Journal of Environmental Management 133 (2014) 268e274
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Anaerobic co-digestion of food waste and dairy manure: Effects of food waste particle size and organic loading rate Fred O. Agyeman, Wendong Tao* Department of Environmental Resources Engineering, College of Environmental Science and Forestry, State University of New York, 1 Forestry Drive, 402 Baker Lab, Syracuse, NY 13210, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2013 Received in revised form 11 December 2013 Accepted 12 December 2013 Available online 7 January 2014
This study was to comprehensively evaluate the effects of food waste particle size on co-digestion of food waste and dairy manure at organic loading rates increased stepwise from 0.67 to 3 g/L/d of volatile solids (VS). Three anaerobic digesters were fed semi-continuously with equal VS amounts of food waste and dairy manure. Food waste was ground to 2.5 mm (fine), 4 mm (medium), and 8 mm (coarse) for the three digesters, respectively. Methane production rate and specific methane yield were significantly higher in the digester with fine food waste. Digestate dewaterability was improved significantly by reducing food waste particle size. Specific methane yield was highest at the organic loading rate of 2 g VS/L/d, being 0.63, 0.56, and 0.47 L CH4/g VS with fine, medium, and coarse food waste, respectively. Methane production rate was highest (1.40e1.53 L CH4/L/d) at the organic loading rate of 3 g VS/L/d. The energy used to grind food waste was minor compared with the heating value of the methane produced. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Anaerobic digestion Dairy manure Dewaterability Food waste Mechanical pretreatment Organic loading rate
1. Introduction In 2011, more than 36 million tons of food waste was generated in the U.S. (U.S. EPA, 2013). Food waste has higher biochemical methane potential. Anaerobic digestion of food waste not only produces methane for energy recovery, but also treats waste for environmental and social benefits (Fuchs and Drosg, 2013; Izumi et al., 2010; Zhang et al., 2013). However, mono-digestion of food waste often leads to digester instability and even failure at higher organic loading rates (OLR), especially under thermophilic conditions, due to accumulation of volatile fatty acids and ammonia (Banks et al., 2012; Banks et al., 2008; Ghanimeh et al., 2012; Nagao et al., 2012; Zhang et al., 2012, 2013). Animal feeding operations generate significant amounts of animal manure, which is typically applied to cropland (ASABE, 2010; USDA, 2009). Concentrated animal feeding operations often do not have adequate land to absorb all of their manure, having to consider on-farm treatment. Anaerobic digestion is increasingly applied to liquid manure to stabilize organic matter, reduce pathogens, eliminate offensive odors, and recover energy from methane (USDA, 2009; U.S. EPA, 2010). However, cattle manure contains high
* Corresponding author. Tel.: þ1 315 470 4928. E-mail address: [email protected] (W. Tao). 0301-4797/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvman.2013.12.016
contents of non-biodegradable substances and has low C/N ratios (Frear et al., 2010; Zhang et al., 2012, 2013), thus having a low methane yield in anaerobic mono-digestion of cattle manure (ElMashad and Zhang, 2010; Frear et al., 2010; Hartmann and Ahring, 2005). Banks et al. (2011a) recommended on-farm codigestion of dairy cattle slurry and source-separated domestic food waste as the most effective means of making dairy cattle slurry digestion economically viable. Co-digestion of cattle manure and food waste can increase biogas production and improve process stability (El-Mashad and Zhang, 2010; Zhang et al., 2012, 2013). Hydrolysis is generally the rate-limiting stage in anaerobic digestion of organic solid waste (Angelidaki and Sanders, 2004; Izumi et al., 2010; Palmowski and Müller, 2000). Good contact between biomass and substrate is a prerequisite for hydrolysis because the organisms secreting hydrolytic enzymes are benefited by adsorption to the surface of particulate substrates (Angelidaki and Sanders, 2004). Methanogens in anaerobic digestion of flushed dairy manure have high affinity to fibrous solids as well (Frear et al., 2010). Reducing substrate particle size through pretreatment such as grinding could increase surface area available for adsorption of hydrolytic enzymes and subsequently produce more biogas (Izumi et al., 2010; Kim et al., 2000; Palmowski and Müller, 2000). However, excessive particle size reduction could overstimulate hydrolysis and acidogenesis, resulting in accumulation of ammonia and volatile fatty acids which could become inhibitory
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to methanogens. The effects of particle size on anaerobic digestion of food waste were investigated in two studies only through short batch tests (Izumi et al., 2010; Kim et al., 2000). The effects of food waste particle size have neither been addressed in continuously fed flow-through anaerobic digesters, nor in co-digestion of food waste and dairy manure. Moreover, the additional energy consumption to produce finer particles and dewaterability of digester effluent has not been reported along with the effect of particle size on methane production. The major objective of this study was to assess the effects of food waste particle size on anaerobic co-digestion of food waste and dairy manure in continuously fed anaerobic digesters at different OLRs. The effects were assessed comprehensively in terms of energy consumption for grinding food waste, biogas production rate, specific methane yield, reduction efficiency for volatile solids (VS), and digestate dewaterability over four periods as OLRs were increased stepwise from 0.67 g VS/L/d to 3 g VS/L/d. Successful long-term mono-digestion of food waste has been typically limited to OLRs below 2.5 g VS/L/d unless enhancement measures such as supplementation of trace elements, solids return and co-digestion are taken (Banks et al., 2011b, 2012; Ghanimeh et al., 2012; Nagao et al., 2012; Zhang et al., 2012). A number of studies have addressed methane production and ammonia inhibition in co-digestion of food waste and cattle manure at different substrate combination ratios and OLRs (El-Mashad and Zhang, 2010; Hartmann and Ahring, 2005; Marañón et al., 2012; Zhang et al., 2012, 2013). Nevertheless, the combined effect of OLR and food waste particle size in stable co-digestion of food waste and dairy manure is unknown. Treatment and disposal of digestate account for a great portion of the operational cost of pilot- and full-scale anaerobic digestion projects (Fuchs and Drosg, 2013). Digestate processing can become a bottleneck to scaled-up applications (Gebrezgabhera et al., 2010). Typically, digestate is separated into liquid and solids by filtration, screw pressing, or centrifugation. However, dewaterability of digestate has rarely been addressed. This paper evaluates digestate dewaterability along with methane production and solid removal. 2. Materials and methods
269
and drive its impeller at 140 rpm. The digestate temperature was targeted at 36 C. The digesters were initially filled with bacterial inoculum to a working volume of 1.8 L. The inoculum was made from anaerobically digested sludge from a municipal wastewater treatment plant and anaerobically digested dairy manure with coarse materials (>2.06 mm) sieved out. The inoculum had a VS concentration of 1.33%, with one half (by mass) from the digested sludge and the other half from the digested manure. Compared with food waste and dairy manure separately, it had a slightly basic pH and generally balanced concentrations of macro- and micronutrients (Table 1). Based on earlier studies (El-Mashad and Zhang, 2010; Zhang et al., 2012, 2013), it appears that a VS ratio of manure to food waste around 1 is the optimum combination for co-digestion of cattle manure and food waste. The feedstock used in this study was prepared by combining domestic food waste and dairy manure at a VS ratio of 1 and stored frozen at 21 C in plastic tubes. Table 1 summarizes the characteristics of the feedstock and its two components. The food waste was collected from a Sheraton Hotel’s restaurant over five days and ground through a MG800 Waring Pro Professional meat grinder with three cutting plates having different aperture diameters (2.5, 4, and 8 mm) for the three digesters, called fine, medium and coarse food waste, respectively. Energy consumption to grind food waste was recorded with a Watts up? PRO electricity watt meter (Electronic Educational Devices, Inc., Denver, CO, USA). Dairy manure was taken from a storage vessel of liquid manure which was scraped from concrete lots of a large-size dairy farm at Cayuga County of New York, USA. The digesters were operated in a semi-continuous mode. The feedstock was thawed in a refrigerator at 4 C and fed to the digesters every 2 d. OLR was increased from 0.67 g VS/L/d to 1, 2, and 3 g VS/L/d stepwise over 178 d of operation. Digestate (67e90 mL) was discharged every 6 d at the OLRs of 0.67 and 1 g VS/L/d, 4 d at 2 g VS/L/d, and 2 d at 3 g VS/L/d. This study aimed at dry digestion. Only 25e45 mL of tap water was used to wash the feedstock storage tubes and maintain the working volume after discharge. Hydraulic retention time or solids retention time was 160 d at the OLRs of 0.67 and 1 g VS/L/d, 80 d at 2 g VS/L/d, and 54 d at 3 g VS/L/d.
2.1. Setup and operation of anaerobic digesters Three 2-L complete-mix anaerobic digesters were set up in a laboratory. Each digester as shown in Fig. 1 was built with a modified Duran GLS80 glass reactor with a magnetic impeller. A Thermo scientific hotplate/stirrer was used to heat each digester
Gas meter
Biogas sampling Measuring
Gas exit Feeding
Magnetically stirred reactor Discharge Hotplate/ stirrer Fig. 1. Sketch of a bench-scale, semi-continuously fed anaerobic digester.
Table 1 Characteristics of inoculum and feedstock made from dairy manure and food waste.
pH Total volatile solids, % Total dissolved solids, g/L Crude protein, g/kg VS Fat, g/kg VS Non-fiber carbohydrate, g/kg VS Neutral detergent fiber, g/kg VS Total N, %TS Total C, %TS Orthophosphate, g P/L Total ammonia, g N/L Sulfur, g/kg TS Total Ca, g/kg TS Total Mg, g/kg TS Total K, g/kg TS Total Na, g/kg TS Total Fe, mg/kg TS Total Zn, mg/kg TS Total Cu, mg/kg TS Mn, mg/kg TS Mo, mg/kg TS
Dairy manure
Food waste
Feedstock
Inoculum
6.6 9.68 16.9 167 40 623
4.4 29.3 16.9 266 350 325
6.6 14.6 16.8 273 231 380
7.7 1.33 7.52 335 90 543
616
196
291
543
1.9 39.9 0.78 1.71 6.1 20.6 8.5 23.8 7.25 705 233 123 176 1.6
3.8 48.4 No data No data 3.4 1.7 0.7 9.6 10.1 41 32 5 8 0.3
3.6 46.3 No data No data 4.8 13.3 5.2 18.0 8.9 374 136 46 98 1.2
3.0 32.4 0.33 1.68 11.9 26.0 12.2 20.3 12.1 18300 900 569 269 11.3
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measured with a Hach HQ40d meter. After separating suspended solids in the digester effluent via centrifugation at 1600 g for 30 min, total ammonia concentration in the centrate was determined colorimetrically with a Hach DR 2800 spectrophotometer (Hach Company, Loveland, Colorado, USA). Free ammonia concentration in digestate was calculated with measured total ammonia concentration, temperature and pH (Pitk et al., 2013). Time-to-filter was determined with the small-volume Standard Method 2710H (APHA, 1999) to reveal dewaterability of digester effluent. The feedstock and its components as well as the inoculum were measured for pH, total solids, total volatile solids, total dissolved solids, and total ammonia, using the same methods as mentioned above. Crude proteins, fats, neutral detergent fiber, and non-fiber carbohydrates were determined at Dairy One Forage Laboratory (Ithaca, New York), following AOAC International standards. Total carbon and nitrogen contents were determined for oven-dried samples with an elemental analyzer (Calo Erba NC2500, Costech Analytical Technologies Inc., Valencia, California, USA). Orthophosphate in centrate of the inoculum and dairy manure was determined colorimetrically with the Hach DR2800
2.2. In-situ measurements and laboratory analyses Each reactor had a head space of 0.69 L at the working volume of 1.8 L. Biogas production was recorded with in-line gas meters and converted to daily production rate under standard conditions (stp: 0 C and 760 mm Hg). While sampling biogas, headspace temperature was measured with a Hach H160 pH meter connected to an ISFET pH stainless steel NMR tube probe. Biogas samples (0.1 mL each) were collected with a gas-tight syringe through rubber septa and diluted with air in 10-mL Wheaton serum vials. The biogas samples along with air samples were analyzed for CH4 and CO2 percentages using a Shimadzu GC-2014 gas chromatograph system with a flame ionization detector (Shimadzu Corporation, Tokyo, Japan). Helium was used as carrier gas. The detection limits were 0.1 ppm for CH4 and 10 ppm for CO2. Digestate temperature and pH were measured with the pH meter while collecting biogas samples. Digester effluent samples were collected for determination of total solids and total volatile solids concentrations, following Standard Methods 2540 B and E, respectively (APHA, 1998). Total dissolved solids concentration was
Biogas production rate (L/L/d)
a)
3.0 OLR = 0.67 g VS/L/d
1.0 g VS/L/d
2.0 g VS/L/d
3.0 g VS/L/d
2.5 2.0 1.5 1.0 Fine
0.5
Medium Coarse
0.0
0
20
40
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120
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Time in operation (d)
b)
1.2 OLR = 0.67 g VS/L/d
1.0 g VS/L/d
2.0 g VS/L/d
3.0 g VS/L/d
Biogas yield (L/g VS)
1.0 0.8 0.6
0.4 Fine 0.2
Medium Coarse
0.0 0
20
40
60
80
100
120
140
160
180
Time in operation (d) Fig. 2. Variations of (a) biogas production rate and (b) specific biogas yield with food waste particle size (fine, medium, and coarse) and organic loading rate (OLR) in mesophilic codigestion of dairy manure and food waste.
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with a backward elimination approach to identify the period of stable operation at a given OLR (Townend, 2002).
8 Fine Medium Coarse
14
7
12
6
10
5
8
4
6
3
4
2
2 0.67
1
OLR (g VS/L/d) = 1.0 2.0
Total volatile solids in digester effluent (%)
Total dissolved solids in digester effluent (g/L)
16
3. Results and discussion The first 52 days of digester operation at the OLR of 0.67 g VS/L/ d was taken as a startup phase and not monitored regularly. Results from 42 d of anaerobic digestion at each of the successive OLRs were used to diagnose the effects of food waste particle size, evaluate methane production and treatment performance, and identify the optimum OLR. As illustrated in Fig. 2 for biogas production rate and specific biogas yield, trend tests showed that it took more than 19, 15, and 10 d to reach stable operation at the OLRs of 1, 2, and 3 g VS/L/d, respectively. Whenever OLR was increased, there was an increase in microbial biomass as reflected by the effluent VS concentrations (Fig. 3). Both biogas production rate and specific yield increased over time initially and became stable, likely as microorganisms were acclimated, at given OLRs (Fig. 2).
3.0
0
0 0
30
60
90
271
120 150 180
3.1. Performance of co-digestion
Time in operation (d)
The co-digestion in this study attained higher methane contents in biogas (Fig. 4) compared with earlier studies on co-digestion of cattle manure and food waste (El-Mashad and Zhang, 2010; Hartmann and Ahring, 2005; Zhang et al., 2012, 2013). Moreover, this study achieved higher specific methane yields during the stable operation periods (0.46e0.63 L CH4/g VS) compared with those in most earlier studies on co-digestion of food waste and cattle manure (0.14e0.46 L CH4/g VS). The higher methane yield in this study relative to those in the earlier studies could be attributed to the long solids retention times associated with dry digestion and the relatively higher lipid content of the feedstock in this study (Table 1). This study also confirmed the synergistic effect of codigestion with the higher specific methane yields compared with 0.46 L CH4/g VS in mono-digestion of food waste without addition of trace elements (Banks et al., 2011b, 2012; Ghanimeh et al., 2012; Nagao et al., 2012; Zhang et al., 2012, 2013) and 0.25 L CH4/g VS in mono-digestion of cattle manure (El-Mashad and Zhang, 2010; Frear et al., 2010; Hartmann and Ahring, 2005; Zhang et al., 2013). Specific methane yield in anaerobic digestion of food waste has been limited mainly because of the operational
Fig. 3. Effects of food waste particle size (fine, medium, and coarse) on digester effluent solid concentrations in co-digestion of dairy manure and food waste at increasing organic loading rates (OLR).
spectrophotometer. The other macro- and micro-nutrients were analyzed with an inductively coupled plasma radial spectrometer after microwave digestion. 2.3. Statistical analysis One-way analysis of variance (ANOVA) was performed to determine whether there were statistically significant differences among the three digesters. The significance level (p value) was set at 5%. If there were significant differences, least significant difference (LSD) was calculated to further identify the pairs of means that had significant differences (Townend, 2002). Spearman’s rank correlation analysis was performed to assess the relationship of biogas production with digestate ammonia concentration, giving correlation coefficient r. Spearman rank trend test was performed
90 Fine
CH4 content in biogas (%) CO2 content in biogas (%)
80
Medium
70
Coarse
60 50 40 30 20
10 OLR = 0.67 g VS/L/d
1.0 g VS/L/d
2.0 g VS/L/d
3.0 g VS/L/d
0
0
20
40
60
80
100
120
140
160
180
Time in operation (d) Fig. 4. Dynamics of biogas composition in anaerobic co-digestion of dairy manure and different particle sizes of food waste (fine, medium, and coarse).
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instability at higher organic loading rates (Banks et al., 2012; Zhang et al., 2012). High fiber content is the main reason for low methane potential of cow manure (El-Mashad and Zhang, 2010; Frear et al., 2010). The methane production rates during the stable operation periods at the final OLR of 3 g VS/L/d were 1.53, 1.41, and 1.40 L CH4/L/ d in the digesters with fine, medium and coarse food waste, respectively. Like specific methane yield, the methane production rates of co-digestion in this study were higher compared to those in mono-digestion of food waste, 1.39 L CH4/L/d (Banks et al., 2011b, 2012; Ghanimeh et al., 2012; Zhang et al., 2013), and monodigestion of dairy manure, 0.10 L CH4/L/d (Zhang et al., 2013). Most enzymes and co-enzymes need a minimal amount of certain trace elements for their activation and activity (Appels et al., 2008). As Table 1 shows, dairy manure generally has higher concentrations of macro- and micro-nutrients than food waste. Combination of dairy manure and food waste improves availability of nutrients for anaerobic digestion of food waste. Such combination of digester feed also resulted in pH values (Fig. 5a) non-
9
Digestate pH
8 7
Fine Medium Coarse
6 5 0.67
4
0
Total ammonia in digestate (mg N/L)
b)
0.67
OLR (g VS/L/d) = 1.0 2.0
3.0
3000 2000
Table 2 presents biogas production during the stable operation periods. The average biogas production rates of the three digesters were significantly different (p 0.05) at the OLRs of 1 g VS/L/ d (LSD ¼ 0.06 L/L/d), 2 g VS/L/d (LSD ¼ 0.08 L/L/d), and 3 g VS/L/ d (LSD ¼ 0.06 L/L/d). The three digesters had significantly different specific biogas yields (p 0.05) at the OLRs of 1 g VS/L/ d (LSD ¼ 0.06 L/g VS), 2 g VS/L/d (LSD ¼ 0.04 L/g VS), and 3 g VS/L/ d (LSD ¼ 0.02 L/g VS) as well. The digester with fine food waste had methane production rate 10e29% higher and specific methane yield 9e34% higher than those with coarse food waste. Although the biogas production rate and specific yield in the digester with medium food waste fell between those with fine and coarse food waste, the differences were only statistically significant at the OLR of 2 g VS/L/d. Food waste particle size did not make significant differences in methane content of biogas (p ¼ 0.30e0.63) except for a significantly lower average methane content in the digester with coarse food waste at the OLR of 2 g VS/L/d (Table 2). Energy consumed for grinding food waste to fine particles was 0.130 Wh/g VS, 0.069 Wh/g VS to medium particles, and 0.054 Wh/g VS to coarse particles. The lower heating value of methane is 10.67 Wh/L under the standard conditions (Metcalf and Eddy Inc, 2003). Considering the specific methane yields at the OLR of 3 g VS/L/d (Table 2), energy consumption for grinding amounted to only 1.1e2.4% of energy carried by methane produced from anaerobic digestion. It is, therefore, cost-effective to grind food
Fine Medium Coarse
1000 0
0
c) Free ammonia in digestate (mg N/L)
30 60 90 120 150 180 Time in operation (d)
5000 4000
OLR (g VS/L/d) = 1.0 2.0 3.0
3.2. Effects of food waste particle size
30 60 90 120 150 180 Time in operation (d)
500
OLR (g VS/L/d) = 1.0 2.0
400
0.67
300
Fine Medium Coarse
200
3.0
100
0 0
30
60
90
120 150 180
Time in operation (d) Fig. 5. Dynamics of (a) digestate pH; (b) total ammonia concentration; and (c) free ammonia concentration in co-digestion of dairy manure and different particle sizes of food waste (fine, medium, and coarse) at increasing organic loading rates (OLR).
300
Time-to-filter of digester effluent (min)
a)
inhibitory to methanogens (Angelidaki and Sanders, 2004) and balanced C/N ratios. It is challenging to separate solids from liquid in anaerobically digested dairy manure, which has time-to-filter at 246e348 min (Xia et al., 2012). Anaerobically digested sludge has time-to-filter at a few minutes (Cheumbarn and Pagilla, 2000; Zhang et al., 2010). The digesters in this study started up at their full working volumes with anaerobically digested dairy manure and digested sludge, which had time-to-filter (initial values in Fig. 6) slightly shorter than that of anaerobically digested dairy manure. Time-to-filter decreased considerably across the OLRs of 1, 2 and 3 g VS/L/d in the co-digestion (Fig. 6), indicating improved dewaterability of digester effluent compared with mono-digestion of dairy manure.
Fine Medium Coarse
250 200
150 100 50 0.67
OLR (g VS/L/d) = 1.0 2.0 3.0
0 0
30
60
90
120 150 180
Time in operation (d) Fig. 6. Effect of food waste particle size (fine, medium, and coarse) on digester effluent dewaterability in co-digestion of dairy manure and food waste.
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Table 2 Biogas production in co-digestion of dairy manure with different particle sizes of food waste during stable operation periods at increasing organic loading ratesa. Specific biogas yield (stp L/g VS)
CH4 content in biogas (%)
Specific CH4 yield (stp L/g VS)
Organic loading rate ¼ 1 g VS/L/d from day 52 to day 94 Fine food waste 0.79 0.06 Medium food waste 0.74 0.08 Coarse food waste 0.72 0.05
0.79 0.06 0.74 0.08 0.72 0.05
67.5 6.9 63.7 11.7 64.2 4.2
0.53 0.04 0.47 0.05 0.46 0.03
Organic loading rate ¼ 2 g VS/L/d from day 94 to day 136 Fine food waste 1.69 0.05 Medium food waste 1.60 0.06 Coarse food waste 1.45 0.14
0.85 0.02 0.80 0.03 0.73 0.07
74.1 5.3 70.3 5.8 64.9 5.6
0.63 0.02 0.56 0.02 0.47 0.05
Organic loading rate ¼ 3 g VS/L/d from day 136 to day 178 Fine food waste 2.12 0.07 Medium food waste 2.03 0.06 Coarse food waste 2.00 0.09
0.71 0.02 0.68 0.02 0.67 0.03
72.2 4.0 69.5 5.4 69.8 4.5
0.51 0.02 0.47 0.01 0.47 0.02
Biogas production rate (stp L/L/d)
a
Mean standard deviation.
waste into finer particles for greater methane yield in co-digestion of food waste with dairy manure. The effects of particle size on biogas production are attributed to the larger specific surface area provided by smaller particles for enhanced hydrolysis (Izumi et al., 2010; Palmowski and Müller, 2000). The optimum particle size for anaerobic mono-digestion of food waste has been examined in different ranges. Kim et al. (2000) reported that the maximum substrate utilization rate coefficient doubled as the average particle size decreased from 2.14 to 1.02 mm in thermophilic batch digestion tests. Izumi et al. (2010) investigated the effect of particle size in a narrow range (0.391e 0.888 mm) and found that the optimum particle size (0.718 mm) resulted in 28% more biogas than that with the worst particle size (0.888 mm) in mesophilic batch digestion tests. The effects of particle size should be further addressed in a wider range with regard to not only methane production, but also energy consumption for particle size reduction and digestate dewaterability in the future. The long time-to-filter as presented in Fig. 6 manifests the difficulty to dewater the digester effluent. Nevertheless, there were significant differences in time-to-filter among the three digesters at all the three OLRs (p 0.001; LSD ¼ 7e10 min). The shortest was recorded in co-digestion with fine food waste and the longest in codigestion with coarse food waste. This was consistent with the positive impact of organic waste particle size on dewaterability of digestate as Mata-Alvarez et al. (2000) reported. 3.3. Optimum organic loading rate As Fig. 2 shows, biogas production rate increased with increasing OLR. As Table 2 shows for the periods of stable operation, however, biogas production rate increased by 101e116% when OLR was increased from 1 to 2 g VS/L/d and only by 25e38% when OLR was further increased from 2 to 3 g VS/L/d. Specific methane yield peaked at the OLR of 2 g VS/L/d in the digesters with fine and medium food waste. Similarly, earlier studies (Hartmann and Ahring, 2005; Zhang et al., 2013, 2012) on co-digestion of cattle manure and food waste at higher organic loading rates (3.3e 16 g VS/L/d) attained lower specific methane yield (0.14e0.41 CH4/ g VS). Therefore, the optimum OLR for deep co-digestion of dairy manure and food waste was close to 3 g VS/L/d, although OLR could be increased further for higher methane production rates. Theoretical methane potential under the standard conditions was estimated to be 0.53 L CH4/g VS for the feedstock in this study, based on biochemical composition of the feedstock (Table 1) and methane potentials of proteins, lipids and carbohydrates as suggested by Angelidaki and Sanders (2004). The methane yields experimentally determined in this study were 89e119% of the
estimated methane potential, suggesting little limitation of trace elements and inhibition due to ammonia and volatile fatty acids. Anaerobic digestion converts organic nitrogen to ammonia, which exists in ionized ammonium and free ammonia, depending on pH and temperature. Free ammonia inhibits more than ionized ammonium to methanogens (Yenigun and Demirel, 2013). The hydrophobic free ammonia may diffuse into cells, causing proton imbalance and potassium deficiency in microorganisms, particularly in methanogens (Pitk et al., 2013). Ammonia could be carried with liquid dairy manure and inoculum into mixed liquor and produced in anaerobic digestion, resulting in inhibition to methanogenesis at high OLRs. The inoculum and dairy manure in this study had high total ammonia concentrations, 1.7 g N/L (Table 1). As Fig. 5b shows, total ammonia concentration in the digester effluent tended to increase up to 3090e3420 mg N/L after 178 d of operation. Free ammonia concentration increased rapidly to 202e 340 mg N/L at the OLR of 1 g VS/L/d, decreased at the OLR of 2 g VS/ L/d, then stabilized between 148 and 237 mg N/L at the OLR of 3 g VS/L/d (Fig. 5c). As reviewed by Yenigun and Demirel (2013), ammonia inhibition to mesophilic anaerobic digestion with acclimated inoculum is triggered at very different concentrations, mostly from 2800 to 6000 mg N/L total ammonia and 337e 800 mg N/L free ammonia (Yenigun and Demirel, 2013). Therefore, ammonia concentration in this study might not be high enough to significantly affect biogas production. There were insignificant correlations between biogas production rate and biogas yield with either total ammonium or free ammonia concentrations (r < critical r). There were insignificant differences in effluent solids concentrations between the digesters (p ¼ 0.08e0.64). Total dissolved solids concentrations were reduced by 40.7e42.6% and total volatile solids concentrations by 80.9e82.7% on average in the three digestersat at the OLR of 1 g VS/L/d. The concentrations of total dissolved solids and total volatile solids in digester effluent increased with the increasing OLRs (Fig. 3). At the end of the period with the OLR of 3 g VS/L/d, the reduction efficiencies decreased to 18.2e23.0% for total dissolved solids and 66.6e69.6% for total volatile solids through the three digesters. Solids removal efficiency at the end of this study became moderate compared with that in mono-digestion of food waste and dairy manure at similar OLRs (Hartmann and Ahring, 2005; Nagao et al., 2012). This suggests again that the optimum OLR is approximately 3 g VS/L/d for mesophilic co-digestion of dairy manure and food waste. 4. Conclusions Reduction of food waste particle size from 8 to 2.5 mm increased methane production rate by 10e29% and specific methane yield by
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9e34% in co-digestion of dairy manure and food waste. Dewaterability of digester effluent was significantly improved by reducing food waste particle size. The energy consumed to grind food waste down to 2.5 mm was minor compared to the heating value of the methane produced. The co-digestion could be loaded up to 3 g VS/L/d without ammonia inhibition while reducing more than 67% of volatile solids and producing 1.40e1.53 L CH4/L/d. The highest specific methane yields, however, were achieved at the OLR of 2 g VS/L/d. Acknowledgments This study was supported by a fellowship to Fred Agyeman from Ford Foundation-IFP and Association of African Universities. The research was partially supported by a U.S. EPA grant to Dr. Tao (SU835331). We would like to thank Dr. Philippe Vidon, Mr. David Kiemle, and Pat Rook for their help with biogas analysis. Our thanks also go to Mr. Steve McGlynn at Twin Birch Diary and Mr. Paul Eno at Sheraton Syracuse University Hotel for providing with the feedstock materials. References Angelidaki, I., Sanders, W., 2004. Assessment of the anaerobic biodegradability of macropollutants. Rev. Environ. Sci. Biotechnol. 3, 117e129. APHA, AWWA, WEF, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, American Water Works Association, Water Environment Federation, Washington, DC. Appels, L., Baeyens, J., Degreve, J., Dewil, R., 2008. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energ. Combust. 34, 755e781. ASABE, 2010. Manure Production and Characteristics, ASABE Standards 2010: ASAE D384.2 MAR2005 (R2010). American Society of Agricultural and Biological Engineers, St. Joseph, MI. 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, 1475e1481. Banks, C.J., Salter, A.M., Heaven, S., Riley, K., 2011a. Energetic and environmental benefits of co-digestion of food waste and cattle slurry: a preliminary assessment. Resour. Conserv. Recycl. 56, 71e79. Banks, C.J., Chesshire, M., Heaven, S., Arnold, R., 2011b. Anaerobic digestion of source segregated domestic food waste: performance assessment by mass and energy balance. Bioresour. Technol. 102, 612e620. Banks, C.J., Zhang, Y., Jiang, Y., Heaven, S., 2012. Trace element requirements for stable food waste digestion at elevated ammonia concentrations. Bioresour. Technol. 104, 127e135. Cheumbarn, T., Pagilla, K.R., 2000. Anaerobic thermophilic/mesophilic dual-stage sludge treatment. J. Environ. Eng. ASCE 126, 796e801. El-Mashad, H.M., Zhang, R., 2010. Biogas production from co-digestion of dairy manure and food waste. Bioresour. Technol. 101, 4021e4028.
Frear, C., Wang, Z.-W., Li, C., Chen, S., 2010. Biogas potential and microbial population distributions in flushed dairy manure and implications on anaerobic digestion technology. J. Chem. Technol. Biotechnol. 86, 145e152. Fuchs, W., Drosg, B., 2013. Assessment of the state of the art of technologies for the processing of digestate residue from anaerobic digesters. Water Sci. Technol. 67, 1984e1993. Gebrezgabhera, S.A., Meuwissen, M.P.M., Prins, B.A.M., Oude Lansink, A.G.J.M., 2010. Economic analysis of anaerobic digestion e a case of green power biogas plant in The Netherlands. NJAS Wageningen J. Life Sci. 57, 109e115. Ghanimeh, S., Fadel, M.E., Saikaly, P., 2012. Mixing effect on thermophilic anaerobic digestion of source-sorted organic fraction of municipal solid waste. Bioresour. Technol. 117, 63e71. Hartmann, H., Ahring, B.K., 2005. Anaerobic digestion of the organic fraction of municipal solid waste: influence of co-digestion with manure. Water Res. 39, 1543e1552. Izumi, K., Okishio, Y., Nagao, N., Niwa, C., Yamamoto, S., Toda, T., 2010. Effects of particle size on anaerobic digestion of food waste. Int. Biodeter. Biodegr 64, 601e608. Kim, D.H., Kim, I.S., Hyun, S.H., 2000. Effect of particle size and sodium ion concentration on anaerobic thermophilic food waste digestion. Water Sci. Technol. 41 (3), 67e73. Marañón, E., Castrillón, L., Quiroga, G., Fernàndez-Nava, Y., Gómez, L., Garcìa, M.M., 2012. Co-digestion of cattle manure with food waste and sludge to increase biogas production. Waste Manage. 32, 1821e1825. Mata-Alvarez, J., Macé, S., Llabrés, P., 2000. Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Bioresour. Technol. 74, 3e16. Metcalf and Eddy, Inc, 2003. Wastewater Engineering: Treatment and Reuse, fourth ed. McGraw Hill, New York. Nagao, N., Tajima, N., Kawai, M., Niwa, C., Kurosawa, N., Matsuyama, T., Yusoff, F.M., Toda, T., 2012. Maximum organic loading rate for the single-stage wet anaerobic digestion of food waste. Bioresour. Technol. 118, 210e218. Palmowski, L.M., Müller, J.A., 2000. Influence of the size reduction of organic waste on their anaerobic digestion. Water Sci. Technol. 41 (3), 155e162. Pitk, P., Kaparaju, P., Palatsi, J., Affes, R., Vilu, R., 2013. Co-digestion of sewage sludge and sterilized solid slaughterhouse waste: Methane production efficiency and process limitations. Bioresour. Technol. 134, 227e232. Townend, J., 2002. Practical Statistics for Environmental and Biological Scientists. John Wiley & Sons, West Sussex, England. USDA, 2009. Manure Use for Fertilizer and for Energy, Report to Congress. U.S. Department of Agriculture, Washington, DC. U.S. EPA, 2010. Guide to Anaerobic Digesters. U.S. Environmental Protection Agency, Washington, DC. http://www.epa.gov/agstar/operational.html (last updated on April 20th, 2010). U.S. EPA, 2013. Municipal Solid Wastes in the US: Facts and Figures. http://www. epa.gov/epawaste/nonhaz/municipal/index.htm (last updated on June 17, 2013). Xia, M., Tao, W., Shayya, W., Lu, Z., 2012. Passive solid-liquid separation of anaerobically digested dairy manure using two-stage filtration. Biosyst. Eng. 111, 392e397. Yenigun, O., Demirel, B., 2013. Ammonia inhibition in anaerobic digestion: A review. Process Biochem. 48, 901e911. Zhang, Z., Xia, S., Zhang, J., 2010. Enhanced dewatering of waste sludge with microbial flocculant TJ-F1 as a novel conditioner. Water Res. 44, 3087e3092. Zhang, Y., Banks, C.J., Heaven, S., 2012. Co-digestion of source segregated domestic food waste to improve process stability. Bioresour. Technol. 114, 168e178. Zhang, C., Xiao, G., Peng, L., Su, H., Tan, T., 2013. The anaerobic co-digestion of food waste and cattle manure. Bioresour. Technol. 129, 170e176.
Contents lists available at ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Anaerobic co-digestion of food waste and dairy manure: Effects of food waste particle size and organic loading rate Fred O. Agyeman, Wendong Tao* Department of Environmental Resources Engineering, College of Environmental Science and Forestry, State University of New York, 1 Forestry Drive, 402 Baker Lab, Syracuse, NY 13210, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2013 Received in revised form 11 December 2013 Accepted 12 December 2013 Available online 7 January 2014
This study was to comprehensively evaluate the effects of food waste particle size on co-digestion of food waste and dairy manure at organic loading rates increased stepwise from 0.67 to 3 g/L/d of volatile solids (VS). Three anaerobic digesters were fed semi-continuously with equal VS amounts of food waste and dairy manure. Food waste was ground to 2.5 mm (fine), 4 mm (medium), and 8 mm (coarse) for the three digesters, respectively. Methane production rate and specific methane yield were significantly higher in the digester with fine food waste. Digestate dewaterability was improved significantly by reducing food waste particle size. Specific methane yield was highest at the organic loading rate of 2 g VS/L/d, being 0.63, 0.56, and 0.47 L CH4/g VS with fine, medium, and coarse food waste, respectively. Methane production rate was highest (1.40e1.53 L CH4/L/d) at the organic loading rate of 3 g VS/L/d. The energy used to grind food waste was minor compared with the heating value of the methane produced. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Anaerobic digestion Dairy manure Dewaterability Food waste Mechanical pretreatment Organic loading rate
1. Introduction In 2011, more than 36 million tons of food waste was generated in the U.S. (U.S. EPA, 2013). Food waste has higher biochemical methane potential. Anaerobic digestion of food waste not only produces methane for energy recovery, but also treats waste for environmental and social benefits (Fuchs and Drosg, 2013; Izumi et al., 2010; Zhang et al., 2013). However, mono-digestion of food waste often leads to digester instability and even failure at higher organic loading rates (OLR), especially under thermophilic conditions, due to accumulation of volatile fatty acids and ammonia (Banks et al., 2012; Banks et al., 2008; Ghanimeh et al., 2012; Nagao et al., 2012; Zhang et al., 2012, 2013). Animal feeding operations generate significant amounts of animal manure, which is typically applied to cropland (ASABE, 2010; USDA, 2009). Concentrated animal feeding operations often do not have adequate land to absorb all of their manure, having to consider on-farm treatment. Anaerobic digestion is increasingly applied to liquid manure to stabilize organic matter, reduce pathogens, eliminate offensive odors, and recover energy from methane (USDA, 2009; U.S. EPA, 2010). However, cattle manure contains high
* Corresponding author. Tel.: þ1 315 470 4928. E-mail address: [email protected] (W. Tao). 0301-4797/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvman.2013.12.016
contents of non-biodegradable substances and has low C/N ratios (Frear et al., 2010; Zhang et al., 2012, 2013), thus having a low methane yield in anaerobic mono-digestion of cattle manure (ElMashad and Zhang, 2010; Frear et al., 2010; Hartmann and Ahring, 2005). Banks et al. (2011a) recommended on-farm codigestion of dairy cattle slurry and source-separated domestic food waste as the most effective means of making dairy cattle slurry digestion economically viable. Co-digestion of cattle manure and food waste can increase biogas production and improve process stability (El-Mashad and Zhang, 2010; Zhang et al., 2012, 2013). Hydrolysis is generally the rate-limiting stage in anaerobic digestion of organic solid waste (Angelidaki and Sanders, 2004; Izumi et al., 2010; Palmowski and Müller, 2000). Good contact between biomass and substrate is a prerequisite for hydrolysis because the organisms secreting hydrolytic enzymes are benefited by adsorption to the surface of particulate substrates (Angelidaki and Sanders, 2004). Methanogens in anaerobic digestion of flushed dairy manure have high affinity to fibrous solids as well (Frear et al., 2010). Reducing substrate particle size through pretreatment such as grinding could increase surface area available for adsorption of hydrolytic enzymes and subsequently produce more biogas (Izumi et al., 2010; Kim et al., 2000; Palmowski and Müller, 2000). However, excessive particle size reduction could overstimulate hydrolysis and acidogenesis, resulting in accumulation of ammonia and volatile fatty acids which could become inhibitory
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to methanogens. The effects of particle size on anaerobic digestion of food waste were investigated in two studies only through short batch tests (Izumi et al., 2010; Kim et al., 2000). The effects of food waste particle size have neither been addressed in continuously fed flow-through anaerobic digesters, nor in co-digestion of food waste and dairy manure. Moreover, the additional energy consumption to produce finer particles and dewaterability of digester effluent has not been reported along with the effect of particle size on methane production. The major objective of this study was to assess the effects of food waste particle size on anaerobic co-digestion of food waste and dairy manure in continuously fed anaerobic digesters at different OLRs. The effects were assessed comprehensively in terms of energy consumption for grinding food waste, biogas production rate, specific methane yield, reduction efficiency for volatile solids (VS), and digestate dewaterability over four periods as OLRs were increased stepwise from 0.67 g VS/L/d to 3 g VS/L/d. Successful long-term mono-digestion of food waste has been typically limited to OLRs below 2.5 g VS/L/d unless enhancement measures such as supplementation of trace elements, solids return and co-digestion are taken (Banks et al., 2011b, 2012; Ghanimeh et al., 2012; Nagao et al., 2012; Zhang et al., 2012). A number of studies have addressed methane production and ammonia inhibition in co-digestion of food waste and cattle manure at different substrate combination ratios and OLRs (El-Mashad and Zhang, 2010; Hartmann and Ahring, 2005; Marañón et al., 2012; Zhang et al., 2012, 2013). Nevertheless, the combined effect of OLR and food waste particle size in stable co-digestion of food waste and dairy manure is unknown. Treatment and disposal of digestate account for a great portion of the operational cost of pilot- and full-scale anaerobic digestion projects (Fuchs and Drosg, 2013). Digestate processing can become a bottleneck to scaled-up applications (Gebrezgabhera et al., 2010). Typically, digestate is separated into liquid and solids by filtration, screw pressing, or centrifugation. However, dewaterability of digestate has rarely been addressed. This paper evaluates digestate dewaterability along with methane production and solid removal. 2. Materials and methods
269
and drive its impeller at 140 rpm. The digestate temperature was targeted at 36 C. The digesters were initially filled with bacterial inoculum to a working volume of 1.8 L. The inoculum was made from anaerobically digested sludge from a municipal wastewater treatment plant and anaerobically digested dairy manure with coarse materials (>2.06 mm) sieved out. The inoculum had a VS concentration of 1.33%, with one half (by mass) from the digested sludge and the other half from the digested manure. Compared with food waste and dairy manure separately, it had a slightly basic pH and generally balanced concentrations of macro- and micronutrients (Table 1). Based on earlier studies (El-Mashad and Zhang, 2010; Zhang et al., 2012, 2013), it appears that a VS ratio of manure to food waste around 1 is the optimum combination for co-digestion of cattle manure and food waste. The feedstock used in this study was prepared by combining domestic food waste and dairy manure at a VS ratio of 1 and stored frozen at 21 C in plastic tubes. Table 1 summarizes the characteristics of the feedstock and its two components. The food waste was collected from a Sheraton Hotel’s restaurant over five days and ground through a MG800 Waring Pro Professional meat grinder with three cutting plates having different aperture diameters (2.5, 4, and 8 mm) for the three digesters, called fine, medium and coarse food waste, respectively. Energy consumption to grind food waste was recorded with a Watts up? PRO electricity watt meter (Electronic Educational Devices, Inc., Denver, CO, USA). Dairy manure was taken from a storage vessel of liquid manure which was scraped from concrete lots of a large-size dairy farm at Cayuga County of New York, USA. The digesters were operated in a semi-continuous mode. The feedstock was thawed in a refrigerator at 4 C and fed to the digesters every 2 d. OLR was increased from 0.67 g VS/L/d to 1, 2, and 3 g VS/L/d stepwise over 178 d of operation. Digestate (67e90 mL) was discharged every 6 d at the OLRs of 0.67 and 1 g VS/L/d, 4 d at 2 g VS/L/d, and 2 d at 3 g VS/L/d. This study aimed at dry digestion. Only 25e45 mL of tap water was used to wash the feedstock storage tubes and maintain the working volume after discharge. Hydraulic retention time or solids retention time was 160 d at the OLRs of 0.67 and 1 g VS/L/d, 80 d at 2 g VS/L/d, and 54 d at 3 g VS/L/d.
2.1. Setup and operation of anaerobic digesters Three 2-L complete-mix anaerobic digesters were set up in a laboratory. Each digester as shown in Fig. 1 was built with a modified Duran GLS80 glass reactor with a magnetic impeller. A Thermo scientific hotplate/stirrer was used to heat each digester
Gas meter
Biogas sampling Measuring
Gas exit Feeding
Magnetically stirred reactor Discharge Hotplate/ stirrer Fig. 1. Sketch of a bench-scale, semi-continuously fed anaerobic digester.
Table 1 Characteristics of inoculum and feedstock made from dairy manure and food waste.
pH Total volatile solids, % Total dissolved solids, g/L Crude protein, g/kg VS Fat, g/kg VS Non-fiber carbohydrate, g/kg VS Neutral detergent fiber, g/kg VS Total N, %TS Total C, %TS Orthophosphate, g P/L Total ammonia, g N/L Sulfur, g/kg TS Total Ca, g/kg TS Total Mg, g/kg TS Total K, g/kg TS Total Na, g/kg TS Total Fe, mg/kg TS Total Zn, mg/kg TS Total Cu, mg/kg TS Mn, mg/kg TS Mo, mg/kg TS
Dairy manure
Food waste
Feedstock
Inoculum
6.6 9.68 16.9 167 40 623
4.4 29.3 16.9 266 350 325
6.6 14.6 16.8 273 231 380
7.7 1.33 7.52 335 90 543
616
196
291
543
1.9 39.9 0.78 1.71 6.1 20.6 8.5 23.8 7.25 705 233 123 176 1.6
3.8 48.4 No data No data 3.4 1.7 0.7 9.6 10.1 41 32 5 8 0.3
3.6 46.3 No data No data 4.8 13.3 5.2 18.0 8.9 374 136 46 98 1.2
3.0 32.4 0.33 1.68 11.9 26.0 12.2 20.3 12.1 18300 900 569 269 11.3
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measured with a Hach HQ40d meter. After separating suspended solids in the digester effluent via centrifugation at 1600 g for 30 min, total ammonia concentration in the centrate was determined colorimetrically with a Hach DR 2800 spectrophotometer (Hach Company, Loveland, Colorado, USA). Free ammonia concentration in digestate was calculated with measured total ammonia concentration, temperature and pH (Pitk et al., 2013). Time-to-filter was determined with the small-volume Standard Method 2710H (APHA, 1999) to reveal dewaterability of digester effluent. The feedstock and its components as well as the inoculum were measured for pH, total solids, total volatile solids, total dissolved solids, and total ammonia, using the same methods as mentioned above. Crude proteins, fats, neutral detergent fiber, and non-fiber carbohydrates were determined at Dairy One Forage Laboratory (Ithaca, New York), following AOAC International standards. Total carbon and nitrogen contents were determined for oven-dried samples with an elemental analyzer (Calo Erba NC2500, Costech Analytical Technologies Inc., Valencia, California, USA). Orthophosphate in centrate of the inoculum and dairy manure was determined colorimetrically with the Hach DR2800
2.2. In-situ measurements and laboratory analyses Each reactor had a head space of 0.69 L at the working volume of 1.8 L. Biogas production was recorded with in-line gas meters and converted to daily production rate under standard conditions (stp: 0 C and 760 mm Hg). While sampling biogas, headspace temperature was measured with a Hach H160 pH meter connected to an ISFET pH stainless steel NMR tube probe. Biogas samples (0.1 mL each) were collected with a gas-tight syringe through rubber septa and diluted with air in 10-mL Wheaton serum vials. The biogas samples along with air samples were analyzed for CH4 and CO2 percentages using a Shimadzu GC-2014 gas chromatograph system with a flame ionization detector (Shimadzu Corporation, Tokyo, Japan). Helium was used as carrier gas. The detection limits were 0.1 ppm for CH4 and 10 ppm for CO2. Digestate temperature and pH were measured with the pH meter while collecting biogas samples. Digester effluent samples were collected for determination of total solids and total volatile solids concentrations, following Standard Methods 2540 B and E, respectively (APHA, 1998). Total dissolved solids concentration was
Biogas production rate (L/L/d)
a)
3.0 OLR = 0.67 g VS/L/d
1.0 g VS/L/d
2.0 g VS/L/d
3.0 g VS/L/d
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Medium Coarse
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Time in operation (d)
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1.2 OLR = 0.67 g VS/L/d
1.0 g VS/L/d
2.0 g VS/L/d
3.0 g VS/L/d
Biogas yield (L/g VS)
1.0 0.8 0.6
0.4 Fine 0.2
Medium Coarse
0.0 0
20
40
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100
120
140
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180
Time in operation (d) Fig. 2. Variations of (a) biogas production rate and (b) specific biogas yield with food waste particle size (fine, medium, and coarse) and organic loading rate (OLR) in mesophilic codigestion of dairy manure and food waste.
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with a backward elimination approach to identify the period of stable operation at a given OLR (Townend, 2002).
8 Fine Medium Coarse
14
7
12
6
10
5
8
4
6
3
4
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2 0.67
1
OLR (g VS/L/d) = 1.0 2.0
Total volatile solids in digester effluent (%)
Total dissolved solids in digester effluent (g/L)
16
3. Results and discussion The first 52 days of digester operation at the OLR of 0.67 g VS/L/ d was taken as a startup phase and not monitored regularly. Results from 42 d of anaerobic digestion at each of the successive OLRs were used to diagnose the effects of food waste particle size, evaluate methane production and treatment performance, and identify the optimum OLR. As illustrated in Fig. 2 for biogas production rate and specific biogas yield, trend tests showed that it took more than 19, 15, and 10 d to reach stable operation at the OLRs of 1, 2, and 3 g VS/L/d, respectively. Whenever OLR was increased, there was an increase in microbial biomass as reflected by the effluent VS concentrations (Fig. 3). Both biogas production rate and specific yield increased over time initially and became stable, likely as microorganisms were acclimated, at given OLRs (Fig. 2).
3.0
0
0 0
30
60
90
271
120 150 180
3.1. Performance of co-digestion
Time in operation (d)
The co-digestion in this study attained higher methane contents in biogas (Fig. 4) compared with earlier studies on co-digestion of cattle manure and food waste (El-Mashad and Zhang, 2010; Hartmann and Ahring, 2005; Zhang et al., 2012, 2013). Moreover, this study achieved higher specific methane yields during the stable operation periods (0.46e0.63 L CH4/g VS) compared with those in most earlier studies on co-digestion of food waste and cattle manure (0.14e0.46 L CH4/g VS). The higher methane yield in this study relative to those in the earlier studies could be attributed to the long solids retention times associated with dry digestion and the relatively higher lipid content of the feedstock in this study (Table 1). This study also confirmed the synergistic effect of codigestion with the higher specific methane yields compared with 0.46 L CH4/g VS in mono-digestion of food waste without addition of trace elements (Banks et al., 2011b, 2012; Ghanimeh et al., 2012; Nagao et al., 2012; Zhang et al., 2012, 2013) and 0.25 L CH4/g VS in mono-digestion of cattle manure (El-Mashad and Zhang, 2010; Frear et al., 2010; Hartmann and Ahring, 2005; Zhang et al., 2013). Specific methane yield in anaerobic digestion of food waste has been limited mainly because of the operational
Fig. 3. Effects of food waste particle size (fine, medium, and coarse) on digester effluent solid concentrations in co-digestion of dairy manure and food waste at increasing organic loading rates (OLR).
spectrophotometer. The other macro- and micro-nutrients were analyzed with an inductively coupled plasma radial spectrometer after microwave digestion. 2.3. Statistical analysis One-way analysis of variance (ANOVA) was performed to determine whether there were statistically significant differences among the three digesters. The significance level (p value) was set at 5%. If there were significant differences, least significant difference (LSD) was calculated to further identify the pairs of means that had significant differences (Townend, 2002). Spearman’s rank correlation analysis was performed to assess the relationship of biogas production with digestate ammonia concentration, giving correlation coefficient r. Spearman rank trend test was performed
90 Fine
CH4 content in biogas (%) CO2 content in biogas (%)
80
Medium
70
Coarse
60 50 40 30 20
10 OLR = 0.67 g VS/L/d
1.0 g VS/L/d
2.0 g VS/L/d
3.0 g VS/L/d
0
0
20
40
60
80
100
120
140
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180
Time in operation (d) Fig. 4. Dynamics of biogas composition in anaerobic co-digestion of dairy manure and different particle sizes of food waste (fine, medium, and coarse).
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instability at higher organic loading rates (Banks et al., 2012; Zhang et al., 2012). High fiber content is the main reason for low methane potential of cow manure (El-Mashad and Zhang, 2010; Frear et al., 2010). The methane production rates during the stable operation periods at the final OLR of 3 g VS/L/d were 1.53, 1.41, and 1.40 L CH4/L/ d in the digesters with fine, medium and coarse food waste, respectively. Like specific methane yield, the methane production rates of co-digestion in this study were higher compared to those in mono-digestion of food waste, 1.39 L CH4/L/d (Banks et al., 2011b, 2012; Ghanimeh et al., 2012; Zhang et al., 2013), and monodigestion of dairy manure, 0.10 L CH4/L/d (Zhang et al., 2013). Most enzymes and co-enzymes need a minimal amount of certain trace elements for their activation and activity (Appels et al., 2008). As Table 1 shows, dairy manure generally has higher concentrations of macro- and micro-nutrients than food waste. Combination of dairy manure and food waste improves availability of nutrients for anaerobic digestion of food waste. Such combination of digester feed also resulted in pH values (Fig. 5a) non-
9
Digestate pH
8 7
Fine Medium Coarse
6 5 0.67
4
0
Total ammonia in digestate (mg N/L)
b)
0.67
OLR (g VS/L/d) = 1.0 2.0
3.0
3000 2000
Table 2 presents biogas production during the stable operation periods. The average biogas production rates of the three digesters were significantly different (p 0.05) at the OLRs of 1 g VS/L/ d (LSD ¼ 0.06 L/L/d), 2 g VS/L/d (LSD ¼ 0.08 L/L/d), and 3 g VS/L/ d (LSD ¼ 0.06 L/L/d). The three digesters had significantly different specific biogas yields (p 0.05) at the OLRs of 1 g VS/L/ d (LSD ¼ 0.06 L/g VS), 2 g VS/L/d (LSD ¼ 0.04 L/g VS), and 3 g VS/L/ d (LSD ¼ 0.02 L/g VS) as well. The digester with fine food waste had methane production rate 10e29% higher and specific methane yield 9e34% higher than those with coarse food waste. Although the biogas production rate and specific yield in the digester with medium food waste fell between those with fine and coarse food waste, the differences were only statistically significant at the OLR of 2 g VS/L/d. Food waste particle size did not make significant differences in methane content of biogas (p ¼ 0.30e0.63) except for a significantly lower average methane content in the digester with coarse food waste at the OLR of 2 g VS/L/d (Table 2). Energy consumed for grinding food waste to fine particles was 0.130 Wh/g VS, 0.069 Wh/g VS to medium particles, and 0.054 Wh/g VS to coarse particles. The lower heating value of methane is 10.67 Wh/L under the standard conditions (Metcalf and Eddy Inc, 2003). Considering the specific methane yields at the OLR of 3 g VS/L/d (Table 2), energy consumption for grinding amounted to only 1.1e2.4% of energy carried by methane produced from anaerobic digestion. It is, therefore, cost-effective to grind food
Fine Medium Coarse
1000 0
0
c) Free ammonia in digestate (mg N/L)
30 60 90 120 150 180 Time in operation (d)
5000 4000
OLR (g VS/L/d) = 1.0 2.0 3.0
3.2. Effects of food waste particle size
30 60 90 120 150 180 Time in operation (d)
500
OLR (g VS/L/d) = 1.0 2.0
400
0.67
300
Fine Medium Coarse
200
3.0
100
0 0
30
60
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Time in operation (d) Fig. 5. Dynamics of (a) digestate pH; (b) total ammonia concentration; and (c) free ammonia concentration in co-digestion of dairy manure and different particle sizes of food waste (fine, medium, and coarse) at increasing organic loading rates (OLR).
300
Time-to-filter of digester effluent (min)
a)
inhibitory to methanogens (Angelidaki and Sanders, 2004) and balanced C/N ratios. It is challenging to separate solids from liquid in anaerobically digested dairy manure, which has time-to-filter at 246e348 min (Xia et al., 2012). Anaerobically digested sludge has time-to-filter at a few minutes (Cheumbarn and Pagilla, 2000; Zhang et al., 2010). The digesters in this study started up at their full working volumes with anaerobically digested dairy manure and digested sludge, which had time-to-filter (initial values in Fig. 6) slightly shorter than that of anaerobically digested dairy manure. Time-to-filter decreased considerably across the OLRs of 1, 2 and 3 g VS/L/d in the co-digestion (Fig. 6), indicating improved dewaterability of digester effluent compared with mono-digestion of dairy manure.
Fine Medium Coarse
250 200
150 100 50 0.67
OLR (g VS/L/d) = 1.0 2.0 3.0
0 0
30
60
90
120 150 180
Time in operation (d) Fig. 6. Effect of food waste particle size (fine, medium, and coarse) on digester effluent dewaterability in co-digestion of dairy manure and food waste.
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273
Table 2 Biogas production in co-digestion of dairy manure with different particle sizes of food waste during stable operation periods at increasing organic loading ratesa. Specific biogas yield (stp L/g VS)
CH4 content in biogas (%)
Specific CH4 yield (stp L/g VS)
Organic loading rate ¼ 1 g VS/L/d from day 52 to day 94 Fine food waste 0.79 0.06 Medium food waste 0.74 0.08 Coarse food waste 0.72 0.05
0.79 0.06 0.74 0.08 0.72 0.05
67.5 6.9 63.7 11.7 64.2 4.2
0.53 0.04 0.47 0.05 0.46 0.03
Organic loading rate ¼ 2 g VS/L/d from day 94 to day 136 Fine food waste 1.69 0.05 Medium food waste 1.60 0.06 Coarse food waste 1.45 0.14
0.85 0.02 0.80 0.03 0.73 0.07
74.1 5.3 70.3 5.8 64.9 5.6
0.63 0.02 0.56 0.02 0.47 0.05
Organic loading rate ¼ 3 g VS/L/d from day 136 to day 178 Fine food waste 2.12 0.07 Medium food waste 2.03 0.06 Coarse food waste 2.00 0.09
0.71 0.02 0.68 0.02 0.67 0.03
72.2 4.0 69.5 5.4 69.8 4.5
0.51 0.02 0.47 0.01 0.47 0.02
Biogas production rate (stp L/L/d)
a
Mean standard deviation.
waste into finer particles for greater methane yield in co-digestion of food waste with dairy manure. The effects of particle size on biogas production are attributed to the larger specific surface area provided by smaller particles for enhanced hydrolysis (Izumi et al., 2010; Palmowski and Müller, 2000). The optimum particle size for anaerobic mono-digestion of food waste has been examined in different ranges. Kim et al. (2000) reported that the maximum substrate utilization rate coefficient doubled as the average particle size decreased from 2.14 to 1.02 mm in thermophilic batch digestion tests. Izumi et al. (2010) investigated the effect of particle size in a narrow range (0.391e 0.888 mm) and found that the optimum particle size (0.718 mm) resulted in 28% more biogas than that with the worst particle size (0.888 mm) in mesophilic batch digestion tests. The effects of particle size should be further addressed in a wider range with regard to not only methane production, but also energy consumption for particle size reduction and digestate dewaterability in the future. The long time-to-filter as presented in Fig. 6 manifests the difficulty to dewater the digester effluent. Nevertheless, there were significant differences in time-to-filter among the three digesters at all the three OLRs (p 0.001; LSD ¼ 7e10 min). The shortest was recorded in co-digestion with fine food waste and the longest in codigestion with coarse food waste. This was consistent with the positive impact of organic waste particle size on dewaterability of digestate as Mata-Alvarez et al. (2000) reported. 3.3. Optimum organic loading rate As Fig. 2 shows, biogas production rate increased with increasing OLR. As Table 2 shows for the periods of stable operation, however, biogas production rate increased by 101e116% when OLR was increased from 1 to 2 g VS/L/d and only by 25e38% when OLR was further increased from 2 to 3 g VS/L/d. Specific methane yield peaked at the OLR of 2 g VS/L/d in the digesters with fine and medium food waste. Similarly, earlier studies (Hartmann and Ahring, 2005; Zhang et al., 2013, 2012) on co-digestion of cattle manure and food waste at higher organic loading rates (3.3e 16 g VS/L/d) attained lower specific methane yield (0.14e0.41 CH4/ g VS). Therefore, the optimum OLR for deep co-digestion of dairy manure and food waste was close to 3 g VS/L/d, although OLR could be increased further for higher methane production rates. Theoretical methane potential under the standard conditions was estimated to be 0.53 L CH4/g VS for the feedstock in this study, based on biochemical composition of the feedstock (Table 1) and methane potentials of proteins, lipids and carbohydrates as suggested by Angelidaki and Sanders (2004). The methane yields experimentally determined in this study were 89e119% of the
estimated methane potential, suggesting little limitation of trace elements and inhibition due to ammonia and volatile fatty acids. Anaerobic digestion converts organic nitrogen to ammonia, which exists in ionized ammonium and free ammonia, depending on pH and temperature. Free ammonia inhibits more than ionized ammonium to methanogens (Yenigun and Demirel, 2013). The hydrophobic free ammonia may diffuse into cells, causing proton imbalance and potassium deficiency in microorganisms, particularly in methanogens (Pitk et al., 2013). Ammonia could be carried with liquid dairy manure and inoculum into mixed liquor and produced in anaerobic digestion, resulting in inhibition to methanogenesis at high OLRs. The inoculum and dairy manure in this study had high total ammonia concentrations, 1.7 g N/L (Table 1). As Fig. 5b shows, total ammonia concentration in the digester effluent tended to increase up to 3090e3420 mg N/L after 178 d of operation. Free ammonia concentration increased rapidly to 202e 340 mg N/L at the OLR of 1 g VS/L/d, decreased at the OLR of 2 g VS/ L/d, then stabilized between 148 and 237 mg N/L at the OLR of 3 g VS/L/d (Fig. 5c). As reviewed by Yenigun and Demirel (2013), ammonia inhibition to mesophilic anaerobic digestion with acclimated inoculum is triggered at very different concentrations, mostly from 2800 to 6000 mg N/L total ammonia and 337e 800 mg N/L free ammonia (Yenigun and Demirel, 2013). Therefore, ammonia concentration in this study might not be high enough to significantly affect biogas production. There were insignificant correlations between biogas production rate and biogas yield with either total ammonium or free ammonia concentrations (r < critical r). There were insignificant differences in effluent solids concentrations between the digesters (p ¼ 0.08e0.64). Total dissolved solids concentrations were reduced by 40.7e42.6% and total volatile solids concentrations by 80.9e82.7% on average in the three digestersat at the OLR of 1 g VS/L/d. The concentrations of total dissolved solids and total volatile solids in digester effluent increased with the increasing OLRs (Fig. 3). At the end of the period with the OLR of 3 g VS/L/d, the reduction efficiencies decreased to 18.2e23.0% for total dissolved solids and 66.6e69.6% for total volatile solids through the three digesters. Solids removal efficiency at the end of this study became moderate compared with that in mono-digestion of food waste and dairy manure at similar OLRs (Hartmann and Ahring, 2005; Nagao et al., 2012). This suggests again that the optimum OLR is approximately 3 g VS/L/d for mesophilic co-digestion of dairy manure and food waste. 4. Conclusions Reduction of food waste particle size from 8 to 2.5 mm increased methane production rate by 10e29% and specific methane yield by
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9e34% in co-digestion of dairy manure and food waste. Dewaterability of digester effluent was significantly improved by reducing food waste particle size. The energy consumed to grind food waste down to 2.5 mm was minor compared to the heating value of the methane produced. The co-digestion could be loaded up to 3 g VS/L/d without ammonia inhibition while reducing more than 67% of volatile solids and producing 1.40e1.53 L CH4/L/d. The highest specific methane yields, however, were achieved at the OLR of 2 g VS/L/d. Acknowledgments This study was supported by a fellowship to Fred Agyeman from Ford Foundation-IFP and Association of African Universities. The research was partially supported by a U.S. EPA grant to Dr. Tao (SU835331). We would like to thank Dr. Philippe Vidon, Mr. David Kiemle, and Pat Rook for their help with biogas analysis. Our thanks also go to Mr. Steve McGlynn at Twin Birch Diary and Mr. Paul Eno at Sheraton Syracuse University Hotel for providing with the feedstock materials. References Angelidaki, I., Sanders, W., 2004. Assessment of the anaerobic biodegradability of macropollutants. Rev. Environ. Sci. Biotechnol. 3, 117e129. APHA, AWWA, WEF, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, American Water Works Association, Water Environment Federation, Washington, DC. Appels, L., Baeyens, J., Degreve, J., Dewil, R., 2008. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energ. Combust. 34, 755e781. ASABE, 2010. Manure Production and Characteristics, ASABE Standards 2010: ASAE D384.2 MAR2005 (R2010). American Society of Agricultural and Biological Engineers, St. Joseph, MI. 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, 1475e1481. Banks, C.J., Salter, A.M., Heaven, S., Riley, K., 2011a. Energetic and environmental benefits of co-digestion of food waste and cattle slurry: a preliminary assessment. Resour. Conserv. Recycl. 56, 71e79. Banks, C.J., Chesshire, M., Heaven, S., Arnold, R., 2011b. Anaerobic digestion of source segregated domestic food waste: performance assessment by mass and energy balance. Bioresour. Technol. 102, 612e620. Banks, C.J., Zhang, Y., Jiang, Y., Heaven, S., 2012. Trace element requirements for stable food waste digestion at elevated ammonia concentrations. Bioresour. Technol. 104, 127e135. Cheumbarn, T., Pagilla, K.R., 2000. Anaerobic thermophilic/mesophilic dual-stage sludge treatment. J. Environ. Eng. ASCE 126, 796e801. El-Mashad, H.M., Zhang, R., 2010. Biogas production from co-digestion of dairy manure and food waste. Bioresour. Technol. 101, 4021e4028.
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