Waste Management xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure Ningning Zhai a,b, Tong Zhang a,b, Dongxue Yin a,c, Gaihe Yang a,b,⇑, Xiaojiao Wang a,b, Guangxin Ren a,b, Yongzhong Feng a,b a b c
College of Agronomy, Northwest A&F University, Shaanxi, Yangling 712100, China Research Center for Recycling Agriculture Engineering Technology of Shaanxi Province, Shaanxi, Yangling 712100, China College of Forestry, Northwest A&F University, Shaanxi, Yangling 712100, China
a r t i c l e
i n f o
Article history: Received 9 August 2014 Accepted 16 December 2014 Available online xxxx Keywords: Initial pH Kitchen waste Cow manure Anaerobic co-digestion Modified Gompertz equation
a b s t r a c t This study investigated the effects of different initial pH (6.0, 6.5, 7.0, 7.5 and 8.0) and uncontrolled initial pH (CK) on the lab-scale anaerobic co-digestion of kitchen waste (KW) with cow manure (CM). The variations of pH, alkalinity, volatile fatty acids (VFAs) and total ammonia nitrogen (NH+4–N) were analyzed. The modified Gompertz equation was used for selecting the optimal initial pH through comprehensive evaluation of methane production potential, degradation of volatile solids (VS), and lag-phase time. The results showed that CK and the fermentation with initial pH of 6.0 failed. The pH values of the rest treatments reached 7.7–7.9 with significantly increased methane production. The predicted lag-phase times of treatments with initial pH of 6.5 and 7.5 were 21 and 22 days, which were 10 days shorter than the treatments with initial pH of 7.0 and 8.0, respectively. The maximum methane production potential (8579 mL) and VS degradation rate (179.8 mL/g VS) were obtained when the initial pH was 7.5, which is recommended for co-digestion of KW and CM. Ó 2015 Published by Elsevier Ltd.
1. Introduction Kitchen waste (KW) is a widely produced municipal solid waste (MSW) with high yield, and it accounts for about 60% of the total MSW collected and transported in China in 2006 (Zhang et al., 2010). KW is nutrient-rich organic material with a tremendous potential of energy recovery by anaerobic digestion (AD) (Jiang et al., 2010, 2013; Lee et al., 2008). Due to its high water content, producing biogas through the AD of KW benefits more than nonbiological disposal methods such as incineration and landfill (Hecht and Griehl, 2009). In addition, after further chemical, biological and physical processing (Arthurson, 2009), the nutrients in digested residues could be recycled back to agriculture and horticulture, thus creating resource recycling society (Banks et al., 2011; Yamashiro et al., 2013) because the organic nitrogen is converted to ammonia, which can be directly used by plants (Luostarinen and Rintala, 2007).
⇑ Corresponding author. Addresses: College of Agronomy, Northwest A&F University, Yangling, 712100 Shaanxi, China and The Research Center of Recycle Agricultural Engineering and Technology of Shaanxi Province, Yangling 712100 Shaanxi, China. Tel./fax: +86 029 8709 2265. E-mail address: [email protected] (G. Yang).
However, the high moisture content in KW makes it easier to become sour and smell (Shahriari et al., 2013), and the high contents of fat and protein in KW and degradation compounds (for example, inhibitory long-chain fatty acids, ammonia or sulphides) affect the utilization of degradable components in the anaerobic process (Braun et al., 2003; Hanaki et al., 1981; Koster and Cramer, 1987). Cho et al. (1995) showed that at an early stage of AD, the soluble organics were converted to VFAs rapidly, resulting in a drastic pH drop and lower biogas production if no efficient pretreatments were adopted. Thus, AD of KW often performs poor buffering capacity and lower biogas production (Banks et al., 2008). To stabilize the anaerobic process and increase methane yields, co-digestion has been considered as an effective approach. Studies reported that co-digestion of KW with other substrates could increase alkalinity and buffering capacity, reduce the inhibition caused by VFAs concentrations, and increase methane yield (Alatriste-Mondragon et al., 2006; Li et al., 2009; Yang et al., 2013). However, mixing KW with other substrates for digestion is not a perfect method for improving methane production. Additional measures can be applied to increase methane production of co-digestion process (Esposito et al., 2012). The culture pH affects the activities of specific acidogenic microbial populations (Zhang et al., 2012) and methanogenic bacteria (Ghosh et al.,
http://dx.doi.org/10.1016/j.wasman.2014.12.027 0956-053X/Ó 2015 Published by Elsevier Ltd.
Please cite this article in press as: Zhai, N., et al. Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.027
2
N. Zhai et al. / Waste Management xxx (2015) xxx–xxx
2000), thereby the stability of co-digestion process. Thus, pH is a pivotal factor that affects the methane production efficiency (Jiang et al., 2010; Liu et al., 2008). Methods for pH control would effectively increase methane production. A study has shown that the optimal range of pH for methane production in AD was 6.5– 7.5 (Liu et al., 2008). But the range is relative wide and the optimal pH value varies with different substrates and digestion techniques. Furthermore, the initial pH is not controlled in almost studies, although an appropriate initial pH value is important for hydrolysis reaction of AD, especially for substrates like KW, for which acidification happens easily at the beginning of digestion (Cho et al., 1995). Thus, a suitable initial pH value should be chosen for digestion process to enhance methane production rate and VS reduction rate. This study aimed to investigate the effects of five different initial pH values (6.0, 6.5, 7.0, 7.5, and 8.0) on co-digestion of KW and CM in order to obtain the most stable digestion environment and the maximum methane production. The variations of pH, total alkalinity, VFAs, and NH+4–N were analyzed and compared. Methane production potential, degradation of volatile solids and lagphase time were also assessed by the ‘‘modified Gompertz equation’’. The optimal initial pH value was then obtained. 2. Methods 2.1. Substrates and inoculant KW used in the study was collected from the student union cafeterias of the Northwest A&F University in Yangling, China. It consisted of rice, noodle, fried vegetables, meat, and fish, etc. After the removal of bones, napkins, spoons, and other inorganic matters, the KW was crushed into slurry state using a food waste disposer, and subsequently stored in a refrigerator at 4 °C for later use. CM was gathered from a livestock farm located in Yangling, China. Mesophilic anaerobic digested sludge used for inoculant was obtained from a normal operation anaerobic digester in a village in Yangling, China. The CM and the inoculant were individually homogenized for later use. The characteristics of KW, CM and inoculant are presented in Table 1. 2.2. Experimental set-up All experiments were conducted with the total solid content of 8%, and the mixing ratio of KW and CM was 1:1 (wet weight basis). Lab-scale anaerobic digesters with working volume of 1 L contained 700 mL total mixture with 200 mL inoculant. The digestion tests were conducted in triplicate for 45 days based on the method described in Wang et al. (2012). The effect of initial pH was investigated by operating the reactors at 35 °C with pH of 6.0, 6.5, 7.0, 7.5, 8.0, and uncontrolled initial pH, labeled as R6.0, R6.5, R7.0,
Fig. 1. Curve of Modified Gompertz Model.
R7.5, R8.0 and CK, respectively. Concentrated NaOH was used to adjust the initial pH to the values previously selected (Lay et al., 1997). All reactors were tightly closed with rubber septa and screw caps. The head space of each reactor was flushed with N2 for approximately 3 min to stabilize anaerobic condition prior to start of the digestion tests. The temperature was controlled using a water circulator. Moreover, all reactors were manually shaken for approximately 1 min daily to mix the reactor contents prior to the measurement of biogas volume. 2.3. Analytical methods TS, VS, total kjeldahl nitrogen (TKN), total organic carbon, alkalinity and VFAs were measured according to the procedures described in the Standard Methods (APHA, 1998). The pH and methane proportion were determined by biogas analyzer (Gasboard-3200P). The contents of lignin, cellulose, and hemicelluloses were determined by raw fiber determination analyzer (Model CXC06, Shanghai). Biogas volume was measured every 24 h by the water displacement method, and pH, alkalinity, and NH+4–N were determined every 5 days (alkalinity, VFAs and NH+4–N, were detected starting from the fifth day). For the sake of analysis, the fermentation process was divided into 3 phases: phase A with no methane production (1–15 days), phase B with growing methane production (16–30 days) and phase C with stable methane production (31–45 days). 2.4. Modified Gompertz equation The cumulative methane production rate was described using the modified Gompertz equation developed by Lay et al. (1996, 1997):
Re M ¼ P exp exp ðk t Þ þ 1 P
Table 1 Characteristics of KW, CM, and Inoculant used in this study. Parameter
KW
CM
Inoculant
pH TS (%) VSa (%) Organic carbona (g/kg VS) TKNa (g/kg VS) C/N Cellulose (%) Hemicelluloses (%) Lignin (%)
4.2 ± 0.23 23.19 ± 0.54 95.69 ± 1.27 56.76 ± 2.21 1.82 ± 0.15 31.18 ± 1.37 2.84 ± 0.16 31.22 ± 0.55 1.77 ± 0.02
7.3 ± 0.12 14.99 ± 0.19 77.2 ± 0.38 67.35 ± 0.045 3.01 ± 0.02 22.37 ± 0.14 16.5 ± 0.37 42.66 ± 1.04 14.36 ± 0.95
7.9 ± 0.10 6.59 ± 0.45 53.56 ± 1.05 26.38 ± 0.27 1.87 ± 0.045 14.10 ± 0.19 7.68 ± 0.22 12.76 ± 0.90 16.81 ± 1.82
Note: The values are average ± standard deviation of three samples. a (Dry basis).
ð1Þ
where M is cumulative methane production, P is methane production potential, k is lag-phase time, R is methane production rate and e is Euler’s constant. 3. Results and discussion 3.1. Reactor performance of the whole process 3.1.1. Phase A The pH of the mixed substrates used in this study without adjustment was 5.4, which was between KW (4.2) and CM (7.3).
Please cite this article in press as: Zhai, N., et al. Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.027
N. Zhai et al. / Waste Management xxx (2015) xxx–xxx
3
pH was increased. But when initial pH exceeded 7.0, the change of the capacity was minor as the minimum pH values of R8.0 (5.5), R7.5 (5.5) and R7.0 (5.4) were very similar. In the next 10 days, the pH values increased quickly except for R6.0 and CK, which rose to 5.9 and 5.6, respectively. However, the pH values at the end of phase A did not exceed their initial values except for R6.5, which reached 6.7. Nevertheless, the order of pH values was the same as the beginning. Researches stated that pH is strongly dependent on the buffering capacity in the AD processes (Espinoza-Escalante et al., 2009). Lowest alkalinity indicated lowest pH of the uncontrolled group. From the 5th to the 10th day, alkalinity of all the groups increased quickly with the same trend of pH as shown in Fig. 2b. In the 15th day, the slight drop of alkalinity was corresponding to the slow rise of pH. At the end of phase A, R7.0, R7.5 and R8.0 had higher alkalinity values than the rest. The major factor affecting the intermediate alkalinity is the concentration of undissociated VFAs (Banks et al., 2011). The descent stage of pH was due to the production and accumulation of VFAs, which were from insoluble macromolecular organic polymers such as carbohydrates, proteins, and fats by hydrolyzing microorganisms. Thus, before VFAs were consumed by methanogens, higher concentration of VFA would lead to inhibition of degradation reactions (Hecht and Griehl, 2009). As shown in Fig. 2c, VFAs concentrations of all treatments were over 2900 mg/L before the 15th day. The VFAs concentrations of treatments with controlled initial pH were higher than that of CK, indicating that the activity of hydrolyzing microorganisms in CK was lower than the others. It was likely owing to the lowest pH (5.6) of CK. Another factor that impacts pH is NH+4–N concentration, which is an important nitrogen source of microorganisms. High concentration of NH+4–N would inhibit the activity of methane bacteria; Angelidaki et al. (2005) found decreases in efficiency when total ammonia was above 4000 mg/L in the co-digestion of manure and organic waste. However, proper concentration of NH+4–N could neutralize VFAs produced in AD process. As reported by Banks et al. (2011), high ammonia concentrations gave stable biogas production at alkaline conditions over extended periods under continuous loading conditions. In the 5th day (Fig. 1d), the NH+4–N concentration of R6.5 was the highest (1100 mg/L), that of R8.0 was the lowest (849 mg/L), and those of other treatments were 924 mg/ L. However, in the next 5 days, the concentrations diverged, treatments with higher initial pH (R7.0, R7.5 and R8.0) increased and treatments with lower initial pH (uncontrolled, R6.0 and R6.5) decreased. At the end of phase A, NH+4–N concentrations had the same trend as pH values. The result showed that produced NH+4– N might act as the buffer under higher VFAs.
Fig. 2. The variations of pH and Alkalinity, VFA, NH+4–N concentrations at phase A, B, and C of the co-digestion process.
It reflected the good buffering capacity of CM, which was consistent with Shilton et al. (2013) and Yamashiro et al. (2013). Fig. 2a shows the variation of pH in each treatment. All pH values dropped quickly to their own lowest values in the 5th day, the order of the minimum values was R8.0 (5.5) > R7.5 (5.5) > R7.0 (5.4) > R6.0 (5.3) > R6.5 (5.2) > CK (4.8). It indicates that pH control in the initial stage of fermentation had effect on the buffering capacity compared to CK. The capacity increased when the initial
3.1.2. Phase B The pH value of CK reached 5.9 at the end of Phase B, and the pH values of R6.0 and CK maintained to the end of the whole fermentation process. Their alkalinity concentrations fluctuated along with NH+4–N concentrations. Meanwhile, their VFAs concentrations were higher than 3440 mg/L throughout the whole process. The greatest VFAs production occurred in R6.0 and CK, which was similar to Li et al. (2008). This is because VFAs were not consumed at very low pH and inhibited microbial growth by passing through the cell membrane of microbes, and led to high concentrations of VFAs (Parawira et al., 2005; Warnecke and Gill, 2005). Thus, R6.0 and CK were no longer discussed in phase C. The pH values of other four treatments were tending to increase slowly. R7.0 was the first to reach a pH of 7.5 in the 20th day, and it increased to 7.8 at the end of phase B. pH of R6.5 reached 7.7, 1.0 greater than that in phase A. pH values of R7.5 and R8.0 changed by less than 0.2, and their final pH values in phase B were 7.4 and 7.1, respectively. The VFAs concentrations of R6.0 and R7.0 remained
Please cite this article in press as: Zhai, N., et al. Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.027
4
N. Zhai et al. / Waste Management xxx (2015) xxx–xxx
high, and those of R7.5 and R8.0 started to decline at the 25th day. The downward trend suggests that the bacteria community gradually adapted to the new conditions and degraded VFAs to biogas. The NH+4–N concentrations of the four treatments were all increasing during phase B, indicating the existence of active microbes. However, the alkalinity showed a declining tendency, and no longer changed with pH and NH+4–N concentration. Jiang et al. (2010) found that pH was not sensitive to increase of alkalinity after pH and alkalinity reached 7.4 and 5000 mg/L (as CaCO3), respectively. In this study, alkalinity concentrations of all cases were over 5000 mg/L, so the critical point of pH was 7.0 as showed in Fig. 2a, b. Therefore, the variation of pH in the later stages did not have a strong association with alkalinity. The fluctuation of alkalinity would guarantee the relatively constant pH by adapting to the variation of VFAs and NH+4–N concentrations. 3.1.3. Phase C During days 31–45, the pH values of R6.5, R7.0, R7.5 and R8.0 remained at stable levels with small fluctuation between 7.7 and 7.9 at the end of digestion process. pH values of R6.5 and R7.0 were unstable until the 30th day, which was 10 days earlier than R7.5 and R8.0. The time needed for pH to become stable depended on the difference between controlled initial pH and original pH of digestion substrates (4.8). Longer time is needed for bacteria to adapt to a case with greater difference in pH. As shown in Fig. 2c, VFAs of the four treatments began to fell dramatically from the 35th day due to the rapid utilization of bacteria, which indicated that the rate of VFAs consumption was greater than its formation rate in fermentation liquor. The result showed that methane bacteria began to grow rapidly under suitable pH environment. NH+4–N concentrations of the treatments also fell almost at the same time, which demonstrated the existence of active bacteria. 3.1.4. Daily methane production with different initial pH values Fig. 3 shows that the methane productions of R6.5 and R7.5 were similar, because methane production started at the 16th day and it started to fall after three peaks in R6.5 and R7.5 (on about the 36th day). Judging from the variation of alkalinity, VFA and NH+4–N (Fig. 2), the rules were basically the same. However, the peak values of R7.5 (416.97, 419.15 and 438.59 mL/d) were higher than those of R6.5 (232.22, 278.40 and 301.27 mL/d) due to higher pH value of R7.5.
650 600 550
Methane production (mL/d)
500 450
6.0 6.5 7.0 7.5 8.0
400 350 300 250 200 150 100 50 0 0
5
10
15
20
25
30
35
40
Time (d) Fig. 3. Daily methane production of different initial pH treatments.
45
By contrast, the treatments of R7.0 and R8.0 produced methane lately, especially for R8.0, which indicates they had longer buffering time and poor buffering capacity at starting stage, although pH, alkalinity and NH+4–N of them were higher in this period (Fig. 2). In the experimental period, only one peak occurred for R7.0 and R8.0, and the peak value (627.44 mL/d) of R8.0 was the highest of all. The methane productions began to decline from the 37th day. As shown in Fig. 2, the pH of R6.0 and CK maintained low during the whole process, and there was no sign of methane for CK, and only little methane was produced in R6.0. All these phenomena indicate poor buffering capacity in R6.0 and CK. Thus, proper initial pH values certainly promoted the methane production. 3.2. Methane production 3.2.1. Parameters of cumulative methane production curve Based on the experimental data of cumulative methane production, parameters in Eq. (1), namely P, R and k, were evaluated by SPSS 12.0. Table 2 summarizes the optimal values and correlation coefficients (r2). R6.0 and CK failed to produce methane, so they were not covered in following discussions. 3.2.2. Influence of initial pH on cumulative methane production, lagphase time and P (mL/gVSfed) Fig. 4 shows that the Gompertz curves of all the treatments can be classified into lag stage, growth stage and stable stage. The experiment period in this study (45 days) only covered lag-stage and growth stage. Values of r2 (>0.99) and Fig. 4 demonstrated that Eq. (1) was suitable to describe the progress of cumulative methane production in batch co-digestion of KW and CM under different initial pH. However, the lag-phase time, maximum methane production rate and methane production potential were different. The lagphase times of R6.5 (21.1d) and R7.5 (22.1d) were almost the same, while those of R7.0 (31.0) and R8.0 (32.6) were longer. The length of lag-phase time indicates the buffering capacity of fermentation liquor, so R6.5 and R7.5 had better buffering capacity than the other two. R7.5 had the highest methane production potential of 8579 mL. R6.5, R7.0 and R8.0 had almost similar potential productions, which were 5792, 5984 and 5958 mL, respectively. The maximum methane production rate was obtained in R8.0 (573.6 mL/d), followed by R7.5 (366.5 mL/d), while R6.5 had the minimum (243.6 mL/d). All the analyzable substrates were used up when the fermentation time reached T(100%)/d. The shortest T(100%)/d among the four treatments was in R8.0, showing that it had the fastest degradation rate, which was proved by the fact that R8.0 had the maximum methane production rate. The degradation rate of R6.5 was the slowest with T(100%)/d of 122d and maximum methane production rate of 243.6 mL/d. The T(100%)/d of R7.0 and R7.5 were similar (Table 2), but the maximum methane production rate of R7.5 (366.5 mL/d) was higher than that of R7.0 (295.4 mL/d). Owing to a small methane production rate during the stable stage, T(90%)/d might be a good indicator for efficient utilization. All the four treatments completed 90% methane production in 45d–56d. In order to achieve the highest utilization efficiency of substrates and fermentation tank, P, R, T(90%)/d and P (mL/gVSfed) should be taken into consideration in actual methane production. Higher methane production potential and P (mL/gVSfed), and shorter lag-phase time and T(90%)/d are the best choice. By comprehensive evaluation and contrast analysis, R7.5 was recommended. This conclusion is consistent with Dinamarca et al. (2003) and Zhang et al. (2005), who suggested that a pH range of 7–8 was suitable for obtaining higher biogas production and degradation of VS.
Please cite this article in press as: Zhai, N., et al. Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.027
5
N. Zhai et al. / Waste Management xxx (2015) xxx–xxx Table 2 Calculated parameters and their correlation coefficient r2, T(90%)/d, T(100%)/d and P (mL/gVSfed). Initial pH
P (mL)
R (mL/d)
k (d)
r2
T(90%)/d
T(100%)/d
P (mL/gVSfed)
R6.5 R7.0 R7.5 R8.0
5792 5984 8579 5958
243.6 295.4 366.5 573.6
21.1 31.0 22.1 32.6
0.9975 0.9977 0.9988 0.9994
47 56 51 45
122 115 111 71
121.4 125.4 179.8 124.9
Note: The total VS was 47.71 g; T(90%)/d is the time for methane production to reach 90% of methane production potential; T(100%)/d is the time for methane production to reach methane production potential; P (mL/gVSfed) is methane production per gram VS fed.
Fig. 4. Modified Gompertz model fit with experimental data at four different initial pH values.
However, the predicted lag-phase times for all treatments were >20 days, further experiments should be conducted to investigate how to shorten it.
R7.0, R7.5 and R8.0 well fit the modified Gompertz equation. R7.5 was the best treatment for co-digestion of KW with CM. Acknowledgements
4. Conclusions This study found that changing initial pH had significant effects on the variations of pH, alkalinity, VFA and NH+4–N except for R6.0 case, which behaved similarly as CK. Second, methane production was improved effectively by changing pH at the beginning of codigestion. Third, the cumulative methane productions of R6.5,
This work was supported by science and technology support projects, the biological technology integration and demonstration of high yield biogas digestion from the mix ingredients (2011BAD15B03) from Ministry of Science and Technology Department of the People’s Republic of China and Research Fund for the Doctoral Program of Higher Education of Northwest A & F University, China (2013BSJJ057).
Please cite this article in press as: Zhai, N., et al. Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.027
6
N. Zhai et al. / Waste Management xxx (2015) xxx–xxx
References Alatriste-Mondragon, F., Samar, P., Cox, H.H., Ahring, B.K., Iranpour, R., 2006. Anaerobic codigestion of municipal, farm, and industrial organic wastes: a survey of recent literature. Water Environ. R 78 (6), 607–636. Angelidaki, I., Boe, K., Ellegaard, L., 2005. Effect of operating conditions and reactor configuration on efficiency of full-scale biogas plants. Water Sci Technol. 52 (1– 2), 189–194. APHA, 1998. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC. Arthurson, V., 2009. Closing the global energy and nutrient cycles through application of biogas residue to agricultural land–potential benefits and drawback. Energies 2, 226–242. Banks, C.J., Chesshire, M., Stringfellow, A., 2008. A pilot-scale comparison of mesophilic and thermophilic digestion of source segregated domestic food waste. Water Sci. Technol. 58 (7), 1475–1481. Banks, C.J., Chesshire, M., Heaven, S., Arnold, R., 2011. Anaerobic digestion of sourcesegregated domestic food waste: performance assessment by mass and energy balance. Bioresour. Technol. 102 (2), 612–620. Braun, R., Brachtl, E., Grasmug, M., 2003. Codigestion of proteinaceous industrial waste. Appl. Biochem. Biotechnol. 109 (1–3), 139–153. Cho, J.K., Park, S.C., Chang, H.N., 1995. Biochemical methane potential and solid state anaerobic digestion of Korean food wastes. Bioresour. Technol. 52 (3), 245–253. Dinamarca, S., Aroca, G., Chamy, R., Guerrero, L., 2003. The influence of pH in the hydrolytic stage of anaerobic digestion of the organic fraction of urban solid waste. Water Sci. Technol. 48 (6), 249–254. Espinoza-Escalante, F.M., Pelayo-Ortíz, C., Navarro-Corona, J., González-García, Y., Bories, A., Gutiérrez-Pulido, H., 2009. Anaerobic digestion of the vinasses from the fermentation of Agave tequilana Weber to tequila: the effect of pH, temperature and hydraulic retention time on the production of hydrogen and methane. Biomass Bioenergy 33 (1), 14–20. Esposito, G., Frunzo, L., Giordano, A., Liotta, F., Panico, A., Pirozzi, F., 2012. Anaerobic co-digestion of organic wastes. Rev. Environ. Sci. Bio. 11 (4), 325–341. Ghosh, S., Henry, M., Sajjad, A., Mensinger, M., Arora, J., 2000. Pilot-scale gasification of municipal solid wastes by high-rate and two-phase anaerobic digestion (TPAD). Water Sci. Technol. 41 (3), 101–110. Hanaki, K., Matsuo, T., Nagase, M., 1981. Mechanism of inhibition caused by longchain fatty acids in anaerobic digestion process. Biotechnol. Bioengy 23 (7), 1591–1610. Hecht, C., Griehl, C., 2009. Investigation of the accumulation of aromatic compounds during biogas production from kitchen waste. Bioresour Technol. 100 (2), 654– 658. Jiang, J., Du, X., Ng, S., Zhang, C., 2010. Comparison of atmospheric pressure effects on the anaerobic digestion of municipal solid waste. Bioresour Technol. 101 (16), 6361–6367. Jiang, J., Zhang, Y., Li, K., Wang, Q., Gong, C., Li, M., 2013. Volatile fatty acids production from food waste: effects of pH, temperature, and organic loading rate. Bioresour. Technol. 143, 525–530. Koster, I.W., Cramer, A., 1987. Inhibition of methanogenesis from acetate in granular sludge by long-chain fatty acids. Appl. Environ. Microb. 53 (2), 403– 409.
Lay, J.-J., Li, Y.-Y., Noike, T., 1996. Effect of moisture content and chemical nature on methane fermentation characteristics of municipal solid wastes. P. Jpn. Soc. Civil Eng., 101–108 Lay, J.J., Li, Y.Y., Noike, T., 1997. Influences of pH and moisture content on the methane production in high-solids sludge digestion. Water Res. 31 (6), 1518– 1524. Lee, M., Hidaka, T., Tsuno, H., 2008. Effect of temperature on performance and microbial diversity in hyperthermophilic digester system fed with kitchen garbage. Bioresour. Technol. 99 (15), 6852–6860. Li, M., Zhao, Y., Guo, Q., Qian, X., Niu, D., 2008. Bio-hydrogen production from food waste and sewage sludge in the presence of aged refuse excavated from refuse landfill. Renew. Energy 33 (12), 2573–2579. Li, R., Chen, S., Li, X., Lar, J.S., He, Y., Zhu, B., 2009. Anaerobic codigestion of kitchen waste with cattle manure for biogas production. Energy Fule. 23, 2225–2228. Liu, C.F., Yuan, X.Z., Zeng, G.M., Li, W.W., Li, J., 2008. Prediction of methane yield at optimum pH for anaerobic digestion of organic fraction of municipal solid waste. Bioresour. Technol. 99 (4), 882–888. Luostarinen, S., Rintala, J., 2007. Anaerobic on-site treatment of kitchen waste in combination with black water in UASB-septic tanks at low temperatures. Bioresour. Technol. 98 (9), 1734–1740. Parawira, W., Murto, M., Read, J., Mattiasson, B., 2005. Profile of hydrolases and biogas production during two-stage mesophilic anaerobic digestion of solid potato waste. Process. Biochem. 40 (9), 2945–2952. Shahriari, H., Warith, M., Hamoda, M., Kennedy, K., 2013. Evaluation of single vs. staged mesophilic anaerobic digestion of kitchen waste with and without microwave pretreatment. J. Environ. Manage. 125, 74–84. Shilton, A., Powell, N., Broughton, A., Pratt, C., Pratt, S., Pepper, C., 2013. Enhanced biogas production using cow manure to stabilize co-digestion of whey and primary sludge. Environ. Technol. 34 (17), 2491–2496. Wang, X., Yang, G., Feng, Y., Ren, G., Han, X., 2012. Optimizing feeding composition and carbon-nitrogen ratios for improved methane yield during anaerobic codigestion of dairy, chicken manure and wheat straw. Bioresour. Technol. 120, 78–83. Warnecke, T., Gill, R.T., 2005. Organic acid toxicity, tolerance, and production in Escherichia coli biorefining applications. Microb. Cell Fact. 4 (1), 25. Yamashiro, T., Lateef, S.A., Ying, C., Beneragama, N., Lukic, M., Masahiro, I., Ihara, I., Nishida, T., Umetsu, K., 2013. Anaerobic co-digestion of dairy cow manure and high concentrated food processing waste. J. Mater. Cycles Waste. 15 (4), 539– 547. Yang, Y.Q., Shen, D.G., Li, N., Xu, D., Long, Y.Y., Lu, X.Y., 2013. Co-digestion of kitchen waste and fruit-vegetable waste by two-phase anaerobic digestion. Environ. Sci. Pollut. Res. 20 (4), 2162–2171. Zhang, B., Zhang, L., Zhang, S., Shi, H., Cai, W., 2005. The influence of pH on hydrolysis and acidogenesis of kitchen wastes in two-phase anaerobic digestion. Environ. Technol. 26 (3), 329–340. Zhang, D.Q., Tan, S.K., Gersberg, R.M., 2010. Municipal solid waste management in China: status, problems and challenges. J. Environ. Manage. 91 (8), 1623–1633. Zhang, X., Qiu, W., Chen, H., 2012. Enhancing the hydrolysis and acidification of steam-exploded cornstalks by intermittent pH adjustment with an enriched microbial community. Bioresour. Technol. 123, 30–35.
Please cite this article in press as: Zhai, N., et al. Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.027
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure Ningning Zhai a,b, Tong Zhang a,b, Dongxue Yin a,c, Gaihe Yang a,b,⇑, Xiaojiao Wang a,b, Guangxin Ren a,b, Yongzhong Feng a,b a b c
College of Agronomy, Northwest A&F University, Shaanxi, Yangling 712100, China Research Center for Recycling Agriculture Engineering Technology of Shaanxi Province, Shaanxi, Yangling 712100, China College of Forestry, Northwest A&F University, Shaanxi, Yangling 712100, China
a r t i c l e
i n f o
Article history: Received 9 August 2014 Accepted 16 December 2014 Available online xxxx Keywords: Initial pH Kitchen waste Cow manure Anaerobic co-digestion Modified Gompertz equation
a b s t r a c t This study investigated the effects of different initial pH (6.0, 6.5, 7.0, 7.5 and 8.0) and uncontrolled initial pH (CK) on the lab-scale anaerobic co-digestion of kitchen waste (KW) with cow manure (CM). The variations of pH, alkalinity, volatile fatty acids (VFAs) and total ammonia nitrogen (NH+4–N) were analyzed. The modified Gompertz equation was used for selecting the optimal initial pH through comprehensive evaluation of methane production potential, degradation of volatile solids (VS), and lag-phase time. The results showed that CK and the fermentation with initial pH of 6.0 failed. The pH values of the rest treatments reached 7.7–7.9 with significantly increased methane production. The predicted lag-phase times of treatments with initial pH of 6.5 and 7.5 were 21 and 22 days, which were 10 days shorter than the treatments with initial pH of 7.0 and 8.0, respectively. The maximum methane production potential (8579 mL) and VS degradation rate (179.8 mL/g VS) were obtained when the initial pH was 7.5, which is recommended for co-digestion of KW and CM. Ó 2015 Published by Elsevier Ltd.
1. Introduction Kitchen waste (KW) is a widely produced municipal solid waste (MSW) with high yield, and it accounts for about 60% of the total MSW collected and transported in China in 2006 (Zhang et al., 2010). KW is nutrient-rich organic material with a tremendous potential of energy recovery by anaerobic digestion (AD) (Jiang et al., 2010, 2013; Lee et al., 2008). Due to its high water content, producing biogas through the AD of KW benefits more than nonbiological disposal methods such as incineration and landfill (Hecht and Griehl, 2009). In addition, after further chemical, biological and physical processing (Arthurson, 2009), the nutrients in digested residues could be recycled back to agriculture and horticulture, thus creating resource recycling society (Banks et al., 2011; Yamashiro et al., 2013) because the organic nitrogen is converted to ammonia, which can be directly used by plants (Luostarinen and Rintala, 2007).
⇑ Corresponding author. Addresses: College of Agronomy, Northwest A&F University, Yangling, 712100 Shaanxi, China and The Research Center of Recycle Agricultural Engineering and Technology of Shaanxi Province, Yangling 712100 Shaanxi, China. Tel./fax: +86 029 8709 2265. E-mail address: [email protected] (G. Yang).
However, the high moisture content in KW makes it easier to become sour and smell (Shahriari et al., 2013), and the high contents of fat and protein in KW and degradation compounds (for example, inhibitory long-chain fatty acids, ammonia or sulphides) affect the utilization of degradable components in the anaerobic process (Braun et al., 2003; Hanaki et al., 1981; Koster and Cramer, 1987). Cho et al. (1995) showed that at an early stage of AD, the soluble organics were converted to VFAs rapidly, resulting in a drastic pH drop and lower biogas production if no efficient pretreatments were adopted. Thus, AD of KW often performs poor buffering capacity and lower biogas production (Banks et al., 2008). To stabilize the anaerobic process and increase methane yields, co-digestion has been considered as an effective approach. Studies reported that co-digestion of KW with other substrates could increase alkalinity and buffering capacity, reduce the inhibition caused by VFAs concentrations, and increase methane yield (Alatriste-Mondragon et al., 2006; Li et al., 2009; Yang et al., 2013). However, mixing KW with other substrates for digestion is not a perfect method for improving methane production. Additional measures can be applied to increase methane production of co-digestion process (Esposito et al., 2012). The culture pH affects the activities of specific acidogenic microbial populations (Zhang et al., 2012) and methanogenic bacteria (Ghosh et al.,
http://dx.doi.org/10.1016/j.wasman.2014.12.027 0956-053X/Ó 2015 Published by Elsevier Ltd.
Please cite this article in press as: Zhai, N., et al. Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.027
2
N. Zhai et al. / Waste Management xxx (2015) xxx–xxx
2000), thereby the stability of co-digestion process. Thus, pH is a pivotal factor that affects the methane production efficiency (Jiang et al., 2010; Liu et al., 2008). Methods for pH control would effectively increase methane production. A study has shown that the optimal range of pH for methane production in AD was 6.5– 7.5 (Liu et al., 2008). But the range is relative wide and the optimal pH value varies with different substrates and digestion techniques. Furthermore, the initial pH is not controlled in almost studies, although an appropriate initial pH value is important for hydrolysis reaction of AD, especially for substrates like KW, for which acidification happens easily at the beginning of digestion (Cho et al., 1995). Thus, a suitable initial pH value should be chosen for digestion process to enhance methane production rate and VS reduction rate. This study aimed to investigate the effects of five different initial pH values (6.0, 6.5, 7.0, 7.5, and 8.0) on co-digestion of KW and CM in order to obtain the most stable digestion environment and the maximum methane production. The variations of pH, total alkalinity, VFAs, and NH+4–N were analyzed and compared. Methane production potential, degradation of volatile solids and lagphase time were also assessed by the ‘‘modified Gompertz equation’’. The optimal initial pH value was then obtained. 2. Methods 2.1. Substrates and inoculant KW used in the study was collected from the student union cafeterias of the Northwest A&F University in Yangling, China. It consisted of rice, noodle, fried vegetables, meat, and fish, etc. After the removal of bones, napkins, spoons, and other inorganic matters, the KW was crushed into slurry state using a food waste disposer, and subsequently stored in a refrigerator at 4 °C for later use. CM was gathered from a livestock farm located in Yangling, China. Mesophilic anaerobic digested sludge used for inoculant was obtained from a normal operation anaerobic digester in a village in Yangling, China. The CM and the inoculant were individually homogenized for later use. The characteristics of KW, CM and inoculant are presented in Table 1. 2.2. Experimental set-up All experiments were conducted with the total solid content of 8%, and the mixing ratio of KW and CM was 1:1 (wet weight basis). Lab-scale anaerobic digesters with working volume of 1 L contained 700 mL total mixture with 200 mL inoculant. The digestion tests were conducted in triplicate for 45 days based on the method described in Wang et al. (2012). The effect of initial pH was investigated by operating the reactors at 35 °C with pH of 6.0, 6.5, 7.0, 7.5, 8.0, and uncontrolled initial pH, labeled as R6.0, R6.5, R7.0,
Fig. 1. Curve of Modified Gompertz Model.
R7.5, R8.0 and CK, respectively. Concentrated NaOH was used to adjust the initial pH to the values previously selected (Lay et al., 1997). All reactors were tightly closed with rubber septa and screw caps. The head space of each reactor was flushed with N2 for approximately 3 min to stabilize anaerobic condition prior to start of the digestion tests. The temperature was controlled using a water circulator. Moreover, all reactors were manually shaken for approximately 1 min daily to mix the reactor contents prior to the measurement of biogas volume. 2.3. Analytical methods TS, VS, total kjeldahl nitrogen (TKN), total organic carbon, alkalinity and VFAs were measured according to the procedures described in the Standard Methods (APHA, 1998). The pH and methane proportion were determined by biogas analyzer (Gasboard-3200P). The contents of lignin, cellulose, and hemicelluloses were determined by raw fiber determination analyzer (Model CXC06, Shanghai). Biogas volume was measured every 24 h by the water displacement method, and pH, alkalinity, and NH+4–N were determined every 5 days (alkalinity, VFAs and NH+4–N, were detected starting from the fifth day). For the sake of analysis, the fermentation process was divided into 3 phases: phase A with no methane production (1–15 days), phase B with growing methane production (16–30 days) and phase C with stable methane production (31–45 days). 2.4. Modified Gompertz equation The cumulative methane production rate was described using the modified Gompertz equation developed by Lay et al. (1996, 1997):
Re M ¼ P exp exp ðk t Þ þ 1 P
Table 1 Characteristics of KW, CM, and Inoculant used in this study. Parameter
KW
CM
Inoculant
pH TS (%) VSa (%) Organic carbona (g/kg VS) TKNa (g/kg VS) C/N Cellulose (%) Hemicelluloses (%) Lignin (%)
4.2 ± 0.23 23.19 ± 0.54 95.69 ± 1.27 56.76 ± 2.21 1.82 ± 0.15 31.18 ± 1.37 2.84 ± 0.16 31.22 ± 0.55 1.77 ± 0.02
7.3 ± 0.12 14.99 ± 0.19 77.2 ± 0.38 67.35 ± 0.045 3.01 ± 0.02 22.37 ± 0.14 16.5 ± 0.37 42.66 ± 1.04 14.36 ± 0.95
7.9 ± 0.10 6.59 ± 0.45 53.56 ± 1.05 26.38 ± 0.27 1.87 ± 0.045 14.10 ± 0.19 7.68 ± 0.22 12.76 ± 0.90 16.81 ± 1.82
Note: The values are average ± standard deviation of three samples. a (Dry basis).
ð1Þ
where M is cumulative methane production, P is methane production potential, k is lag-phase time, R is methane production rate and e is Euler’s constant. 3. Results and discussion 3.1. Reactor performance of the whole process 3.1.1. Phase A The pH of the mixed substrates used in this study without adjustment was 5.4, which was between KW (4.2) and CM (7.3).
Please cite this article in press as: Zhai, N., et al. Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.027
N. Zhai et al. / Waste Management xxx (2015) xxx–xxx
3
pH was increased. But when initial pH exceeded 7.0, the change of the capacity was minor as the minimum pH values of R8.0 (5.5), R7.5 (5.5) and R7.0 (5.4) were very similar. In the next 10 days, the pH values increased quickly except for R6.0 and CK, which rose to 5.9 and 5.6, respectively. However, the pH values at the end of phase A did not exceed their initial values except for R6.5, which reached 6.7. Nevertheless, the order of pH values was the same as the beginning. Researches stated that pH is strongly dependent on the buffering capacity in the AD processes (Espinoza-Escalante et al., 2009). Lowest alkalinity indicated lowest pH of the uncontrolled group. From the 5th to the 10th day, alkalinity of all the groups increased quickly with the same trend of pH as shown in Fig. 2b. In the 15th day, the slight drop of alkalinity was corresponding to the slow rise of pH. At the end of phase A, R7.0, R7.5 and R8.0 had higher alkalinity values than the rest. The major factor affecting the intermediate alkalinity is the concentration of undissociated VFAs (Banks et al., 2011). The descent stage of pH was due to the production and accumulation of VFAs, which were from insoluble macromolecular organic polymers such as carbohydrates, proteins, and fats by hydrolyzing microorganisms. Thus, before VFAs were consumed by methanogens, higher concentration of VFA would lead to inhibition of degradation reactions (Hecht and Griehl, 2009). As shown in Fig. 2c, VFAs concentrations of all treatments were over 2900 mg/L before the 15th day. The VFAs concentrations of treatments with controlled initial pH were higher than that of CK, indicating that the activity of hydrolyzing microorganisms in CK was lower than the others. It was likely owing to the lowest pH (5.6) of CK. Another factor that impacts pH is NH+4–N concentration, which is an important nitrogen source of microorganisms. High concentration of NH+4–N would inhibit the activity of methane bacteria; Angelidaki et al. (2005) found decreases in efficiency when total ammonia was above 4000 mg/L in the co-digestion of manure and organic waste. However, proper concentration of NH+4–N could neutralize VFAs produced in AD process. As reported by Banks et al. (2011), high ammonia concentrations gave stable biogas production at alkaline conditions over extended periods under continuous loading conditions. In the 5th day (Fig. 1d), the NH+4–N concentration of R6.5 was the highest (1100 mg/L), that of R8.0 was the lowest (849 mg/L), and those of other treatments were 924 mg/ L. However, in the next 5 days, the concentrations diverged, treatments with higher initial pH (R7.0, R7.5 and R8.0) increased and treatments with lower initial pH (uncontrolled, R6.0 and R6.5) decreased. At the end of phase A, NH+4–N concentrations had the same trend as pH values. The result showed that produced NH+4– N might act as the buffer under higher VFAs.
Fig. 2. The variations of pH and Alkalinity, VFA, NH+4–N concentrations at phase A, B, and C of the co-digestion process.
It reflected the good buffering capacity of CM, which was consistent with Shilton et al. (2013) and Yamashiro et al. (2013). Fig. 2a shows the variation of pH in each treatment. All pH values dropped quickly to their own lowest values in the 5th day, the order of the minimum values was R8.0 (5.5) > R7.5 (5.5) > R7.0 (5.4) > R6.0 (5.3) > R6.5 (5.2) > CK (4.8). It indicates that pH control in the initial stage of fermentation had effect on the buffering capacity compared to CK. The capacity increased when the initial
3.1.2. Phase B The pH value of CK reached 5.9 at the end of Phase B, and the pH values of R6.0 and CK maintained to the end of the whole fermentation process. Their alkalinity concentrations fluctuated along with NH+4–N concentrations. Meanwhile, their VFAs concentrations were higher than 3440 mg/L throughout the whole process. The greatest VFAs production occurred in R6.0 and CK, which was similar to Li et al. (2008). This is because VFAs were not consumed at very low pH and inhibited microbial growth by passing through the cell membrane of microbes, and led to high concentrations of VFAs (Parawira et al., 2005; Warnecke and Gill, 2005). Thus, R6.0 and CK were no longer discussed in phase C. The pH values of other four treatments were tending to increase slowly. R7.0 was the first to reach a pH of 7.5 in the 20th day, and it increased to 7.8 at the end of phase B. pH of R6.5 reached 7.7, 1.0 greater than that in phase A. pH values of R7.5 and R8.0 changed by less than 0.2, and their final pH values in phase B were 7.4 and 7.1, respectively. The VFAs concentrations of R6.0 and R7.0 remained
Please cite this article in press as: Zhai, N., et al. Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.027
4
N. Zhai et al. / Waste Management xxx (2015) xxx–xxx
high, and those of R7.5 and R8.0 started to decline at the 25th day. The downward trend suggests that the bacteria community gradually adapted to the new conditions and degraded VFAs to biogas. The NH+4–N concentrations of the four treatments were all increasing during phase B, indicating the existence of active microbes. However, the alkalinity showed a declining tendency, and no longer changed with pH and NH+4–N concentration. Jiang et al. (2010) found that pH was not sensitive to increase of alkalinity after pH and alkalinity reached 7.4 and 5000 mg/L (as CaCO3), respectively. In this study, alkalinity concentrations of all cases were over 5000 mg/L, so the critical point of pH was 7.0 as showed in Fig. 2a, b. Therefore, the variation of pH in the later stages did not have a strong association with alkalinity. The fluctuation of alkalinity would guarantee the relatively constant pH by adapting to the variation of VFAs and NH+4–N concentrations. 3.1.3. Phase C During days 31–45, the pH values of R6.5, R7.0, R7.5 and R8.0 remained at stable levels with small fluctuation between 7.7 and 7.9 at the end of digestion process. pH values of R6.5 and R7.0 were unstable until the 30th day, which was 10 days earlier than R7.5 and R8.0. The time needed for pH to become stable depended on the difference between controlled initial pH and original pH of digestion substrates (4.8). Longer time is needed for bacteria to adapt to a case with greater difference in pH. As shown in Fig. 2c, VFAs of the four treatments began to fell dramatically from the 35th day due to the rapid utilization of bacteria, which indicated that the rate of VFAs consumption was greater than its formation rate in fermentation liquor. The result showed that methane bacteria began to grow rapidly under suitable pH environment. NH+4–N concentrations of the treatments also fell almost at the same time, which demonstrated the existence of active bacteria. 3.1.4. Daily methane production with different initial pH values Fig. 3 shows that the methane productions of R6.5 and R7.5 were similar, because methane production started at the 16th day and it started to fall after three peaks in R6.5 and R7.5 (on about the 36th day). Judging from the variation of alkalinity, VFA and NH+4–N (Fig. 2), the rules were basically the same. However, the peak values of R7.5 (416.97, 419.15 and 438.59 mL/d) were higher than those of R6.5 (232.22, 278.40 and 301.27 mL/d) due to higher pH value of R7.5.
650 600 550
Methane production (mL/d)
500 450
6.0 6.5 7.0 7.5 8.0
400 350 300 250 200 150 100 50 0 0
5
10
15
20
25
30
35
40
Time (d) Fig. 3. Daily methane production of different initial pH treatments.
45
By contrast, the treatments of R7.0 and R8.0 produced methane lately, especially for R8.0, which indicates they had longer buffering time and poor buffering capacity at starting stage, although pH, alkalinity and NH+4–N of them were higher in this period (Fig. 2). In the experimental period, only one peak occurred for R7.0 and R8.0, and the peak value (627.44 mL/d) of R8.0 was the highest of all. The methane productions began to decline from the 37th day. As shown in Fig. 2, the pH of R6.0 and CK maintained low during the whole process, and there was no sign of methane for CK, and only little methane was produced in R6.0. All these phenomena indicate poor buffering capacity in R6.0 and CK. Thus, proper initial pH values certainly promoted the methane production. 3.2. Methane production 3.2.1. Parameters of cumulative methane production curve Based on the experimental data of cumulative methane production, parameters in Eq. (1), namely P, R and k, were evaluated by SPSS 12.0. Table 2 summarizes the optimal values and correlation coefficients (r2). R6.0 and CK failed to produce methane, so they were not covered in following discussions. 3.2.2. Influence of initial pH on cumulative methane production, lagphase time and P (mL/gVSfed) Fig. 4 shows that the Gompertz curves of all the treatments can be classified into lag stage, growth stage and stable stage. The experiment period in this study (45 days) only covered lag-stage and growth stage. Values of r2 (>0.99) and Fig. 4 demonstrated that Eq. (1) was suitable to describe the progress of cumulative methane production in batch co-digestion of KW and CM under different initial pH. However, the lag-phase time, maximum methane production rate and methane production potential were different. The lagphase times of R6.5 (21.1d) and R7.5 (22.1d) were almost the same, while those of R7.0 (31.0) and R8.0 (32.6) were longer. The length of lag-phase time indicates the buffering capacity of fermentation liquor, so R6.5 and R7.5 had better buffering capacity than the other two. R7.5 had the highest methane production potential of 8579 mL. R6.5, R7.0 and R8.0 had almost similar potential productions, which were 5792, 5984 and 5958 mL, respectively. The maximum methane production rate was obtained in R8.0 (573.6 mL/d), followed by R7.5 (366.5 mL/d), while R6.5 had the minimum (243.6 mL/d). All the analyzable substrates were used up when the fermentation time reached T(100%)/d. The shortest T(100%)/d among the four treatments was in R8.0, showing that it had the fastest degradation rate, which was proved by the fact that R8.0 had the maximum methane production rate. The degradation rate of R6.5 was the slowest with T(100%)/d of 122d and maximum methane production rate of 243.6 mL/d. The T(100%)/d of R7.0 and R7.5 were similar (Table 2), but the maximum methane production rate of R7.5 (366.5 mL/d) was higher than that of R7.0 (295.4 mL/d). Owing to a small methane production rate during the stable stage, T(90%)/d might be a good indicator for efficient utilization. All the four treatments completed 90% methane production in 45d–56d. In order to achieve the highest utilization efficiency of substrates and fermentation tank, P, R, T(90%)/d and P (mL/gVSfed) should be taken into consideration in actual methane production. Higher methane production potential and P (mL/gVSfed), and shorter lag-phase time and T(90%)/d are the best choice. By comprehensive evaluation and contrast analysis, R7.5 was recommended. This conclusion is consistent with Dinamarca et al. (2003) and Zhang et al. (2005), who suggested that a pH range of 7–8 was suitable for obtaining higher biogas production and degradation of VS.
Please cite this article in press as: Zhai, N., et al. Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.027
5
N. Zhai et al. / Waste Management xxx (2015) xxx–xxx Table 2 Calculated parameters and their correlation coefficient r2, T(90%)/d, T(100%)/d and P (mL/gVSfed). Initial pH
P (mL)
R (mL/d)
k (d)
r2
T(90%)/d
T(100%)/d
P (mL/gVSfed)
R6.5 R7.0 R7.5 R8.0
5792 5984 8579 5958
243.6 295.4 366.5 573.6
21.1 31.0 22.1 32.6
0.9975 0.9977 0.9988 0.9994
47 56 51 45
122 115 111 71
121.4 125.4 179.8 124.9
Note: The total VS was 47.71 g; T(90%)/d is the time for methane production to reach 90% of methane production potential; T(100%)/d is the time for methane production to reach methane production potential; P (mL/gVSfed) is methane production per gram VS fed.
Fig. 4. Modified Gompertz model fit with experimental data at four different initial pH values.
However, the predicted lag-phase times for all treatments were >20 days, further experiments should be conducted to investigate how to shorten it.
R7.0, R7.5 and R8.0 well fit the modified Gompertz equation. R7.5 was the best treatment for co-digestion of KW with CM. Acknowledgements
4. Conclusions This study found that changing initial pH had significant effects on the variations of pH, alkalinity, VFA and NH+4–N except for R6.0 case, which behaved similarly as CK. Second, methane production was improved effectively by changing pH at the beginning of codigestion. Third, the cumulative methane productions of R6.5,
This work was supported by science and technology support projects, the biological technology integration and demonstration of high yield biogas digestion from the mix ingredients (2011BAD15B03) from Ministry of Science and Technology Department of the People’s Republic of China and Research Fund for the Doctoral Program of Higher Education of Northwest A & F University, China (2013BSJJ057).
Please cite this article in press as: Zhai, N., et al. Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.027
6
N. Zhai et al. / Waste Management xxx (2015) xxx–xxx
References Alatriste-Mondragon, F., Samar, P., Cox, H.H., Ahring, B.K., Iranpour, R., 2006. Anaerobic codigestion of municipal, farm, and industrial organic wastes: a survey of recent literature. Water Environ. R 78 (6), 607–636. Angelidaki, I., Boe, K., Ellegaard, L., 2005. Effect of operating conditions and reactor configuration on efficiency of full-scale biogas plants. Water Sci Technol. 52 (1– 2), 189–194. APHA, 1998. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC. Arthurson, V., 2009. Closing the global energy and nutrient cycles through application of biogas residue to agricultural land–potential benefits and drawback. Energies 2, 226–242. Banks, C.J., Chesshire, M., Stringfellow, A., 2008. A pilot-scale comparison of mesophilic and thermophilic digestion of source segregated domestic food waste. Water Sci. Technol. 58 (7), 1475–1481. Banks, C.J., Chesshire, M., Heaven, S., Arnold, R., 2011. Anaerobic digestion of sourcesegregated domestic food waste: performance assessment by mass and energy balance. Bioresour. Technol. 102 (2), 612–620. Braun, R., Brachtl, E., Grasmug, M., 2003. Codigestion of proteinaceous industrial waste. Appl. Biochem. Biotechnol. 109 (1–3), 139–153. Cho, J.K., Park, S.C., Chang, H.N., 1995. Biochemical methane potential and solid state anaerobic digestion of Korean food wastes. Bioresour. Technol. 52 (3), 245–253. Dinamarca, S., Aroca, G., Chamy, R., Guerrero, L., 2003. The influence of pH in the hydrolytic stage of anaerobic digestion of the organic fraction of urban solid waste. Water Sci. Technol. 48 (6), 249–254. Espinoza-Escalante, F.M., Pelayo-Ortíz, C., Navarro-Corona, J., González-García, Y., Bories, A., Gutiérrez-Pulido, H., 2009. Anaerobic digestion of the vinasses from the fermentation of Agave tequilana Weber to tequila: the effect of pH, temperature and hydraulic retention time on the production of hydrogen and methane. Biomass Bioenergy 33 (1), 14–20. Esposito, G., Frunzo, L., Giordano, A., Liotta, F., Panico, A., Pirozzi, F., 2012. Anaerobic co-digestion of organic wastes. Rev. Environ. Sci. Bio. 11 (4), 325–341. Ghosh, S., Henry, M., Sajjad, A., Mensinger, M., Arora, J., 2000. Pilot-scale gasification of municipal solid wastes by high-rate and two-phase anaerobic digestion (TPAD). Water Sci. Technol. 41 (3), 101–110. Hanaki, K., Matsuo, T., Nagase, M., 1981. Mechanism of inhibition caused by longchain fatty acids in anaerobic digestion process. Biotechnol. Bioengy 23 (7), 1591–1610. Hecht, C., Griehl, C., 2009. Investigation of the accumulation of aromatic compounds during biogas production from kitchen waste. Bioresour Technol. 100 (2), 654– 658. Jiang, J., Du, X., Ng, S., Zhang, C., 2010. Comparison of atmospheric pressure effects on the anaerobic digestion of municipal solid waste. Bioresour Technol. 101 (16), 6361–6367. Jiang, J., Zhang, Y., Li, K., Wang, Q., Gong, C., Li, M., 2013. Volatile fatty acids production from food waste: effects of pH, temperature, and organic loading rate. Bioresour. Technol. 143, 525–530. Koster, I.W., Cramer, A., 1987. Inhibition of methanogenesis from acetate in granular sludge by long-chain fatty acids. Appl. Environ. Microb. 53 (2), 403– 409.
Lay, J.-J., Li, Y.-Y., Noike, T., 1996. Effect of moisture content and chemical nature on methane fermentation characteristics of municipal solid wastes. P. Jpn. Soc. Civil Eng., 101–108 Lay, J.J., Li, Y.Y., Noike, T., 1997. Influences of pH and moisture content on the methane production in high-solids sludge digestion. Water Res. 31 (6), 1518– 1524. Lee, M., Hidaka, T., Tsuno, H., 2008. Effect of temperature on performance and microbial diversity in hyperthermophilic digester system fed with kitchen garbage. Bioresour. Technol. 99 (15), 6852–6860. Li, M., Zhao, Y., Guo, Q., Qian, X., Niu, D., 2008. Bio-hydrogen production from food waste and sewage sludge in the presence of aged refuse excavated from refuse landfill. Renew. Energy 33 (12), 2573–2579. Li, R., Chen, S., Li, X., Lar, J.S., He, Y., Zhu, B., 2009. Anaerobic codigestion of kitchen waste with cattle manure for biogas production. Energy Fule. 23, 2225–2228. Liu, C.F., Yuan, X.Z., Zeng, G.M., Li, W.W., Li, J., 2008. Prediction of methane yield at optimum pH for anaerobic digestion of organic fraction of municipal solid waste. Bioresour. Technol. 99 (4), 882–888. Luostarinen, S., Rintala, J., 2007. Anaerobic on-site treatment of kitchen waste in combination with black water in UASB-septic tanks at low temperatures. Bioresour. Technol. 98 (9), 1734–1740. Parawira, W., Murto, M., Read, J., Mattiasson, B., 2005. Profile of hydrolases and biogas production during two-stage mesophilic anaerobic digestion of solid potato waste. Process. Biochem. 40 (9), 2945–2952. Shahriari, H., Warith, M., Hamoda, M., Kennedy, K., 2013. Evaluation of single vs. staged mesophilic anaerobic digestion of kitchen waste with and without microwave pretreatment. J. Environ. Manage. 125, 74–84. Shilton, A., Powell, N., Broughton, A., Pratt, C., Pratt, S., Pepper, C., 2013. Enhanced biogas production using cow manure to stabilize co-digestion of whey and primary sludge. Environ. Technol. 34 (17), 2491–2496. Wang, X., Yang, G., Feng, Y., Ren, G., Han, X., 2012. Optimizing feeding composition and carbon-nitrogen ratios for improved methane yield during anaerobic codigestion of dairy, chicken manure and wheat straw. Bioresour. Technol. 120, 78–83. Warnecke, T., Gill, R.T., 2005. Organic acid toxicity, tolerance, and production in Escherichia coli biorefining applications. Microb. Cell Fact. 4 (1), 25. Yamashiro, T., Lateef, S.A., Ying, C., Beneragama, N., Lukic, M., Masahiro, I., Ihara, I., Nishida, T., Umetsu, K., 2013. Anaerobic co-digestion of dairy cow manure and high concentrated food processing waste. J. Mater. Cycles Waste. 15 (4), 539– 547. Yang, Y.Q., Shen, D.G., Li, N., Xu, D., Long, Y.Y., Lu, X.Y., 2013. Co-digestion of kitchen waste and fruit-vegetable waste by two-phase anaerobic digestion. Environ. Sci. Pollut. Res. 20 (4), 2162–2171. Zhang, B., Zhang, L., Zhang, S., Shi, H., Cai, W., 2005. The influence of pH on hydrolysis and acidogenesis of kitchen wastes in two-phase anaerobic digestion. Environ. Technol. 26 (3), 329–340. Zhang, D.Q., Tan, S.K., Gersberg, R.M., 2010. Municipal solid waste management in China: status, problems and challenges. J. Environ. Manage. 91 (8), 1623–1633. Zhang, X., Qiu, W., Chen, H., 2012. Enhancing the hydrolysis and acidification of steam-exploded cornstalks by intermittent pH adjustment with an enriched microbial community. Bioresour. Technol. 123, 30–35.
Please cite this article in press as: Zhai, N., et al. Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.027
