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Production of methane by co-digestion of cassava pulp with various concentrations of pig manure
biomass and bioenergy 34 (2010) 1117–1124
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http://www.elsevier.com/locate/biombioe
Production of methane by co-digestion of cassava pulp with various concentrations of pig manure Pornpan Panichnumsin a,b, Annop Nopharatana c, Birgitte Ahring d, Pawinee Chaiprasert e,* a
The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, Thungkru, Bangkok 10140, Thailand b Excellent Center of Waste Utilization and Management, National Center for Genetic Engineering and Biotechnology, Bangkhuntien, Bangkok 10150, Thailand c Pilot Plant Development and Training Institute, King Mongkut’s University of Technology Thonburi, Bangkhuntien, Bangkok 10150, Thailand d AAU, Copenhagen Institute of Technology, Lautrupvang 15, 2750 Ballerup, Denmark e School of Bioresources and Technology, King Mongkut’s University of Technology Thonburi, Bangkhuntien, Bangkok 10150, Thailand
article info
abstract
Article history:
Cassava pulp is a major by-product produced in a cassava starch factory, containing
Received 18 March 2009
50–60% of starch (dry basis). Therefore, in this study we are considering its potential as
Received in revised form
a raw material substrate for the production of methane. To ensure sufficient amounts of
19 February 2010
nutrients for the anaerobic digestion process, the potential of co-digestion of cassava pulp
Accepted 26 February 2010
(CP) with pig manure (PM) was further examined. The effect of the co-substrate mixture
Available online 31 March 2010
ratio was carried out in a semi-continuously fed stirred tank reactor (CSTR) operated under mesophilic condition (37 C) and at a constant OLR of 3.5 kg VS m3 d1 and a HRT of 15
Keywords:
days. The results showed that co-digestion resulted in higher methane production and
Anaerobic digestion
reduction of volatile solids (VS) but lower buffering capacity. Compared to the digestion of
Cassava pulp
PM alone, the specific methane yield increased 41% higher when co-digested with CP in
C:N ratio
concentrations up to 60% of the incoming VS. This was probably due to an increase in
Pig manure
available easily degradable carbohydrates as the CP ratio in feedstock increased. The
Manihot esculenta Crantz
highest methane yield and VS removal of 306 mL g1 VSadded and 61%, respectively, were
Methane potential
achieved with good process stability (VFA:Alkalinity ratio < 0.1) when CP accounted for 60% of the feedstock VS. A further increase of CP of the feedstock led to a decrease in methane yield and solid reductions. This appeared to be caused by an extremely high C:N ratio of the feedstock resulting in a deficiency of ammonium nitrogen for microbial growth and buffering capacity. ª 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
During starch production from cassava (Manihot esculenta Crantz), approximately 5.2 Mt of fresh cassava pulp residue is generated annually in Thailand [1]. At present, some of the residue has application as a low cost animal feed (60 US$ t1
dry weight) and some of them has become a major solid waste problem with contamination of soil and ground water [1]. Since cassava pulp comprises 50–60% starch in dry matter and 60–70% moisture content [2], it has a major potential as raw material for biogas production. Therefore, utilizing this waste effectively could contribute as a valuable source of future
* Corresponding author. Tel.: þ66 2 470 7525; fax: þ66 2 452 3455. E-mail address: [email protected] (P. Chaiprasert). 0961-9534/$ – see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2010.02.018
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energy and could minimize environmental management costs. However, the low concentrations of nutrients such as nitrogen and the low buffering capacity of this waste is a limitation in the conversion to methane. Similar to fruit and vegetable wastes (FVW), using cassava pulp alone might quickly result in the failure of the process due to the rise of volatile fatty acid (VFA). The problem could then be worsen as a result of poor buffering capacity and a drastic drop of pH which consequently inhibits the methanogenesis step [3,4]. Anaerobic co-digestion is a promising technology widely applied to many waste treatments, especially pig and cattle manure. Manure is an excellent co-substrate due to its high buffering capacity and is rich in a wide variety of nutrients necessary for optimum bacterial growth [5]. Thus, co-digestion with manure would give the balance of nutrients, at an appropriate C:N ratio and a stable pH needed to increase methane production [6]. The co-digestion technology of manure has been successfully applied with food industrial waste [7], potato tuber plus its industrial by-products [8] and energy crops and crop residues [9]. Thailand has an extensive pig industry with 8 million heads produced per year with an approximate amount of 2.16 Mt of excreta [10]. Even though there is a well established use of anaerobic digestion, the typical methane yield is low and is associated with the high nitrogen and lignocellulosic content [11]. Co-digestion of pig manure with cassava pulp could offer an efficient solution to both substrates. Here we study the effect of different mixture ratios on the methane production potential and the stability of the CSTR.
2.
Materials and methods
2.1.
Feedstocks and inoculum
Approximately 200 kg of fresh cassava pulp (CP) was collected directly from a pulp pressing machine of a cassava starch factory in Rayong Province [12 400 4800 N, 101 160 4800 E], Thailand. Approximately 100 kg of pig manure (PM) was taken directly from excretion of pig fatteners in a medium-scale farm in Nakornprathom Province [13 490 1100 N, 100 30 5700 E], Thailand. The CP and PM were kept in ice boxes and transported from origins within 1–2 hours. Then, they were aliquot into 1-kg zip-lock bags and stored at 20 C until used. The characteristics of the feedstocks are shown in Table 1. Partially digested pig manure taken from an industrial anaerobic plug flow reactor was used as the starting inoculum. Its pH value was 7.5 and contained 31 g L1 of volatile solid (VS) and 53 g L1 of total solid (TS) with an estimated VS:TS ratio of 0.59. The inoculum was kept at 37 C until the major part of the remaining organic compound had been digested prior to usage.
2.2.
Methane potential assay
The ultimate methane productions of CP, PM and five mixtures were determined in triplicate using 120-mL vials with 45 mL of working volume. CP, PM and five mixtures in different CP:PM ratios of 20:80, 40:60, 50:50, 60:40 and 80:20 were applied based on VS (w/w). The inoculum-substrate ratio
Table 1 – Chemical and elemental compositions of fresh cassava pulp and pig manure (values are the mean ± S.D. of 3 determinations). Composition Moisture (% of fresh waste) TS (g kg1 fresh waste) VS (g kg1 TS) Ash (g kg1 TS) COD (g kg1 VS) Starch (g kg1 VS) Crude fiber (g kg1 VS) Cellulose (g kg1 VS) Hemi-cellulose (g kg1 VS) Lignin (g kg1 VS) TKN (g kg1 TS) 1 TS) NHþ 4 -N (g kg Protein (g kg1 VS) Lipid (g kg1 VS) VFA (g COD kg1 VS) C (% of TS) H (% of TS) O (% of TS) N (% of TS) S (% of TS) C:N ratio
Cassava pulp
Pig manure
69.37 0.23 305 2 984 1 16 1 1050 5 655 5 320 20 165 8 116 5 40 5 1.73 0.01 N.D.b 10.8 0.3 10.1 1.0 2.1 0.3 40.02 7.52 37.51 0.19 N.D. 210
74.7 0.44 253 4 716 10 284 10 1400 200 N.A.a 210 20 108 15 54 7 83 18 44.23 0.8 6.29 0.5 276.8 4.5 123.6 6.2 67.3 5.1 35.42 5.43 26.07 2.57 0.08 14
a N.A. ¼ no analysis. b N.D. ¼ not detectable.
in a total of 20 mL was supplemented with 15 mL inoculum and 10 mL distilled water. When CP was used as a sole substrate, pH in the vials was maintained between 7.0 and 7.2 by an addition of 10 g L1 of sodium bicarbonate. The vials were flushed with a gas mixture of 70% N2 and 30% CO2 before being sealed off with rubber stoppers and aluminum crimps. The biogas composition and production were monitored throughout the 120 days of incubation at 37 C. The control set containing only inoculum was used as a baseline comparison of methane production.
2.3. Semi-continuous single stage digestion of different mixtures Single-stage anaerobic co-digestion of CP and PM was carried out in a 5 L semi-continuously stirred glass reactor with 3 L working volume. The reactor was operated at a constant organic loading rate (OLR) of 3.5 kg VS m3 d1 and a hydraulic retention time (HRT) of 15 days. The reactor was operated with a withdraw/feed method once a day and mechanically stirred at 1.67 Hz by an electric motor for 15 minutes each half hour. The reactor temperature was maintained at 37 C by circulating water through a water jacket by a temperaturecontrolled water bath. Daily biogas production was measured by a liquid displacement measurement system connected to the reactor, logging the gas production automatically. The CSTR was started up using PM as a sole substrate. After the reactor reached a steady state at the designed OLR and HRT, characterized by stable pH, gas production and composition, soluble COD concentration, for 2–3 cycles of HRT, it was fed with various mixture ratios of CP and PM instead. The
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CP:PM ratios of 20:80, 40:60, 50:50, 60:40, 80:20 and 100:0 were consecutively applied based on VS (w/w). The stocks of CP and PM were stored at 20 C in 1-kg zip-lock bags until used. The feedstocks were prepared every 2–3 days by diluting the CP and PM with tap water, such that their volatile solid content was brought to 5.25%, and stored at 4 C. The characteristics of substrates are shown in Table 2. The technical compositions of influent and effluent were determined three times a week except pH which was monitored daily. The results from the analysis of each mixture at steady state were used for evaluating the effect of co-digestion on biodegradability and biogas production efficiency as well as process stability.
2.4.
Analysis
Total solid (TS), volatile solid (VS), chemical oxygen demand (COD), pH, total nitrogen (TKN) and ammonium-nitrogen (NH4-N) were analyzed according to the Standard Method [12]. Moisture, crude fiber and ash content were determined as described by AOAC standard methods [13]. Protein content was calculated from TKN using a multiplied factor of 6.25. The remaining starch content was analyzed using glucoamylase method according to AACC [14]. Cellulose, hemicellulose and Klason lignin were determined by a two-step acid hydrolysis according to the procedure published by the NREL [15]. The lipid content was analyzed by Soxhlet with petroleum ether extraction according to AOAC methods [13]. The C, H, O, N and S elemental analysis was performed on ThermoFinnigan Flash EA 1112 elemental analyzer, following the manufacturer’s standard procedures. Alkalinity was measured by titration to pH 4 with 0.1 mol L1 H2SO4. The biogas production was measured using a liquid displacement method [12]. The percentages of methane and carbon dioxide in the biogas were analyzed using gas chromatography (Shimadzu, Class-GC 14B, Japan), using a Porapak-N column equipped with a thermal conductivity detector (TCD). The oven, injector and detector temperatures were 70, 120 and 120 C, respectively. Helium was used as the carrier gas at a flow rate of 30 mL min1. The individual VFA concentrations (acetic acid, propionic acid, butyric/isobutyric acids and valeric/isovaleric acids) were
analyzed using the Shimadzu gas chromatography (Class-GC 14B) equipped with a flame ionization detector (FID) and Carbowax B-DA column. The oven, injector and detector temperatures were 170, 200 and 200 C, respectively. Helium was used as the carrier gas at a flow rate of 50 mL min1, and nitrogen was used as a makeup gas at a flow rate of 50 mL min1.
2.5.
Calculations
Theoretical methane yield (BU) was calculated based on the elemental composition of CP and PM using Eqs. (1) and (2). The biodegradability was calculated by Eq. (3) in which BO is the ultimate methane yield obtained from the methane potential studied in a batch test [16]. Carbohydrate was calculated by Eq. (4) according to Moller et al. [17]. Ca Hb Oc Nd þ 1=4ð4a b 2c þ 3dÞH2 O/1=8ð4a þ b 2c 3dÞCH4 þ 1=8ð4a b þ 2c þ 3dÞCO2 þ dNH3
(1)
BU ¼ ða=2 þ b=8 c=4 3d=8Þ=ð12a þ b þ 16c þ 14dÞ 22:4 mL CH4 g1 VS
(2)
Biodegradability ð%Þ ¼ ðBO =BU Þ 100
(3)
VScarbohydrate ¼ VS VSprotein VSlipid VSVFA
(4)
3.
Results and discussion
3.1.
Feedstocks characteristics
Table 1, about here, shows analyzed compositions of CP and PM. CP contained starch and fiber as major components and small amounts of protein and lipid. The fiber content of CP comprised 50% cellulose, 34% hemi-cellulose and 16% lignin. The major components of PM were protein, lipid, fiber and VFA. The fiber composition of PM comprised 47% cellulose, 17% hemi-cellulose and 36% lignin. The C, H, O content in CP was higher than PM whereas the N content was much lower. The C:N ratios were about 210 in CP and 14 in PM.
Table 2 – Characteristics of feedstocks at different mixture ratio used in CSTR experiment (values are the mean ± S.D. of 5 determinations). CP:PM ratios
TS (%) VS (%) COD-T (g L1) COD-S (g L1) pH VFA (g COD L1) Ammonium N (g L1) TKN (g L1) Total Alkalinity (g CaCO3 L1) C:N ratio a N.D. ¼ not detectable.
0:100
20:80
40:60
50:50
60:40
80:20
100:0
7.2 0.2 5.2 0.2 85.9 3.1 9.1 0.3 7.42 0.3 3.26 0.1 0.18 0.04 2.30 0.08 3.3 0.3 14
6.8 0.1 5.2 0.1 80.5 2.2 7.5 0.3 7.32 0.2 2.63 0.1 0.15 0.03 1.92 0.07 2.7 0.2 17
6.5 0.2 5.2 0.1 75.1 2.4 5.9 0.2 7.25 0.2 1.99 0.05 0.11 0.02 1.48 0.06 2.0 0.2 23
6.2 0.3 5.2 0.1 72.4 1.8 5.2 0.2 7.22 0.2 1.68 0.05 0.09 0.01 1.18 0.05 1.7 0.2 27
6.1 0.2 5.2 0.1 69.7 2.4 4.4 0.2 7.16 0.1 1.37 0.05 0.08 0.01 0.95 0.05 1.3 0.2 33
5.7 0.2 5.2 0.1 64.3 2.7 2.75 0.1 6.88 0.3 0.74 0.05 0.04 0.01 0.54 0.07 0.7 0.1 59
5.3 0.1 5.2 0.1 58.9 1.7 1.16 0.1 4.49 0.3 0.11 0.05 N.D.a 0.10 0.04 0.2 0.1 210
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3.2.
biomass and bioenergy 34 (2010) 1117–1124
Methane potential and biodegradability
The ultimate methane potentials (BO) of CP, PM and five mixtures are shown in Table 3. For a single substrate, the methane production per kg of VS in PM (362 mL g1 VSadded) was higher than that of CP (344 mL g1 VSadded). In contrast, the methane production per wet weight of PM (66 mL g1 ww) was much lower than that of CP (103 mL g1 ww). The theoretical methane yield (BU) of CP and PM were 530 and 541 mL g1 VSadded, respectively. However, the biodegradability of CP and PM was not significantly different. Our results are similar to Moller et al. [17] who reported the BU, BO and biodegradability of PM were 516, 356 mL g1 VSadded and 69%, respectively. The partial methane yield (B15), the yield produced within 15 days, of CP was 23% higher than that of PM. The B15/BU ratios of CP and PM were 51% and 43%, respectively. According to the biodegradability and B15/BU ratio, the proportion of easily and slowly degradable fraction in VS of CP and PM could be estimated. Since the biodegradability of CP was 65%, it might be deduced that VS of CP contains 51% of easily degradable fraction and 14% of slowly degradable fraction. For PM, the biodegradability was 67%, therefore, its VS contains 43% of easily degradable fraction and 24% of slowly degradable fraction. These indicate that the composition of CP was more easily degraded than that of PM. For the co-substrate, the methane yield and biodegradability increased with increasing CP ratio up to 50% compared to PM alone. The highest methane yield and biodegradability of 391 mL g1 VSadded and 73%, respectively, were obtained at CP ratio of 20%. The co-substrate improved specific methane yield by 8% and 14% compared to PM and CP, respectively. The increase in the proportion of CP in the co-substrate increased the partial methane yield by 19% compared to PM alone. This was probably due to the increase of easily degradable fraction (7–9%) in the co-substrates. For CP ratios in the range of 80–100%, although the biodegradability was similar to that of PM alone, the ultimate methane yield was lower. There is generally agreement in literature that the biodegradability and methane potential of a complex substrate are dependent on its composition, for instance, the content of biodegradable carbohydrates, proteins, and lipids [9,18,19]. As shown in Table 1, CP contains almost 100% of carbohydrate while PM contains a wide variety of components, such as protein, lipid, carbohydrate and VFA.
The carbohydrate content in feedstock increased whereas the protein and lipid content decreased when the ratio of CP increased. Theoretically, the methane yield obtains from carbohydrates (415 mL g1 VS) is lower than that of proteins (496 mL g1 VS) and lipids (1014 mL g1 VS) [17].
3.3. Semi-continuous single-stage digestion of different mixtures The CSTR was initially fed with PM at OLR of 3.5 kg VS m3 d1 and 15 days of HRT for 49 days and subsequently fed with the mixtures of CP and PM. The proportion of CP in feedstock sequentially increased up to 100% while maintaining constant OLR and HRT. The summarized values of the parameters monitored during steady state conditions are given in Table 4. The results indicate that the maximum ratio of CP in feedstock achieved the highest methane production at 60%. As can be seen from Table 4 and Fig. 1, the specific biogas and methane yields of PM alone were 316 and 217 mL g1 VSadded while the volumetric biogas and methane productions were 1110 and 740 mL L1 d1 and these increased to the maximum production of 1800 and 1060 mL L1 d1 at 60% CP ratio in feedstock. Correspondingly, the maximum biogas and methane yield achieved 514 and 306 mL g1 VSadded, respectively. In contrast, the methane content slightly decreased from 64% with PM alone to 57% with 60% CP ratio. When the ratio of CP in feedstock increased to 80%, the amounts of TS and VS in the digestates decreased (Fig. 2). Likewise, the removal efficiencies of VS, TS, and COD gradually increased (Table 4). The VS, TS and COD reductions of the reactor fed with PM alone were 46%, 39% and 50%, respectively. The maximum reductions of VS, TS, and COD obtained at CP ratio of 80% were 64%, 55% and 57%, respectively. The results showed that co-digestion improved the efficiencies up to 33% (VS) and 14% (COD) compared to PM alone. When the reactor was fed with 100% CP, the efficiencies of gas production and solids reduction considerably decreased. In comparison with PM alone, the increase in methane yield (41%) and VS removal (33%) obtained in co-digestion with 60% CP ratio of feedstock VS was apparently due to the increased amount of easily degradable compound in the feedstock. The easily degradable fraction contained in these co-substrates was 7-9% higher than that in PM. The increase of the easily degradable fraction resulted from the increase of
Table 3 – Methane yield and biodegradability of CP, PM, and five mixtures (Values are the mean ± S.D. of 3 determinations). CP
PM
CP:PM ratio 20:80
40:60
50:50
60:40
80:20
Ultimate methane yield mL CH4 g1 VS; BO mL CH4 g1 wet weight
344 24 103 7
362 8 66 1
391 3 80 1
383 6 88 1
388 22 93 5
365 16 92 4
357 15 99 4
Partial methane yield (15 day) mL CH4 g1 VS; B15
270 14
232 6
265 8
268 6
277 20
266 12
268 10
Theoretical methane yield mL CH4 g1 VS; BU B15/BU (%) Biodegradability (%)
530 51 65
541 43 67
539 49 73
536 50 71
535 52 73
534 50 68
532 50 67
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Table 4 – Steady-state performance data of single stage CSTR treating PM, CP, and five mixtures. CP:PM ratios 20:80
40:60
50:50
60:40
80:20
100:0
4.4 0.1 2.8 0.1 42.5 2.6 2.1 0.1 7.53 0.13 0.22 0.10 1.46 0.06 4.92 0.55 5.1 0.2 39 2 46 2 50 3 1110 110 316 30 740 60 217 16 64 1 33 1
4.2 0.1 2.6 0.1 39.5 1.4 1.9 0.1 7.50 0.02 0.21 0.02 1.22 0.05 4.75 0.28 4.5 0.2 39 1 49 2 51 2 1230 50 350 15 790 30 227 10 63 1 34 1
3.7 0.2 2.4 0.1 36.2 1.3 1.8 0.2 7.50 0.08 0.22 0.01 0.780 0.03 3.97 0.37 3.8 0.2 43 2 54 1 52 2 1490 50 426 13 930 20 266 10 61 1 37 1
3.2 0.1 2.2 0.1 31.0 1.7 1.5 0.1 7.11 0.10 0.20 0.02 0.65 0.04 3.13 0.32 3.4 0.2 48 2 59 1 57 1 1670 60 478 18 1010 40 290 16 59 1 38 1
3.0 0.1 2.0 0.1 30.0 1.6 1.7 0.2 7.10 0.05 0.24 0.02 0.53 0.04 2.65 0.16 2.0 0.2 51 2 61 1 57 1 1800 90 514 18 1060 30 306 13 57 0 39 1
2.6 0.1 1.9 0.2 27.7 2.4 1.4 0.2 6.73 0.10 0.60 0.20 0.21 0.09 1.89 0.15 1.1 0.2 55 1 64 4 57 4 1620 280 463 81 920 180 263 50 55 1 41 1
3.4 0.3 2.9 0.3 38.6 4.8 1.8 0.6 5.80 0.60 1.10 0.20 0.03 0.02 0.90 0.18 0.7 0.3 36 8 44 6 35 8 730 320 208 91 410 170 118 50 43 8 41 3
carbohydrate fraction especially composed of starch in the feedstock. As shown in Table 1, when the ratio of CP in the feedstock changed from 0% to 100%, the carbohydrate fraction increased from 45% to 94%. In contrast, protein and lipid decreased from 28% to 1% and from 12% to 1%, respectively. As reviewed by Neves et al. [19], carbohydrates are hydrolyzed faster than proteins and lipids under anaerobic conditions.
20:80
40:60
50:50
60:40
80:20
CP
500
1200
400
1000 800
300
600 200 400 100
200
Biogas yield (mL g-1 VSadded)
0
0
700
80
600
70 60
500
50
400
40 300
30
200
20
100
10
0
1
16
31
46
61
76
91 106 121 136 151 166 181 196 211 Time (day)
VS reduction (%)
Methane yield (mL g-1 VSadded)
PM (0:100)
Carbohydrates had higher hydrolysis rate constants (0.5–2.0 d1) than proteins and lipids (0.25–0.8 d1 and 0.1–0.7 d1, respectively). The composition of co-substrates also affects the stability of the digestion process. For anaerobic digestion, CP is considered as a poor substrate in terms of nutrient availability and buffering capacity but it contains high amount of easily
Volumetric methane production (mL L-1reactor d-1)
TS (%) VS (%) COD-T (g L1) COD-S (g L1) pH VFA (g COD L1) Ammonium N (g L1) TKN (g L1) Total Alkalinity (g CaCO3 L1) TS reduction (%) VS reduction (%) COD-T reduction (%) Volumetric biogas production (mL L1 d1) Specific biogas yield (mL g1 VS) Volumetric CH4 production (mL L1d1) Specific CH4 yield (mL g1 VS) CH4 content (%) CO2 content (%)
0:100
0
Fig. 1 – Performance of single-stage CSTR fed with seven CP:PM ratios of 0:100, 20:80, 40:60, 50:50, 60:40, 80:20 and 100:0, respectively. (B) Specific methane yield; (:) Volumetric methane production; (-) Specific biogas yield and (A) VS reduction. Dashed lines represent the changes in the CP:PM ratio.
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biomass and bioenergy 34 (2010) 1117–1124
PM (0:100)
20:80
40:60
50:50
60:40
80:20
CP
5
TS, VS (%)
4 3 2 1 0
2000
8
7 1000
6.5
pH
NH4 -N (mg L-1 )
7.5 1500
6 500 5.5 5
6000
2.5
5000
2
4000 1.5 3000 1
2000
0.5
1000 0
VFA:Alkalinity ratio
-1
VFA, Alkalinity (mg L )
0
0 1
16
31
46
61
76
91
106 121
136 151 166 181 196 211
Time (d) Fig. 2 – Characteristics of digestate of single-stage CSTR fed with seven CP:PM ratios of 0:100, 20:80, 40:60, 50:50, 60:40, 80:20 and 100:0, respectively. (>) TS; (D) VS; (:) Ammonium-nitrogen; (*) pH; (,) VFA; (A) Alkalinity and (6) VFA:Alkalinity ratio. Dashed lines represent the changes in the CP:PM ratio.
degradable matter. Thus, the high ratio of CP in the feedstock associated with an improper C:N ratio could cause co-digestion process imbalance and failure. The results indicate that for stable co-digestion of CP and PM, the maximum ratio of CP in feedstock was 60%. At CP ratio of up to 60%, VFA was rapidly consumed by methanogens. The VFA concentration in the digestate was around 200–240 mg L1 and the soluble COD concentration was 1400 mg L1 while the VFA:Alkalinity ratio was below 0.1 (Table 4 and Fig. 2). The further increase in the CP ratio resulted in the reduction of methane yield and the accumulation of VFA. The first HRT of feeding 80% CP of the feedstock resulted in the best reactor performance with the highest efficiencies of both VS reduction and methane production (Fig. 1). However, gas production and solid reductions significantly deteriorated at the second HRT and as
shown in Table 4, Figs. 1 and 2, the biogas and methane yield decreased from 580 and 330 to 460 and 260 mL g1 VSadded, respectively. The VFA accumulation was observed at the concentration of 620 mg L1 and the VFA:Alkalinity ratio significantly increased to 0.6. The failure of the reactor fed with CP alone was indicated by a sharp drop in both removal efficiency and methane yield. The VS removal and specific methane yield decreased by 20% and 70%, respectively, compared to PM alone. The soluble COD decreased to 1000 mg L1 while the VFA increased to 980 mg L1 and the major VFA accumulated in the reactor was propionate (data not shown). The VFA:Alkalinity ratio gradually increased to 2 which showed significant process instability. Callaghan et al. [6] reported that at a VFA:Alkalinity ratio less than 0.4, the reactor should be considered as stable. However, the VFA level
biomass and bioenergy 34 (2010) 1117–1124
was much lower than the considered inhibitory concentration (4000 mg L1) for anaerobic digestion [20]. The results showed that pH, alkalinity, NH4–N, and TKN decreased with increasing CP ratio which related to the C:N ratio (Table 4 and Fig. 2). The C:N ratio of the feedstock increased from 14 in PM alone to 17, 23, 27, 33, 59, and 210 while the proportion of CP increased from 20% to 100% (Table 2). As the C:N ratio increased from 14 to 210, the TKN and NH4–N in the digestate decreased from 4920 to 900 mg L1 and 1460 to 10 mg L1, respectively (Table 4 and Fig. 2). The decrease in the proportion of PM resulted in a reduction of NH4–N and a lowering of the buffering capacity within the process. The alkalinity and pH values throughout the experiment were aligned with the ammonium concentration. The alkalinity significantly decreased from 5300 to 600 mg L1 when the CP ratio in feedstock changed from 0% to 100%. The pH was around 7.5 in the reactor-fed CP up to 40%. At further increased proportion of CP, the pH was gradually dropped from 7.1 (50% CP ratio) to 5.9 (100% CP ratio) which is lower than the optimum range for good growth of methanogenic bacteria [4]. The results demonstrated that the drastic drop in pH was mainly due to a lack of sufficient buffering capacity. One aim of co-digestion is to improve the C:N ratio up to the optimum range for methanogenesis. The C:N ratio is an important factor that could be the limitation of anaerobic digestion. The optimum ratio frequently reported is in the range of 25–30 [21]. In this study, the maximum methane yield (306 mL g1 VSadded) was obtained when the feedstock contained 60% CP (C:N ratio ¼ 33). Even though, this ratio is a bit higher than previously reported as optimal, it is apparently sufficient to provide NH4–N for microbial growth and buffering capacity for good performance and stability in the conditions studied. At higher CP ratios (80–100%), the C:N ratios were 59 and 210 which are much higher than the optimum ratio for methanogenesis. The deterioration of performance and stability of the reactor was most likely due to the lack of NH4– N resulting in microbial wash out especially of methanogenic bacteria. At these C:N ratios, NH4–N in the reactor slowly decreased from 100 mg L1 to less than 10 mg L1 (Fig. 2). Lee et al. [22] reported that a decrease in ammonia concentration from 700 to 220 mg NH4–N L1 made methanogenesis unsuccessful in the co-digestion of kitchen garbage and excess sludge in mesophilic condition. Speece [23] also stated that an excess of 40–70 mg NH4–N L1 must be maintained in the reactor to prevent a reduction of microbial activity and the acetate utilization rate was only 54% of maximum potential at 12 mg L1 of NH4–N in the reactor. The maximum specific methane yield (306 mL g1 VSadded) and the VS reduction (61%) obtained in this study are comparable with other works on co-digestion of similar waste materials. Kaparaju and Rintala [8] who studied anaerobic codigestion of potato tuber and its industrial by-products with pig manure in a semi-continuously stirred tank reactor reported methane yields between 210 and 330 mL g1 VSadded. Alvarez and Liden [24] reported a methane yield in the range of 270–350 mL g1 VSadded and a 50–67% VS reduction in the codigestion of FVW with SCSM (cattle and pig manure mixture) and SCSSW (solid cattle-swine slaughterhouse waste). The present study illustrates that anaerobic digestion of CP without external addition of nutrients and buffering agents is
1123
limited by the lack of nitrogen. In contrast, PM contains very high concentrations of ammonia which may be inhibitory to methanogenesis. Therefore, co-digestion of CP and PM could reduce both chemical usage and operating costs for nitrogen and buffering agent supplementation for biomethanation from CP. Furthermore, digestate from co-digestion contained lower amounts of ammonium nitrogen than that from digestion of PM alone. This might resolve the inhibitory effects of ammonia on anaerobic digestion and make a posttreatment system for nitrogen removal simpler and cheaper. Therefore, co-digestion could make possible more costeffective and ecological biogas plants.
4.
Conclusions
The present study demonstrates the biodegradability and methane potential of CP, PM and co-substrates as well as the effects of a mixture ratio on the performance and stability of the continuous co-digestion process. The anaerobic biodegradability of CP and PM was similar even though their compositions were widely different. Since CP contained higher concentrations of easily degradable organic matter, an increase in the proportion of CP in co-digestion improved the reactor performance in terms of methane yield and solids reduction. However, the high level of easily degradable fraction in feedstock could affect the reactor stability, especially when the reactor was fed with feedstock containing an inappropriate C:N ratio. The deterioration of the reactor at a high CP ratio (80–100%) was caused by rapid VFA accumulation and insufficient buffering capacity. Co-digestion with pig manure helps to increase the buffering capacity and provide a nitrogen source for microbial synthesis that will result in a stable anaerobic digestion process.
Acknowledgements The financial support from Joint Graduate School of Energy and Environment and the Royal Jubilee Ph.D Program is gratefully acknowledged. The authors also wish to thank Nunsurakit’s Tapioca Flour Ltd., Part and K P Farm for supporting raw materials used in this study.
references
[1] Paepatung N, Songkasiri W, Kullavanijaya P, Leethochawalit M, Chaiprasert P, Noppharatana A. Assessment of biogas potential and technology for agricultural residues and energy crops in Thailand. Pilot Plant Development and Training Institute. Thonburi: King Mongut’s University of Technology; 2008 [in Thai]. [2] Sriroth K, Chollakup R, Chotineeranat S, Piyachomkwan K, Oates CG. Processing of cassava waste for improved biomass utilization. Bioresource Technology 2000;71(1):63–9. [3] Parawira W, Read JS, Mattiasson B, Bjornsson L. Energy production from agricultural residues: high methane yields in pilot-scale two-stage anaerobic digestion. Biomass and Bioenergy 2008;32(1):44–50.
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[4] Ward AJ, Hobbs PJ, Holliman PJ, Jones DL. Optimisation of the anaerobic digestion of agricultural resources. Bioresource Technology 2008;99(17):7928–40. [5] Hartmann H, Angelidaki I, Ahring BK. Co-digestion of the organic fraction of municipal waste. In: Mata-Alvarez J, editor. Biomethanization of the organic fraction of municipal solid waste. London: IWA Publishing; 2002. p. 181–200. [6] Callaghan FJ, Wase DAJ, Thayanithy K, Forster CF. Continuous co-digestion of cattle slurry with fruit and vegetable wastes and chicken manure. Biomass and Bioenergy 2002;22(1):71–7. [7] Murto M, Bjornsson L, Mattiasson B. Impact of food industrial waste on anaerobic co-digestion of sewage sludge and pig manure. Journal of Environmental Management 2004;70(2): 101–7. [8] Kaparaju P, Rintala J. Anaerobic co-digestion of potato tuber and its industrial by-products with pig manure. Resources, Conservation and Recycling 2005;43(2):175–88. [9] Lehtomaki A, Huttunen S, Rintala JA. Laboratory investigations on co-digestion of energy crops and crop residues with cow manure for methane production: effect of crop to manure ratio. Resources, Conservation and Recycling 2007;51(3):591–609. [10] Paepatung N, Kullavanijaya P, Loapitinan O, Songkasiri W, Noppharatana A, Chaiprasert P. Assessment of biomass potential for biogas production in Thailand. Pilot Plant Development and Training Institute. Thonburi: King Mongut’s University of Technology; 2006 [in Thai]. [11] Mladenovska Z, Dabrowski S, Ahring BK. Anaerobic digestion of manure and mixture of manure with lipids: biogas reactor performance and microbial community analysis. Water Science and Technology 2003;48(6):271–8. [12] APHA. Standard methods for the examination of water and waste water. 19th ed. Washington, DC: American Public Health Association; 1995. [13] AOAC. Official method of analysis. 16th ed. Virginia: Association of Official Agricultural Chemists; 1995. [14] AACC. Approved methods of the AACC. 8th ed. Minnesota: American Association of Cereal Chemists; 1990.
[15] Sluiter A, Hames B, Ruiz R, Scarlata C, Sluite J, Templeton D, et al. Determination of structural carbohydrates and lignin in biomass. Laboratory Analytical Procedure, http://devafdc. nrel.gov/pdfs/9572.pdf; 2006. [16] Heo NH, Park SC, Kang H. Effects of mixture ratio and hydraulic retention time on single-stage anaerobic co-digestion of food waste and waste activated sludge. Journal of Environmental Science and Health Part A-Toxic/ Hazardous Substances & Environmental Engineering 2004; A39(7):1739–56. [17] Moller HB, Sommer SG, Ahring BK. Methane productivity of manure, straw and solid fractions of manure. Biomass and Bioenergy 2004;26(5):485–95. [18] Amon T, Amon B, Kryvoruchko V, Zollitsch W, Mayer K, Gruber L. Biogas production from maize and dairy cattle manure: influence of biomass composition on the methane yield. Agriculture, Ecosystems & Environment 2007;118(1–4):173–82. [19] Neves L, Goncalo E, Oliveira R, Alves MM. Influence of composition on the biomethanation potential of restaurant waste at mesophilic temperatures. Waste Management 2008; 28(6):965–72. [20] Siegert I, Banks C. The effect of volatile fatty acid additions on the anaerobic digestion of cellulose and glucose in batch reactors. Process Biochemistry 2005;40(11):3412–8. [21] Kayhanian MHS. The impact of four design parameters on the performance of a high-solids anaerobic digestion of municipal solid waste for fuel gas production. Environmental Technology 1994;15(6):557–67. [22] Lee MY, Hidaka T, Hagiwara W, Tsuno H. Comparative performance and microbial diversity of hyperthermophilic and thermophilic co-digestion of kitchen garbage and excess sludge. Bioresource Technology 2009;100(2):578–85. [23] Speece RG. Anaerobic biotechnology for industrial wastewaters. Vanderbilt University, Tennessee: Archae Press; 1996. [24] Alvarez R, Liden G. Semi-continuous co-digestion of solid slaughterhouse waste, manure, and fruit and vegetable waste. Renewable Energy 2008;33(4):726–34.
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Production of methane by co-digestion of cassava pulp with various concentrations of pig manure Pornpan Panichnumsin a,b, Annop Nopharatana c, Birgitte Ahring d, Pawinee Chaiprasert e,* a
The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, Thungkru, Bangkok 10140, Thailand b Excellent Center of Waste Utilization and Management, National Center for Genetic Engineering and Biotechnology, Bangkhuntien, Bangkok 10150, Thailand c Pilot Plant Development and Training Institute, King Mongkut’s University of Technology Thonburi, Bangkhuntien, Bangkok 10150, Thailand d AAU, Copenhagen Institute of Technology, Lautrupvang 15, 2750 Ballerup, Denmark e School of Bioresources and Technology, King Mongkut’s University of Technology Thonburi, Bangkhuntien, Bangkok 10150, Thailand
article info
abstract
Article history:
Cassava pulp is a major by-product produced in a cassava starch factory, containing
Received 18 March 2009
50–60% of starch (dry basis). Therefore, in this study we are considering its potential as
Received in revised form
a raw material substrate for the production of methane. To ensure sufficient amounts of
19 February 2010
nutrients for the anaerobic digestion process, the potential of co-digestion of cassava pulp
Accepted 26 February 2010
(CP) with pig manure (PM) was further examined. The effect of the co-substrate mixture
Available online 31 March 2010
ratio was carried out in a semi-continuously fed stirred tank reactor (CSTR) operated under mesophilic condition (37 C) and at a constant OLR of 3.5 kg VS m3 d1 and a HRT of 15
Keywords:
days. The results showed that co-digestion resulted in higher methane production and
Anaerobic digestion
reduction of volatile solids (VS) but lower buffering capacity. Compared to the digestion of
Cassava pulp
PM alone, the specific methane yield increased 41% higher when co-digested with CP in
C:N ratio
concentrations up to 60% of the incoming VS. This was probably due to an increase in
Pig manure
available easily degradable carbohydrates as the CP ratio in feedstock increased. The
Manihot esculenta Crantz
highest methane yield and VS removal of 306 mL g1 VSadded and 61%, respectively, were
Methane potential
achieved with good process stability (VFA:Alkalinity ratio < 0.1) when CP accounted for 60% of the feedstock VS. A further increase of CP of the feedstock led to a decrease in methane yield and solid reductions. This appeared to be caused by an extremely high C:N ratio of the feedstock resulting in a deficiency of ammonium nitrogen for microbial growth and buffering capacity. ª 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
During starch production from cassava (Manihot esculenta Crantz), approximately 5.2 Mt of fresh cassava pulp residue is generated annually in Thailand [1]. At present, some of the residue has application as a low cost animal feed (60 US$ t1
dry weight) and some of them has become a major solid waste problem with contamination of soil and ground water [1]. Since cassava pulp comprises 50–60% starch in dry matter and 60–70% moisture content [2], it has a major potential as raw material for biogas production. Therefore, utilizing this waste effectively could contribute as a valuable source of future
* Corresponding author. Tel.: þ66 2 470 7525; fax: þ66 2 452 3455. E-mail address: [email protected] (P. Chaiprasert). 0961-9534/$ – see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2010.02.018
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energy and could minimize environmental management costs. However, the low concentrations of nutrients such as nitrogen and the low buffering capacity of this waste is a limitation in the conversion to methane. Similar to fruit and vegetable wastes (FVW), using cassava pulp alone might quickly result in the failure of the process due to the rise of volatile fatty acid (VFA). The problem could then be worsen as a result of poor buffering capacity and a drastic drop of pH which consequently inhibits the methanogenesis step [3,4]. Anaerobic co-digestion is a promising technology widely applied to many waste treatments, especially pig and cattle manure. Manure is an excellent co-substrate due to its high buffering capacity and is rich in a wide variety of nutrients necessary for optimum bacterial growth [5]. Thus, co-digestion with manure would give the balance of nutrients, at an appropriate C:N ratio and a stable pH needed to increase methane production [6]. The co-digestion technology of manure has been successfully applied with food industrial waste [7], potato tuber plus its industrial by-products [8] and energy crops and crop residues [9]. Thailand has an extensive pig industry with 8 million heads produced per year with an approximate amount of 2.16 Mt of excreta [10]. Even though there is a well established use of anaerobic digestion, the typical methane yield is low and is associated with the high nitrogen and lignocellulosic content [11]. Co-digestion of pig manure with cassava pulp could offer an efficient solution to both substrates. Here we study the effect of different mixture ratios on the methane production potential and the stability of the CSTR.
2.
Materials and methods
2.1.
Feedstocks and inoculum
Approximately 200 kg of fresh cassava pulp (CP) was collected directly from a pulp pressing machine of a cassava starch factory in Rayong Province [12 400 4800 N, 101 160 4800 E], Thailand. Approximately 100 kg of pig manure (PM) was taken directly from excretion of pig fatteners in a medium-scale farm in Nakornprathom Province [13 490 1100 N, 100 30 5700 E], Thailand. The CP and PM were kept in ice boxes and transported from origins within 1–2 hours. Then, they were aliquot into 1-kg zip-lock bags and stored at 20 C until used. The characteristics of the feedstocks are shown in Table 1. Partially digested pig manure taken from an industrial anaerobic plug flow reactor was used as the starting inoculum. Its pH value was 7.5 and contained 31 g L1 of volatile solid (VS) and 53 g L1 of total solid (TS) with an estimated VS:TS ratio of 0.59. The inoculum was kept at 37 C until the major part of the remaining organic compound had been digested prior to usage.
2.2.
Methane potential assay
The ultimate methane productions of CP, PM and five mixtures were determined in triplicate using 120-mL vials with 45 mL of working volume. CP, PM and five mixtures in different CP:PM ratios of 20:80, 40:60, 50:50, 60:40 and 80:20 were applied based on VS (w/w). The inoculum-substrate ratio
Table 1 – Chemical and elemental compositions of fresh cassava pulp and pig manure (values are the mean ± S.D. of 3 determinations). Composition Moisture (% of fresh waste) TS (g kg1 fresh waste) VS (g kg1 TS) Ash (g kg1 TS) COD (g kg1 VS) Starch (g kg1 VS) Crude fiber (g kg1 VS) Cellulose (g kg1 VS) Hemi-cellulose (g kg1 VS) Lignin (g kg1 VS) TKN (g kg1 TS) 1 TS) NHþ 4 -N (g kg Protein (g kg1 VS) Lipid (g kg1 VS) VFA (g COD kg1 VS) C (% of TS) H (% of TS) O (% of TS) N (% of TS) S (% of TS) C:N ratio
Cassava pulp
Pig manure
69.37 0.23 305 2 984 1 16 1 1050 5 655 5 320 20 165 8 116 5 40 5 1.73 0.01 N.D.b 10.8 0.3 10.1 1.0 2.1 0.3 40.02 7.52 37.51 0.19 N.D. 210
74.7 0.44 253 4 716 10 284 10 1400 200 N.A.a 210 20 108 15 54 7 83 18 44.23 0.8 6.29 0.5 276.8 4.5 123.6 6.2 67.3 5.1 35.42 5.43 26.07 2.57 0.08 14
a N.A. ¼ no analysis. b N.D. ¼ not detectable.
in a total of 20 mL was supplemented with 15 mL inoculum and 10 mL distilled water. When CP was used as a sole substrate, pH in the vials was maintained between 7.0 and 7.2 by an addition of 10 g L1 of sodium bicarbonate. The vials were flushed with a gas mixture of 70% N2 and 30% CO2 before being sealed off with rubber stoppers and aluminum crimps. The biogas composition and production were monitored throughout the 120 days of incubation at 37 C. The control set containing only inoculum was used as a baseline comparison of methane production.
2.3. Semi-continuous single stage digestion of different mixtures Single-stage anaerobic co-digestion of CP and PM was carried out in a 5 L semi-continuously stirred glass reactor with 3 L working volume. The reactor was operated at a constant organic loading rate (OLR) of 3.5 kg VS m3 d1 and a hydraulic retention time (HRT) of 15 days. The reactor was operated with a withdraw/feed method once a day and mechanically stirred at 1.67 Hz by an electric motor for 15 minutes each half hour. The reactor temperature was maintained at 37 C by circulating water through a water jacket by a temperaturecontrolled water bath. Daily biogas production was measured by a liquid displacement measurement system connected to the reactor, logging the gas production automatically. The CSTR was started up using PM as a sole substrate. After the reactor reached a steady state at the designed OLR and HRT, characterized by stable pH, gas production and composition, soluble COD concentration, for 2–3 cycles of HRT, it was fed with various mixture ratios of CP and PM instead. The
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CP:PM ratios of 20:80, 40:60, 50:50, 60:40, 80:20 and 100:0 were consecutively applied based on VS (w/w). The stocks of CP and PM were stored at 20 C in 1-kg zip-lock bags until used. The feedstocks were prepared every 2–3 days by diluting the CP and PM with tap water, such that their volatile solid content was brought to 5.25%, and stored at 4 C. The characteristics of substrates are shown in Table 2. The technical compositions of influent and effluent were determined three times a week except pH which was monitored daily. The results from the analysis of each mixture at steady state were used for evaluating the effect of co-digestion on biodegradability and biogas production efficiency as well as process stability.
2.4.
Analysis
Total solid (TS), volatile solid (VS), chemical oxygen demand (COD), pH, total nitrogen (TKN) and ammonium-nitrogen (NH4-N) were analyzed according to the Standard Method [12]. Moisture, crude fiber and ash content were determined as described by AOAC standard methods [13]. Protein content was calculated from TKN using a multiplied factor of 6.25. The remaining starch content was analyzed using glucoamylase method according to AACC [14]. Cellulose, hemicellulose and Klason lignin were determined by a two-step acid hydrolysis according to the procedure published by the NREL [15]. The lipid content was analyzed by Soxhlet with petroleum ether extraction according to AOAC methods [13]. The C, H, O, N and S elemental analysis was performed on ThermoFinnigan Flash EA 1112 elemental analyzer, following the manufacturer’s standard procedures. Alkalinity was measured by titration to pH 4 with 0.1 mol L1 H2SO4. The biogas production was measured using a liquid displacement method [12]. The percentages of methane and carbon dioxide in the biogas were analyzed using gas chromatography (Shimadzu, Class-GC 14B, Japan), using a Porapak-N column equipped with a thermal conductivity detector (TCD). The oven, injector and detector temperatures were 70, 120 and 120 C, respectively. Helium was used as the carrier gas at a flow rate of 30 mL min1. The individual VFA concentrations (acetic acid, propionic acid, butyric/isobutyric acids and valeric/isovaleric acids) were
analyzed using the Shimadzu gas chromatography (Class-GC 14B) equipped with a flame ionization detector (FID) and Carbowax B-DA column. The oven, injector and detector temperatures were 170, 200 and 200 C, respectively. Helium was used as the carrier gas at a flow rate of 50 mL min1, and nitrogen was used as a makeup gas at a flow rate of 50 mL min1.
2.5.
Calculations
Theoretical methane yield (BU) was calculated based on the elemental composition of CP and PM using Eqs. (1) and (2). The biodegradability was calculated by Eq. (3) in which BO is the ultimate methane yield obtained from the methane potential studied in a batch test [16]. Carbohydrate was calculated by Eq. (4) according to Moller et al. [17]. Ca Hb Oc Nd þ 1=4ð4a b 2c þ 3dÞH2 O/1=8ð4a þ b 2c 3dÞCH4 þ 1=8ð4a b þ 2c þ 3dÞCO2 þ dNH3
(1)
BU ¼ ða=2 þ b=8 c=4 3d=8Þ=ð12a þ b þ 16c þ 14dÞ 22:4 mL CH4 g1 VS
(2)
Biodegradability ð%Þ ¼ ðBO =BU Þ 100
(3)
VScarbohydrate ¼ VS VSprotein VSlipid VSVFA
(4)
3.
Results and discussion
3.1.
Feedstocks characteristics
Table 1, about here, shows analyzed compositions of CP and PM. CP contained starch and fiber as major components and small amounts of protein and lipid. The fiber content of CP comprised 50% cellulose, 34% hemi-cellulose and 16% lignin. The major components of PM were protein, lipid, fiber and VFA. The fiber composition of PM comprised 47% cellulose, 17% hemi-cellulose and 36% lignin. The C, H, O content in CP was higher than PM whereas the N content was much lower. The C:N ratios were about 210 in CP and 14 in PM.
Table 2 – Characteristics of feedstocks at different mixture ratio used in CSTR experiment (values are the mean ± S.D. of 5 determinations). CP:PM ratios
TS (%) VS (%) COD-T (g L1) COD-S (g L1) pH VFA (g COD L1) Ammonium N (g L1) TKN (g L1) Total Alkalinity (g CaCO3 L1) C:N ratio a N.D. ¼ not detectable.
0:100
20:80
40:60
50:50
60:40
80:20
100:0
7.2 0.2 5.2 0.2 85.9 3.1 9.1 0.3 7.42 0.3 3.26 0.1 0.18 0.04 2.30 0.08 3.3 0.3 14
6.8 0.1 5.2 0.1 80.5 2.2 7.5 0.3 7.32 0.2 2.63 0.1 0.15 0.03 1.92 0.07 2.7 0.2 17
6.5 0.2 5.2 0.1 75.1 2.4 5.9 0.2 7.25 0.2 1.99 0.05 0.11 0.02 1.48 0.06 2.0 0.2 23
6.2 0.3 5.2 0.1 72.4 1.8 5.2 0.2 7.22 0.2 1.68 0.05 0.09 0.01 1.18 0.05 1.7 0.2 27
6.1 0.2 5.2 0.1 69.7 2.4 4.4 0.2 7.16 0.1 1.37 0.05 0.08 0.01 0.95 0.05 1.3 0.2 33
5.7 0.2 5.2 0.1 64.3 2.7 2.75 0.1 6.88 0.3 0.74 0.05 0.04 0.01 0.54 0.07 0.7 0.1 59
5.3 0.1 5.2 0.1 58.9 1.7 1.16 0.1 4.49 0.3 0.11 0.05 N.D.a 0.10 0.04 0.2 0.1 210
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3.2.
biomass and bioenergy 34 (2010) 1117–1124
Methane potential and biodegradability
The ultimate methane potentials (BO) of CP, PM and five mixtures are shown in Table 3. For a single substrate, the methane production per kg of VS in PM (362 mL g1 VSadded) was higher than that of CP (344 mL g1 VSadded). In contrast, the methane production per wet weight of PM (66 mL g1 ww) was much lower than that of CP (103 mL g1 ww). The theoretical methane yield (BU) of CP and PM were 530 and 541 mL g1 VSadded, respectively. However, the biodegradability of CP and PM was not significantly different. Our results are similar to Moller et al. [17] who reported the BU, BO and biodegradability of PM were 516, 356 mL g1 VSadded and 69%, respectively. The partial methane yield (B15), the yield produced within 15 days, of CP was 23% higher than that of PM. The B15/BU ratios of CP and PM were 51% and 43%, respectively. According to the biodegradability and B15/BU ratio, the proportion of easily and slowly degradable fraction in VS of CP and PM could be estimated. Since the biodegradability of CP was 65%, it might be deduced that VS of CP contains 51% of easily degradable fraction and 14% of slowly degradable fraction. For PM, the biodegradability was 67%, therefore, its VS contains 43% of easily degradable fraction and 24% of slowly degradable fraction. These indicate that the composition of CP was more easily degraded than that of PM. For the co-substrate, the methane yield and biodegradability increased with increasing CP ratio up to 50% compared to PM alone. The highest methane yield and biodegradability of 391 mL g1 VSadded and 73%, respectively, were obtained at CP ratio of 20%. The co-substrate improved specific methane yield by 8% and 14% compared to PM and CP, respectively. The increase in the proportion of CP in the co-substrate increased the partial methane yield by 19% compared to PM alone. This was probably due to the increase of easily degradable fraction (7–9%) in the co-substrates. For CP ratios in the range of 80–100%, although the biodegradability was similar to that of PM alone, the ultimate methane yield was lower. There is generally agreement in literature that the biodegradability and methane potential of a complex substrate are dependent on its composition, for instance, the content of biodegradable carbohydrates, proteins, and lipids [9,18,19]. As shown in Table 1, CP contains almost 100% of carbohydrate while PM contains a wide variety of components, such as protein, lipid, carbohydrate and VFA.
The carbohydrate content in feedstock increased whereas the protein and lipid content decreased when the ratio of CP increased. Theoretically, the methane yield obtains from carbohydrates (415 mL g1 VS) is lower than that of proteins (496 mL g1 VS) and lipids (1014 mL g1 VS) [17].
3.3. Semi-continuous single-stage digestion of different mixtures The CSTR was initially fed with PM at OLR of 3.5 kg VS m3 d1 and 15 days of HRT for 49 days and subsequently fed with the mixtures of CP and PM. The proportion of CP in feedstock sequentially increased up to 100% while maintaining constant OLR and HRT. The summarized values of the parameters monitored during steady state conditions are given in Table 4. The results indicate that the maximum ratio of CP in feedstock achieved the highest methane production at 60%. As can be seen from Table 4 and Fig. 1, the specific biogas and methane yields of PM alone were 316 and 217 mL g1 VSadded while the volumetric biogas and methane productions were 1110 and 740 mL L1 d1 and these increased to the maximum production of 1800 and 1060 mL L1 d1 at 60% CP ratio in feedstock. Correspondingly, the maximum biogas and methane yield achieved 514 and 306 mL g1 VSadded, respectively. In contrast, the methane content slightly decreased from 64% with PM alone to 57% with 60% CP ratio. When the ratio of CP in feedstock increased to 80%, the amounts of TS and VS in the digestates decreased (Fig. 2). Likewise, the removal efficiencies of VS, TS, and COD gradually increased (Table 4). The VS, TS and COD reductions of the reactor fed with PM alone were 46%, 39% and 50%, respectively. The maximum reductions of VS, TS, and COD obtained at CP ratio of 80% were 64%, 55% and 57%, respectively. The results showed that co-digestion improved the efficiencies up to 33% (VS) and 14% (COD) compared to PM alone. When the reactor was fed with 100% CP, the efficiencies of gas production and solids reduction considerably decreased. In comparison with PM alone, the increase in methane yield (41%) and VS removal (33%) obtained in co-digestion with 60% CP ratio of feedstock VS was apparently due to the increased amount of easily degradable compound in the feedstock. The easily degradable fraction contained in these co-substrates was 7-9% higher than that in PM. The increase of the easily degradable fraction resulted from the increase of
Table 3 – Methane yield and biodegradability of CP, PM, and five mixtures (Values are the mean ± S.D. of 3 determinations). CP
PM
CP:PM ratio 20:80
40:60
50:50
60:40
80:20
Ultimate methane yield mL CH4 g1 VS; BO mL CH4 g1 wet weight
344 24 103 7
362 8 66 1
391 3 80 1
383 6 88 1
388 22 93 5
365 16 92 4
357 15 99 4
Partial methane yield (15 day) mL CH4 g1 VS; B15
270 14
232 6
265 8
268 6
277 20
266 12
268 10
Theoretical methane yield mL CH4 g1 VS; BU B15/BU (%) Biodegradability (%)
530 51 65
541 43 67
539 49 73
536 50 71
535 52 73
534 50 68
532 50 67
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Table 4 – Steady-state performance data of single stage CSTR treating PM, CP, and five mixtures. CP:PM ratios 20:80
40:60
50:50
60:40
80:20
100:0
4.4 0.1 2.8 0.1 42.5 2.6 2.1 0.1 7.53 0.13 0.22 0.10 1.46 0.06 4.92 0.55 5.1 0.2 39 2 46 2 50 3 1110 110 316 30 740 60 217 16 64 1 33 1
4.2 0.1 2.6 0.1 39.5 1.4 1.9 0.1 7.50 0.02 0.21 0.02 1.22 0.05 4.75 0.28 4.5 0.2 39 1 49 2 51 2 1230 50 350 15 790 30 227 10 63 1 34 1
3.7 0.2 2.4 0.1 36.2 1.3 1.8 0.2 7.50 0.08 0.22 0.01 0.780 0.03 3.97 0.37 3.8 0.2 43 2 54 1 52 2 1490 50 426 13 930 20 266 10 61 1 37 1
3.2 0.1 2.2 0.1 31.0 1.7 1.5 0.1 7.11 0.10 0.20 0.02 0.65 0.04 3.13 0.32 3.4 0.2 48 2 59 1 57 1 1670 60 478 18 1010 40 290 16 59 1 38 1
3.0 0.1 2.0 0.1 30.0 1.6 1.7 0.2 7.10 0.05 0.24 0.02 0.53 0.04 2.65 0.16 2.0 0.2 51 2 61 1 57 1 1800 90 514 18 1060 30 306 13 57 0 39 1
2.6 0.1 1.9 0.2 27.7 2.4 1.4 0.2 6.73 0.10 0.60 0.20 0.21 0.09 1.89 0.15 1.1 0.2 55 1 64 4 57 4 1620 280 463 81 920 180 263 50 55 1 41 1
3.4 0.3 2.9 0.3 38.6 4.8 1.8 0.6 5.80 0.60 1.10 0.20 0.03 0.02 0.90 0.18 0.7 0.3 36 8 44 6 35 8 730 320 208 91 410 170 118 50 43 8 41 3
carbohydrate fraction especially composed of starch in the feedstock. As shown in Table 1, when the ratio of CP in the feedstock changed from 0% to 100%, the carbohydrate fraction increased from 45% to 94%. In contrast, protein and lipid decreased from 28% to 1% and from 12% to 1%, respectively. As reviewed by Neves et al. [19], carbohydrates are hydrolyzed faster than proteins and lipids under anaerobic conditions.
20:80
40:60
50:50
60:40
80:20
CP
500
1200
400
1000 800
300
600 200 400 100
200
Biogas yield (mL g-1 VSadded)
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700
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30
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91 106 121 136 151 166 181 196 211 Time (day)
VS reduction (%)
Methane yield (mL g-1 VSadded)
PM (0:100)
Carbohydrates had higher hydrolysis rate constants (0.5–2.0 d1) than proteins and lipids (0.25–0.8 d1 and 0.1–0.7 d1, respectively). The composition of co-substrates also affects the stability of the digestion process. For anaerobic digestion, CP is considered as a poor substrate in terms of nutrient availability and buffering capacity but it contains high amount of easily
Volumetric methane production (mL L-1reactor d-1)
TS (%) VS (%) COD-T (g L1) COD-S (g L1) pH VFA (g COD L1) Ammonium N (g L1) TKN (g L1) Total Alkalinity (g CaCO3 L1) TS reduction (%) VS reduction (%) COD-T reduction (%) Volumetric biogas production (mL L1 d1) Specific biogas yield (mL g1 VS) Volumetric CH4 production (mL L1d1) Specific CH4 yield (mL g1 VS) CH4 content (%) CO2 content (%)
0:100
0
Fig. 1 – Performance of single-stage CSTR fed with seven CP:PM ratios of 0:100, 20:80, 40:60, 50:50, 60:40, 80:20 and 100:0, respectively. (B) Specific methane yield; (:) Volumetric methane production; (-) Specific biogas yield and (A) VS reduction. Dashed lines represent the changes in the CP:PM ratio.
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biomass and bioenergy 34 (2010) 1117–1124
PM (0:100)
20:80
40:60
50:50
60:40
80:20
CP
5
TS, VS (%)
4 3 2 1 0
2000
8
7 1000
6.5
pH
NH4 -N (mg L-1 )
7.5 1500
6 500 5.5 5
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4000 1.5 3000 1
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VFA:Alkalinity ratio
-1
VFA, Alkalinity (mg L )
0
0 1
16
31
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91
106 121
136 151 166 181 196 211
Time (d) Fig. 2 – Characteristics of digestate of single-stage CSTR fed with seven CP:PM ratios of 0:100, 20:80, 40:60, 50:50, 60:40, 80:20 and 100:0, respectively. (>) TS; (D) VS; (:) Ammonium-nitrogen; (*) pH; (,) VFA; (A) Alkalinity and (6) VFA:Alkalinity ratio. Dashed lines represent the changes in the CP:PM ratio.
degradable matter. Thus, the high ratio of CP in the feedstock associated with an improper C:N ratio could cause co-digestion process imbalance and failure. The results indicate that for stable co-digestion of CP and PM, the maximum ratio of CP in feedstock was 60%. At CP ratio of up to 60%, VFA was rapidly consumed by methanogens. The VFA concentration in the digestate was around 200–240 mg L1 and the soluble COD concentration was 1400 mg L1 while the VFA:Alkalinity ratio was below 0.1 (Table 4 and Fig. 2). The further increase in the CP ratio resulted in the reduction of methane yield and the accumulation of VFA. The first HRT of feeding 80% CP of the feedstock resulted in the best reactor performance with the highest efficiencies of both VS reduction and methane production (Fig. 1). However, gas production and solid reductions significantly deteriorated at the second HRT and as
shown in Table 4, Figs. 1 and 2, the biogas and methane yield decreased from 580 and 330 to 460 and 260 mL g1 VSadded, respectively. The VFA accumulation was observed at the concentration of 620 mg L1 and the VFA:Alkalinity ratio significantly increased to 0.6. The failure of the reactor fed with CP alone was indicated by a sharp drop in both removal efficiency and methane yield. The VS removal and specific methane yield decreased by 20% and 70%, respectively, compared to PM alone. The soluble COD decreased to 1000 mg L1 while the VFA increased to 980 mg L1 and the major VFA accumulated in the reactor was propionate (data not shown). The VFA:Alkalinity ratio gradually increased to 2 which showed significant process instability. Callaghan et al. [6] reported that at a VFA:Alkalinity ratio less than 0.4, the reactor should be considered as stable. However, the VFA level
biomass and bioenergy 34 (2010) 1117–1124
was much lower than the considered inhibitory concentration (4000 mg L1) for anaerobic digestion [20]. The results showed that pH, alkalinity, NH4–N, and TKN decreased with increasing CP ratio which related to the C:N ratio (Table 4 and Fig. 2). The C:N ratio of the feedstock increased from 14 in PM alone to 17, 23, 27, 33, 59, and 210 while the proportion of CP increased from 20% to 100% (Table 2). As the C:N ratio increased from 14 to 210, the TKN and NH4–N in the digestate decreased from 4920 to 900 mg L1 and 1460 to 10 mg L1, respectively (Table 4 and Fig. 2). The decrease in the proportion of PM resulted in a reduction of NH4–N and a lowering of the buffering capacity within the process. The alkalinity and pH values throughout the experiment were aligned with the ammonium concentration. The alkalinity significantly decreased from 5300 to 600 mg L1 when the CP ratio in feedstock changed from 0% to 100%. The pH was around 7.5 in the reactor-fed CP up to 40%. At further increased proportion of CP, the pH was gradually dropped from 7.1 (50% CP ratio) to 5.9 (100% CP ratio) which is lower than the optimum range for good growth of methanogenic bacteria [4]. The results demonstrated that the drastic drop in pH was mainly due to a lack of sufficient buffering capacity. One aim of co-digestion is to improve the C:N ratio up to the optimum range for methanogenesis. The C:N ratio is an important factor that could be the limitation of anaerobic digestion. The optimum ratio frequently reported is in the range of 25–30 [21]. In this study, the maximum methane yield (306 mL g1 VSadded) was obtained when the feedstock contained 60% CP (C:N ratio ¼ 33). Even though, this ratio is a bit higher than previously reported as optimal, it is apparently sufficient to provide NH4–N for microbial growth and buffering capacity for good performance and stability in the conditions studied. At higher CP ratios (80–100%), the C:N ratios were 59 and 210 which are much higher than the optimum ratio for methanogenesis. The deterioration of performance and stability of the reactor was most likely due to the lack of NH4– N resulting in microbial wash out especially of methanogenic bacteria. At these C:N ratios, NH4–N in the reactor slowly decreased from 100 mg L1 to less than 10 mg L1 (Fig. 2). Lee et al. [22] reported that a decrease in ammonia concentration from 700 to 220 mg NH4–N L1 made methanogenesis unsuccessful in the co-digestion of kitchen garbage and excess sludge in mesophilic condition. Speece [23] also stated that an excess of 40–70 mg NH4–N L1 must be maintained in the reactor to prevent a reduction of microbial activity and the acetate utilization rate was only 54% of maximum potential at 12 mg L1 of NH4–N in the reactor. The maximum specific methane yield (306 mL g1 VSadded) and the VS reduction (61%) obtained in this study are comparable with other works on co-digestion of similar waste materials. Kaparaju and Rintala [8] who studied anaerobic codigestion of potato tuber and its industrial by-products with pig manure in a semi-continuously stirred tank reactor reported methane yields between 210 and 330 mL g1 VSadded. Alvarez and Liden [24] reported a methane yield in the range of 270–350 mL g1 VSadded and a 50–67% VS reduction in the codigestion of FVW with SCSM (cattle and pig manure mixture) and SCSSW (solid cattle-swine slaughterhouse waste). The present study illustrates that anaerobic digestion of CP without external addition of nutrients and buffering agents is
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limited by the lack of nitrogen. In contrast, PM contains very high concentrations of ammonia which may be inhibitory to methanogenesis. Therefore, co-digestion of CP and PM could reduce both chemical usage and operating costs for nitrogen and buffering agent supplementation for biomethanation from CP. Furthermore, digestate from co-digestion contained lower amounts of ammonium nitrogen than that from digestion of PM alone. This might resolve the inhibitory effects of ammonia on anaerobic digestion and make a posttreatment system for nitrogen removal simpler and cheaper. Therefore, co-digestion could make possible more costeffective and ecological biogas plants.
4.
Conclusions
The present study demonstrates the biodegradability and methane potential of CP, PM and co-substrates as well as the effects of a mixture ratio on the performance and stability of the continuous co-digestion process. The anaerobic biodegradability of CP and PM was similar even though their compositions were widely different. Since CP contained higher concentrations of easily degradable organic matter, an increase in the proportion of CP in co-digestion improved the reactor performance in terms of methane yield and solids reduction. However, the high level of easily degradable fraction in feedstock could affect the reactor stability, especially when the reactor was fed with feedstock containing an inappropriate C:N ratio. The deterioration of the reactor at a high CP ratio (80–100%) was caused by rapid VFA accumulation and insufficient buffering capacity. Co-digestion with pig manure helps to increase the buffering capacity and provide a nitrogen source for microbial synthesis that will result in a stable anaerobic digestion process.
Acknowledgements The financial support from Joint Graduate School of Energy and Environment and the Royal Jubilee Ph.D Program is gratefully acknowledged. The authors also wish to thank Nunsurakit’s Tapioca Flour Ltd., Part and K P Farm for supporting raw materials used in this study.
references
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