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Minimizing asynchronism to improve the performances of anaerobic co-digestion of food waste and corn stover
Accepted Manuscript Minimizing asynchronism to improve the performances of anaerobic co-digestion of food waste and corn stover Qi Zhou, Fei Shen, Hairong Yuan, Dexun Zou, Yanping Liu, Baoning Zhu, Muhanmad Jaffu, Akiber Chufo, Xiujin Li PII: DOI: Reference:
S0960-8524(14)00614-2 http://dx.doi.org/10.1016/j.biortech.2014.04.074 BITE 13370
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
19 January 2014 13 April 2014 21 April 2014
Please cite this article as: Zhou, Q., Shen, F., Yuan, H., Zou, D., Liu, Y., Zhu, B., Jaffu, M., Chufo, A., Li, X., Minimizing asynchronism to improve the performances of anaerobic co-digestion of food waste and corn stover, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.04.074
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Minimizing asynchronism to improve the performances of anaerobic co-digestion of food waste and corn stover Qi Zhoua , Fei Shena,b, Hairong Yuana, Dexun Zoua, Yanping Liua, Baoning Zhua, Muhanmad Jaffua, Akiber Chufoa, Xiujin Lia* a
Centre for Resource and Environmental Research, Beijing University of Chemical Technology, No. 15 Beisanhuan East Road,
Chaoyang District, Beijing 100029, P. R. China b
Institute of Ecological and Environmental Sciences, Sichuan Agricultural University, Chengdu, Sichuan 611130, P. R. China
Abstract: To investigate the existence of the asynchronism during the anaerobic co-digestion of different substrates, two typical substrates of food waste and corn stover were anaerobically digested with altering organic loadings (OL). The results indicated that
the biodegradability of food waste and corn stover was calculated to be 81.5% and 55.1%, respectively, which was main reason
causing the asynchronism in the co-digestion. The asynchronism was minimized by NaOH-pretreatment for corn stover, which could
improve the biodegradability by 36.6%. The co-digestion with pretreatment could increase the biomethane yield by 12.2%, 3.2% and
0.6% comparing with the co-digestion without pretreatment at C/N ratios of 20, 25 and 30 at OL of 35 g-VS/L, respectively. The
results indicated that the digestibility synchronism of food waste and corn stover was improved through enhancing the accessibility
and digestibility of corn stover. The biomethane production could be increased by minimizing the asynchronism of two substrates in
co-digestion.
Keywords: Asynchronism minimization; Anaerobic co-digestion; Food waste; Corn stover 1. Introduction Food waste mainly come from hotels, restaurants, and canteens of universities and enterprises, etc.. Food waste amount kept increasing over last decades due to the rapid growth of population in cities and fast economy development in China (Jie et al., 2012). Taking Beijing city as an example, approximately 1600 tons of food waste was collected daily in 2012. Currently, food waste either goes to animal farms as feedstock or to landfill in most cities. Serious health threat associated with food waste as animal feedstock has attracted great public attentions. Food waste landfill has also caused serious environmental problems, such as increased leachate amount and *
Corresponding author. Addresses: No. 15 Beisanhuan East Road, Chaoyang District, Beijing, 100029, P. R. China. Tel/fax.: +86 10 64432281. E-mail addresses: [email protected] or [email protected]. 1
treatment cost, and offensive odor emission, due to high moisture content and readily biodegradable components with food waste. Anaerobic digestion (AD) is extensively acceptable as an efficient process to treat and utilize food waste because it has been proven to be a promising method for waste reduction and energy recycling (MacLellan et al., 2013). Food waste contains rich nutrients and is a superior substrate for AD. However, as a single organic waste stream, food waste is rich in nitrogen sources such as protein, leading to low carbon-nitrogen ratio (C/N) (13.8~18.2) (Khalid et al., 2011; Pan et al., 2008; Wang et al., 2012; Zhang et al., 2012). Generally, low C/N ratio is not proper for high efficient AD due to the inhibition from extra total ammonia nitrogen in the food waste (Pan et al., 2008; Chen et al., 2008). On the other hand, AD of mono-substrate food waste could also suffer sensitive acidification and instability as organic loading rate (OLR) was increased over 4 kg-VS/(m3·d) or 35 g-VS/L(Liu and Liu, 2011; Nagao et al., 2012). Anaerobic co-digestion could offer the possibility of balancing carbon and nitrogen in the system when food waste is digested with other substrate. Correspondingly, the digestion performances and operations could be maintained more efficiently and stably (Ganesh et al., 2013). To seek some stable carbon sources, lignocellulosic biomass should be a better option, especially for crop residues, because of their easy availability and collectability in China. Thus, the co-digestion of food waste with crop residues could be potentially digested to improve the performance of digestion system. However, crop residues are lignocellulosic biomass, which presented the characteristics and composition of difficult digestibility comparing with food waste. When co-digested, each substrate would pose different biodegradable rate, which could cause asynchronism of digestion rate, and may potentially affect mass bioconversion and biogas production adversely. Xie et al (2011) evaluated the operation stability and methane production potential of pig manure and grass silage co-digestion, and obtained a high specific methane yield in a specific ratio. They also found appearance of several biogas peaks in the digestion process and attributed it to the asynchronism of characteristics and composition between pig manure and grass 2
silage. Similar phenomena were found in other cases of co-digestion (Lin et al., 2011; Riaño et al., 2011; Zhang et al., 2011). Although C and N could be balanced by mixing two feedstocks for digestion, the asynchronism of digestibility could pose adverse effect on the stability of digestion system and yield of methane. Therefore, it is necessary to go further to investigate asynchronism and effect in the co-digestion with different substrates. This study was to evaluate the performances of anaerobic co-digestion of food waste and corn stover. Corn stover was used to adjust C/N ratio at different proportions. NaOH was applied to pretreat corn stover to improve biodegradability of corn stover, and to minimize asynchronism of both substrates for efficient digestion. 2. Materials and methods 2.1 Feedstock characteristics Food waste was collected from the canteen of Beijing University of Chemical Technology. After removing the indigestible materials such as bones, plastic bags, egg shells, and chopsticks, it was homogenized into slurry by a food waste homogenizer (SS2600, Meijiamada Co., Zhenjiang, China). The homogenizer was run at the speed of 2600 rpm. Corn stover was collected from rural area in Shunyi District, Beijing, China. The corn stover was chopped into 30 to 40 mm in length and then ground into a desired size of 5~10 mm by a hammer mill (FE130, Staida Co., Tianjin, China). The homogenized food waste was stored in a freezer at -20℃ for later digestion tests. The characteristics of food waste and corn stover are listed in Table 1. 2.2 Inoculum The activated sludge for seeding was collected from an AD system for swine manure treatment in Shunyi, Beijing. The total solid (TS), volatile solid (VS), and mixed liquid suspended solids (MLSS) were 11.13%, 5.36%, and 82.11 g/L, respectively. The seeding sludge was stored at 4℃ after collection. The sludge was incubated at 35℃ for a week before it was inoculated for AD. The characteristics and composition of the seeding sludge are also presented in Table 1. 2.3 Biochemical methane potential (BMP) test 3
Biochemical methane potential tests in this study were based on the Elbeshbishy’s protocols (Elbeshbishy et al., 2012). The tests were performed in the 250 mL serum bottles with 200 mL working volume. Food waste and inoculum were added to the bottles at the mixture ratio of 1:4 (VS/VS) with the initial pH of 7.0. The bottles were sealed by rubber plugs and parafilm. Afterwards, the bottles were incubated at 35±1℃, and shaken in a frequency of 120 rpm twice a day. The daily biomethane yield and the methane content were monitored every day. All tests were run in triplicate and the control group (inoculum was added only) was set. The BMP tests for corn stover were performed with similar protocols. 2.4 AD in batch Batch digestions were carried out on 1 L serum bottles with the working volume of 0.8 L. After the substrates and seeding sludge were loaded in the bottles, the sealed digestion bottles were operated in a 35±1℃ thermostat water bath with shaking frequency of 120 rpm twice per day. The co-digestion of food waste and corn stover and their corresponding mono-digestion were all performed using the mentioned digestion system. Each run in this work was repeated in triplicate. The biogas production and the methane content were recorded every day and employed for evaluating the digestion performances. 2.5 Chemical pretreatment to corn stover In order to improve the co-digestion, the NaOH-pretreated corn stover was also employed in this work. According to the referred work (Zheng et al., 2009), the ground stover was pretreated in sealed bags at ambient temperature for three days. The mixing ratio of corn stover (dry basis), NaOH, and tap water were controlled at 1:0.02:6 (wt. /wt.) for the pretreatment. The pretreated corn stover was washed 10~15 times with tap water till the pH of the washed water was in the range of 7.3~7.6. Afterwards, the washed corn stover was stored in 4℃ for digestion tests (Dongyan et al., 2003). 2.6 Analysis methods The element composition in the employed substrates was analyzed by a Vario EL/micro cube elemental 4
analyzer (Elementar, Germany). The organic fraction of the food waste and corn stover can be expressed in the molecular formula of
. Theoretical methane yield (TMY) is the maximum volume of methane when
the substrate was converted completely in standard temperature and pressure. Based on the element analysis, the TMY of one substrate can be calculated by Eq. 1 and Eq. 2 (Sosnowski et al., 2003). The biodegradability of the substrate can be calculated by Eq.3, correspondingly.
where VT is theoretical methane production, and VC is biomethane production. Biogas production in BMP tests and batch digestion was both measured by water displacement method. Methane content in the biogas was determined by gas chromatography (SP-2100, China) equipped with a molecular sieve packed stainless-steel column with the length and diameter of 2.0 m×3.0 mm (TDX-01) and a thermal conductivity detector (TCD). The detailed determination protocol for the methane content was in accord with the reference (Zheng et al., 2009). The standard gas with 5.0% N2, 60.1% CH4, and 34.9% CO2 was used for the standard calibration for the determination. The biomethane production was calculated based on the methane content and biogas production. In order to assess the biomethane production rate, the Gompertz model (Eq.4) was employed to fit the curve of cumulative methane yield (Li et al., 2010). Based on the Gompertz model, the lag phase during the digestion could be determined. Subsequently, the digestion rate can be clarified by the first-order kinetic model (Eq.5) (Ching-Shyung et al., 1996; Group, 2002)
5
where, M was the biomethane yield at t time, mL/(g-VS·d), P was the final biomethane yield, mL/g-VS, R m was the maximum daily biomethane yield, mL/(g-VS·d),
was the lag phase, d.
where k is the first order kinetic constant (d-1), t is the digestion time (d), S represents the residual substrate (organics) concentration (mg/L). 3. Results and discussion 3.1 Evaluation of biochemical methane potential and biodegradability The elemental compositions of food waste and corn stover are summarized in Table 2. The organic fraction of food waste and corn stover were represented with formulation
and
,
respectively. According to Eq. 2, the TMYs of food waste and corn stover were 622 mL-CH4/g-VS and 564 mL-CH4/g-VS, respectively, which were similar to the results of 360~551 mL-CH4/g-VS in the referred work (Liu et al., 2009; Raposo et al., 2012). As shown in Table 2, the actual biomethane yields of 507 mL/g-VS and 311 mL/g-VS were obtained for food waste and corn stover, respectively, corresponding to 81.5% and 55.1% of TMYs of two substrates. It can be found that food waste presented a higher biodegradability comparing with corn stover. This was mainly attributed to the readily-biodegradable ingredients of starch, protein, and fat in food waste in contrast to the hard-biodegradable compositions of cellulose, hemicellulose, and lignin in corn stover (Wang et al., 2007). In addition, the C/N ratio of food waste was 18.9, which was much closer to the generally-defined range (20~30) compared with the corn stover (63.5) (Estevez et al., 2012; Yadvika et al., 2004). Previous research was mainly focused on balancing C and N in co-digestion, while neglecting the effect of difference in biodegradability of substrates applied (Zhong et al., 2012; Yen and Brune, 2007). Actually, the different biodegradability of the employed substrates would cause possible asynchronism in co-digestion, and potentially affect mass
6
bioconversion and biogas production adversely. 3.2 Investigation on asynchronism in co-digestion As external carbon source, corn stover was employed to adjust C/N ratio of substrates to the optimal range of 20~30. Corn stover was mixed with food waste in three different proportions to make the C/N ratio of the substrate mixture at 20, 25, and 30, respectively. The mixture substrate of food waste and corn stover were anaerobically co-digested at organic loadings of 35 g-VS/L and 45 g-VS/L. The results are shown in Fig. 1 and Fig. 2. Fig. 1 presents the daily biomethane yields for co-digestion at organic loading of 35 g-VS/L. Similar trends were observed for three C/N ratios. Two obvious peaks of daily biomethane yield were observed during the digestion process. The appearance of two peaks could be attributed to the asynchronism of biodegradability of food waste and corn stover. The first peak was mainly contributed by food waste as it was readily biodegradable and digested in relatively early stage, while the second peak by corn stover as it was relatively hard biodegradable and took longer time for digesting. The finding was agreed with previous research, reporting that several biogas peaks appeared during co-digestion processes (Lin et al., 2011; Xie et al., 2011). Furthermore, the gap between two peaks was changed for different C/N ratios. The gap between two peaks was 5 days for C/N ratio of 20, and increased to 10 and 8 days for C/N ratios of 25 and 30, respectively. Correspondingly, the biomethane yield per unit VS decreased from 311.5 mL/g-VS to 216.3 mL/g-VS (Table 3). In addition, the biomethane yield per unit VS for co-digestion were generally lower than those of food waste mono-digestion. The relatively lower biomethane yield with co-digestion indicated the adverse effect by asynchronism of digestion substrates, although all C/N ratios were within the optimal ranges. As shown in Fig. 2, the organic loading was increased to 45 g-VS/L, while the C/N ratio was also adjusted by corn stover within the optimal range of 20~30. The biomethane yield per unit VS in co-digestion were generally higher than those in mono-digestion. It may be attributed to the improvement of buffer capability of co-digestion 7
system (Marañón et al., 2012; Zhang et al., 2013). AD of mono-substrate food waste at high OLR often suffered serious acidification due to the lower buffer capability. More importantly, although the C/N ratio was adjusted to the optimal range (20~30), the biomethane yield per unit-VS descended significantly when the C/N ratio was increased from 20 to 30. This trend was similar to that at organic loading of 35 g-VS/L. According to the difference between the peaks of daily biomethane yield, the gap at 45 g-VS/L extended to 7 d, 12 d, and 10 d for C/N ratio of 20, 25, and 30, respectively. Compared with the runs with 35 g-VS/L, the gaps were extended. The result again proved that the increased digestibility asynchronism of substrates could adversely impacted the synergism from the nutrition balance of carbon and nitrogen sources in co-digestion. As a result, the asynchronism of the different substrates indeed existed in the anaerobic co-digestion, which would adversely affect digestion performances. Therefore, it was necessary to consider the digestibility synchronism of different substrates in anaerobic co-digestion, and it should be a possible method to improve digestion performance by minimizing the asynchronism. 3.3 Minimizing asynchronism by corn stover pretreatment Asynchronism needs to be minimized in order to achieve high digestion efficiency and biomethane yield. According to the referred work, the digestibility of lignocellulosic biomass can be improved significantly through pretreatment (Zheng et al., 2009; Chandra et al., 2012). NaOH was proven to be one of effective pretreatment agent, therefore, applied for corn stover in this study (Luo et al., 2005). The purpose of NaOH pretreatment was to enhance corn stover biodegradability, and to narrow the gap and asynchronism between corn stover and food waste. The experiments on the co-digestion of food waste and NaOH-pretreated corn stover were thereby carried out at two organic loadings of 35 and 45 g-VS/L, and the daily biomethane yields are presented in Fig. 3 and Fig. 4, respectively. To compare Fig. 1 with Fig. 3, it can be seen that two typical biomethane peaks were found in co-digestion of food waste and pretreated corn stover at C/N ratios of 20, 25, and 30 for organic loading of 35 g-VS/L. The gap 8
between two peaks became greater with the increase of C/N ratio from 20 to 30, the trends of which were similar to those of the co-digestion of food waste and untreated corn stover. However, the gaps with pretreated corn stover were 1, 4 and 7 days, significantly shorter than those of the co-digestion with untreated corn stover. The narrowed gaps implied that the biodegradability of pretreated corn stover was enhanced, and the digestion asynchronism of pretreated corn stover and food waste was thereby shortened. This was further verified by increased biomethane yield per unit-VS with co-digestion of pretreated corn stover and food waste (Table 3 and 4), which achieved 5.7%, 5.6%, and 10.3% more biomethane yield per unit-VS than those of co-digestion with untreated corn stover at corresponding C/N ratios, respectively. The same results were found for the co-digestion of pretreated corn stover and food waste at organic loading of 45 g-VS/L (see Fig. 4). The gap of two peaks were 4, 7, and 9 days for the C/N ratios of 20, 25, and 30, which was 3, 5, and 1 day shorter than those of the co-digestion of untreated corn stover and food waste, respectively. The above findings indicated that the digestion performance and biomethane yield of co-digestion could be improved by shortening the asynchronism of two different substrates. Kinetic constant is referred to the reaction rate of one substrate. Table 3 shows the biomethane yield per unit-VS and kinetic constant in mono-digestion and co-digestion of food waste and untreated corn stover. The kinetic constants were in the ranges of 0.125~0.183 and 0.128~0.159 for the co-digestion at organic loadings of 35 and 45 g-VS/L, respectively. It can be seen that the co-digestion with C/N ratio of 20 achieved the highest kinetic constants and relatively higher biomethane yield per unit-VS at two organic loadings, implying that optimized C/N ratio of co-digestion could improve digestion rate and biomethane yield. Table 4 shows the biomethane yield per unit-VS and kinetic constant in co-digestion of food waste and pretreated corn stover. The kinetic constants in co-digestion with pretreated corn stover were all higher than those with untreated corn stover at the same organic loadings (see Table 3 and Table 4), correspondingly, biomethane yield per unit-VS were enhanced by 0.5%~10.6%. 4. Conclusions 9
Although the anaerobic co-digestion performance for biomethane production was conventionally regarded as C/N- ratio-dependent, the accessibility and digestibility of different substrates showed even more important effects. Food waste and corn stover have great differences in compositional characteristics and digestibility, and presented obvious asynchronism in co-digestion. The asynchronism could further negatively affected the biomethane production. The digestibility synchronism of food waste and corn stover could be significantly improved through enhancing the accessibility and digestibility of corn stover by NaOH pretreatment. Consequently, the biomethane production performance would be improved by minimizing asynchronism of the two substrates.
Nomenclature AD=Anaerobic digestion BMP=Biochemical methane potential C/N ratio= Carbon to nitrogen ratio DMY=Daily biomethane yield MLSS=Mixed liquid suspended solids OLR=Organic loading rate VS=Volatile solid VFA=Volatile fatty acid TC=Total carbon TCOD=Total chemical oxygen demand TMY=Theoretical methane yield TN=Total nitrogen TS=Total solid
Acknowledgements: The authors are grateful to the fund supports from the University Doctorial Foundation (No. 20120010110004) and Beijing Natural Science Foundation (8142030).
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12
800
co-digestion (C/N=30)
DMY (mL)
600 400 200
8 0 800
co-digestion (C/N=25)
DMY (mL)
600 400 200
10
0 800
co-digestion (C/N=20)
DMY (mL)
600 400
5
200 0 0
10
20
30
40
50
Time (d)
Fig. 1. Daily biomethane yield at 35 g-VS/L from the co-digestion of the food waste and untreated corn stover
13
800
co-digestion (C/N=30)
DMY (mL)
600 400 200
10
0 800
co-digestion (C/N=25)
DMY (mL)
600 400 200
12 0 800
co-digestion (C/N=20)
DMY (mL)
600 400
7
200 0 0
10
20
30
40
50
Time (d)
Fig. 2. Daily biomethane yield at 45 g-VS/L from the co-digestion of the food waste and untreated corn stover
14
800
co-digestion (C/N=30)
DMY (mL)
600 400 200
7
0 800
co-digestion (C/N=25)
DMY (mL)
600 400 200
4
0 800
co-digestion (C/N=20)
DMY (mL)
600 400 200
1
0 0
10
20
30
40
50
Time (d)
Fig. 3. Daily biomethane yield at 35 g-VS/L from the co-digestion of food waste and the pretreated corn stover
15
800
co-digestion (C/N=30)
DMY (mL)
600 400
9
200 0 800
co-digestion (C/N=25)
DMY (mL)
600 400 200
7 0 800
co-digestion (C/N=20)
DMY (mL)
600 400
4
200 0 0
10
20
30
40
50
Time (d)
Fig. 4. Daily biomethane yield at 45 g-VS/L from the co-digestion of food waste and the pretreated corn stover
16
Table 1 Characteristics of food waste, corn stover and inoculum Items pH
a
Food waste
Corn stover
Inoculum
5.02
-
7.44
TS (%)
22.71
94.48
11.13
VS (%)
20.72
86.76
5.36
b
48.30
49.77
19.09
TC (%)
c
TN (%)
2.56
0.78
2.14
C/N
18.9
63.5
8.9
d
19581.14
-
140.65
e
VFAs (mg/L)
Cellulose (%)
4.12
38.81
5.09
Hemi-cellulose (%)e
9.68
29.50
12.56
Lignin (%)e
1.80
7.10
4.18
Crude fat (%)
30.40
-
2.92
TCOD (mg/L)
198951.7
-
17841.3
a. The pH value was measured by pH meter (CHN868, Thermo Orion, America) b. TC was determined by an elemental analyzer (Vario EL/micro cube elemental analyzer, Germany) c. TN was determined by the total Kjeldahl nitrogen analyzer (Model KDN-2C, Shanghai). d. The VFAs were determined by a gas chromatography (GC-2014, Shimadzu, Japan) e. The contents of cellulose, hemi-cellulose and lignin were measured by the extraction unit according to the procedures proposed by Van Soest et al. (Van Soest et al., 1991).
17
Table 2 Elemental compositions, C/N ratios, and theoretical methane yields of food waste and corn stover Elemental composition
C/N
(wt., %, TS)
C
Food waste Corn stover
H
O
Number of atoms
Theoretical methane
N
a
b
c
d
yield (mL/g-VS)
Actual Biometha
Biodegra
ne yield
dability
(mL/g-VS
(%)
)
48.30
6.91
27.7
2.56
18.9
22.01
37.79
9.47
1.00
622
507
81.51
49.77
6.24
34.35
0.78
63.5
74.44
112.00
38.53
1.00
564
311
55.14
18
Table 3 Biomethane yield per unit-VS and kinetic constant in mono-digestion and co-digestion of food waste and untreated corn stover Biomethane yield per
Kinetic constant
unit-VS OL=35
OL=45
g-VS/L
g-VS/L
k1 (OL=35 g-VS/L)
R2
k2 (OL=45 g-VS/L)
R2
Co-digestion (C/N=20)
311.5
311.8
0.183
0.9564
0.159
0.9052
Co-digestion (C/N=25)
283.3
286.9
0.131
0.9342
0.144
0.8558
Co-digestion (C/N=30)
216.3
255.6
0.125
0.9459
0.128
0.9048
330.9
250.5
0.167
0.8881
0.140
0.8770
214.8
220.3
0.152
0.9544
0.115
0.9235
Mono-digestion (food waste) Mono-digestion (corn stover)
19
Table 4 Biomethane yield per unit-VS and kinetic constant in co-digestion of food waste and pretreated corn stover Biomethane yield per
Kinetic constant
unit-VS OL=35
OL=45
g-VS/L
g-VS/L
Co-digestion (C/N=20)
329.4
Co-digestion (C/N=25) Co-digestion (C/N=30)
k1 (OL=35 g-VS/L)
R2
k2 (OL=45 g-VS/L)
R2
345.0
0.198
0.9505
0.176
0.9164
299.2
288.5
0.161
0.8799
0.162
0.9113
238.7
263.7
0.147
0.9361
0.139
0.9207
20
Highlights
Food waste and corn stover were anaerobically digested with altering OLRs at 35℃.
The asynchronism in co-digestion of two substrates was investigated in this work.
Corn stover was pretreated to enhance the accessibility and digestibility.
The digestibility synchronism of food waste and corn stover was improved.
0.55%-12.23% enhancement was achieved in co-digestion with pretreated corn stover.
21
Equations C +
4 − − 2 + 3 4 + − 2 − 3 4 − + 2 + 3 → + . 1 4 8 8 =
4 + − 2 − 3 × 2.8 $ ,% 12 + + 16 + 14 ' ( Biodegradability =
8 = 9 × :;< =−:;< >
&
. 2
5 × 100% . 3
?@ × : A − B + 1CD . 4 9
( = −E( . 5 B
22
S0960-8524(14)00614-2 http://dx.doi.org/10.1016/j.biortech.2014.04.074 BITE 13370
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
19 January 2014 13 April 2014 21 April 2014
Please cite this article as: Zhou, Q., Shen, F., Yuan, H., Zou, D., Liu, Y., Zhu, B., Jaffu, M., Chufo, A., Li, X., Minimizing asynchronism to improve the performances of anaerobic co-digestion of food waste and corn stover, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.04.074
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Minimizing asynchronism to improve the performances of anaerobic co-digestion of food waste and corn stover Qi Zhoua , Fei Shena,b, Hairong Yuana, Dexun Zoua, Yanping Liua, Baoning Zhua, Muhanmad Jaffua, Akiber Chufoa, Xiujin Lia* a
Centre for Resource and Environmental Research, Beijing University of Chemical Technology, No. 15 Beisanhuan East Road,
Chaoyang District, Beijing 100029, P. R. China b
Institute of Ecological and Environmental Sciences, Sichuan Agricultural University, Chengdu, Sichuan 611130, P. R. China
Abstract: To investigate the existence of the asynchronism during the anaerobic co-digestion of different substrates, two typical substrates of food waste and corn stover were anaerobically digested with altering organic loadings (OL). The results indicated that
the biodegradability of food waste and corn stover was calculated to be 81.5% and 55.1%, respectively, which was main reason
causing the asynchronism in the co-digestion. The asynchronism was minimized by NaOH-pretreatment for corn stover, which could
improve the biodegradability by 36.6%. The co-digestion with pretreatment could increase the biomethane yield by 12.2%, 3.2% and
0.6% comparing with the co-digestion without pretreatment at C/N ratios of 20, 25 and 30 at OL of 35 g-VS/L, respectively. The
results indicated that the digestibility synchronism of food waste and corn stover was improved through enhancing the accessibility
and digestibility of corn stover. The biomethane production could be increased by minimizing the asynchronism of two substrates in
co-digestion.
Keywords: Asynchronism minimization; Anaerobic co-digestion; Food waste; Corn stover 1. Introduction Food waste mainly come from hotels, restaurants, and canteens of universities and enterprises, etc.. Food waste amount kept increasing over last decades due to the rapid growth of population in cities and fast economy development in China (Jie et al., 2012). Taking Beijing city as an example, approximately 1600 tons of food waste was collected daily in 2012. Currently, food waste either goes to animal farms as feedstock or to landfill in most cities. Serious health threat associated with food waste as animal feedstock has attracted great public attentions. Food waste landfill has also caused serious environmental problems, such as increased leachate amount and *
Corresponding author. Addresses: No. 15 Beisanhuan East Road, Chaoyang District, Beijing, 100029, P. R. China. Tel/fax.: +86 10 64432281. E-mail addresses: [email protected] or [email protected]. 1
treatment cost, and offensive odor emission, due to high moisture content and readily biodegradable components with food waste. Anaerobic digestion (AD) is extensively acceptable as an efficient process to treat and utilize food waste because it has been proven to be a promising method for waste reduction and energy recycling (MacLellan et al., 2013). Food waste contains rich nutrients and is a superior substrate for AD. However, as a single organic waste stream, food waste is rich in nitrogen sources such as protein, leading to low carbon-nitrogen ratio (C/N) (13.8~18.2) (Khalid et al., 2011; Pan et al., 2008; Wang et al., 2012; Zhang et al., 2012). Generally, low C/N ratio is not proper for high efficient AD due to the inhibition from extra total ammonia nitrogen in the food waste (Pan et al., 2008; Chen et al., 2008). On the other hand, AD of mono-substrate food waste could also suffer sensitive acidification and instability as organic loading rate (OLR) was increased over 4 kg-VS/(m3·d) or 35 g-VS/L(Liu and Liu, 2011; Nagao et al., 2012). Anaerobic co-digestion could offer the possibility of balancing carbon and nitrogen in the system when food waste is digested with other substrate. Correspondingly, the digestion performances and operations could be maintained more efficiently and stably (Ganesh et al., 2013). To seek some stable carbon sources, lignocellulosic biomass should be a better option, especially for crop residues, because of their easy availability and collectability in China. Thus, the co-digestion of food waste with crop residues could be potentially digested to improve the performance of digestion system. However, crop residues are lignocellulosic biomass, which presented the characteristics and composition of difficult digestibility comparing with food waste. When co-digested, each substrate would pose different biodegradable rate, which could cause asynchronism of digestion rate, and may potentially affect mass bioconversion and biogas production adversely. Xie et al (2011) evaluated the operation stability and methane production potential of pig manure and grass silage co-digestion, and obtained a high specific methane yield in a specific ratio. They also found appearance of several biogas peaks in the digestion process and attributed it to the asynchronism of characteristics and composition between pig manure and grass 2
silage. Similar phenomena were found in other cases of co-digestion (Lin et al., 2011; Riaño et al., 2011; Zhang et al., 2011). Although C and N could be balanced by mixing two feedstocks for digestion, the asynchronism of digestibility could pose adverse effect on the stability of digestion system and yield of methane. Therefore, it is necessary to go further to investigate asynchronism and effect in the co-digestion with different substrates. This study was to evaluate the performances of anaerobic co-digestion of food waste and corn stover. Corn stover was used to adjust C/N ratio at different proportions. NaOH was applied to pretreat corn stover to improve biodegradability of corn stover, and to minimize asynchronism of both substrates for efficient digestion. 2. Materials and methods 2.1 Feedstock characteristics Food waste was collected from the canteen of Beijing University of Chemical Technology. After removing the indigestible materials such as bones, plastic bags, egg shells, and chopsticks, it was homogenized into slurry by a food waste homogenizer (SS2600, Meijiamada Co., Zhenjiang, China). The homogenizer was run at the speed of 2600 rpm. Corn stover was collected from rural area in Shunyi District, Beijing, China. The corn stover was chopped into 30 to 40 mm in length and then ground into a desired size of 5~10 mm by a hammer mill (FE130, Staida Co., Tianjin, China). The homogenized food waste was stored in a freezer at -20℃ for later digestion tests. The characteristics of food waste and corn stover are listed in Table 1. 2.2 Inoculum The activated sludge for seeding was collected from an AD system for swine manure treatment in Shunyi, Beijing. The total solid (TS), volatile solid (VS), and mixed liquid suspended solids (MLSS) were 11.13%, 5.36%, and 82.11 g/L, respectively. The seeding sludge was stored at 4℃ after collection. The sludge was incubated at 35℃ for a week before it was inoculated for AD. The characteristics and composition of the seeding sludge are also presented in Table 1. 2.3 Biochemical methane potential (BMP) test 3
Biochemical methane potential tests in this study were based on the Elbeshbishy’s protocols (Elbeshbishy et al., 2012). The tests were performed in the 250 mL serum bottles with 200 mL working volume. Food waste and inoculum were added to the bottles at the mixture ratio of 1:4 (VS/VS) with the initial pH of 7.0. The bottles were sealed by rubber plugs and parafilm. Afterwards, the bottles were incubated at 35±1℃, and shaken in a frequency of 120 rpm twice a day. The daily biomethane yield and the methane content were monitored every day. All tests were run in triplicate and the control group (inoculum was added only) was set. The BMP tests for corn stover were performed with similar protocols. 2.4 AD in batch Batch digestions were carried out on 1 L serum bottles with the working volume of 0.8 L. After the substrates and seeding sludge were loaded in the bottles, the sealed digestion bottles were operated in a 35±1℃ thermostat water bath with shaking frequency of 120 rpm twice per day. The co-digestion of food waste and corn stover and their corresponding mono-digestion were all performed using the mentioned digestion system. Each run in this work was repeated in triplicate. The biogas production and the methane content were recorded every day and employed for evaluating the digestion performances. 2.5 Chemical pretreatment to corn stover In order to improve the co-digestion, the NaOH-pretreated corn stover was also employed in this work. According to the referred work (Zheng et al., 2009), the ground stover was pretreated in sealed bags at ambient temperature for three days. The mixing ratio of corn stover (dry basis), NaOH, and tap water were controlled at 1:0.02:6 (wt. /wt.) for the pretreatment. The pretreated corn stover was washed 10~15 times with tap water till the pH of the washed water was in the range of 7.3~7.6. Afterwards, the washed corn stover was stored in 4℃ for digestion tests (Dongyan et al., 2003). 2.6 Analysis methods The element composition in the employed substrates was analyzed by a Vario EL/micro cube elemental 4
analyzer (Elementar, Germany). The organic fraction of the food waste and corn stover can be expressed in the molecular formula of
. Theoretical methane yield (TMY) is the maximum volume of methane when
the substrate was converted completely in standard temperature and pressure. Based on the element analysis, the TMY of one substrate can be calculated by Eq. 1 and Eq. 2 (Sosnowski et al., 2003). The biodegradability of the substrate can be calculated by Eq.3, correspondingly.
where VT is theoretical methane production, and VC is biomethane production. Biogas production in BMP tests and batch digestion was both measured by water displacement method. Methane content in the biogas was determined by gas chromatography (SP-2100, China) equipped with a molecular sieve packed stainless-steel column with the length and diameter of 2.0 m×3.0 mm (TDX-01) and a thermal conductivity detector (TCD). The detailed determination protocol for the methane content was in accord with the reference (Zheng et al., 2009). The standard gas with 5.0% N2, 60.1% CH4, and 34.9% CO2 was used for the standard calibration for the determination. The biomethane production was calculated based on the methane content and biogas production. In order to assess the biomethane production rate, the Gompertz model (Eq.4) was employed to fit the curve of cumulative methane yield (Li et al., 2010). Based on the Gompertz model, the lag phase during the digestion could be determined. Subsequently, the digestion rate can be clarified by the first-order kinetic model (Eq.5) (Ching-Shyung et al., 1996; Group, 2002)
5
where, M was the biomethane yield at t time, mL/(g-VS·d), P was the final biomethane yield, mL/g-VS, R m was the maximum daily biomethane yield, mL/(g-VS·d),
was the lag phase, d.
where k is the first order kinetic constant (d-1), t is the digestion time (d), S represents the residual substrate (organics) concentration (mg/L). 3. Results and discussion 3.1 Evaluation of biochemical methane potential and biodegradability The elemental compositions of food waste and corn stover are summarized in Table 2. The organic fraction of food waste and corn stover were represented with formulation
and
,
respectively. According to Eq. 2, the TMYs of food waste and corn stover were 622 mL-CH4/g-VS and 564 mL-CH4/g-VS, respectively, which were similar to the results of 360~551 mL-CH4/g-VS in the referred work (Liu et al., 2009; Raposo et al., 2012). As shown in Table 2, the actual biomethane yields of 507 mL/g-VS and 311 mL/g-VS were obtained for food waste and corn stover, respectively, corresponding to 81.5% and 55.1% of TMYs of two substrates. It can be found that food waste presented a higher biodegradability comparing with corn stover. This was mainly attributed to the readily-biodegradable ingredients of starch, protein, and fat in food waste in contrast to the hard-biodegradable compositions of cellulose, hemicellulose, and lignin in corn stover (Wang et al., 2007). In addition, the C/N ratio of food waste was 18.9, which was much closer to the generally-defined range (20~30) compared with the corn stover (63.5) (Estevez et al., 2012; Yadvika et al., 2004). Previous research was mainly focused on balancing C and N in co-digestion, while neglecting the effect of difference in biodegradability of substrates applied (Zhong et al., 2012; Yen and Brune, 2007). Actually, the different biodegradability of the employed substrates would cause possible asynchronism in co-digestion, and potentially affect mass
6
bioconversion and biogas production adversely. 3.2 Investigation on asynchronism in co-digestion As external carbon source, corn stover was employed to adjust C/N ratio of substrates to the optimal range of 20~30. Corn stover was mixed with food waste in three different proportions to make the C/N ratio of the substrate mixture at 20, 25, and 30, respectively. The mixture substrate of food waste and corn stover were anaerobically co-digested at organic loadings of 35 g-VS/L and 45 g-VS/L. The results are shown in Fig. 1 and Fig. 2. Fig. 1 presents the daily biomethane yields for co-digestion at organic loading of 35 g-VS/L. Similar trends were observed for three C/N ratios. Two obvious peaks of daily biomethane yield were observed during the digestion process. The appearance of two peaks could be attributed to the asynchronism of biodegradability of food waste and corn stover. The first peak was mainly contributed by food waste as it was readily biodegradable and digested in relatively early stage, while the second peak by corn stover as it was relatively hard biodegradable and took longer time for digesting. The finding was agreed with previous research, reporting that several biogas peaks appeared during co-digestion processes (Lin et al., 2011; Xie et al., 2011). Furthermore, the gap between two peaks was changed for different C/N ratios. The gap between two peaks was 5 days for C/N ratio of 20, and increased to 10 and 8 days for C/N ratios of 25 and 30, respectively. Correspondingly, the biomethane yield per unit VS decreased from 311.5 mL/g-VS to 216.3 mL/g-VS (Table 3). In addition, the biomethane yield per unit VS for co-digestion were generally lower than those of food waste mono-digestion. The relatively lower biomethane yield with co-digestion indicated the adverse effect by asynchronism of digestion substrates, although all C/N ratios were within the optimal ranges. As shown in Fig. 2, the organic loading was increased to 45 g-VS/L, while the C/N ratio was also adjusted by corn stover within the optimal range of 20~30. The biomethane yield per unit VS in co-digestion were generally higher than those in mono-digestion. It may be attributed to the improvement of buffer capability of co-digestion 7
system (Marañón et al., 2012; Zhang et al., 2013). AD of mono-substrate food waste at high OLR often suffered serious acidification due to the lower buffer capability. More importantly, although the C/N ratio was adjusted to the optimal range (20~30), the biomethane yield per unit-VS descended significantly when the C/N ratio was increased from 20 to 30. This trend was similar to that at organic loading of 35 g-VS/L. According to the difference between the peaks of daily biomethane yield, the gap at 45 g-VS/L extended to 7 d, 12 d, and 10 d for C/N ratio of 20, 25, and 30, respectively. Compared with the runs with 35 g-VS/L, the gaps were extended. The result again proved that the increased digestibility asynchronism of substrates could adversely impacted the synergism from the nutrition balance of carbon and nitrogen sources in co-digestion. As a result, the asynchronism of the different substrates indeed existed in the anaerobic co-digestion, which would adversely affect digestion performances. Therefore, it was necessary to consider the digestibility synchronism of different substrates in anaerobic co-digestion, and it should be a possible method to improve digestion performance by minimizing the asynchronism. 3.3 Minimizing asynchronism by corn stover pretreatment Asynchronism needs to be minimized in order to achieve high digestion efficiency and biomethane yield. According to the referred work, the digestibility of lignocellulosic biomass can be improved significantly through pretreatment (Zheng et al., 2009; Chandra et al., 2012). NaOH was proven to be one of effective pretreatment agent, therefore, applied for corn stover in this study (Luo et al., 2005). The purpose of NaOH pretreatment was to enhance corn stover biodegradability, and to narrow the gap and asynchronism between corn stover and food waste. The experiments on the co-digestion of food waste and NaOH-pretreated corn stover were thereby carried out at two organic loadings of 35 and 45 g-VS/L, and the daily biomethane yields are presented in Fig. 3 and Fig. 4, respectively. To compare Fig. 1 with Fig. 3, it can be seen that two typical biomethane peaks were found in co-digestion of food waste and pretreated corn stover at C/N ratios of 20, 25, and 30 for organic loading of 35 g-VS/L. The gap 8
between two peaks became greater with the increase of C/N ratio from 20 to 30, the trends of which were similar to those of the co-digestion of food waste and untreated corn stover. However, the gaps with pretreated corn stover were 1, 4 and 7 days, significantly shorter than those of the co-digestion with untreated corn stover. The narrowed gaps implied that the biodegradability of pretreated corn stover was enhanced, and the digestion asynchronism of pretreated corn stover and food waste was thereby shortened. This was further verified by increased biomethane yield per unit-VS with co-digestion of pretreated corn stover and food waste (Table 3 and 4), which achieved 5.7%, 5.6%, and 10.3% more biomethane yield per unit-VS than those of co-digestion with untreated corn stover at corresponding C/N ratios, respectively. The same results were found for the co-digestion of pretreated corn stover and food waste at organic loading of 45 g-VS/L (see Fig. 4). The gap of two peaks were 4, 7, and 9 days for the C/N ratios of 20, 25, and 30, which was 3, 5, and 1 day shorter than those of the co-digestion of untreated corn stover and food waste, respectively. The above findings indicated that the digestion performance and biomethane yield of co-digestion could be improved by shortening the asynchronism of two different substrates. Kinetic constant is referred to the reaction rate of one substrate. Table 3 shows the biomethane yield per unit-VS and kinetic constant in mono-digestion and co-digestion of food waste and untreated corn stover. The kinetic constants were in the ranges of 0.125~0.183 and 0.128~0.159 for the co-digestion at organic loadings of 35 and 45 g-VS/L, respectively. It can be seen that the co-digestion with C/N ratio of 20 achieved the highest kinetic constants and relatively higher biomethane yield per unit-VS at two organic loadings, implying that optimized C/N ratio of co-digestion could improve digestion rate and biomethane yield. Table 4 shows the biomethane yield per unit-VS and kinetic constant in co-digestion of food waste and pretreated corn stover. The kinetic constants in co-digestion with pretreated corn stover were all higher than those with untreated corn stover at the same organic loadings (see Table 3 and Table 4), correspondingly, biomethane yield per unit-VS were enhanced by 0.5%~10.6%. 4. Conclusions 9
Although the anaerobic co-digestion performance for biomethane production was conventionally regarded as C/N- ratio-dependent, the accessibility and digestibility of different substrates showed even more important effects. Food waste and corn stover have great differences in compositional characteristics and digestibility, and presented obvious asynchronism in co-digestion. The asynchronism could further negatively affected the biomethane production. The digestibility synchronism of food waste and corn stover could be significantly improved through enhancing the accessibility and digestibility of corn stover by NaOH pretreatment. Consequently, the biomethane production performance would be improved by minimizing asynchronism of the two substrates.
Nomenclature AD=Anaerobic digestion BMP=Biochemical methane potential C/N ratio= Carbon to nitrogen ratio DMY=Daily biomethane yield MLSS=Mixed liquid suspended solids OLR=Organic loading rate VS=Volatile solid VFA=Volatile fatty acid TC=Total carbon TCOD=Total chemical oxygen demand TMY=Theoretical methane yield TN=Total nitrogen TS=Total solid
Acknowledgements: The authors are grateful to the fund supports from the University Doctorial Foundation (No. 20120010110004) and Beijing Natural Science Foundation (8142030).
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800
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600 400 200
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Fig. 1. Daily biomethane yield at 35 g-VS/L from the co-digestion of the food waste and untreated corn stover
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Fig. 2. Daily biomethane yield at 45 g-VS/L from the co-digestion of the food waste and untreated corn stover
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20
30
40
50
Time (d)
Fig. 3. Daily biomethane yield at 35 g-VS/L from the co-digestion of food waste and the pretreated corn stover
15
800
co-digestion (C/N=30)
DMY (mL)
600 400
9
200 0 800
co-digestion (C/N=25)
DMY (mL)
600 400 200
7 0 800
co-digestion (C/N=20)
DMY (mL)
600 400
4
200 0 0
10
20
30
40
50
Time (d)
Fig. 4. Daily biomethane yield at 45 g-VS/L from the co-digestion of food waste and the pretreated corn stover
16
Table 1 Characteristics of food waste, corn stover and inoculum Items pH
a
Food waste
Corn stover
Inoculum
5.02
-
7.44
TS (%)
22.71
94.48
11.13
VS (%)
20.72
86.76
5.36
b
48.30
49.77
19.09
TC (%)
c
TN (%)
2.56
0.78
2.14
C/N
18.9
63.5
8.9
d
19581.14
-
140.65
e
VFAs (mg/L)
Cellulose (%)
4.12
38.81
5.09
Hemi-cellulose (%)e
9.68
29.50
12.56
Lignin (%)e
1.80
7.10
4.18
Crude fat (%)
30.40
-
2.92
TCOD (mg/L)
198951.7
-
17841.3
a. The pH value was measured by pH meter (CHN868, Thermo Orion, America) b. TC was determined by an elemental analyzer (Vario EL/micro cube elemental analyzer, Germany) c. TN was determined by the total Kjeldahl nitrogen analyzer (Model KDN-2C, Shanghai). d. The VFAs were determined by a gas chromatography (GC-2014, Shimadzu, Japan) e. The contents of cellulose, hemi-cellulose and lignin were measured by the extraction unit according to the procedures proposed by Van Soest et al. (Van Soest et al., 1991).
17
Table 2 Elemental compositions, C/N ratios, and theoretical methane yields of food waste and corn stover Elemental composition
C/N
(wt., %, TS)
C
Food waste Corn stover
H
O
Number of atoms
Theoretical methane
N
a
b
c
d
yield (mL/g-VS)
Actual Biometha
Biodegra
ne yield
dability
(mL/g-VS
(%)
)
48.30
6.91
27.7
2.56
18.9
22.01
37.79
9.47
1.00
622
507
81.51
49.77
6.24
34.35
0.78
63.5
74.44
112.00
38.53
1.00
564
311
55.14
18
Table 3 Biomethane yield per unit-VS and kinetic constant in mono-digestion and co-digestion of food waste and untreated corn stover Biomethane yield per
Kinetic constant
unit-VS OL=35
OL=45
g-VS/L
g-VS/L
k1 (OL=35 g-VS/L)
R2
k2 (OL=45 g-VS/L)
R2
Co-digestion (C/N=20)
311.5
311.8
0.183
0.9564
0.159
0.9052
Co-digestion (C/N=25)
283.3
286.9
0.131
0.9342
0.144
0.8558
Co-digestion (C/N=30)
216.3
255.6
0.125
0.9459
0.128
0.9048
330.9
250.5
0.167
0.8881
0.140
0.8770
214.8
220.3
0.152
0.9544
0.115
0.9235
Mono-digestion (food waste) Mono-digestion (corn stover)
19
Table 4 Biomethane yield per unit-VS and kinetic constant in co-digestion of food waste and pretreated corn stover Biomethane yield per
Kinetic constant
unit-VS OL=35
OL=45
g-VS/L
g-VS/L
Co-digestion (C/N=20)
329.4
Co-digestion (C/N=25) Co-digestion (C/N=30)
k1 (OL=35 g-VS/L)
R2
k2 (OL=45 g-VS/L)
R2
345.0
0.198
0.9505
0.176
0.9164
299.2
288.5
0.161
0.8799
0.162
0.9113
238.7
263.7
0.147
0.9361
0.139
0.9207
20
Highlights
Food waste and corn stover were anaerobically digested with altering OLRs at 35℃.
The asynchronism in co-digestion of two substrates was investigated in this work.
Corn stover was pretreated to enhance the accessibility and digestibility.
The digestibility synchronism of food waste and corn stover was improved.
0.55%-12.23% enhancement was achieved in co-digestion with pretreated corn stover.
21
Equations C +
4 − − 2 + 3 4 + − 2 − 3 4 − + 2 + 3 → + . 1 4 8 8 =
4 + − 2 − 3 × 2.8 $ ,% 12 + + 16 + 14 ' ( Biodegradability =
8 = 9 × :;< =−:;< >
&
. 2
5 × 100% . 3
?@ × : A − B + 1CD . 4 9
( = −E( . 5 B
22