Waste Management xxx (2014) xxx–xxx
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Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies Long Wang a, Fei Shen a,b, Hairong Yuan a, Dexun Zou a, Yanping Liu a, Baoning Zhu a, Xiujin Li a,⇑ a b
Centre for Resource and Environmental Research, Beijing University of Chemical Technology, 15 Beisanhuan East Road, Chaoyang District, Beijing 100029, PR China Institute of Ecological and Environmental Sciences, Sichuan Agricultural University, Chengdu, Sichuan 611130, PR China
a r t i c l e
i n f o
Article history: Received 24 January 2014 Accepted 1 August 2014 Available online xxxx Keywords: Kitchen waste Fruit/vegetable waste Co-digestion Pilot scale Profit analysis
a b s t r a c t The anaerobic digestion performances of kitchen waste (KW) and fruit/vegetable waste (FVW) were investigated for establishing engineering digestion system. The study was conducted from lab-scale to pilot-scale, including batch, single-phase and two-phase experiments. The lab-scale experimental results showed that the ratio of FVW to KW at 5:8 presented higher methane productivity (0.725 L CH4/g VS), and thereby was recommended. Two-phase digestion appeared to have higher treatment capacity and better buffer ability for high organic loading rate (OLR) (up to 5.0 g (VS) L1 d1), compared with the low OLR of 3.5 g (VS) L1 d1 for single-phase system. For two-phase digestion, the pilot-scale system showed similar performances to those of lab-scale one, except slightly lower maximum OLR of 4.5 g (VS) L1 d1 was allowed. The pilot-scale system proved to be profitable with a net profit of 10.173 $/ton as higher OLR (P3.0 g (VS) L1 d1) was used. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Recently, with great economic growth and rapid urbanization in China, the problem of huge municipal solid waste (MSW) disposal has been more serious in most megacities (Liu et al., 2012a). Statistically, approximate 16.4 107 tons of MSW was collected in China in the year of 2011 (National Bureau of Statistics of China, 2011), in which the organic ingredients including fruit/vegetable waste (FVW) and kitchen waste (KW) accounted for 50–60% (Liu et al., 2012b). Traditionally, most of these wastes generally have been treated by composting, landfill, and incineration together with other MSW. Unlike other solid waste, FVW and KW are characterized by high moisture content and rich biodegradable organic ingredients, which may potentially cause some negative issues in the traditional systems for MSW treatment. For example, the spread of odor during composting, the serious greenhouse gas and huge leachate discharge during landfill, and unsteady burning resulting in dioxin production during incineration (Hartmann and Ahring, 2006). By contrast, the anaerobic digestion can convert these wastes into biogas as energy and avoid the mentioned issues, which is rather meaningful to current energy crisis and environmental protection (De Baere, 2006; DiStefano and Belenky, 2009; El Hanandeh and El-Zein, 2010; Kafle et al., 2014). ⇑ Corresponding author. Tel./fax: +86 10 64432281. E-mail addresses: [email protected], [email protected] (X. Li).
Besides, the size of cities in China is extremely gigantic with extremely high population density. Thereby, food consumptions are relatively amassed in communities, canteens of universities or enterprises, and some restaurants. Correspondingly, the produced food waste can be largely and easily collected in these mentioned areas. Thus, a proper scale anaerobic digestion system can be potentially designed on the spot for treating these wastes and offer the biogas or electricity to canteens or restaurants, which will be a beneficial way to reduce logistic costs and pollution risks during the collection and transportation of food waste, meanwhile, the treatment difficulties of MSW also could be alleviated. Technically, high volatile solids content is the common characteristic of FVW and KW, which caused the rapid hydrolysis during the digestion resulting in a severe acidification when the FVW and KW were digested separately. Consequently, the methanogenesis would be seriously inhibited (Ward et al., 2008; Jiang et al., 2012). This issue has been definitely limited the application of anaerobic digestion for treating food waste in industrial-scale. Currently, co-digestion for different organic substrates is generally accepted as an efficient way to balance the nutrients for anaerobic microorganisms and improves the digestion stability and methane production (Gomez et al., 2006; Cavinato et al., 2013). A few studies have proven that the co-digestion of FVW or KW with other organic substrates could achieve more stable digestion performances (Dinsdale et al., 2000; El-Mashad and Zhang, 2010; Kafle et al., 2012). Liu and Alkanok et al. reported that a number of
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Please cite this article in press as: Wang, L., et al. Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.005
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L. Wang et al. / Waste Management xxx (2014) xxx–xxx
different substrates, which have similar compositions to FVW and KW, could be successfully co-digested (Liu et al., 2009; Alkanok et al., 2014). Therefore, it is possible to co-digest FVW with KW to achieve the more stable and applicable digestion. Moreover, it was reported that the anaerobic digestion system of food waste was very difficult to run at higher organic loading rate (OLR) (Mata-Alvarez et al., 1992). It can be aborted completely as the OLR was increased to 3.0 g (VS) L1 d1 due to the serious accumulation of volatile fatty acids (VFA) (Lin et al., 2011). The methanogenic and non-methanogenic microorganisms are significantly different with respect to nutritional requirements, physiology, growth and metabolic characteristics, and sensitivity to environmental stress (Yu et al., 2012). In addition, the digestion failure risks from increasing OLR for FVW and KW digestion could be theoretically decreased by separating the acidification from methanogenesis. Thus, the two-phase digestion for FVW and KW may be potentially feasible to operate the digestion at high OLR. In this study, the batch co-digestion of FVW and KW in lab-scale was carried out to investigate the digestion performances and clarify suitable FVW and KW mixture ratio for efficient co-digestion application. Co-digestion of two-phase and single-phase was also performed to investigate the enhancement potential of OLR. A pilot-scale anaerobic digestion system was built to verify the obtained results from lab-scale. Correspondingly, the operation costs and profits of this pilot-scale system were also evaluated. 2. Materials and methods 2.1. Feedstock and seeding sludge FVW and KW were collected from the canteen in Beijing University of Chemistry Technology, Beijing. The indigestible compositions in FVW and KW, such as plastic and chopsticks were selected out before they were crushed and homogenized. The homogenized substrates were kept in a 20 °C fridge till for anaerobic digestion. Activated sludge for inoculum was taken from the anaerobic stream in the Xiaohongmen Wastewater Treatment Plant in Beijing. The main characteristics of substrates and seeding sludge are shown in Table 1.
2.2.1. Batch co-digestion The batch digestion in lab-scale was carried out in 2-L conical flasks with working volume of 1.5 L at temperature of 35 ± 1 °C Table 1 Characteristics of feedstock and inoculum.
a b c d
2.2.2. Single-phase co-digestion A10-L completely stirred tank reactor (CSTR) with working volume of 8.0 L was employed for anaerobic digestion in the mode of sequencing batch. This anaerobic digestion system was operated at temperature of 35 ± 1 °C and stirring rate of 120 rev./min with the frequency of 8 times per day, and each stirring was lasted for 5 min. The start-up OLR for the digestion was 0.5 g (VS) L1 d1. Afterwards, the OLR was stepwise enhanced from 0.5 to 3.5 g (VS) L1 d1 during the following 129 days digestion. The hydraulic retention time (HRT) of 30 days was maintained in the whole digestion process. The employed FVW to KW ratio was based on the results from batch digestion. During the digestion, biogas production, methane content and effluent pH were also recorded daily. VFA in the effluent at the stable phase of each OLR were sampled and analyzed. 2.2.3. Two-phase co-digestion The two-phase digestion was carried out in two CSTRs with working volume of 5.0 L and 8.0 L for acidification and methanogenesis, respectively. The start-up of acidification phase was at the OLR of 2.0 g (VS) L1 d1 for 10 days in batch. Afterwards, the OLR was gradually increased from 2.0 to 10.0 g (VS) L1 d1 with the HRT of 10 days for acidification phase. The effluent from the acidification phase was pumped to second phase for methanogenesis with the OLR of 1.0, 2.0, 3.0, 4.0 and 5.0 g (VS) L1 d1, and the HRT of 20 days. The start-up for the methanogenic phase was completely same with the single-phase anaerobic digestion. 2.3. Pilot-scale co-digestion
2.2. Lab-scale co-digestion
Water content (%) TSa (wet basis, %) VSa (wet basis, %) VS/TS (%) pH Density (kg/L) Crude fat (dry basis, %) Crude fiber (dry basis, %) Crude protein (dry basis, %) Soluble carbohydrate (dry basis, %) Total carbon (dry basis, %) Total nitrogen (dry basis, %) C/Nd
(Appels et al., 2008). The OLR of 16.5 g VS/L was employed in the digestion with FVW/KW ratio of 0:8, 2:8, 5:8, 8:8, and 8:0 (correspondingly labeled as A1–A5). The initial food (FVW and KW as feedstock) and microorganism (sludge as inoculum) weight for these batch reactors were 24.7 g (VS) and 9.7 g (VS), respectively, with food to microorganism ratio of 2.5. A reactor with the same amount of inoculums with A1–A5 was operated as blank (without feedstock). Shaking rate for digestion were controlled as 120 rev./ min with shaking frequency of 24 times per day and 2 min for every shaking. During the digestion, the biogas production and methane content were monitored daily.
KW
FVW
Seeding sludge
77.83 22.17 ± 1.57b 17.87 ± 1.28 80.60 5.08 ± 0.07 1.05 ± 0.04 33.82 ± 5.04 6.93 ± 0.93 16.88 ± 1.24 21.60 ± 2.79
92.06 7.94 ± 0.83 6.74 ± 0.65 84.89 5.28 ± 0.09 1.09 ± 0.04 3.78 ± 0.88 24.50 ± 2.72 13.80 ± 1.74 11.80 ± 1.64
94.57 5.43 ± 0.30 2.29 ± 0.26 42.17 7.74 ± 0.06 1.03 ± 0.02 2.73 ± 0.91 N.D.c 19.12 ± 1.55 N.D.
32.85 2.35 13.98
28.05 1.63 17.21
18.80 3.06 6.14
TS and VS are the abbreviations of total solids and volatile solids, respectively. The ‘‘±’’ in the table represent standard deviations. N.D. means not detected. C/N means the ratio of total carbon to total nitrogen.
A pilot-scale two-phase digestion system was established on campus at Beijing University of Chemistry Technology (BUCT), Beijing, to treat partial KW and VFW from canteen at BUCT for biogas production. The flow photo and diagram of the two-phase pilotscale system are shown in Fig. 2. The working volume acidogenic phase and methanogenic stage phase were 2.0 m3 and 4.0 m3, respectively, with the daily treatment ability of 100–130 kg waste. The pilot-scale system was mainly integrated by the units of feedstock homogenization, pH adjustment, pumping, effluent reservoir, and biogas storage (see Fig. 2b). In order to make a comparison on the digestion performances between the lab-scale and pilot-scale system, the similar OLR was employed for digestion in the pilotscale system. Other operation conditions for pilot-scale digestion including temperature, HRT and agitation were similar to those of lab-scale. 2.4. Analytical methods Daily produced biogas in batch digestion was determined by water displacement method, and gas meters were used for the single-phase and two-phase digestion. Biogas content was analyzed using a gas chromatograph (GC) (Shimadzu, SP2100) equipped with a TDX-01 stainless steel column and a thermal conductivity detector. VFAs were analyzed by another GC (Shimadzu,
Please cite this article in press as: Wang, L., et al. Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.005
L. Wang et al. / Waste Management xxx (2014) xxx–xxx
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GC-2014) equipped with a flammable ionized detector and a DB-WAX123-7032 capillary column. The detailed protocols for the gas content and VFA were strictly following the reference (De La Rubia et al., 2009). The content of crude fat was tested using a fat analyzer (TOP, SZF-06C). The analysis of TS, VS, soluble chemical oxygen demand (SCOD), alkalinity, ammonia and pH in the effluent were determined according to standard methods of American Public Health Association (APHA) (APHA, 2005). Biogas production was calculated by dividing the volume of daily produced biogas by the working volume of reactor. Methane yield was calculated by dividing the methane production by the amount of added VS. All the gas volumes in this context were converted to the volumes at standard temperature (0 °C) and pressure (1 atm). The significance of differences about methane yields in batch systems were determined by t-test using Excel software 2010. Variance of average biogas production and methane yield were also calculated. A statistical coefficient of variation (Cvar) on biogas production was introduced to analyze the stability of the semicontinuous systems.
C var ¼ ðStandard deviation=AverageÞ 100%
ð1Þ
2.5. Operation costs and profits analysis The operation costs and profits analysis for the pilot-scale case was based on the mass flow and energy flow of this system. The current price of related input and output units was according to the investigation of local market. Net profits of the system were the difference of output profits and input costs. 3. Results and discussion 3.1. Batch anaerobic co-digestion in lab-scale The co-digestion of FVW and KW was conducted in batch to investigate the digestion performances and seek a relatively suitable ratio for the application. The daily biogas production, total biogas production, and the methane production based on VS were employed to evaluate the digestion performances. The batch test with inoculum only as Blank was performed. The biogas production from inoculum has been subtracted for all the presented data. The results are presented in Fig. 1. According to Fig. 1a, there were about 11–19 days for the adaptation of microorganisms to anaerobic digestion system. Afterwards, the digestion process entered typical biogas production stage. The daily biogas production peaks in the runs of A1–A5 appeared at the 17th day, 18th day, 10th day, 13th day, and 25th day, respectively. Similar result was found for the co-digestion of apple waste with swine manure (Kafle and Kim, 2013a). Based on these results, the co-digestion rate of FVW and KW can be obviously improved comparing with their monodigestion. Especially, when the mixture ratio of FVW to KW was 5:8, the duration to achieve the maximum biogas production could be reduced by 41% and 60% compared with their mono-digestion. Moreover, the maximum daily biogas production in the runs of A2, A3, and A4 with mixture of FVW and KW were all apparently higher than the run of A1 and A5 with only KW or FVW. This result again proved the digestion rate could be greatly improved with the co-digestion, especially with the FVW and KW ratio of 5:8. When the total biogas production was considered in the runs of A1–A5, the curves all presented a typical ‘‘S’’ during the 70 days digestion (see Fig. 1b). The total biogas production of A3 was 4.46% and 2.25% higher than that of A1 and A5, although the total
Fig. 1. Biogas production during the batch digestion with various ratios of FVW to KW. (a) Daily biogas production, (b) total biogas production, and (c) methane yield.
biogas production was not improved in A2 and A4. These results indicated that proper ratio for FVW/KW could be beneficial to improve digestion performances. T80, which is defined as the time to achieve 80% total biogas production, was calculated as 25 days, 25 days, 16 days, 21 days, and 29 days for the runs of A1–A5. They meant the digestion rate was significantly accelerated by codigestion. The methane production based on VS with the run of
Please cite this article in press as: Wang, L., et al. Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.005
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(a)
14
(b) 11 9 10
18 8 7
4
6
2
1
13
P-4
5
12
3
16
17
15
Fig. 2. The pilot-scale digestion system. (a) Actual photo, and (b) flowchart. 1-Crusher; 2-Pump; 3-Adjustment reservoir; 4-Motor; 5-One-way valve for liquid; 6-Acidogenic reactor; 7-Heater coil; 8-Sensor for temperature; 9-Pressure meter; 10-Relief valve; 11-Gas meter; 12-Electromagnetic valve; 13-Methanogenic reactor; 14-One-way valve for biogas; 15-Effluent reservoir; 16-Buffer tank of biogas; 17-Compressor; 18-Biogas storage tank.
A1–A5 was plotted in Fig. 1c (error bars are standard deviations). A1–A4, which contained higher fat and protein contents, achieved higher methane productions (0.693–0.725 L/g VS) than A5 (0.620 L/g VS). This result agreed with the finding by Kafle and Kim (2013b). They reported that higher fat and protein contents in one substrate could possibly help increase methane production. The maximum methane production was 0.725 L/g VS at the FVW/ KW ratio of 5:8, which was significant higher (4.6–16.7%) than that of A1 (p < 0.01) and A5 (p < 0.01) based on the t-test. The methane production in A2 and A4 also present a little bit higher than that of A1 and A5. These results indicated that the mixing of FVW and KW could also improve the energy recovery in the digestion comparing with the mono-digestion. According to analysis above, the co-digestion could be beneficial to the improvement of digestion rate, biogas production and energy recovery, and the ratio of FVW to KW of 5:8 was recommended for the engineering application, and the corresponding VS ratio of FVW to KW was calculated as 5:21. 3.2. Single-phase vs. two-phase co-digestion in lab-scale It is always expected to employ high OLR for low cost and high efficiency of one digestion system. However, the operation with high OLR is always a big challenge because of potential rapid acidification and unstable, especially, for readily biodegradable KW and FVW. Therefore, single-phase and two-phase digestion systems were investigated by using different OLRs in the CSTRs. The operation parameters included temperature, HRT and OLR. The digestion performances of single-phase and two-phase systems were compared in Table 2. In the single-phase system, the biogas production was increased with the increasing OLR from 1.0 to 3.0 g (VS) L1 d1. The average biogas production at these OLRs reached 0.80, 1.36 and 1.95 L L1 d1, respectively. Correspondingly, the methane content main-
tained on average of 59.70%, 58.39%, and 59.65%, and no obvious difference in methane content was observed for different OLRs. However, the biogas production presented serious fluctuation as the OLR was raised to 3.5 g (VS) L1 d1, and the digestion process completely ceased (data not shown) when the OLR was further increased to 4.0 g (VS) L1 d1. The OLR of 3.5 g (VS) L1 d1 was determined as one allowed maximum for single-phase digestion. Additionally, pH varied in the range of 7.04–7.20 when the digestion system ran at the OLRs of 1.0–3.0 g (VS) L1 d1, and it decreased to 6.89 as the OLR was increased to 3.5 g (VS) L1 d1. The VFA concentrations in the effluent generally kept increase from 186.6 to 1582.0 mg/L with the OLR increase, and reached to 2302.0 mg/L at the OLR of 3.5 g (VS) L1 d1. The propionic acid content varied from 21.8 mg/L to 1576.3 mg/L, which was positively correlated with the VFA concentration, and the percentage of propionic acid in the VFA was increased from 11.6% to 68.4% when the OLR increased from 1.0 to 3.5 g (VS) L1 d1. Fischer et al. reported that the VFA accumulation mainly resulted from propionic acid (Fischer et al., 1984). Two-phase digestion was also performed with altering OLRs for methanogenesis of 1.0–5.0 g (VS) L1 d1. The biogas production increased greatly from 0.76 L L1 d1 to 1.39, 2.08, and 2.33 L L1 d1as the OLR altered from 1.0 to 4.0 g (VS) L1 d1, respectively. The methane content was 60.88%, 61.13%, 60.67%, 60.41%, and 59.3% for the OLRs applied, and no obvious difference was found for different OLRs. Like the single-phase, the two-phase system also suffered the accumulation of VFA (8905.9 mg/L) as the OLR was increased to high level of 5.0 g (VS) L1 d1. The corresponding propionic acid reached to 6182.7 mg/L in the effluent. The pH decreased from 7.21–7.28 to 6.89. But the digestion process still maintained at stable state at high OLR level of 5.0 g (VS) L1 d1. However, Kafle and Kim (2011) reported a stable digestion of swine manure with constant methane production and pH even the VFA concentration reached up to 10,000 mg/L. The better buffer
Please cite this article in press as: Wang, L., et al. Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.005
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L. Wang et al. / Waste Management xxx (2014) xxx–xxx Table 2 Comparison of single-phase and two-phase digestion (lab scale). OLR for single-phase/two-phase system (g (VS) L1 d1) 1
2
3
3.5a/4.0
N.D./5.0a
Operation conditions Duration (d) TS of feedstock (g/L) VS of feedstock (g/L) HRT (d)
48/52 1.22/1.22 0.99/0.99 30/30b
13/13 2.44/2.44 1.99/1.99 30/30
31/16 3.66/3.66 2.98/2.98 30/30
11/15 4.27/4.88 3.48/3.99 30/30
N.D./28 N.D./6.10 N.D./4.96 N.D./30
Effluent characteristics VFA (mg/L) Propionic acid (mg/L) pH
186.6/246.3 21.8/17.0 7.07/7.28
470.2/353.2 164.0/67.9 7.20/7.42
1582.0/2239.6 951.0/1536.9 7.04/7.21
2302.0/4443.5 1576.3/3093.8 6.89/7.20
N.D./8905.9 N.D./6182.7 N.D./6.67
Digester performances Biogas production (L L1 d1) Methane content (%) Methane yield (L/g VS)
0.80 ± 0.06c/0.76 ± 0.09 59.7/60.9 0.49 ± 0.04/0.47 ± 0.05
1.36 ± 0.04/1.39 ± 0.06 58.4/61.1 0.39 ± 0.01/0.43 ± 0.02
1.95 ± 0.10/2.08 ± 0.20 59.7/60.7 0.38 ± 0.02/0.42 ± 0.04
1.93 ± 0.11/2.33 ± 0.13 59.5/61.2 0.33 ± 0.02/0.36 ± 0.02
N.D./1.63 ± 0.22 N.D./59.3 N.D./0.20 ± 0.02
a The OLR of 3.5 g (VS) L1 d1 and 5.0 g (VS) L1 d1 for the single-phase and two-phase were the maximum loading, and digestion will cease once the OLRs were higher than them. b HRT for the two-phase digestion included two parts, in which the acidification and methanogenesis phase were controlled as 10 days and 20 days, respectively. c The ‘‘±’’ in the table represent standard deviations.
ability to high VFA concentration of their research could probably be explained by the short total operation time (49 days) and exchange of sludge. Based on above results, it can be deduced that the maximum OLR allowed for the two-phase system was 5.0 g (VS) L1 d1. It was observed that the energy recovery did not seem to have obvious difference for single-phase and two-phase systems when OLR was at lower level of 1.0 g (VS) L1 d1, but the two-phase system achieved greater biogas production and methane yield than those of the single-phase when OLR was higher than 2.0 g (VS) L1 d1 (see Table 2), indicating that two-phase system was capable of using higher OLR. This would bring more economic benefit as more KW and VFW can be digested for the same digester volume. Two-phase system could be also operated stably at high OLRs (up to 5.0 g (VS) L1 d1) even the VFA and propionic acid were almost 4 folds higher than those of single-phase, implying that two-phase system had better buffering ability. These results suggested that two-phase system could be more suitable for KW and VFW digestion in engineering application for its greater energy recovery, higher OLR, and better system stability.
lab-scale system. The Cvar of pilot-scale was lower than lab-scale when OLR was 63.0 g (VS) L1 d1, indicating relatively better stability with the pilot-scale system at lower OLRs. When the OLR > 3.0 g (VS) L1 d1, the Cvar of pilot-scale digestion was higher than that of lab-scale system implying potential fluctuating stability with the pilot-scale system. However, the pilot-scale system
3.3. Two-phase anaerobic co-digestion: lab-scale vs. pilot-scale In the pilot-scale system, the biogas production increased rapidly with the increase of OLRs from 1.0 to 3.0 g (VS) L1 d1 (see Fig. 3b). The average biogas production were 0.92, 1.65, and 2.66 L L1 d1, respectively (see Table 3). The biogas production fluctuated when the OLR was further raised to 4.0 and 4.5 g (VS) L1 d1, which was similar to the results in lab-scale experiment. Methane contents at the OLRs were 66.2%, 66.8%, 65.7%, 65.0%, and 64.8% (see Table 3). When the system was operated at the OLR over 4.5 g (VS) L1 d1 (actually increased to about 5.0 g (VS) L1 d1), the digestion system no longer not run stable (data not shown). This indicated that the maximum OLR (4.5 g (VS) L1 d1) for pilot-scale system was slight lower than that (5.0 g (VS) L1 d1) of lab-scale system, which might be attributed to the different characteristics of feedstock fed to pilot-scale system daily. Cvar for biogas production at each OLR was calculated and used to compare the stability between the lab-scale and the pilot-scale system (Shen et al., 2013). The Cvar was 5.08%, 2.71%, 4.16%, 8.49% and 10.54% as the OLR increased from 1.0 to 4.5 g (VS) L1 d1 in the pilot-scale system. Similarly, the Cvar was 9.62%, 4.47%, 4.93%, 5.36% and 8.73% at the OLR of 1.0–5.0 g (VS) L1 d1 in the
Fig. 3. Performances of biogas production and pH with OLR in (a) lab-scale and (b) pilot-scale digestion.
Please cite this article in press as: Wang, L., et al. Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.005
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Table 3 Performance parameters of different OLRs in pilot-scale system. OLR of pilot-scale system/g (VS) L1 d1
a b
1.0
2.0
3.0
4.0
4.5
Operation conditions Duration (d) TS of feedstock (g/L) VS of feedstock (g/L) HRT (d)
29 1.22 0.99 30a
22 2.44 1.98 30
26 3.66 2.98 30
43 4.88 3.97 30
32 5.49 4.47 30
Effluent characteristics TS of effluent (g/L) VS of effluent (g/L) VFA (mg/L) NH+4–N (mg/L) Alkalinity (mg/L) SCOD (mg/L) pH
0.13 0.07 135.6 692.0 4300 996.6 7.15
0.42 0.17 280.4 830.7 3800 1451.3 7.08
0.76 0.26 320.8 868.0 4850 1459.6 7.36
1.04 0.83 1280.7 917.3 5063 4438.8 7.05
1.29 1.24 2582.0 1258.7 6573 6464.8 7.01
Digester performances TS removal rate (%) VS removal rate (%) Biogas production (L L1 d1) Methane content (%) Methane yield (L/g VS)
87.0 91.0 0.92 ± 0.47b 66.2 0.64 ± 0.05
82.3 89.4 1.65 ± 0.21 66.8 0.57 ± 0.03
79.20 89.2 2.66 ± 0.46 65.7 0.61 ± 0.09
78.60 73.9 3.24 ± 0.45 65.0 0.53 ± 0.07
70.60 65.4 3.17 ± 0.43 64.8 0.46 ± 0.06
HRT for the pilot-scale digestion included two parts, in which the acidification and methanogenesis phase were controlled as 10 days and 20 days, respectively. The ‘‘±’’ in the table represent standard deviations.
was still capable of operating stably if OLR was not higher than 4.5 g (VS) L1 d1. The effluent pH varied in the range of 7.08–7.35 when the pilot-scale digestion ran at the OLR of 1.0–3.0 g (VS) L1 d1, and occasionally decreased below 7.0 when OLR > 3.0 g (VS) L1 d1. NH+4–N (692.0–1258.7 mg/L) and SCOD (996.6–6564.8 mg/L) were also accumulated in effluent with the increase of OLR. TS and VS removal rate in the pilot-scale system were negatively correlated with the OLR increase (see Table 3). The VFAs concentration in the effluent kept increasing (135.6–320.8 mg/L) with the OLR and reached 1280.7 and 2582.0 mg/L at the OLRs of 4.0 and 4.5 g (VS) L1 d1, which were lower than lab-scale system (see Tables 2 and 3). Moreover, the energy recovery (methane production) from the pilot-system at the different OLRs was obviously higher (about 29.8–49.6%) than that of the lab-scale system. These results indicated that the pilot-scale two-phase system could maintain stable operation if OLR was kept at reasonable level.
was used for cooking energy and the effluent was used as an organic fertilizer. The price of inputs and outputs elements was according to the statistics of local market in 2012. The costs and profits of the system at different operation OLR were calculated based on the US dollar ($) and results were listed in Table 4. As shown in Table 4, operation profit of this pilot-scale system was 1.980 $/d and 0.671 $/d as the OLR was 1.0 g (VS) L1 d1 and 2.0 g (VS) L1 d1, respectively. Their corresponding net profits based on weigh of feedstocks were 67.813 $/ton, and 11.470 $/ ton. When the operation OLR for the pilot system was increased to 3.0 g (VS) L1 d1, the operation profit from the pilot system was increased to 0.891 $/d (corresponding net profit based on feedstocks of 10.173 $/ton). It also could be found that the operation profit of food waste anaerobic digestion was positively correlated to they employed influent OLR (R2 = 0.9974) implying the influent OLR was very crucial to adjust the system profits of food waste anaerobic digestion. Therefore, the operation of food waste in application was recommended at the OLR of 3.0 g (VS) L1 d1 to achieve the reasonable profits. However, the food waste digestion
3.4. Operation costs and profits analysis based on the pilot-scale system Based on the pilot-scale system established in this work, the analysis of costs and profits for system operation was performed according to the input costs (including consumed electricity, employed labor, and water consumption) and output profits (including the biogas production and effluent production). Biogas production at stable period of each OLR was employed in this section. The input and output units for this pilot system were plotted as a flowchart in Fig. 4. As mentioned above, this pilot-scale anaerobic digestion system was established on BUCT campus for treating FVW and KW from canteen. Therefore, the feedstocks of food waste were offered for free. The labor cost included the transportation of feedstocks and management of the system. The electricity consumption mainly covered the feedstocks homogenization, pumping, agitation, and heating for digesters. They were integrally measured and recorded by an electricity meter. The water consumption was mainly used for the OLR-adjustment for the feedstocks, which could be calculated by the OLR of influent every day. The main outputs mainly included the biogas and effluent production (digestate). The biogas
Inputs
Electricity
Homogenization &Pumping
Raw material
Pretreatment
Collection
Agitation
Heat
Anaerobic digestion
Biogas
Operation
Labor
Water
Effluent
Outputs Fig. 4. The flowchart of input and output elements for the pilot-scale system.
Please cite this article in press as: Wang, L., et al. Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.005
L. Wang et al. / Waste Management xxx (2014) xxx–xxx Table 4 Operation profits and costs analysis of the pilot-scale system for FW digestion. OLR of pilot-scale two-phase system/g (VS) L1 d1
Weigh of feedstock (ton/d) Biogas production of stable period (m3/d) Electricity consumption (kW h/d) Water costa ($/d) Labor costb ($/d) Electricity costc ($/d) Total operation costd ($/d) Biogas profite ($/d) Effluent profitf ($/d) Total operation profitg ($/d) Operation profith ($/d) Net profiti ($/ton)
1.0
2.0
3.0
0.0292 1.00 24 0.109 3.204 1.570 4.883 1.461 1.442 2.903 1.980 67.813
0.0585 7.08 28 0.091 3.204 1.884 5.179 2.586 1.922 4.508 0.671 11.470
0.0876 11.92 33 0.072 3.204 2.590 5.867 4.354 2.403 6.757 0.891 10.173
a
Water cost = water consumption volume water price (0.6408 $/ton). Labor cost = working hours (1.0 h) labor price (3.204 $/h one person) (It is 1.0 h/d for the transportation of feedstocks and management of the system based on the employed labor for this case). c Electricity consumption = electricity consumed electricity price (0.078 $/ kW h). d Total operation cost = water cost + labor cost + electricity cost. e Biogas profit = biogas production biogas price (0.365 $/m3), and price are available at: http://www.bjpc.gov.cn/ywpd/wjgl/cx/jz/201208/t3884350.htm. f Effluent profit = effluent volume effluent price (effluent price is related to the content of fertilizer compositions, and in the range of 7.209–12.015 $/ton according to local market price). g Total operation profit = biogas profit + effluent profit. h Operation profit = total operation profit total operation cost. i Net profit = operation profit weigh of feedstock. b
system has to face a technical barrier in practice to avoid the system instability with increasing OLR. Hence, how to solve the stable operation of food waste digestion at high OLR should correspondingly be the core issue in future research. 4. Conclusions The batch test results of this study demonstrated that the ratio of FVW to KW at 5:8 was more suitable for co-digestion of FVW with KW. Two-phase digestion in lab-scale appeared to have high treatment capacity and better buffer ability for high OLR, and could employ maximum OLR of 5.0 g (VS) L1 d1, compared with that of 3.5 g (VS) L1 d1 for single-phase system. For two-phase digestion, the pilot-scale system showed similar performances to those of lab-scale one, except using slightly lower maximum OLR of more stable than the lab-scale system with OLR 6 4.5 g (VS) L1 d1. The pilot-scale system proved to be profitable with a net profit of 10.173 $/ton as higher OLR (P3.0 g (VS) L1 d1) was used . Acknowledgments The authors are grateful to the fund supports from the University Doctorial Foundation (No. 20120010110004) and Beijing Natural Science Foundation (No. 8142030). References Alkanok, G., Demirel, B., Onay, T.T., 2014. Determination of biogas generation potential as a renewable energy source from supermarket wastes. Waste Manage. 34, 134–140.
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APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, American Water Works Association, Washington, DC, New York, USA. Appels, L., Baeyens, J., Degrève, J., Dewil, R., 2008. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energy Combust. 34, 755– 781. Cavinato, C., Bolzonella, D., Pavan, P., Fatone, F., Cecchi, F., 2013. Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot- and full-scale reactors. Renew. Energy 55, 260–265. De Baere, L., 2006. Will anaerobic digestion of solid waste survive in the future? Water Sci. Technol. 53, 187–194. De La Rubia, M.A., Raposo, F., Rincón, B., Borja, R., 2009. Evaluation of the hydrolytic–acidogenic step of a two-stage mesophilic anaerobic digestion process of sunflower oil cake. Bioresour. Technol. 100, 4133–4138. Dinsdale, R.M., Premier, G.C., Hawkes, F.R., Hawkes, D.L., 2000. Two-stage anaerobic co-digestion of waste activated sludge and fruit/vegetable waste using inclined tubular digesters. Bioresour. Technol. 72, 159–168. DiStefano, T.D., Belenky, L.G., 2009. Life-cycle analysis of energy and greenhouse gas emissions from anaerobic biodegradation of municipal solid waste. J. Environ. Eng. – Asce. 135, 1097–1105. El Hanandeh, A., El-Zein, A., 2010. The development and application of multicriteria decision-making tool with consideration of uncertainty: the selection of a management strategy for the bio-degradable fraction in the municipal solid waste. Bioresour. Technol. 101, 555–561. El-Mashad, H.M., Zhang, R., 2010. Biogas production from co-digestion of dairy manure and food waste. Bioresour. Technol. 101, 4021–4028. Fischer, J.R., Iannotti, E.L., Porter, J.H., 1984. Anaerobic digestion of swine manure at various influent solids concentrations. Agric. Wastes 11, 157–166. Gomez, X., Cuetos, M.J., Cara, J., Moran, A., Garcia, A.I., 2006. 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Energy 103, 61– 72. Kafle, G.K., Kim, S.H., 2013b. Effects of chemical compositions and ensiling on the biogas productivity and degradation rates of agricultural and food processing by-products. Bioresour. Technol. 142, 553–561. Kafle, G.K., Bhattarai, S., Kim, S.H., Chen, L., 2014. Effect of feed to microbe ratios on anaerobic digestion of Chinese cabbage waste under mesophilic and thermophilic conditions: biogas potential and kinetic study. J. Environ. Manage. 133, 293–301. Lin, J., Zuo, J.E., Gan, L.L., Li, P., Liu, F.L., Wang, K.J., Chen, L., Gan, H.N., 2011. Effects of mixture ratio on anaerobic co-digestion with fruit and vegetable waste and food waste of China. J. Environ. Sci.-China 23, 1403–1408. Liu, G.Q., Zhang, R.H., El-Mashad, H.M., Dong, R.J., 2009. Effect of feed to inoculum ratios on biogas yields of food and green wastes. Bioresour. Technol. 100, 5103– 5108. Liu, X., Gao, X.B., Wang, W., Zheng, L., Zhou, Y.J., Sun, Y.F., 2012a. Pilot-scale anaerobic co-digestion of municipal biomass waste: focusing on biogas production and GHG reduction. Renew. Energy 44, 463–468. Liu, X., Wang, W., Shi, Y.C., Zheng, L., Gao, X.B., Qiao, W., Zhou, Y.J., 2012b. Pilot-scale anaerobic co-digestion of municipal biomass waste and waste activated sludge in China: effect of organic loading rate. Waste Manage. 32, 2056–2060. Mata-Alvarez, J., Cecchi, F., Llabrés, P., Pavan, P., 1992. Anaerobic digestion of the Barcelona central food market organic wastes. Plant design and feasibility study. Bioresour. Technol. 42, 33–42. National Bureau of Statistics of China, 2011. China Statistical Yearbook. . Shen, F., Yuan, H.R., Pang, Y.Z., Chen, S.L., Zhu, B.N., Zou, D.X., Liu, Y.P., Ma, J.W., Yu, L., Li, X.J., 2013. Performances of anaerobic co-digestion of fruit & vegetable waste (FVW) and food waste (FW): single-phase vs. two-phase. Bioresour. Technol. 144, 80–85. Ward, A.J., Hobbs, P.J., Holliman, P.J., Jones, D.L., 2008. 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Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies Long Wang a, Fei Shen a,b, Hairong Yuan a, Dexun Zou a, Yanping Liu a, Baoning Zhu a, Xiujin Li a,⇑ a b
Centre for Resource and Environmental Research, Beijing University of Chemical Technology, 15 Beisanhuan East Road, Chaoyang District, Beijing 100029, PR China Institute of Ecological and Environmental Sciences, Sichuan Agricultural University, Chengdu, Sichuan 611130, PR China
a r t i c l e
i n f o
Article history: Received 24 January 2014 Accepted 1 August 2014 Available online xxxx Keywords: Kitchen waste Fruit/vegetable waste Co-digestion Pilot scale Profit analysis
a b s t r a c t The anaerobic digestion performances of kitchen waste (KW) and fruit/vegetable waste (FVW) were investigated for establishing engineering digestion system. The study was conducted from lab-scale to pilot-scale, including batch, single-phase and two-phase experiments. The lab-scale experimental results showed that the ratio of FVW to KW at 5:8 presented higher methane productivity (0.725 L CH4/g VS), and thereby was recommended. Two-phase digestion appeared to have higher treatment capacity and better buffer ability for high organic loading rate (OLR) (up to 5.0 g (VS) L1 d1), compared with the low OLR of 3.5 g (VS) L1 d1 for single-phase system. For two-phase digestion, the pilot-scale system showed similar performances to those of lab-scale one, except slightly lower maximum OLR of 4.5 g (VS) L1 d1 was allowed. The pilot-scale system proved to be profitable with a net profit of 10.173 $/ton as higher OLR (P3.0 g (VS) L1 d1) was used. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Recently, with great economic growth and rapid urbanization in China, the problem of huge municipal solid waste (MSW) disposal has been more serious in most megacities (Liu et al., 2012a). Statistically, approximate 16.4 107 tons of MSW was collected in China in the year of 2011 (National Bureau of Statistics of China, 2011), in which the organic ingredients including fruit/vegetable waste (FVW) and kitchen waste (KW) accounted for 50–60% (Liu et al., 2012b). Traditionally, most of these wastes generally have been treated by composting, landfill, and incineration together with other MSW. Unlike other solid waste, FVW and KW are characterized by high moisture content and rich biodegradable organic ingredients, which may potentially cause some negative issues in the traditional systems for MSW treatment. For example, the spread of odor during composting, the serious greenhouse gas and huge leachate discharge during landfill, and unsteady burning resulting in dioxin production during incineration (Hartmann and Ahring, 2006). By contrast, the anaerobic digestion can convert these wastes into biogas as energy and avoid the mentioned issues, which is rather meaningful to current energy crisis and environmental protection (De Baere, 2006; DiStefano and Belenky, 2009; El Hanandeh and El-Zein, 2010; Kafle et al., 2014). ⇑ Corresponding author. Tel./fax: +86 10 64432281. E-mail addresses: [email protected], [email protected] (X. Li).
Besides, the size of cities in China is extremely gigantic with extremely high population density. Thereby, food consumptions are relatively amassed in communities, canteens of universities or enterprises, and some restaurants. Correspondingly, the produced food waste can be largely and easily collected in these mentioned areas. Thus, a proper scale anaerobic digestion system can be potentially designed on the spot for treating these wastes and offer the biogas or electricity to canteens or restaurants, which will be a beneficial way to reduce logistic costs and pollution risks during the collection and transportation of food waste, meanwhile, the treatment difficulties of MSW also could be alleviated. Technically, high volatile solids content is the common characteristic of FVW and KW, which caused the rapid hydrolysis during the digestion resulting in a severe acidification when the FVW and KW were digested separately. Consequently, the methanogenesis would be seriously inhibited (Ward et al., 2008; Jiang et al., 2012). This issue has been definitely limited the application of anaerobic digestion for treating food waste in industrial-scale. Currently, co-digestion for different organic substrates is generally accepted as an efficient way to balance the nutrients for anaerobic microorganisms and improves the digestion stability and methane production (Gomez et al., 2006; Cavinato et al., 2013). A few studies have proven that the co-digestion of FVW or KW with other organic substrates could achieve more stable digestion performances (Dinsdale et al., 2000; El-Mashad and Zhang, 2010; Kafle et al., 2012). Liu and Alkanok et al. reported that a number of
http://dx.doi.org/10.1016/j.wasman.2014.08.005 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang, L., et al. Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.005
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L. Wang et al. / Waste Management xxx (2014) xxx–xxx
different substrates, which have similar compositions to FVW and KW, could be successfully co-digested (Liu et al., 2009; Alkanok et al., 2014). Therefore, it is possible to co-digest FVW with KW to achieve the more stable and applicable digestion. Moreover, it was reported that the anaerobic digestion system of food waste was very difficult to run at higher organic loading rate (OLR) (Mata-Alvarez et al., 1992). It can be aborted completely as the OLR was increased to 3.0 g (VS) L1 d1 due to the serious accumulation of volatile fatty acids (VFA) (Lin et al., 2011). The methanogenic and non-methanogenic microorganisms are significantly different with respect to nutritional requirements, physiology, growth and metabolic characteristics, and sensitivity to environmental stress (Yu et al., 2012). In addition, the digestion failure risks from increasing OLR for FVW and KW digestion could be theoretically decreased by separating the acidification from methanogenesis. Thus, the two-phase digestion for FVW and KW may be potentially feasible to operate the digestion at high OLR. In this study, the batch co-digestion of FVW and KW in lab-scale was carried out to investigate the digestion performances and clarify suitable FVW and KW mixture ratio for efficient co-digestion application. Co-digestion of two-phase and single-phase was also performed to investigate the enhancement potential of OLR. A pilot-scale anaerobic digestion system was built to verify the obtained results from lab-scale. Correspondingly, the operation costs and profits of this pilot-scale system were also evaluated. 2. Materials and methods 2.1. Feedstock and seeding sludge FVW and KW were collected from the canteen in Beijing University of Chemistry Technology, Beijing. The indigestible compositions in FVW and KW, such as plastic and chopsticks were selected out before they were crushed and homogenized. The homogenized substrates were kept in a 20 °C fridge till for anaerobic digestion. Activated sludge for inoculum was taken from the anaerobic stream in the Xiaohongmen Wastewater Treatment Plant in Beijing. The main characteristics of substrates and seeding sludge are shown in Table 1.
2.2.1. Batch co-digestion The batch digestion in lab-scale was carried out in 2-L conical flasks with working volume of 1.5 L at temperature of 35 ± 1 °C Table 1 Characteristics of feedstock and inoculum.
a b c d
2.2.2. Single-phase co-digestion A10-L completely stirred tank reactor (CSTR) with working volume of 8.0 L was employed for anaerobic digestion in the mode of sequencing batch. This anaerobic digestion system was operated at temperature of 35 ± 1 °C and stirring rate of 120 rev./min with the frequency of 8 times per day, and each stirring was lasted for 5 min. The start-up OLR for the digestion was 0.5 g (VS) L1 d1. Afterwards, the OLR was stepwise enhanced from 0.5 to 3.5 g (VS) L1 d1 during the following 129 days digestion. The hydraulic retention time (HRT) of 30 days was maintained in the whole digestion process. The employed FVW to KW ratio was based on the results from batch digestion. During the digestion, biogas production, methane content and effluent pH were also recorded daily. VFA in the effluent at the stable phase of each OLR were sampled and analyzed. 2.2.3. Two-phase co-digestion The two-phase digestion was carried out in two CSTRs with working volume of 5.0 L and 8.0 L for acidification and methanogenesis, respectively. The start-up of acidification phase was at the OLR of 2.0 g (VS) L1 d1 for 10 days in batch. Afterwards, the OLR was gradually increased from 2.0 to 10.0 g (VS) L1 d1 with the HRT of 10 days for acidification phase. The effluent from the acidification phase was pumped to second phase for methanogenesis with the OLR of 1.0, 2.0, 3.0, 4.0 and 5.0 g (VS) L1 d1, and the HRT of 20 days. The start-up for the methanogenic phase was completely same with the single-phase anaerobic digestion. 2.3. Pilot-scale co-digestion
2.2. Lab-scale co-digestion
Water content (%) TSa (wet basis, %) VSa (wet basis, %) VS/TS (%) pH Density (kg/L) Crude fat (dry basis, %) Crude fiber (dry basis, %) Crude protein (dry basis, %) Soluble carbohydrate (dry basis, %) Total carbon (dry basis, %) Total nitrogen (dry basis, %) C/Nd
(Appels et al., 2008). The OLR of 16.5 g VS/L was employed in the digestion with FVW/KW ratio of 0:8, 2:8, 5:8, 8:8, and 8:0 (correspondingly labeled as A1–A5). The initial food (FVW and KW as feedstock) and microorganism (sludge as inoculum) weight for these batch reactors were 24.7 g (VS) and 9.7 g (VS), respectively, with food to microorganism ratio of 2.5. A reactor with the same amount of inoculums with A1–A5 was operated as blank (without feedstock). Shaking rate for digestion were controlled as 120 rev./ min with shaking frequency of 24 times per day and 2 min for every shaking. During the digestion, the biogas production and methane content were monitored daily.
KW
FVW
Seeding sludge
77.83 22.17 ± 1.57b 17.87 ± 1.28 80.60 5.08 ± 0.07 1.05 ± 0.04 33.82 ± 5.04 6.93 ± 0.93 16.88 ± 1.24 21.60 ± 2.79
92.06 7.94 ± 0.83 6.74 ± 0.65 84.89 5.28 ± 0.09 1.09 ± 0.04 3.78 ± 0.88 24.50 ± 2.72 13.80 ± 1.74 11.80 ± 1.64
94.57 5.43 ± 0.30 2.29 ± 0.26 42.17 7.74 ± 0.06 1.03 ± 0.02 2.73 ± 0.91 N.D.c 19.12 ± 1.55 N.D.
32.85 2.35 13.98
28.05 1.63 17.21
18.80 3.06 6.14
TS and VS are the abbreviations of total solids and volatile solids, respectively. The ‘‘±’’ in the table represent standard deviations. N.D. means not detected. C/N means the ratio of total carbon to total nitrogen.
A pilot-scale two-phase digestion system was established on campus at Beijing University of Chemistry Technology (BUCT), Beijing, to treat partial KW and VFW from canteen at BUCT for biogas production. The flow photo and diagram of the two-phase pilotscale system are shown in Fig. 2. The working volume acidogenic phase and methanogenic stage phase were 2.0 m3 and 4.0 m3, respectively, with the daily treatment ability of 100–130 kg waste. The pilot-scale system was mainly integrated by the units of feedstock homogenization, pH adjustment, pumping, effluent reservoir, and biogas storage (see Fig. 2b). In order to make a comparison on the digestion performances between the lab-scale and pilot-scale system, the similar OLR was employed for digestion in the pilotscale system. Other operation conditions for pilot-scale digestion including temperature, HRT and agitation were similar to those of lab-scale. 2.4. Analytical methods Daily produced biogas in batch digestion was determined by water displacement method, and gas meters were used for the single-phase and two-phase digestion. Biogas content was analyzed using a gas chromatograph (GC) (Shimadzu, SP2100) equipped with a TDX-01 stainless steel column and a thermal conductivity detector. VFAs were analyzed by another GC (Shimadzu,
Please cite this article in press as: Wang, L., et al. Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.005
L. Wang et al. / Waste Management xxx (2014) xxx–xxx
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GC-2014) equipped with a flammable ionized detector and a DB-WAX123-7032 capillary column. The detailed protocols for the gas content and VFA were strictly following the reference (De La Rubia et al., 2009). The content of crude fat was tested using a fat analyzer (TOP, SZF-06C). The analysis of TS, VS, soluble chemical oxygen demand (SCOD), alkalinity, ammonia and pH in the effluent were determined according to standard methods of American Public Health Association (APHA) (APHA, 2005). Biogas production was calculated by dividing the volume of daily produced biogas by the working volume of reactor. Methane yield was calculated by dividing the methane production by the amount of added VS. All the gas volumes in this context were converted to the volumes at standard temperature (0 °C) and pressure (1 atm). The significance of differences about methane yields in batch systems were determined by t-test using Excel software 2010. Variance of average biogas production and methane yield were also calculated. A statistical coefficient of variation (Cvar) on biogas production was introduced to analyze the stability of the semicontinuous systems.
C var ¼ ðStandard deviation=AverageÞ 100%
ð1Þ
2.5. Operation costs and profits analysis The operation costs and profits analysis for the pilot-scale case was based on the mass flow and energy flow of this system. The current price of related input and output units was according to the investigation of local market. Net profits of the system were the difference of output profits and input costs. 3. Results and discussion 3.1. Batch anaerobic co-digestion in lab-scale The co-digestion of FVW and KW was conducted in batch to investigate the digestion performances and seek a relatively suitable ratio for the application. The daily biogas production, total biogas production, and the methane production based on VS were employed to evaluate the digestion performances. The batch test with inoculum only as Blank was performed. The biogas production from inoculum has been subtracted for all the presented data. The results are presented in Fig. 1. According to Fig. 1a, there were about 11–19 days for the adaptation of microorganisms to anaerobic digestion system. Afterwards, the digestion process entered typical biogas production stage. The daily biogas production peaks in the runs of A1–A5 appeared at the 17th day, 18th day, 10th day, 13th day, and 25th day, respectively. Similar result was found for the co-digestion of apple waste with swine manure (Kafle and Kim, 2013a). Based on these results, the co-digestion rate of FVW and KW can be obviously improved comparing with their monodigestion. Especially, when the mixture ratio of FVW to KW was 5:8, the duration to achieve the maximum biogas production could be reduced by 41% and 60% compared with their mono-digestion. Moreover, the maximum daily biogas production in the runs of A2, A3, and A4 with mixture of FVW and KW were all apparently higher than the run of A1 and A5 with only KW or FVW. This result again proved the digestion rate could be greatly improved with the co-digestion, especially with the FVW and KW ratio of 5:8. When the total biogas production was considered in the runs of A1–A5, the curves all presented a typical ‘‘S’’ during the 70 days digestion (see Fig. 1b). The total biogas production of A3 was 4.46% and 2.25% higher than that of A1 and A5, although the total
Fig. 1. Biogas production during the batch digestion with various ratios of FVW to KW. (a) Daily biogas production, (b) total biogas production, and (c) methane yield.
biogas production was not improved in A2 and A4. These results indicated that proper ratio for FVW/KW could be beneficial to improve digestion performances. T80, which is defined as the time to achieve 80% total biogas production, was calculated as 25 days, 25 days, 16 days, 21 days, and 29 days for the runs of A1–A5. They meant the digestion rate was significantly accelerated by codigestion. The methane production based on VS with the run of
Please cite this article in press as: Wang, L., et al. Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.005
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18 8 7
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Fig. 2. The pilot-scale digestion system. (a) Actual photo, and (b) flowchart. 1-Crusher; 2-Pump; 3-Adjustment reservoir; 4-Motor; 5-One-way valve for liquid; 6-Acidogenic reactor; 7-Heater coil; 8-Sensor for temperature; 9-Pressure meter; 10-Relief valve; 11-Gas meter; 12-Electromagnetic valve; 13-Methanogenic reactor; 14-One-way valve for biogas; 15-Effluent reservoir; 16-Buffer tank of biogas; 17-Compressor; 18-Biogas storage tank.
A1–A5 was plotted in Fig. 1c (error bars are standard deviations). A1–A4, which contained higher fat and protein contents, achieved higher methane productions (0.693–0.725 L/g VS) than A5 (0.620 L/g VS). This result agreed with the finding by Kafle and Kim (2013b). They reported that higher fat and protein contents in one substrate could possibly help increase methane production. The maximum methane production was 0.725 L/g VS at the FVW/ KW ratio of 5:8, which was significant higher (4.6–16.7%) than that of A1 (p < 0.01) and A5 (p < 0.01) based on the t-test. The methane production in A2 and A4 also present a little bit higher than that of A1 and A5. These results indicated that the mixing of FVW and KW could also improve the energy recovery in the digestion comparing with the mono-digestion. According to analysis above, the co-digestion could be beneficial to the improvement of digestion rate, biogas production and energy recovery, and the ratio of FVW to KW of 5:8 was recommended for the engineering application, and the corresponding VS ratio of FVW to KW was calculated as 5:21. 3.2. Single-phase vs. two-phase co-digestion in lab-scale It is always expected to employ high OLR for low cost and high efficiency of one digestion system. However, the operation with high OLR is always a big challenge because of potential rapid acidification and unstable, especially, for readily biodegradable KW and FVW. Therefore, single-phase and two-phase digestion systems were investigated by using different OLRs in the CSTRs. The operation parameters included temperature, HRT and OLR. The digestion performances of single-phase and two-phase systems were compared in Table 2. In the single-phase system, the biogas production was increased with the increasing OLR from 1.0 to 3.0 g (VS) L1 d1. The average biogas production at these OLRs reached 0.80, 1.36 and 1.95 L L1 d1, respectively. Correspondingly, the methane content main-
tained on average of 59.70%, 58.39%, and 59.65%, and no obvious difference in methane content was observed for different OLRs. However, the biogas production presented serious fluctuation as the OLR was raised to 3.5 g (VS) L1 d1, and the digestion process completely ceased (data not shown) when the OLR was further increased to 4.0 g (VS) L1 d1. The OLR of 3.5 g (VS) L1 d1 was determined as one allowed maximum for single-phase digestion. Additionally, pH varied in the range of 7.04–7.20 when the digestion system ran at the OLRs of 1.0–3.0 g (VS) L1 d1, and it decreased to 6.89 as the OLR was increased to 3.5 g (VS) L1 d1. The VFA concentrations in the effluent generally kept increase from 186.6 to 1582.0 mg/L with the OLR increase, and reached to 2302.0 mg/L at the OLR of 3.5 g (VS) L1 d1. The propionic acid content varied from 21.8 mg/L to 1576.3 mg/L, which was positively correlated with the VFA concentration, and the percentage of propionic acid in the VFA was increased from 11.6% to 68.4% when the OLR increased from 1.0 to 3.5 g (VS) L1 d1. Fischer et al. reported that the VFA accumulation mainly resulted from propionic acid (Fischer et al., 1984). Two-phase digestion was also performed with altering OLRs for methanogenesis of 1.0–5.0 g (VS) L1 d1. The biogas production increased greatly from 0.76 L L1 d1 to 1.39, 2.08, and 2.33 L L1 d1as the OLR altered from 1.0 to 4.0 g (VS) L1 d1, respectively. The methane content was 60.88%, 61.13%, 60.67%, 60.41%, and 59.3% for the OLRs applied, and no obvious difference was found for different OLRs. Like the single-phase, the two-phase system also suffered the accumulation of VFA (8905.9 mg/L) as the OLR was increased to high level of 5.0 g (VS) L1 d1. The corresponding propionic acid reached to 6182.7 mg/L in the effluent. The pH decreased from 7.21–7.28 to 6.89. But the digestion process still maintained at stable state at high OLR level of 5.0 g (VS) L1 d1. However, Kafle and Kim (2011) reported a stable digestion of swine manure with constant methane production and pH even the VFA concentration reached up to 10,000 mg/L. The better buffer
Please cite this article in press as: Wang, L., et al. Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.005
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L. Wang et al. / Waste Management xxx (2014) xxx–xxx Table 2 Comparison of single-phase and two-phase digestion (lab scale). OLR for single-phase/two-phase system (g (VS) L1 d1) 1
2
3
3.5a/4.0
N.D./5.0a
Operation conditions Duration (d) TS of feedstock (g/L) VS of feedstock (g/L) HRT (d)
48/52 1.22/1.22 0.99/0.99 30/30b
13/13 2.44/2.44 1.99/1.99 30/30
31/16 3.66/3.66 2.98/2.98 30/30
11/15 4.27/4.88 3.48/3.99 30/30
N.D./28 N.D./6.10 N.D./4.96 N.D./30
Effluent characteristics VFA (mg/L) Propionic acid (mg/L) pH
186.6/246.3 21.8/17.0 7.07/7.28
470.2/353.2 164.0/67.9 7.20/7.42
1582.0/2239.6 951.0/1536.9 7.04/7.21
2302.0/4443.5 1576.3/3093.8 6.89/7.20
N.D./8905.9 N.D./6182.7 N.D./6.67
Digester performances Biogas production (L L1 d1) Methane content (%) Methane yield (L/g VS)
0.80 ± 0.06c/0.76 ± 0.09 59.7/60.9 0.49 ± 0.04/0.47 ± 0.05
1.36 ± 0.04/1.39 ± 0.06 58.4/61.1 0.39 ± 0.01/0.43 ± 0.02
1.95 ± 0.10/2.08 ± 0.20 59.7/60.7 0.38 ± 0.02/0.42 ± 0.04
1.93 ± 0.11/2.33 ± 0.13 59.5/61.2 0.33 ± 0.02/0.36 ± 0.02
N.D./1.63 ± 0.22 N.D./59.3 N.D./0.20 ± 0.02
a The OLR of 3.5 g (VS) L1 d1 and 5.0 g (VS) L1 d1 for the single-phase and two-phase were the maximum loading, and digestion will cease once the OLRs were higher than them. b HRT for the two-phase digestion included two parts, in which the acidification and methanogenesis phase were controlled as 10 days and 20 days, respectively. c The ‘‘±’’ in the table represent standard deviations.
ability to high VFA concentration of their research could probably be explained by the short total operation time (49 days) and exchange of sludge. Based on above results, it can be deduced that the maximum OLR allowed for the two-phase system was 5.0 g (VS) L1 d1. It was observed that the energy recovery did not seem to have obvious difference for single-phase and two-phase systems when OLR was at lower level of 1.0 g (VS) L1 d1, but the two-phase system achieved greater biogas production and methane yield than those of the single-phase when OLR was higher than 2.0 g (VS) L1 d1 (see Table 2), indicating that two-phase system was capable of using higher OLR. This would bring more economic benefit as more KW and VFW can be digested for the same digester volume. Two-phase system could be also operated stably at high OLRs (up to 5.0 g (VS) L1 d1) even the VFA and propionic acid were almost 4 folds higher than those of single-phase, implying that two-phase system had better buffering ability. These results suggested that two-phase system could be more suitable for KW and VFW digestion in engineering application for its greater energy recovery, higher OLR, and better system stability.
lab-scale system. The Cvar of pilot-scale was lower than lab-scale when OLR was 63.0 g (VS) L1 d1, indicating relatively better stability with the pilot-scale system at lower OLRs. When the OLR > 3.0 g (VS) L1 d1, the Cvar of pilot-scale digestion was higher than that of lab-scale system implying potential fluctuating stability with the pilot-scale system. However, the pilot-scale system
3.3. Two-phase anaerobic co-digestion: lab-scale vs. pilot-scale In the pilot-scale system, the biogas production increased rapidly with the increase of OLRs from 1.0 to 3.0 g (VS) L1 d1 (see Fig. 3b). The average biogas production were 0.92, 1.65, and 2.66 L L1 d1, respectively (see Table 3). The biogas production fluctuated when the OLR was further raised to 4.0 and 4.5 g (VS) L1 d1, which was similar to the results in lab-scale experiment. Methane contents at the OLRs were 66.2%, 66.8%, 65.7%, 65.0%, and 64.8% (see Table 3). When the system was operated at the OLR over 4.5 g (VS) L1 d1 (actually increased to about 5.0 g (VS) L1 d1), the digestion system no longer not run stable (data not shown). This indicated that the maximum OLR (4.5 g (VS) L1 d1) for pilot-scale system was slight lower than that (5.0 g (VS) L1 d1) of lab-scale system, which might be attributed to the different characteristics of feedstock fed to pilot-scale system daily. Cvar for biogas production at each OLR was calculated and used to compare the stability between the lab-scale and the pilot-scale system (Shen et al., 2013). The Cvar was 5.08%, 2.71%, 4.16%, 8.49% and 10.54% as the OLR increased from 1.0 to 4.5 g (VS) L1 d1 in the pilot-scale system. Similarly, the Cvar was 9.62%, 4.47%, 4.93%, 5.36% and 8.73% at the OLR of 1.0–5.0 g (VS) L1 d1 in the
Fig. 3. Performances of biogas production and pH with OLR in (a) lab-scale and (b) pilot-scale digestion.
Please cite this article in press as: Wang, L., et al. Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.005
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L. Wang et al. / Waste Management xxx (2014) xxx–xxx
Table 3 Performance parameters of different OLRs in pilot-scale system. OLR of pilot-scale system/g (VS) L1 d1
a b
1.0
2.0
3.0
4.0
4.5
Operation conditions Duration (d) TS of feedstock (g/L) VS of feedstock (g/L) HRT (d)
29 1.22 0.99 30a
22 2.44 1.98 30
26 3.66 2.98 30
43 4.88 3.97 30
32 5.49 4.47 30
Effluent characteristics TS of effluent (g/L) VS of effluent (g/L) VFA (mg/L) NH+4–N (mg/L) Alkalinity (mg/L) SCOD (mg/L) pH
0.13 0.07 135.6 692.0 4300 996.6 7.15
0.42 0.17 280.4 830.7 3800 1451.3 7.08
0.76 0.26 320.8 868.0 4850 1459.6 7.36
1.04 0.83 1280.7 917.3 5063 4438.8 7.05
1.29 1.24 2582.0 1258.7 6573 6464.8 7.01
Digester performances TS removal rate (%) VS removal rate (%) Biogas production (L L1 d1) Methane content (%) Methane yield (L/g VS)
87.0 91.0 0.92 ± 0.47b 66.2 0.64 ± 0.05
82.3 89.4 1.65 ± 0.21 66.8 0.57 ± 0.03
79.20 89.2 2.66 ± 0.46 65.7 0.61 ± 0.09
78.60 73.9 3.24 ± 0.45 65.0 0.53 ± 0.07
70.60 65.4 3.17 ± 0.43 64.8 0.46 ± 0.06
HRT for the pilot-scale digestion included two parts, in which the acidification and methanogenesis phase were controlled as 10 days and 20 days, respectively. The ‘‘±’’ in the table represent standard deviations.
was still capable of operating stably if OLR was not higher than 4.5 g (VS) L1 d1. The effluent pH varied in the range of 7.08–7.35 when the pilot-scale digestion ran at the OLR of 1.0–3.0 g (VS) L1 d1, and occasionally decreased below 7.0 when OLR > 3.0 g (VS) L1 d1. NH+4–N (692.0–1258.7 mg/L) and SCOD (996.6–6564.8 mg/L) were also accumulated in effluent with the increase of OLR. TS and VS removal rate in the pilot-scale system were negatively correlated with the OLR increase (see Table 3). The VFAs concentration in the effluent kept increasing (135.6–320.8 mg/L) with the OLR and reached 1280.7 and 2582.0 mg/L at the OLRs of 4.0 and 4.5 g (VS) L1 d1, which were lower than lab-scale system (see Tables 2 and 3). Moreover, the energy recovery (methane production) from the pilot-system at the different OLRs was obviously higher (about 29.8–49.6%) than that of the lab-scale system. These results indicated that the pilot-scale two-phase system could maintain stable operation if OLR was kept at reasonable level.
was used for cooking energy and the effluent was used as an organic fertilizer. The price of inputs and outputs elements was according to the statistics of local market in 2012. The costs and profits of the system at different operation OLR were calculated based on the US dollar ($) and results were listed in Table 4. As shown in Table 4, operation profit of this pilot-scale system was 1.980 $/d and 0.671 $/d as the OLR was 1.0 g (VS) L1 d1 and 2.0 g (VS) L1 d1, respectively. Their corresponding net profits based on weigh of feedstocks were 67.813 $/ton, and 11.470 $/ ton. When the operation OLR for the pilot system was increased to 3.0 g (VS) L1 d1, the operation profit from the pilot system was increased to 0.891 $/d (corresponding net profit based on feedstocks of 10.173 $/ton). It also could be found that the operation profit of food waste anaerobic digestion was positively correlated to they employed influent OLR (R2 = 0.9974) implying the influent OLR was very crucial to adjust the system profits of food waste anaerobic digestion. Therefore, the operation of food waste in application was recommended at the OLR of 3.0 g (VS) L1 d1 to achieve the reasonable profits. However, the food waste digestion
3.4. Operation costs and profits analysis based on the pilot-scale system Based on the pilot-scale system established in this work, the analysis of costs and profits for system operation was performed according to the input costs (including consumed electricity, employed labor, and water consumption) and output profits (including the biogas production and effluent production). Biogas production at stable period of each OLR was employed in this section. The input and output units for this pilot system were plotted as a flowchart in Fig. 4. As mentioned above, this pilot-scale anaerobic digestion system was established on BUCT campus for treating FVW and KW from canteen. Therefore, the feedstocks of food waste were offered for free. The labor cost included the transportation of feedstocks and management of the system. The electricity consumption mainly covered the feedstocks homogenization, pumping, agitation, and heating for digesters. They were integrally measured and recorded by an electricity meter. The water consumption was mainly used for the OLR-adjustment for the feedstocks, which could be calculated by the OLR of influent every day. The main outputs mainly included the biogas and effluent production (digestate). The biogas
Inputs
Electricity
Homogenization &Pumping
Raw material
Pretreatment
Collection
Agitation
Heat
Anaerobic digestion
Biogas
Operation
Labor
Water
Effluent
Outputs Fig. 4. The flowchart of input and output elements for the pilot-scale system.
Please cite this article in press as: Wang, L., et al. Anaerobic co-digestion of kitchen waste and fruit/vegetable waste: Lab-scale and pilot-scale studies. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.005
L. Wang et al. / Waste Management xxx (2014) xxx–xxx Table 4 Operation profits and costs analysis of the pilot-scale system for FW digestion. OLR of pilot-scale two-phase system/g (VS) L1 d1
Weigh of feedstock (ton/d) Biogas production of stable period (m3/d) Electricity consumption (kW h/d) Water costa ($/d) Labor costb ($/d) Electricity costc ($/d) Total operation costd ($/d) Biogas profite ($/d) Effluent profitf ($/d) Total operation profitg ($/d) Operation profith ($/d) Net profiti ($/ton)
1.0
2.0
3.0
0.0292 1.00 24 0.109 3.204 1.570 4.883 1.461 1.442 2.903 1.980 67.813
0.0585 7.08 28 0.091 3.204 1.884 5.179 2.586 1.922 4.508 0.671 11.470
0.0876 11.92 33 0.072 3.204 2.590 5.867 4.354 2.403 6.757 0.891 10.173
a
Water cost = water consumption volume water price (0.6408 $/ton). Labor cost = working hours (1.0 h) labor price (3.204 $/h one person) (It is 1.0 h/d for the transportation of feedstocks and management of the system based on the employed labor for this case). c Electricity consumption = electricity consumed electricity price (0.078 $/ kW h). d Total operation cost = water cost + labor cost + electricity cost. e Biogas profit = biogas production biogas price (0.365 $/m3), and price are available at: http://www.bjpc.gov.cn/ywpd/wjgl/cx/jz/201208/t3884350.htm. f Effluent profit = effluent volume effluent price (effluent price is related to the content of fertilizer compositions, and in the range of 7.209–12.015 $/ton according to local market price). g Total operation profit = biogas profit + effluent profit. h Operation profit = total operation profit total operation cost. i Net profit = operation profit weigh of feedstock. b
system has to face a technical barrier in practice to avoid the system instability with increasing OLR. Hence, how to solve the stable operation of food waste digestion at high OLR should correspondingly be the core issue in future research. 4. Conclusions The batch test results of this study demonstrated that the ratio of FVW to KW at 5:8 was more suitable for co-digestion of FVW with KW. Two-phase digestion in lab-scale appeared to have high treatment capacity and better buffer ability for high OLR, and could employ maximum OLR of 5.0 g (VS) L1 d1, compared with that of 3.5 g (VS) L1 d1 for single-phase system. For two-phase digestion, the pilot-scale system showed similar performances to those of lab-scale one, except using slightly lower maximum OLR of more stable than the lab-scale system with OLR 6 4.5 g (VS) L1 d1. The pilot-scale system proved to be profitable with a net profit of 10.173 $/ton as higher OLR (P3.0 g (VS) L1 d1) was used . Acknowledgments The authors are grateful to the fund supports from the University Doctorial Foundation (No. 20120010110004) and Beijing Natural Science Foundation (No. 8142030). References Alkanok, G., Demirel, B., Onay, T.T., 2014. Determination of biogas generation potential as a renewable energy source from supermarket wastes. Waste Manage. 34, 134–140.
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