 0.11 0.14 ± 0.01 2.16 ± 0.29 766 ± 402 60 ± 30 76 ± 22
5.2 ± 0.3 18.5 ± 0.1 17.0 ± 0.1 0.92 46.5 ± 1.5 2.2 ± 0.3 21.1 22.8 3.45 ± 0.20 2.30 ± 0.04 0.16 ± 0.01 0.03 ± 0.01 100 ± 23 110 ± 95 160 ± 30
– 17.1 ± 0.6 14.4 ± 0.2 0.84 28 ± 2.0 4.8 ± 0.5 5.8 – – – – – – – –
9.2 ± 0.3 16.3 ± 0.1 13.2 ± 0.1 0.81 26.7 ± 1.8 5.1 ± 0.6 5.2 – 1.44 ± 0.1 1.27 ± 0.1 4.99 ± 0.4 2.27 ± 0.3 150 ± 30 950 ± 232 250 ± 42
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Table 2 Experimental conditions and results in batch tests. Item Unit
Digestion type –
FW g-VS/L
CM g-VS/L
FW/CM –
C/N –
Biogas production mL/g-VS
Methane yield mL/g-VS
Total methane mL
Initial pH –
Final pH –
R1 R2 R3 R4 R5 R6 R7
Co-digestion Co-digestion Co-digestion Mono-digestion Mono-digestion Mono-digestion Mono-digestion
8.0 8.0 8.0 8.0 0 0 0
4.0 2.7 2 0 4.0 2.7 2
2 3 4 – – – –
15.8 17.1 17.9 21.1 5.2 5.2 5.2
570 526 537 621 19 28 38
388 352 343 410 5 6 9
3725 3003 2744 2624 16 13 14
7.5 7.7 7.4 7.5 7.6 7.6 7.5
7.5 7.4 7.4 7.3 7.9 7.8 7.8
Table 3 Experimental conditions and results in semi-continuous tests. Item Unit
FW g-VS/L/d
CW g-VS/L/d
Methane yield mL/g-VS/d
Methane content %
Total methane L
Initial pH –
Final pH –
NHþ 4 mg/L
R8 R9 R10 R11 R12 R13 R14 R15 R16
8 10 12 8 10 12 0 0 0
0 0 0 4 5 6 4 5 6
347 277 96 388 317 139 69 60 55
61.2 58.0 35.1 62.3 60.2 39.7 33.5 32.9 32.7
33.3 33.2 5.5 55.9 57.1 14 3.3 3.6 4.0
7.5 7.6 7.6 7.4 7.7 7.6 7.4 7.4 7.4
7.2 7.0 4.0 7.3 7.1 4.3 7.6 7.6 7.7
487 512 471 630 677 632 937 1213 1570
nitrogen, hydrogen and air were 50 mL/min, 30 mL/min and 500 mL/min, respectively. 2.4. Chemical analysis The pH, TS and VS of food waste and cattle manure samples were measured according to the standard methods (APHA, 1998). Total carbon was determined by a TOC analyzer (TOC-V, Shimadzu in Japan). Total nitrogen was determined by Kjeltec Nitrogen Analyzer. Fat was measured by Soxhlet extraction method. The metal elements of food waste and cattle manure including K, Na Mg, Ca, Fe, Mn and Zn were analyzed before anaerobic digestion by AAS (atomic absorption spectrophotometer, VarianSpectrAA55-B, Palo Alto, USA). 3. Results and discussion 3.1. Anaerobic digestion in batch tests 3.1.1. Mono- and co-digestion of food waste and cattle manure The total biogas production and the corresponding CH4 and CO2 contents are shown in Fig. 1. Fig. 1A shows that the total biogas production increased gradually in the digestion from R1 to R4, and 90% of the final biogas production was produced in the initial 10 days. After 18 days of digestion, the total biogas production of R1, R2, R3 and R4 were 5472, 4488, 4300 and 3974 mL, respectively. Lower methane production was obtained in monodigestions R5 to R7. Compared with mono-digestion of group 1 (R4–R5), group 2 (R4–R6) and group 3 (R4–R7), the total biogas production in the co-digestion of R1, R2 and R3 were all enhanced. The highest biogas production was obtained in digestion R1 where the FW/CM ratio is 2 (Table 2), accepted as the optimum FW/CM ratio for co-digestion of food waste and cattle manure. Fig. 1B and C show the productions of CH4 and CO2 in batch tests. The CH4 content increased sharply during the initial 2 days. The CH4 content of biogas increased to 65% in co-digestion of R2–R5 until day 2. Thereafter, it increased gradually in the following days. The highest CH4 content was 79% in co-digestion of R2. By contrast, a higher CO2 content was obtained at the initial of digestion. The
CO2 content decreased gradually along with the increase of CH4 content. The lowest CO2 content could reach a level of 21% in batch tests. Table 2 shows the results obtained from batch tests. The methane yields of R1 to R4 were 388, 352, 343 and 410 mL/g-VS, respectively. Although the methane yield of co-digestion was lower than that obtained in mono-digestion, the total methane produced in co-digestion was obviously higher. The total methane production of R1–R4 (Table 2) were 3725, 3003, 2744 and 2624 mL, respectively. The methane production in R1, R2 and R3 was enhanced by 41.1%, 13.9% and 4.0%, respectively. This indicated that codigestion of food waste and cattle manure is an effective approach for methane improvement. Anaerobic co-digestion of two or more substrates to increase biogas production/methane yield has been reported previously. Addition of press water and food waste in bio-waste digester not only increased biogas production rates but also improved the total biogas production (Nayono et al., 2010). A higher methane production of 603 mLCH4/g-VS was obtained in the co-digestion of a mixture of 70% manure, 20% food waste and 10% sewage sludge at the OLR of 1.2 g-VS/L day (Marañón et al., 2012). Table 2 shows the FW/CM and C/N ratios of feedstock in batch digestion. The C/N ratio of feedstock in R1 was 15.8 where the FW/CM ratio is 2. At this C/N ratio, the maximum biogas production and methane yield were obtained. The results indicated that the optimum C/N ratio was 15.8, in-line with previous findings of C/N = 15 (Huang et al., 2004), or 13.9–19.6 (Kumar et al., 2010). The C/N ratio of FW and CM were 21.1 and 5.2 (Table 2), respectively, and hence obviously and individually either higher or lower than the recommended optimum. The correct C/N ratio of the feedstock is important towards optimizing the biogas and methane production. Table 1 shows the metal elements of food waste and cattle manure. The sodium and potassium content in food waste and cattle manure, on dry base, were 3.45%, 2.30% and 1.44%, 1.27%, respectively. McCarthy and McKinney (1961) reported the inhibition concentration of sodium and potassium was at 10 g/L when acetate is used as pure substrate. Based on the data in Table 1, the total sodium and potassium concentration at the OLR of 8 g-VSFW/L/d,
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A
R1 R4
6000
Total biogas production (mL)
anaerobic system. Many enzymes and co-enzymes are in need of a minimal amount of certain trace element for their activation and activity (Appels et al., 2008). The trace elements in cattle manure may have played an important role in enhancing methanogen activity in anaerobic digestion. Co-digestion of food waste with piggery wastewater, Zhang et al. (2011) also found that the trace element is the reason for improvement of co-digestion performance.
7000
R2 R5
R3 R6
R7
5000
4000
3000
2000
1000
0 0
5
10
15
20
25
30
Time (days)
B
90
R1
R2
R3
R4
80
CH4 (%)
70
3.1.2. Stability of co-digestion Fig. 2 shows the variations of methane yield and pH at high organic loads of 24 g-VSFW/L and 32 g-VSFW/L. Fig. 2A indicates that anaerobic co-digestion of food waste and cattle manure could proceed at high organic loads. However, digestion failure was observed in mono-digestion of food waste at the two selected organic loads. Fig. 2B shows that the pH value in both monodigestion and co-digestion decreased gradually at day 1. By contrast, the pH value of mono-digestion further decreased in the following days while, pH recovery was observed in co-digestion at day 2. The pH values of food waste and cattle manure were 5.2 and 9.2 (Table 1), respectively. The alkaline components of cattle manure might have neutralized the pH, and thus enhanced the buffer capacity and stability of anaerobic system, allowing high organic load of 32 g-VSFW/L/d, corresponding to the total organic load of 48 g-VS/L in batch digestion without specific pH control.
60
50
A
600
40
30 0
5
10
15
20
25
30
Time (days)
C
80
R1
70
R2
R3
R4
Methane yield (ml/gVS)
500
CO2 (%)
60
400
300
200
24 g-VSFW/L/d + 12 g-VSCM/L/d 24 g-VSFW/L/d 32 g-VSFW/L/d + 16 g-VSCM/L/d
100
32 g-VSFW/L/d
50 0 0
5
10
40
15
20
25
30
Time (days)
B
30
20
8.4 7.8 7.2
10 0
5
10
15
20
25
30
Time (days)
pH
Fig. 1. Total biogas production (A), CH4 content (B) and CO2 content (C) in batch tests (R1: 8 g-VSFW/L + 4 g-VSCM/L; R2: 8 g-VSFW/L + 2.7 g-VSCM/L; R3: 8 g-VSFW/ L + 2 g-VSCM/L; R4: 8 g-VSFW/L; R5: 4 g-VSCM/L; R6: 2.7 g-VSCM/L; R7: 2 g-VSCM/L).
6.6 6.0
24 g-VSFW/L/d + 12 g-VSCM/L/d
5.4
24 g-VSFW/L/d 32 g-VSFW/L/d + 16 g-VSCM/L/d
4.8
with the C/N ratio of 2, were 0.33 g/L and 0.23 g/L, respectively. The sodium and potassium concentration was far lower than the inhibition concentration. Moreover, the concentrations of Mg, Ca, Mn, Zn in cattle manure were 4.99%, 2.27%, 950 ppm, 250 ppm, respectively. Compared with cattle manure, the concentrations of trace elements were obviously lower in food waste. Addition of cattle manure enhanced the trace elements concentration in
32 g-VSFW/L/d 4.2
0
5
10
15
20
25
30
Time (days) Fig. 2. Variations of methane yield (A) and pH (B) with time at high organic loads (24 and 32 g-VSFW/L/d).
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3.2.2. VFAs and ammonia Fig. 4 shows the variation of total VFAs during the semi-continuous process. The VFAs concentration of R8–R13 increased steadily during the initial 3 days. Thereafter, the total VFAs concentration increased continuously at the OLR of 12 g-VSFW/L/d (R10, R13), reaching 15.2 g/L and 13.5 g/L, respectively, at day 8. VFAs accumulated due to the overloading of the system. A lower pH value (Table 2) as well as the inhibition on biogas production (Fig. 3) was observed due to VFAs accumulation at the OLR of 12 g-VSFW/L/d. By contrast, the VFAs concentration maintained at a stable level at the OLR of 8 and 10 g-VSFW/L/d (R8, R9, R11, R12), e.g., at the OLR of 10 g-VSFW/L/d, the VFAs concentration was in the range from 6 to 7 g/L in co-digestion. Compared with mono-digestion, the VFAs concentration in co-digestion was higher, indicating that more VFAs were generated in co-digestion system. However, a higher final pH value (Table 2) was measured in the co-digestion system compared with mono-digestion. This result again shows a higher stability of the co-digestion system due to the increased buffer capacity, indicating that the addition of cattle manure to food waste could stabilize anaerobic system. Ammonium ions or ammonia were produced from proteins and amino acids during anaerobic digestion. The ammonia concentration during semi-continuous digestion is shown in Table 3. The ammonia concentrations at the OLR of 8, 10 and 12 g-VS/L/d in mono-digestion were 487, 512 and 471 mg/L, respectively. The corresponding ammonia concentrations were 630, 677 and 632 mg/L in co-digestion. Compared with mono-digestion, the ammonia concentration in co-digestion effluent was obviously higher. An ammonia concentration of 700 mg/L was considered to be an inhibitory concentration for methanogenic activity (Lay et al., 1998). According to the ammonia concentration in Table 3, no ammonia inhibition occurred in co-digestion. However, the ammonia concentration in
3.2. Anaerobic digestion in semi-continuous mode 3.2.1. Methane yield Fig. 3 shows the response surface and contour plot of daily methane yield of mixture, food waste and cattle manure in semicontinuous mode. Fig. 3A and B indicate that anaerobic co-digestion could proceed at the OLR of 15 g-VS/L/d. The average methane yield, at the OLR of 15 g-VS/L/d, was 317 mL/g-VS (Table 3). The daily methane yield decreased rapidly with the increase of OLR, e.g., at the OLR of 18 g-VS/L/d, the daily methane yield decreased to 67 mL/g-VS at day 7. This result indicated that the maximum OLR for anaerobic co-digestion is 15 g-VS/L/d in semi-continuous mode. Fig. 3C and D indicate that the maximum OLR for food waste is 10 gVS/L/d, corresponding to the average methane yield of 277 mL/g-VS (Table 3). Anaerobic digestion failed at the OLR of 12 g-VS/L/d. Fig. 3E and F illustrate that the daily methane yield varied from 113 mL/gVS to 50 mL/g-VS when cattle manure was digested at the OLR from 3 g-VS/L/d to 6 g-VS/L/d. Compared with co-digestion, both the methane yield and the maximum OLR of mono-digestion were lower, indicating that anaerobic co-digestion of food waste and cattle manure could significantly improve the maximum OLR and the methane yield. Table 3 shows the average methane yield of mono-digestion and co-digestion. The methane yield of mono-digestion R8 (8 g-VSFW/L/d) and R9 (10 g-VSFW/L/d) were 347 and 277 mL/g-VS, respectively. With the same OLR of food waste (Table 3), the corresponding methane yield of co-digestion were 388 (R11) and 317 mL/g-VS (R12), respectively. The result indicated that addition of cattle manure could increase the methane yield in semi-continuous mode. The total methane production in co-digestion increased by 52.7% and 55.2%, respectively, corresponding to the OLR of 8 and 10 g-VSFW/L/d.
C Methane yield (mL/g-VS)
Methane yield (mL/g-VS)
470 390 310 230 150 70 -10
E
430
320
210
100
-10
15 13 11 9 7
X: Time (days)
5 3 1
18
17
16
15
14
13
12
11
10
9
15 13
11
9
7
5
3
X: Time (days)
Y: OLR (g-VS/L/d)
120
Methane yield (g-VS/L/d)
A
1
12
11
10
9
8
7
100 80 60 40 20 0
6
15 13 11
Y: OLR (g-VS/L/d)
3 9
4
7
X: Time (days)
5
5
3 1
B 18
D
Methane yield (mL/g-VS)
Methane yield (g-VS/L/d) 6 69
63
142
17
F
Methane yield (mL/g-VS) 12
67
50
135
218
11
207
16
72
5 257
299
15
Y: OLR (g-VS/L/d)
6
10
14
309
9
5
50
365
13
341
82
8
12 393
4
11 7
381
98
415
10
1
5
8
X: Time (days) Y: OLR (g-VS/L/d)
12
15
113
3
6
9
1
3
5
7
9
11
X: Time (days) Y: OLR (g-VS/L/d)
13
15
1
3
5
7
9
11
X: Time (days) Y: OLR (g-VS/L/d)
Fig. 3. Response surface and contour plot of daily methane yield from mixture (A and B), food waste (C and D) and cattle manure (E and F).
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22.8% (Table 1). Lot of fat may have lost due to the formation of the agglomerated lipid particle in mono-digestion. By contrast, no agglomerated particles were observed in the effluent of co-digestion, tentatively explained by the dilution of the FW lipids when CM was added: more lipids could be digested by Syntrophomonadaceae and Clostridiaceae families due to the dilution (Hatamoto et al., 2007), increasing the total biogas production. The lipid-rich waste such as oil could be diluted in co-digestion has been reported previously. Angelidaki and Ahring (1997) reported that oil mill effluent needed to be diluted before it was added to anaerobic digesters because of the rapid decrease of pH in acidification. By contrast, by co-digestion with animal manure, oil mill effluent could be digested without preliminary dilution. The higher biodegradability of lipids has been recognized as a main reason for the increased biogas production.
18
R8 R11
R9 R12
R10 R13
Total VFAs (g/L)
15
12
9
6
3
0 0
2
4
6
8
175
10
12
14
16
Time (days)
4. Conclusions
Fig. 4. Variation of total VFA concentration with time in the semi-continuous process (R8: 8 g-VSFW/L/d; R9: 10 g-VSFW/L/d; R10: 12 g-VSFW/L/d; R11: 8 g-VSFW/L/ d + 4 g-VSCM/L/d; R12: 10 g-VSFW/L/d + 5 g-VSCM/L/d; R13: 12 g-VSFW/L/d + 6 g-VSCM/L/d).
R14, R15 and R16 was beyond 700 mg/L, meaning the inhibition from ammonia might have occurred when cattle manure was digested alone. Tables 2 and 3 illustrate that the final pH value of effluent in co-digestion was higher than the value in mono-digestion. These results indicated that co-digestion of food waste and cattle manure improved the buffer capacity of anaerobic system. In addition, the total organic nitrogen of FW and CM were 2.2% and 5.1% (Table 1), respectively. Addition of cattle manure could increase the total organic nitrogen in co-digestion system, resulting in the higher concentration of ammonia in co-digestion system. The higher ammonia neutralized the VFAs in acidification. The ammonia and VFAs may have ionized in liquid phase. The reaction can be defined as follows:
CxHyCOOH CxHyCOO þ Hþ
ð1Þ
NH3 H2 O NHþ4 þ OH
ð2Þ
where CxHyCOOH presents the VFAs. Combine Eqs. (1) and (2), Eq. (3) was obtained
CxHyCOOH þ NH3 H2 O ! CxHyCOO þ NHþ4 þ H2 O
ð3Þ
More VFAs will be neutralized at higher concentration of ammonia. Thus, the buffer system formed in co-digestion, allowing high concentration of VFAs without pH decrease. Therefore, the higher ammonia concentration may be the reason for buffer capacity improvement in co-digestion. Co-substrate could enhance buffer capacity has been reported by the previous report. Nayono et al. (2010) found that addition of press water or food waste to biowaste digester lead to high buffer capacity. Digestion could be carried out at very high loadings without pH control. 3.2.3. Characteristic of effluent During the mono-digestion of food waste, yellow and white particles, of size from 1 to 5 mm, were observed in the effluent. Liu et al. (2011) reported the particles to be agglomerated by calcium salts of long chain fatty acids (LCFAs). According to Appels et al. (2008), LCFAs are well known to be inhibitory at low concentrations. The inhibition from LCFAs could be alleviated in monodigestion when the agglomerated particles formed. Nevertheless, lipid-rich waste such as fat and oil are considered to be most attractive organic source for biogas production due to its high methane potential. In theory, the methane potential of lipids is 1014 mL/g-VS (Wan et al., 2011). The fat content of food waste is
Anaerobic co-digestion of food waste and cattle manure could enhance the biogas production and the methane yield. The optimum C/N ratio was 15.8, corresponding to the FW/CM ratio of 2. The total methane production at the optimum FW/CM ratio was enhanced by 41.1%, corresponding to the methane yield of 388 mL/g-VS. Addition of cattle manure enhanced the buffer capacity in the digesters, allowing high organic loads in batch digestion without pH control. The C/N ratio and the higher biodegradation of lipids might be the main reasons for the improved biogas production and methane yield during co-digestion. Acknowledgements This work was funded by the Hi-Tech Research and Development Program of China (Grant number: 2008AA062401), the Natural Science Foundation of China (20876008, 21076009), the Project-sponsored by SRF for ROCS, SEM (LXJJ2012-001) and the Program for New Century Excellent Talents in University (NCET-100212). References Angelidaki, I., Ahring, B.K., 1997. Co-digestion of olive oil mill wastewaters with manure, household waste or sewage sludge. Biodegradation 8, 221–226. Angelidaki, I., Ellegaard, L., Ahring, B.K., 2003. Application of the anaerobic digestion process. Adv. Biochem. Eng./Biotechnol. Biomethanat. II 82, 1–33. APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 18th ed. American Public Health Association, Washington, DC, USA. Appels, L., Baeyens, J., Degreve, J., Dewil, R., 2008. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energy Combust. 34, 755– 781. El-Mashad, H.M., Zhang, R.H., 2010. Biogas production from co-digestion of dairy manure and food waste. Bioresour. Technol. 101, 4021–4028. Hanaki, K., Nagase, M., Matsuo, T., 1981. Mechanism of inhibition caused by longchain fatty-acids in anaerobic-digestion process. Biotechnol. Bioeng. 23 (7), 1591–1610. Hartmann, H., Angelidaki, I., Ahring, B.K., 2003. Co-digestion of the organic fraction of municipal solid waste with other waste types. In: Mata-Alvarez, J. (Ed.), Biomethanisation of the Organic Fraction of Municipal Solid Wastes. IWA Publishing Company, Amsterdam. Hatamoto, M., Imachi, H., Yashiro, Y., Ohashi, A., Harada, H., 2007. Diversity of anaerobic microorganisms involved in LCFA degradation in methanogenic sludges revealed by RNA-based stable isotope probing. Appl. Environ. Microbiol. 73 (13), 4119–4127. Huang, G.F., Wong, J.W.C., Wu, Q.T., Nagar, B.B., 2004. Effect of C/N on composting of pig manure with sawdust. Waste Manage. 24, 805–813. Kumar, M., Ou, Y.L., Lin, J.G., 2010. Co-composting of green waste and food waste at low C/N ratio. Waste Manage. 30, 602–609. Lay, J.J., Li, Y.Y., Noike, T., 1998. The influence of pH and ammonia concentration on the methane production in high-solids digestion processes. Water Environ. Res. 70 (5), 1075–1082. Li, R.P., Ge, Y.J., Wang, K.S., Li, X.J., Pang, Y.Z., 2010. Characteristics and anaerobic digestion performances of kitchen wastes. Renewable Energy, Resources 28, 76–80 (Chinese).
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Li, X.J., Li, L.Q., Zheng, M.X., Fu, G.Z., Lar, J.S., 2009. Anaerobic co-digestion of cattle manure with corn stover pretreated by sodium hydroxide for efficient biogas production. Energy Fuels 23, 4635–4639. Liu, Y.P., Chen, X., Zhu, B.N., Yuan, H.R., Zhou, Q., Xia, Y., Li, X.J., 2011. Formation and function of calcium stearate in anaerobic digestion of food waste. Chin. J. Environ. Eng. 5 (12), 2844–2848 (in Chinese). Marañón, E., Castrillón, L., Quiroga, G., Fernández-Nava, Y., Gómez, L., García, M.M., 2012. Co-digestion of cattle manure with food waste and sludge to increase biogas production. Waste Manage. 32, 1821–1825. McCarthy, P.L., McKinney, R.E., 1961. Salt toxicity in anaerobic digestion. J. Water Pollut. Control Fed. 33, 399–408. Mshandete, A., Kivaisi, A., Rubindamayugi, M., Mattiasson, B., 2004. Anaerobic batch co-digestion of sisal pulp and fish wastes. Bioresour. Technol. 95 (1), 19–24. Nayono, S.E., Gallert, C., Winter, J., 2010. Co-digestion of press water and food waste in a biowaste digester for improvement of biogas production. Bioresour. Technol. 101, 6987–6993. Palatsi, J., Viñas, M., Guivernau, M., Fernandez, B., Flotats, X., 2011. Anaerobic digestion of slaughterhouse waste: main process limitations and microbial community interactions. Bioresour. Technol. 102, 2219–2227.
Pereira, M.A., Pires, O.C., Mota, M., Alves, M.M., 2005. Anaerobic biodegradation of oleic and palmitic acids: evidence of mass transfer limitations caused by long chain fatty acid accumulation onto the anaerobic sludge. Biotechnol. Bioeng. 92 (1), 15–23. Resch, C., Worl, A., Waltenberger, R., Braun, R., Kirchmayr, R., 2011. Enhancement options for the utilisation of nitrogen rich animal by-products in anaerobic digestion. Bioresour. Technol. 102, 2503–2510. Wan, C.X., Zhou, Q.C., Fu, G.M., Li, Y.B., 2011. Semi-continuous anaerobic codigestion of thickened waste activated sludge and fat, oil and grease. Waste Manage. 31, 1752–1758. Zhang, Y., Banks, C.J., Heaven, S., 2012. Co-digestion of source segregated domestic food waste to improve process stability. Bioresour. Technol. 114, 168–178. Zhang, L., Lee, Y.W., Jahng, D., 2011. Anaerobic co-digestion of food waste and piggery wastewater: focusing on the role of trace elements. Bioresour. Technol. 102, 5048–5059. Zhang, R.H., El-Mashad, H.M., Hartman, K., Wang, F., Liu, G., Choate, C., Gamble, P., 2007. Characterization of food waste as feedstock for anaerobic digestion. Bioresour. Technol. 98 (4), 929–935. Zhu, N., 2007. Effect of low initial C/N ratio on aerobic composting of swine manure with rice straw. Bioresour. Technol. 98, 9–13.
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
The anaerobic co-digestion of food waste and cattle manure Cunsheng Zhang, Gang Xiao, Liyu Peng, Haijia Su ⇑, Tianwei Tan Beijing Key Laboratory of Bioprocess, Beijing University of Chemical Technology, Beijing 100029, PR China
h i g h l i g h t s " The separate anaerobic digestion of food waste or cattle manure was hardly feasible. " Co-digestion produced significant quantities of biogas, with high methane content. " Addition of cattle manure enhanced the buffer capacity of anaerobic system. " In co-digestion, the C/N ratio contributed to the improving biogas production.
a r t i c l e
i n f o
Article history: Available online 5 November 2012 Keywords: Food waste Cattle manure Methane Co-digestion Buffer capacity
a b s t r a c t This study assessed the anaerobic co-digestion of food waste and cattle manure, in order to identify the key parameters that determine the biogas and methane yield. Results of both batch and semi-continuous tests indicated that the total methane production is enhanced in co-digestion, with an optimum food waste (FM) to cattle manure (CM) ratio of 2. At this ratio, the total methane production in batch tests was enhanced by 41.1%, and the corresponding methane yield was 388 mL/g-VS. In the semi-continuous mode, the total methane production in co-digestion, at the organic loading rate (OLR) of 10 g-VSFW/L/d, increased by 55.2%, corresponding to the methane yield of 317 mL/g-VS. Addition of cattle manure enhanced the buffer capacity (created by NHþ 4 and VFAs), allowing high organic load without pH control. The C/N ratio and the higher biodegradation of lipids might be the main reasons for the biogas production improvement. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Anaerobic digestion has been proven to be an efficient and green technology in disposing of sewage sludge, crop residues, food waste and animal manure (Wan et al., 2011; Li et al., 2009). Advantages are the production of renewable energy in the form of biogas and the possibility to recycle valuable nutrients, concentrated in the digestion residue (Zhang et al., 2012; Angelidaki et al., 2003). Food waste has already been considered as a very attractive feedstock for anaerobic digestion due to its high methane potential (Zhang et al., 2011). Li et al. (2010) reported that the fat content of food waste is about 23%. Under specific operating conditions, lipidrich waste such as fat and oil will significantly contribute to the methane production (Wan et al., 2011). However, long-chain fatty acids (LCFAs) are formed during the degradation of fat and lipids: the 18-C LCFAs (such as oleic and stearic acid) are inhibitory at concentrations exceeding 1.0 g/L (Appels et al., 2008). LCFAs can moreover be toxic to both syntrophic acetogens and methanogens (Hanaki et al., 1981) and limit the transport of nutrients to cells ⇑ Corresponding author. Tel.: +86 10 644452756; fax: +86 10 64414268. E-mail address: [email protected] (H. Su). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.10.138
due to being adsorbed on the microbial surfaces (Pereira et al., 2005). It was therefore difficult if not impossible to treat only FW by anaerobic digestion (Resch et al., 2011; Zhang et al., 2011; Palatsi et al., 2011). Anaerobic digestion was also found to be unstable when the cattle manure is used as mono-substrate due to the low C/N ratio (5–8) (Li et al., 2009). It is therefore important to examine an alternative approach for anaerobic digestion of FW or CM, co-digestion possibly helping to overcome the deficiencies of mono-digestion. This co-digestion is studied in the present research. The carbon to nitrogen (C/N) ratio is one of the important parameters influencing the digestion process (Kumar et al., 2010). Zhu (2007) suggested that anaerobic digestion could be carried out efficiently when the C/N ratio is 15. Kumar et al. (2010) found that a C/N ratio range from 13.9 to 19.6 is acceptable for digestion. Anaerobic co-digestion of different organic materials may enhance the stability of the anaerobic process because of a better carbon to nitrogen (C/N) balance (El-Mashad and Zhang, 2010; Mshandete et al., 2004). Co-digestion may moreover alleviate the inhibitory effect of high ammonia and sulfide concentrations (Hartmann et al., 2003), and exhibit a more stable biogas production
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due to an improved buffer capacity (Nayono et al., 2010). Although the increasing biogas production through anaerobic co-digestion of food waste and cattle manure has been reported, no study has been carried out to address the C/N ratio, the buffer capacity and the removal of LCFAs in co-digestion of food waste and cattle manure. The aim of this study is therefore threefold: (i) assess the performance of anaerobic co-digestion of food waste and cattle manure; (ii) identify the key parameters influencing the increase of biogas and methane yield, including the effect of the C/N ratio; and (iii) examine the extent of the buffer capacity and the removal of LCFAs.
was digested alone at the load of 8 g-VS/L, as the control group. Cattle manure was digested as mono-substrate in digestion R5, R6 and R7 with the amounts of 4.0, 2.7 and 2.0 g-VS/L, respectively. Thus, to determine the performance of co-digestion, the codigestion of R1, R2 and R3 was compared with mono-digestion groups of R4–R5 (group 1), R4–R6 (group 2) and R4–R7 (group 3), respectively. To evaluate the performance of co-digestion, anaerobic co-digestion was further carried out at high organic loads of 24 and 32 g-VSFW/L, with the optimum FW/CM ratio determined from preliminary experimental results. The control group was also designed for comparison of co-digestion and mono-digestion.
Cattle manure was collected at a farm in Zaozhuang, Shandong province. Food waste was provided by the mess of the Beijing University of Chemical Technology. The organic substrates of FW were selected manually and ground into small particles (<3 mm) by a mill (SS3300, Waste King in USA). Both cattle manure and food waste were stored in the freezer before digestion. The properties of the food waste and cattle manure are shown in Table 1, and compared with literature data.
2.2.2. Anaerobic digestion in semi-continuous mode The digester, the effective volume of each digester and the temperature in semi-continuous tests were the same as during the batch tests. The FW/CM ratio in co-digestion was 2, determined as optimum ratio from the preliminary results in batch operation. The experimental conditions in semi-continuous tests are shown in Table 3. The OLR for co-digestion were 8, 10 and 12 g-VSFW/L/d, respectively, along with 4, 5 and 6 g-VSCM/L/d added to each digester. The mono-digestion at the same OLR of food waste/cattle manure was performed to compare its performance with co-digestion. Before each discharge and after feeding, the activated sludge in digesters was fully mixed. The samples of effluent of each digester were taken for pH and VFAs measurements.
2.2. Anaerobic digestion tests
2.3. Biogas and VFAs measurements
2.2.1. Batch tests Batch digestion tests were carried out in 1-L glass digesters at mesophilic temperature (35 ± 1 °C). The effective volume of each digester was 0.8 L. In each digester, anaerobically treated activated sludge was used as inoculum. It had been acclimated for 14 months in a 20-L tank in the laboratory. The TS and VS/TS of inoculum were 3.2% (wet basis) and 46.9%, respectively. The inoculum and substrates were fully mixed before being added to the digesters. Each digester was flushed for 5 min (300 mL/min) with inert gas (N2) to create an anaerobic environment. Food waste, cattle manure, and their mixtures were separately examined, in mono-digestion or co-digestion respectively. The characteristics of the different experiments are shown in Table 2. In co-digestion, the amount of food waste in each digester was kept constant (8 g-VS/L), while varying the amount of cattle manure added. The FW/CM ratios (based on VS) of digestion R1, R2, R3 were designed as 2, 3 and 4, respectively, corresponding to the cattle manure amounts of 4.0, 2.7 and 2.0 g-VS/L. In digestion R4, food waste
Biogas was collected by water displacement method. The biogas volume was calculated daily and was transformed into the volume at STP condition. Biogas samples were examined by gas chromatography (GC-2014C, Shimadzu in Japan) to determine the CH4 and CO2 content. A thermal conductivity detector (TCD) and a stainless steel column of TDX-01 (packed with carbon molecular sieve, 2 m 3 mm) were used in the GC for biogas measurements. The temperatures of injection port, column and TCD were 160 °C, 160 °C and 180 °C, respectively. Argon was used as carrier gas with a pressure of 0.3 MPa and a flow rate of 25 mL/min. The VFAs in the effluent were also measured by GC, using a hydrogen flame ionization detector (FID) and a capillary column (DB-WAX, 30 m 0.32 mm, Agilent). The temperatures of the injection port and FID were both 250 °C. The temperature programming was: 95 °C during 2 min; increase to 210 °C at a rate of 30 °C/min; keep at 210 °C for 2 min; increase to 230 °C at a rate of 30 °C/min; and maintain at 230 °C during 3.5 min. Nitrogen was used as carrier gas at a pressure of 0.4 MPa. The flow rates of
2. Methods 2.1. Collection of substrates and inoculums
Table 1 Properties of food waste and cattle manure (mean value ± standard deviation). Parameter
pH TS (wt.%) VS (wt.%) VS/TS ratio Carbon, C (%, d.b.) Nitrogen, N (%, d.b.) C/N ratio Fat (%, d.b.) Na+ (%, d.b.) K+ (%, d.b.) Mg2+ (%, d.b.) Ca2+ (%, d.b.) Fe3+ (ppm) Mn2+ (ppm) Zn2+ (ppm)
Food waste
Cattle manure
Zhang et al. (2007)
This study
Li et al. (2009)
This study
– 30.9 ± 0.1 26.4 ± 0.1 0.85 46.78 ± 1.15 3.16 ± 0.22 14.8 – – 0.9 ± 0.11 0.14 ± 0.01 2.16 ± 0.29 766 ± 402 60 ± 30 76 ± 22
5.2 ± 0.3 18.5 ± 0.1 17.0 ± 0.1 0.92 46.5 ± 1.5 2.2 ± 0.3 21.1 22.8 3.45 ± 0.20 2.30 ± 0.04 0.16 ± 0.01 0.03 ± 0.01 100 ± 23 110 ± 95 160 ± 30
– 17.1 ± 0.6 14.4 ± 0.2 0.84 28 ± 2.0 4.8 ± 0.5 5.8 – – – – – – – –
9.2 ± 0.3 16.3 ± 0.1 13.2 ± 0.1 0.81 26.7 ± 1.8 5.1 ± 0.6 5.2 – 1.44 ± 0.1 1.27 ± 0.1 4.99 ± 0.4 2.27 ± 0.3 150 ± 30 950 ± 232 250 ± 42
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Table 2 Experimental conditions and results in batch tests. Item Unit
Digestion type –
FW g-VS/L
CM g-VS/L
FW/CM –
C/N –
Biogas production mL/g-VS
Methane yield mL/g-VS
Total methane mL
Initial pH –
Final pH –
R1 R2 R3 R4 R5 R6 R7
Co-digestion Co-digestion Co-digestion Mono-digestion Mono-digestion Mono-digestion Mono-digestion
8.0 8.0 8.0 8.0 0 0 0
4.0 2.7 2 0 4.0 2.7 2
2 3 4 – – – –
15.8 17.1 17.9 21.1 5.2 5.2 5.2
570 526 537 621 19 28 38
388 352 343 410 5 6 9
3725 3003 2744 2624 16 13 14
7.5 7.7 7.4 7.5 7.6 7.6 7.5
7.5 7.4 7.4 7.3 7.9 7.8 7.8
Table 3 Experimental conditions and results in semi-continuous tests. Item Unit
FW g-VS/L/d
CW g-VS/L/d
Methane yield mL/g-VS/d
Methane content %
Total methane L
Initial pH –
Final pH –
NHþ 4 mg/L
R8 R9 R10 R11 R12 R13 R14 R15 R16
8 10 12 8 10 12 0 0 0
0 0 0 4 5 6 4 5 6
347 277 96 388 317 139 69 60 55
61.2 58.0 35.1 62.3 60.2 39.7 33.5 32.9 32.7
33.3 33.2 5.5 55.9 57.1 14 3.3 3.6 4.0
7.5 7.6 7.6 7.4 7.7 7.6 7.4 7.4 7.4
7.2 7.0 4.0 7.3 7.1 4.3 7.6 7.6 7.7
487 512 471 630 677 632 937 1213 1570
nitrogen, hydrogen and air were 50 mL/min, 30 mL/min and 500 mL/min, respectively. 2.4. Chemical analysis The pH, TS and VS of food waste and cattle manure samples were measured according to the standard methods (APHA, 1998). Total carbon was determined by a TOC analyzer (TOC-V, Shimadzu in Japan). Total nitrogen was determined by Kjeltec Nitrogen Analyzer. Fat was measured by Soxhlet extraction method. The metal elements of food waste and cattle manure including K, Na Mg, Ca, Fe, Mn and Zn were analyzed before anaerobic digestion by AAS (atomic absorption spectrophotometer, VarianSpectrAA55-B, Palo Alto, USA). 3. Results and discussion 3.1. Anaerobic digestion in batch tests 3.1.1. Mono- and co-digestion of food waste and cattle manure The total biogas production and the corresponding CH4 and CO2 contents are shown in Fig. 1. Fig. 1A shows that the total biogas production increased gradually in the digestion from R1 to R4, and 90% of the final biogas production was produced in the initial 10 days. After 18 days of digestion, the total biogas production of R1, R2, R3 and R4 were 5472, 4488, 4300 and 3974 mL, respectively. Lower methane production was obtained in monodigestions R5 to R7. Compared with mono-digestion of group 1 (R4–R5), group 2 (R4–R6) and group 3 (R4–R7), the total biogas production in the co-digestion of R1, R2 and R3 were all enhanced. The highest biogas production was obtained in digestion R1 where the FW/CM ratio is 2 (Table 2), accepted as the optimum FW/CM ratio for co-digestion of food waste and cattle manure. Fig. 1B and C show the productions of CH4 and CO2 in batch tests. The CH4 content increased sharply during the initial 2 days. The CH4 content of biogas increased to 65% in co-digestion of R2–R5 until day 2. Thereafter, it increased gradually in the following days. The highest CH4 content was 79% in co-digestion of R2. By contrast, a higher CO2 content was obtained at the initial of digestion. The
CO2 content decreased gradually along with the increase of CH4 content. The lowest CO2 content could reach a level of 21% in batch tests. Table 2 shows the results obtained from batch tests. The methane yields of R1 to R4 were 388, 352, 343 and 410 mL/g-VS, respectively. Although the methane yield of co-digestion was lower than that obtained in mono-digestion, the total methane produced in co-digestion was obviously higher. The total methane production of R1–R4 (Table 2) were 3725, 3003, 2744 and 2624 mL, respectively. The methane production in R1, R2 and R3 was enhanced by 41.1%, 13.9% and 4.0%, respectively. This indicated that codigestion of food waste and cattle manure is an effective approach for methane improvement. Anaerobic co-digestion of two or more substrates to increase biogas production/methane yield has been reported previously. Addition of press water and food waste in bio-waste digester not only increased biogas production rates but also improved the total biogas production (Nayono et al., 2010). A higher methane production of 603 mLCH4/g-VS was obtained in the co-digestion of a mixture of 70% manure, 20% food waste and 10% sewage sludge at the OLR of 1.2 g-VS/L day (Marañón et al., 2012). Table 2 shows the FW/CM and C/N ratios of feedstock in batch digestion. The C/N ratio of feedstock in R1 was 15.8 where the FW/CM ratio is 2. At this C/N ratio, the maximum biogas production and methane yield were obtained. The results indicated that the optimum C/N ratio was 15.8, in-line with previous findings of C/N = 15 (Huang et al., 2004), or 13.9–19.6 (Kumar et al., 2010). The C/N ratio of FW and CM were 21.1 and 5.2 (Table 2), respectively, and hence obviously and individually either higher or lower than the recommended optimum. The correct C/N ratio of the feedstock is important towards optimizing the biogas and methane production. Table 1 shows the metal elements of food waste and cattle manure. The sodium and potassium content in food waste and cattle manure, on dry base, were 3.45%, 2.30% and 1.44%, 1.27%, respectively. McCarthy and McKinney (1961) reported the inhibition concentration of sodium and potassium was at 10 g/L when acetate is used as pure substrate. Based on the data in Table 1, the total sodium and potassium concentration at the OLR of 8 g-VSFW/L/d,
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A
R1 R4
6000
Total biogas production (mL)
anaerobic system. Many enzymes and co-enzymes are in need of a minimal amount of certain trace element for their activation and activity (Appels et al., 2008). The trace elements in cattle manure may have played an important role in enhancing methanogen activity in anaerobic digestion. Co-digestion of food waste with piggery wastewater, Zhang et al. (2011) also found that the trace element is the reason for improvement of co-digestion performance.
7000
R2 R5
R3 R6
R7
5000
4000
3000
2000
1000
0 0
5
10
15
20
25
30
Time (days)
B
90
R1
R2
R3
R4
80
CH4 (%)
70
3.1.2. Stability of co-digestion Fig. 2 shows the variations of methane yield and pH at high organic loads of 24 g-VSFW/L and 32 g-VSFW/L. Fig. 2A indicates that anaerobic co-digestion of food waste and cattle manure could proceed at high organic loads. However, digestion failure was observed in mono-digestion of food waste at the two selected organic loads. Fig. 2B shows that the pH value in both monodigestion and co-digestion decreased gradually at day 1. By contrast, the pH value of mono-digestion further decreased in the following days while, pH recovery was observed in co-digestion at day 2. The pH values of food waste and cattle manure were 5.2 and 9.2 (Table 1), respectively. The alkaline components of cattle manure might have neutralized the pH, and thus enhanced the buffer capacity and stability of anaerobic system, allowing high organic load of 32 g-VSFW/L/d, corresponding to the total organic load of 48 g-VS/L in batch digestion without specific pH control.
60
50
A
600
40
30 0
5
10
15
20
25
30
Time (days)
C
80
R1
70
R2
R3
R4
Methane yield (ml/gVS)
500
CO2 (%)
60
400
300
200
24 g-VSFW/L/d + 12 g-VSCM/L/d 24 g-VSFW/L/d 32 g-VSFW/L/d + 16 g-VSCM/L/d
100
32 g-VSFW/L/d
50 0 0
5
10
40
15
20
25
30
Time (days)
B
30
20
8.4 7.8 7.2
10 0
5
10
15
20
25
30
Time (days)
pH
Fig. 1. Total biogas production (A), CH4 content (B) and CO2 content (C) in batch tests (R1: 8 g-VSFW/L + 4 g-VSCM/L; R2: 8 g-VSFW/L + 2.7 g-VSCM/L; R3: 8 g-VSFW/ L + 2 g-VSCM/L; R4: 8 g-VSFW/L; R5: 4 g-VSCM/L; R6: 2.7 g-VSCM/L; R7: 2 g-VSCM/L).
6.6 6.0
24 g-VSFW/L/d + 12 g-VSCM/L/d
5.4
24 g-VSFW/L/d 32 g-VSFW/L/d + 16 g-VSCM/L/d
4.8
with the C/N ratio of 2, were 0.33 g/L and 0.23 g/L, respectively. The sodium and potassium concentration was far lower than the inhibition concentration. Moreover, the concentrations of Mg, Ca, Mn, Zn in cattle manure were 4.99%, 2.27%, 950 ppm, 250 ppm, respectively. Compared with cattle manure, the concentrations of trace elements were obviously lower in food waste. Addition of cattle manure enhanced the trace elements concentration in
32 g-VSFW/L/d 4.2
0
5
10
15
20
25
30
Time (days) Fig. 2. Variations of methane yield (A) and pH (B) with time at high organic loads (24 and 32 g-VSFW/L/d).
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3.2.2. VFAs and ammonia Fig. 4 shows the variation of total VFAs during the semi-continuous process. The VFAs concentration of R8–R13 increased steadily during the initial 3 days. Thereafter, the total VFAs concentration increased continuously at the OLR of 12 g-VSFW/L/d (R10, R13), reaching 15.2 g/L and 13.5 g/L, respectively, at day 8. VFAs accumulated due to the overloading of the system. A lower pH value (Table 2) as well as the inhibition on biogas production (Fig. 3) was observed due to VFAs accumulation at the OLR of 12 g-VSFW/L/d. By contrast, the VFAs concentration maintained at a stable level at the OLR of 8 and 10 g-VSFW/L/d (R8, R9, R11, R12), e.g., at the OLR of 10 g-VSFW/L/d, the VFAs concentration was in the range from 6 to 7 g/L in co-digestion. Compared with mono-digestion, the VFAs concentration in co-digestion was higher, indicating that more VFAs were generated in co-digestion system. However, a higher final pH value (Table 2) was measured in the co-digestion system compared with mono-digestion. This result again shows a higher stability of the co-digestion system due to the increased buffer capacity, indicating that the addition of cattle manure to food waste could stabilize anaerobic system. Ammonium ions or ammonia were produced from proteins and amino acids during anaerobic digestion. The ammonia concentration during semi-continuous digestion is shown in Table 3. The ammonia concentrations at the OLR of 8, 10 and 12 g-VS/L/d in mono-digestion were 487, 512 and 471 mg/L, respectively. The corresponding ammonia concentrations were 630, 677 and 632 mg/L in co-digestion. Compared with mono-digestion, the ammonia concentration in co-digestion effluent was obviously higher. An ammonia concentration of 700 mg/L was considered to be an inhibitory concentration for methanogenic activity (Lay et al., 1998). According to the ammonia concentration in Table 3, no ammonia inhibition occurred in co-digestion. However, the ammonia concentration in
3.2. Anaerobic digestion in semi-continuous mode 3.2.1. Methane yield Fig. 3 shows the response surface and contour plot of daily methane yield of mixture, food waste and cattle manure in semicontinuous mode. Fig. 3A and B indicate that anaerobic co-digestion could proceed at the OLR of 15 g-VS/L/d. The average methane yield, at the OLR of 15 g-VS/L/d, was 317 mL/g-VS (Table 3). The daily methane yield decreased rapidly with the increase of OLR, e.g., at the OLR of 18 g-VS/L/d, the daily methane yield decreased to 67 mL/g-VS at day 7. This result indicated that the maximum OLR for anaerobic co-digestion is 15 g-VS/L/d in semi-continuous mode. Fig. 3C and D indicate that the maximum OLR for food waste is 10 gVS/L/d, corresponding to the average methane yield of 277 mL/g-VS (Table 3). Anaerobic digestion failed at the OLR of 12 g-VS/L/d. Fig. 3E and F illustrate that the daily methane yield varied from 113 mL/gVS to 50 mL/g-VS when cattle manure was digested at the OLR from 3 g-VS/L/d to 6 g-VS/L/d. Compared with co-digestion, both the methane yield and the maximum OLR of mono-digestion were lower, indicating that anaerobic co-digestion of food waste and cattle manure could significantly improve the maximum OLR and the methane yield. Table 3 shows the average methane yield of mono-digestion and co-digestion. The methane yield of mono-digestion R8 (8 g-VSFW/L/d) and R9 (10 g-VSFW/L/d) were 347 and 277 mL/g-VS, respectively. With the same OLR of food waste (Table 3), the corresponding methane yield of co-digestion were 388 (R11) and 317 mL/g-VS (R12), respectively. The result indicated that addition of cattle manure could increase the methane yield in semi-continuous mode. The total methane production in co-digestion increased by 52.7% and 55.2%, respectively, corresponding to the OLR of 8 and 10 g-VSFW/L/d.
C Methane yield (mL/g-VS)
Methane yield (mL/g-VS)
470 390 310 230 150 70 -10
E
430
320
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100
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Methane yield (g-VS/L/d) 6 69
63
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Methane yield (mL/g-VS) 12
67
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Fig. 3. Response surface and contour plot of daily methane yield from mixture (A and B), food waste (C and D) and cattle manure (E and F).
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22.8% (Table 1). Lot of fat may have lost due to the formation of the agglomerated lipid particle in mono-digestion. By contrast, no agglomerated particles were observed in the effluent of co-digestion, tentatively explained by the dilution of the FW lipids when CM was added: more lipids could be digested by Syntrophomonadaceae and Clostridiaceae families due to the dilution (Hatamoto et al., 2007), increasing the total biogas production. The lipid-rich waste such as oil could be diluted in co-digestion has been reported previously. Angelidaki and Ahring (1997) reported that oil mill effluent needed to be diluted before it was added to anaerobic digesters because of the rapid decrease of pH in acidification. By contrast, by co-digestion with animal manure, oil mill effluent could be digested without preliminary dilution. The higher biodegradability of lipids has been recognized as a main reason for the increased biogas production.
18
R8 R11
R9 R12
R10 R13
Total VFAs (g/L)
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6
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4. Conclusions
Fig. 4. Variation of total VFA concentration with time in the semi-continuous process (R8: 8 g-VSFW/L/d; R9: 10 g-VSFW/L/d; R10: 12 g-VSFW/L/d; R11: 8 g-VSFW/L/ d + 4 g-VSCM/L/d; R12: 10 g-VSFW/L/d + 5 g-VSCM/L/d; R13: 12 g-VSFW/L/d + 6 g-VSCM/L/d).
R14, R15 and R16 was beyond 700 mg/L, meaning the inhibition from ammonia might have occurred when cattle manure was digested alone. Tables 2 and 3 illustrate that the final pH value of effluent in co-digestion was higher than the value in mono-digestion. These results indicated that co-digestion of food waste and cattle manure improved the buffer capacity of anaerobic system. In addition, the total organic nitrogen of FW and CM were 2.2% and 5.1% (Table 1), respectively. Addition of cattle manure could increase the total organic nitrogen in co-digestion system, resulting in the higher concentration of ammonia in co-digestion system. The higher ammonia neutralized the VFAs in acidification. The ammonia and VFAs may have ionized in liquid phase. The reaction can be defined as follows:
CxHyCOOH CxHyCOO þ Hþ
ð1Þ
NH3 H2 O NHþ4 þ OH
ð2Þ
where CxHyCOOH presents the VFAs. Combine Eqs. (1) and (2), Eq. (3) was obtained
CxHyCOOH þ NH3 H2 O ! CxHyCOO þ NHþ4 þ H2 O
ð3Þ
More VFAs will be neutralized at higher concentration of ammonia. Thus, the buffer system formed in co-digestion, allowing high concentration of VFAs without pH decrease. Therefore, the higher ammonia concentration may be the reason for buffer capacity improvement in co-digestion. Co-substrate could enhance buffer capacity has been reported by the previous report. Nayono et al. (2010) found that addition of press water or food waste to biowaste digester lead to high buffer capacity. Digestion could be carried out at very high loadings without pH control. 3.2.3. Characteristic of effluent During the mono-digestion of food waste, yellow and white particles, of size from 1 to 5 mm, were observed in the effluent. Liu et al. (2011) reported the particles to be agglomerated by calcium salts of long chain fatty acids (LCFAs). According to Appels et al. (2008), LCFAs are well known to be inhibitory at low concentrations. The inhibition from LCFAs could be alleviated in monodigestion when the agglomerated particles formed. Nevertheless, lipid-rich waste such as fat and oil are considered to be most attractive organic source for biogas production due to its high methane potential. In theory, the methane potential of lipids is 1014 mL/g-VS (Wan et al., 2011). The fat content of food waste is
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