Waste Management 36 (2015) 86–92
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Psychrophilic dry anaerobic digestion of dairy cow feces: Long-term operation Daniel I. Massé ⇑, Noori M. Cata Saady Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, 2000 College Street, Sherbrooke, Québec, Canada J1M 0C8
a r t i c l e
i n f o
Article history: Received 20 March 2014 Accepted 30 October 2014 Available online 27 November 2014 Keywords: Psychrophilic Dry Anaerobic Manure Straw
a b s t r a c t This paper reports experimental results which demonstrate psychrophilic dry anaerobic digestion of cow feces during long-term operation in sequence batch reactor. Cow feces (13–16% total solids) has been anaerobically digested in 12 successive cycles (252 days) at 21 days treatment cycle length (TCL) and temperature of 20 °C using psychrotrophic anaerobic mixed culture. An average specific methane yield (SMY) of 184.9 ± 24.0, 189.9 ± 27.3, and 222 ± 27.7 NL CH4 kg 1 of VS fed has been achieved at an organic loading rate of 3.0, 4.0, and 5.0 g TCOD kg 1 inoculum d 1 and TCL of 21 days, respectively. The corresponding substrate to inoculum ratio (SIR) was 0.39 ± 0.06, 0.48 ± .02, 0.53 ± 0.05, respectively. Average methane production rate of 10 ± 1.4 NL CH4 kg 1 VS fed d 1 has been obtained. The low concentration of volatile fatty acids indicated that hydrolysis was the reaction limiting step. Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
1. Introduction Current livestock production practices generate large amount of organic wastes in the form of manure, feed refusal, spoiled feeds and livestock mortalities. These wastes need to be stabilized prior valorisation. Livestock manure is responsible for about 6.6% and 1.1% of total greenhouse gas (GHG) emissions in the United States and Canada, respectively (Baylis and Paulson, 2011). Storage of dairy cow manure slurry in cool climate regions in the European countries and the U.S. contributes about 12% and 23% of the total CH4 emission, respectively (Hindrichsen et al., 2005; IPCC, 1996). Cattle alone produce between 86% and 75% of the manure generated by Canadian and USA livestock, respectively (Hofmann and Beaulieu, 2006; Wen et al., 2004). Management of animal manure translates into cost which could be compensated by converting the biodegradable components of manure into biofuel (methane) through anaerobic digestion with positive economic return. Currently, on-farm applications of manure anaerobic digestion are under intensive research to introduce technological advancement which decrease the environmental impact, increase bioenergy yield, and reduce cost. Fresh cow feces has a total solids content of about 12–16% and requires dilution before conventional wet anaerobic digestion to decrease the solids content in order to allow liquid handling and ⇑ Corresponding author. Tel.: +1 819 780 7128; fax: +1 819 564 5507. E-mail address: [email protected] (D.I. Massé). http://dx.doi.org/10.1016/j.wasman.2014.10.032 0956-053X/Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
processing (El-Mashad et al., 2004). However, dilution substantially increases the digester volume. Increasing the total solids of the substrate fed to anaerobic digestion (AD) is an important engineering design objective in order to decrease the bioreactor volume as well as the volume of the bioreactor effluent to store and to apply on the land (Luning et al., 2003), increase the energy yield per bioreactor unit volume, and reduce its construction and operation costs. Psychrophilic AD of cow feces has been developed recently at Agriculture and Agri-Food, Dairy and Swine Research and Development Centre (DSRDC) in Sherbrooke, Quebec–Canada (Massé et al., 1997, 1996). In comparison to mesophilic and thermophilic wet anaerobic digestion, psychrophilic dry AD of cow feces at its TS (13–16%) offers more advantages such less energy input for mixing and heating, increase the specific volumetric energy output of bioreactor. Cow feces is a complex substrate composed of soluble and particulate or fibrous components. Usually, it contains carbohydrates (cellulosic and hemicellulosic fibers and volatile fatty acids), lipids and fats, and proteins. Approximately 40–50% of the volatile solids (VS) in dairy manure is biodegradable lingocellulosics biomass containing reduced carbon which can be converted to CH4 (AbbassiGuendouz et al., 2012). Saady and Massé (2013) have shown that psychrophilic anaerobic digestion of particulate cellulose and hemicellulose (xylan) is feasible in static batch system. In addition, they demonstrated the feasibility of psychrophilic anaerobic digestion of dilute cow feces. Mesophilic and thermophilic dry anaerobic digestion (DAD) of the organic fraction of municipal solid wastes
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(OFMSW) have been demonstrated (Challen Urbanic et al., 2011; Li et al., 2011; Ramasamy and Abbasi, 2000). Generally, mesophilic or thermophilic DAD of high-solids agricultural wastes is relatively a new biotechnology (Ahn et al., 2010; Di Maria et al., 2012; Kusch et al., 2008). Farmers in cold-climate regions would be interested in psychrophilic DAD of livestock manure because of its obvious advantages over mesophilic and thermophilic operations. Unfortunately, lack of research and development of DAD of animal manure is behind its current low acceptance. Therefore, more work is needed to demonstrate its feasibility, to optimize its operation, to evaluate its economic benefits, develop, and adopt more suitable technologies. Ultimately, to make on-farm DAD acceptable it has to be an integral part of farm waste management systems (Schäfer et al., 2006). DAD of rice straw resulted in a greater specific biogas yield at a shorter treatment cycle compared to a wet process (Li et al., 2006); hence, DAD was considered more efficient than wet anaerobic digestion for rice straw (Sun et al., 1987). Luning et al. (2003) reported that dry and wet anaerobic digestion of the organic solid waste fraction were identical based on the specific gas yield and the organic loading rates. The psychrophilic anaerobic digestion (PAD) in sequential batch reactor (SBR), to digest agricultural wastes with TS contents lower than 12%, such as swine manure, has been developed at Agriculture and Agri-Food, Dairy and Swine Research and Development Centre (DSRDC) in Sherbrooke, Quebec–Canada. The SBR offers a simple operating system consisting of four consecutive steps for wet anaerobic digestion: fill, react, settle, and draw. For a dry anaerobic digestion where no liquid–solid phase separation occurs, the settle step is eliminated. Another modification is the draw step where digestate rather than decant is withdrawn as effluent. The SBR is charged with the feed (cow feces and wheat straw) during the fill step. The length of the react period should be sufficient to meet pre-determined treatment objectives (specific methane yield, VS and TCOD removal, etc.). During the draw period, some of the digestate is removed as effluent and some is used as inoculum for the next cycle. PAD offers several advantages such as it successfully reduces odors, decreases the organic pollution load by more than 70% (Massé et al., 1996), produces high quality biogas, significantly diminishes pathogens survival (Massé et al., 2011), and improves the agronomic value of digestate (Massé et al., 2007). The process offers the competitive advantages of great stability, robustness, maximum performance, and minimum supervision (Massé et al., 1997). Research is still needed to adapt this kind of process for agricultural wastes with high solids such as dairy manure with bedding or solid fraction of dairy manure without bedding. With the addition of bedding, the manure is usually under solid or semi-solid state (15–20% TS). To the best of the author’s knowledge, no previous reports are available in the accessible literature on psychrophilic DAD. This is the first report on successful psychrophilic (20 °C) dry anaerobic digestion of cow feces over long-term operation in sequence batch redactor. However, it still needs more development and optimization studies. The principal objective of this study was to assess the performance of psychrophilic dry anaerobic digestion of dairy cow manure (TS 13–16%) in long-term operation using sequence batch bioreactor.
reactor to the next feeding) in a temperature controlled room (20 °C). The reactors were fitted with two gas lines; one for purging with nitrogen gas immediately after feeding the substrate to maintain the anaerobic condition, and the second to release and measure the biogas produced into the bioreactor. A schematic showing the details of the reactor is shown in Fig. 1. A total of 12 cycles have been conducted at organic loading rate (OLR, g TCOD kg 1 inoculum d 1) of 3.0 (5 cycles), 4.0 (3 cycles), and 5.0 (4 cycles). The mass of inoculum and the cow feces fed to each bioreactor at the beginning of the successive cycles and the organic loading rate (OLR) are given in Table 1. Physico-chemical characteristics of the inoculum and substrates before feeding bioreactors were analyzed and are given in Table 2. The initial inoculum which has been used to initiate the reaction in the first cycle was obtained from a semi-industrial scale (11.4 m3; TS = 9%) psychrophilic (20 °C) anaerobic reactor fed with fresh dairy manure (12% TS), and operated as a SBR. Starting from the second cycle forwards, 6 kg of the digestate from the previous cycle has been used as inoculum for the next cycle in each reactor. Fresh feces from dairy cows was collected at the experimental farm of the DSRDC. Feces were collected on wood boards, before getting in contact with urine and bedding, transferred into a plastic drum, stored at 4 °C, before being fed to the reactors. At the TS content of the digesting matrix maintained during the entire operation no solid–liquid separation has been observed thus no decanting step was required in the operation of the SBR reactors. At the end of each cycle, the effluent has been withdrawn manually using a scoop since the digestate was in solid state. The remaining contents of the replicate reactors have been mixed, homogenized, and each empty reactor received 6 kg of the digestate as inoculum for the next cycle. The substrate and inoculum has been mixed manually for 5 min during feeding. Every week the content of the bioreactor is mixed manually for 5 min before sampling the content to ensure a homogenous and representative sample is taken. No mixing took place during other time of incubation; therefore, the process can be considered as a static dry anaerobic digestion. 2.2. Organic loading rate Organic loading rate (OLR) has been calculated based on the masses of TCOD and VS and the composition of the substrate fed (Table 2). OLR was expressed in g of VS fed per kg inoculum per day and g of TCOD fed per kg of inoculum per day. The substrate to inoculum ratio (SRI; based on mass) ranged between 0.36 and 0.48. 2.3. Biogas measurement Biogas production was measured daily with calibrated wet tip gas meters while the biogas components (CH4, H2S, CO2) were
7 6
1 1- 40 L plastic barrel 2- Barrel lid with metal lock
5
3- Gas release line 4- N2 purging line with valve
2. Materials and methods
5- Substrate-inoculum mixture
2.1. Experimental setup Triplicate 40-L cylindrical barrels bioreactors were set-up and operated as pseudo sequential batch reactors (PSBR) at a Treatment Cycle Length (TCL) of 21 days (measured from feeding the
2 4
3
6- Head space 7- Wet tip gas meter
Fig. 1. A schematic of the sequence batch reactor.
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Table 1 Organic loading rate and total solids of the feed. Cycle Inoculum (kg) Feces (kg) Substrate TS (%) Substrate to Solids retention Substrate TS (%) Organic loading rate time (SRT) (day) inoculum TCOD fed (g) VS fed (g) g TCOD kg ratio (SIR)
1
inoculum d
1
g VS fed kg inoculum d
1 2 3 4 5
6 6 6 6 6
2.35 2.8 2.554 1.94 2.185
12.7 12.8 13.3 13.2 13.2
0.39 0.47 0.43 0.32 0.36
74.6 66.0 70.3 85.9 78.7
12.7 12.8 13.3 13.2 13.2
389.4 413.3 378.2 378.7 379.1
267.9 319.2 303.9 230.9 257.8
3.1 3.3 3.0 3.0 3.0
2.1 2.5 2.4 1.8 2.0
6 7 8
6 6 6
2.913 2.913 2.739
13.2 13.2 12.5
0.49 0.49 0.46
64.3 64.3 67.0
13.2 13.2 12.5
505.4 533.7 504.5
343.7 343.7 306.8
4.0 4.2 4.0
2.7 2.7 2.4
9 10 11 12
6 6 6 6
3.419 3.419 2.857 2.857
12.5 12.5 16.5 16.5
0.57 0.57 0.48 0.48
57.9 57.9 65.1 65.1
12.5 12.5 16.5 16.5
629.8 629.8 630.0 630.0
382.9 382.9 408.6 408.6
5.0 5.0 5.0 5.0
3.0 3.0 3.2 3.2
1 1
Table 2 Physicochemical characteristics of the inoculum, cow feces and mixed liquor at the beginning of each digestion cycle. Cycle
Substrate
pH
TCOD (g kg
1
Inoculum Feces Mixed liquor
7.2 6.0 6.9 ± 0.0
2
Inoculum Feces Mixed liquor
3
1
)
1
VS (%)
Acetate (g kg
– 165.7 –
10.3 13.2 10.5 ± 0.2
8.8 11.8 9.0 ± 0.2
0.19 ± 0.02 3.4 1.95 ± 0.02
0.03 ± 0.0 0.88 0.61 ± 0.01
0.0 ± 0.0 1.39 0.57 ± 0.01
7.2 ± 0.0 5.9 6.9 ± 0.1
– 147.6 –
9.0 ± 0.2 12.8 10.2 ± 0.3
7.5 ± 0.2 11.4 8.7 ± 0.3
1.13 ± 0.09 2.8 1.61 ± 0.10
0.39 ± 0.03 1.3 0.50 ± 0.03
0.41 ± 0.01 0.8 1.10 ± 0.10
Inoculum Feces Mixed liquor
7.2 ± 0.1 5.9 7.0 ± 0.0
– 148.1 –
9.1 ± 0.3 13.3 10.2 ± 0.2
7.6 ± 0.3 11.9 8.7 ± 0.3
0.96 ± 0.05 3.4 1.50 ± 0.07
0.38 ± 0.02 0.88 0.50 ± 0.04
0.26 ± 0.03 0.11 0.26 ± 0.02
4
Inoculum Feces Mixed liquor
7.2 ± 0.1 6.0 7.0 ± 0.0
– 195.2 –
9.1 ± 0.1 13.24 10.4 ± 0.5
7.4 ± 0.3 11.9 8.9 ± 0.5
0.23 ± 0.02 3.82 1.47 ± 0.06
0.00 ± 0.00 1.15 0.38 ± 0.03
0.00 ± 0.00 0.80 0.13 ± 0.00
5
Inoculum Feces Mixed liquor
7.5 ± 0.0 6.6 7.3 ± 0.0
– 173.5 –
8.7 ± 0.1 13.2 9.9 ± 0.4
7.2 ± 0.2 11.8 8.4 ± 0.4
0.24 ± 0.03 4.0 0.99 ± 0.63
0.0 ± 0.0 1.05 0.28 ± 0.19
0.20 ± 0.05 0.65 0.10 ± 0.09
6
Inoculum Feces Mixed liquor
– 6.6 7.2 ± 0.2
– 173.5 –
9.3 ± 0.2 12.8 10.4 ± 0.1
7.6 ± 0.3 11.3 8.9 ± 0.1
0.13 ± 0.02 4.3 1.46 ± 0.09
0.02 ± 0.0 1.10 0.38 ± 0.05
0.0 ± 0.0 0.78 0.64 ± 0.09
7
Inoculum Feces Mixed liquor
7.2 ± 0.0 6.5 7.0 ± 1.0
– 183.2 –
9.3 ± 0.2 12.8 10.8 ± 0.5
7.7 ± 0.3 11.3 9.3 ± 0.4
0.14 ± 0.01 3.5 1.40 ± 0.16
0.02 ± 0.01 1.5 0.49 ± 0.07
0.00 ± 0.00 2.36 0.83 ± 0.10
8
Inoculum Feces Mixed liquor
7.3 ± 0.0 7.0 7.1 ± 0.0
– 184.2 –
9.1 ± 0.1 12.5 10.6 ± 0.7
7.6 ± 0.1 11.2 9.1 ± 0.7
0.18 ± 0.03 1.54 1.54 ± 0.12
0.05 ± 0.02 0.46 0.40 ± 0.03
0.05 ± 0.01 0.26 0.18 ± 0.02
9
Inoculum Feces Mixed liquor
7.3 ± 0.1 7.0 7.0 ± 0.0
– 184.2 –
9.0 ± 0.0 12.5 10.8 ± 0.2
7.5 ± 0.1 12.5 9.3 ± 0.2
0.16 ± 0.03 11.2 1.57 ± 0.12
0.03 ± 0.01 0.46 0.41 ± 0.03
0.16 ± 0.04 0.26 0.65 ± 0.06
10
Inoculum Feces Mixed liquor
7.3 ± 0.0 7.0 7.0 ± 0.0
– 184.2 –
9.2 ± 0.2 12.5 10.5 ± 0.1
7.8 ± 0.1 12.5 9.1 ± 0.1
0.19 ± 0.10 11.2 1.49 ± 0.15
0.05 ± 0.03 0.46 0.47 ± 0.02
0.18 ± 0.02 0.26 1.53 ± 0.08
11
Inoculum Feces Mixed liquor
7.1 ± 0.0 6.8 7.0 ± 0.0
– 220.5 –
9.2 ± 0.1 16.2 11.5 ± 0.0
7.8 ± 0.1 14.3 9.9 ± 0.0
0.18 ± 0.02 2.4 1.84 ± 0.19
0.03 ± 0.00 1.3 0.58 ± 0.04
0.11 ± 0.03 0.7 0.38 ± 0.03
12
Inoculum Feces Mixed liquor
7.2 ± 0.0 6.8 7.2 ± 0.0
– 220.5 –
10.1 ± 0.2 16.2 11.8 ± 0.1
8.4 ± 0.2 14.3 10.0 ± 0.1
0.11 ± 0.03 2.4 1.66 ± 0.38
0.01 ± 0.00 1.3 0.51 ± 0.12
0.01 ± 0.0 0.7 0.29 ± 0.07
determined weekly using a Hach Carle 400 AGC gas chromatograph (GC) (Chandler Engineering, Houston, TX) at 85 °C with a helium gas flow rate of 30 mL min 1. The GC calibration was performed weekly with a standard gas (27.3% CO2, 1.01% N2, 71.16% CH4, 0.53% H2S). Methane production is reported in normalized litres (NL CH4). Total cumulative CH4 yield was established at the end of each digestion cycle. Specific CH4 yield in each cycle was calculated as the ratio of CH4 produced over the mass of volatile solids (VS) fed to the reactor at the beginning of the cycle.
)
Propionate (g kg
1
TS (%)
)
Butyrate (g kg
1
)
2.4. Analytical methods Samples were collected from each bioreactor and analyzed weekly for volatile fatty acids (VFAs), total solids (TS), volatile solids (VS), pH, and alkalinity. Total chemical oxygen demand (TCOD) was determined before and after each treatment cycle. TCOD, TS, VS, alkalinity and pH were determined using standard methods (APHA, 1992). The cow feces samples taken for TCOD determination have been prepared using a blender and each sample has been
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taken under vigorous mixing to ensure a representative sample of the homogenized liquid and solid fractions is collected. VFAs concentration was measured with a Perkin Elmer gas chromatograph model 8310 (Perkin Elmer, Waltham, Mass.), equipped with a DB-FFAP high resolution column.
(11.3%). Similarly, wheat straw fibers composed of cellulose (38.61%), hemicellulose (25.14%) and lignin (7.3%). 3.1. Methane production The methane yield and stability of psychrophilic anaerobic bioreactors were evaluated in 12 successive cycles (252 days). The profiles of methane production expressed as specific methane yield (SMY; NL CH4 kg 1 VS fed) in the replicate bioreactors are shown in Fig. 2. The maximum SMYs calculated during the successive cycles are given in Table 3. Except for the SMY during cycle 5 (161.8 ± 11 NL CH4 kg 1 VS fed), the average SMY calculated increased gradually 171.9 ± 32 (cycle 1), 178.4 ± 19 (cycle 2), 185.5 ± 17 (cycle 3), and 224.4 ± 14 (cycle 4) NL CH4 kg 1 VS fed at OLR of 3.0 g TCOD kg 1 inoculum d 1 and TCL of 21 days (Fig 2 and Table 3). Tukey’s multiple comparison test at 95% confidence interval revealed that the SMY means during cycles 1–4 were not significantly different from each other likely because of the large standard deviations, however, the SMY during cycle 5 was significantly different from those obtained during the first four cycles is likely due to the variation in the quality of the cow feces fed (Table 2). Cow feces contains various types of material such as fibers (carbohydrate), protein, lipid, and amino acids. Notice that the theoretical methane potential per gram of VS is significantly greater for fat compared to proteins and carbohydrate (1014, 496, and 415 NL CH4 kg 1 VS, respectively). Although the physico-chemical characteristics shown in Table 2 (pH, TCOD, VS, TS, VFA were within the same range for cow feces used in the different cycles. The contents and biodegradability of carbohydrate, protein, lipid, amino acids, etc. might have been different and affected the measured specific methane yield from cycle to cycle. At OLR of 4.0 g TCOD kg 1 VS d 1, the SMY increased from 169 ± 13 (cycle 6), to 180 ± 17 (cycle 7), and to 221 ± 17 (cycle 8) with an overall average of 190 ± 27 NL CH4 kg 1 VS fed. At OLR of 5.0 g TCOD kg 1 inoculum d 1, significant increase in the SMY was observed with an overall average 222 ± 27.7 NL CH4 kg 1 VS fed. Tukey’s multiple comparison test at 95% confidence interval revealed that the averages of the SMY (based on total VS fed) during the cycles operated at OLR of 3.0, 4.0, and 5.0 g TCOD kg 1 inoculum d 1 were, in general, not significantly different from each other except for cycles 4, 8, and 12, respectively. Notice that the variation in the calculated SMY among the replicate bioreactors also decreased successively. The large value of the standard deviation of the SMY during cycle 1 is likely due to a gas leak which developed in one of the replicate bioreactor (R3, Fig 2.). Notice that after each increase in the OLR the SMY dropped slightly and then resumed its normal levels after two or three cycles; this indicates an adaptation phenomenon of the inoculum to the new OLR. The
2.5. Fiber analysis Cow feces is a complex substrate which contains fibers. The fiber content in cellulose, hemicellulose, and lignin was determined. Hemicellulose can be calculated as the difference between neutral detergent fiber (NDF) and acid detergent fiber (ADF), cellulose as the difference between acid detergent fiber and acid detergent lignin (ADL) (Bauer et al., 2009). 2.6. Statistical methods Data given in tables and shown in figures are the means and standard deviations. Statistical analysis was carried out using Minitab v. 14 statistical software (Minitab Inc., PA, USA). Data were analyzed by one-way ANOVA, followed by post hoc multiple comparisons (Tukey’s test) with a confidence level of 95% (i.e., p < 0.05). 3. Results and discussion
400
Specific CH 4 yield (NL CH 4 kg -1 VS fed )
350
Cycle number 1
2
3
4
5
6
7
8
9
10
11
12
5.0 4.0 3.0
300 250 200 150 100 50
Organic loading rate (g TCOD kg -1 inoculum d -1)
The percent of H2S in the biogas was less than 0.06% in all samples of gas analyzed during the successive cycles. The cow feces fed during the whole duration of the experiment contained fibers composed of cellulose (23.61%), hemicellulose (18.71%) and lignin
0 0 21 42 63 84 105 126 147168 189 210 231 252 273
Time (days) R1
R2
R3
Fig. 2. Specific methane yield profiles for the psychrophilic anaerobic digestion of cow feces.
Table 3 Rate and specific methane yield for the psychrophilic anaerobic digestion of cow feces. Retention time
Cycle
21 21 21 21 21 21 21 21 21 21 21 21
1 2 3 4 5 6 7 8 9 10 11 12
OLR (g TCOD kg 3 3 3 3 3 4 4 4 5 5 5 5
Feed TS (%) 1
inoculum d
1
) 12.7 12.8 13.3 13.2 13.2 13.2 13.2 12.5 12.5 12.5 16.5 16.5
SMY (NL CH4 kg
1
VS)
171.9 ± 32.2a 178.9 ± 19.4a 185.5 ± 17.0a 224.4 ± 13.7 b 161.8 ± 11.2a 168.8 ± 13.1a 180.1 ± 16.9a 220.7 ± 16.6 b 202.2 ± 13.0a,b 205.2 ± 7.1a,b 218.6 ± 9.6a,b 262.2 ± 21.4 c
SMY (NL CH4 kg
1
116.2 ± 21.8 135.8 ± 14.7 149.2 ± 13.7 136.8 ± 8.3 110.3 ± 7.6 115.1 ± 8.9 122.5 ± 11.5 134.3 ± 10.1 128.4 ± 8.3 130.3 ± 4.5 116.6 ± 5.1 154.6 ± 12.6
TCOD)
Rate of CH4 production (NL CH4 kg 1 VS d 1) 8.2 ± 1.5 8.5 ± 0.9 8.8 ± 0.8 10.7 ± 0.7 7.7 ± 0.5 8.0 ± 0.6 8.6 ± 0.8 10.5 ± 0.8 9.6 ± 0.6 9.8 ± 0.3 10.4 ± 0.5 12.5 ± 1.0
Note: SMY’s with different superscripts indicate that they are significantly different at 95% confidence level according to Tukey’s multiple comparison test.
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The average yield of 184.5 ± 24 NL CH4 kg 1 VS of cow feces (at OLR 3.0 g TCOD kg 1 inoculum d 1 or 2.12 g VSfed kg 1 inoculum d 1) obtained in this study after 21 days of psychrophilic (20 °C) incubation during 5 successive cycles is greater than the yield 161 NL CH4 kg 1 VS of dairy cattle feces (at OLR of 1.4 kg VS substrate kg 1 VS inoculum) reported by Møller et al. (2004) at 35 °C and a hydraulic retention time (HRT) of 40 days. For the same incubation time, Schäfer et al. (2006) reported a SMY of 85 L CH4 kg 1 VS from beef cattle manure in two-stage (hydrolysis–methanogensis) reactor with OLR of 6 and 3 kg VS m 3 d 1 for hydrolysis and methanogensis stages, respectively. The SMYs from cow feces at a TCL of 21 days in any of the PDAD 12 successive cycles obtained in this study are higher than the 135 and 164 L CH4 kg 1 VS obtained during mesophilic (30 °C) anaerobic digestion of dairy cattle manure reported by Somayaji and Khanna (1994) and Shyam (2002) at HRT of 40 and 50 days, respectively. The average SMY (184.5 ± 24 NL CH4 kg 1 VS) obtained at OLR 3.0 g TCOD kg 1 inoculum d 1 is greater than the 165 L CH4 kg 1 VS reported by Ahring et al. (2001) during thermophilic (65 °C) operation at OLR of 3.0 kg VS m 3 d 1 and HRT of 15 days. Interestingly, the average SMY (208.7 ± 8.7 NL CH4 kg 1 VS) obtained at OLR 5.0 g TCOD kg 1 inoculum d 1 is comparable to the 202 L CH4 kg 1 VS reported by Ahring et al. (2001) during thermophilic (55 °C) operation at the same OLR of 3.0 kg VS m 3 d 1 and HRT of 15 days. The average SMY obtained in this study (182.9 ± 16.9 NL CH4 kg 1 VS) at OLR of 3.0, 4.0, and 5.0 g TCOD kg 1 inoculum d 1 is less than those reported by Varel et al. (1980) [240–280 L CH4 kg 1 VS] at temperatures between 35 and 65 °C, HRT of 18 days, and OLR of 3.3 kg VS m 3 d 1 (Table 4). Compared to Varel’s et al. (1980) results for operations at 35–65 °C, the current study demonstrated a potential technology which minimises energy required for heating bioreactor. Achieving a stable anaerobic digestion of cow manure at psychrophilic condition over long-term operation is a significant improvement with potential market in cold climate area. The next objectives of this research would be shortening the retention time and increasing the organic loading rate. The specific methane yields obtained in this study provide evidence that PDAD of cow feces is practically feasible and as efficient as mesophilic and thermophilic anaerobic digestion given that a well-acclimatized inoculum is developed and maintained.
large values of the standard deviations of the SMY during cycle 2, 3, 6, 7, 8, and 9 are likely due to variation in the composition of the cow feces fed, slight gas leak which might have been undetected, or experimental error. Generally, the pattern of performance consistency of the replicate bioreactors indicates a stable reproducible process during the 252 days of operation because of the development of well-adapted inoculum to the dry psychrophilic conditions. Usually, two segments are recognized in the SMY curve during batch anaerobic digestion: an initial exponential phase followed by a slowdown phase (plateau) in CH4 production for the rest of the incubation period. The 12 cycles showed only the initial straight line part of the CH4 production curve because the TCL was limited to 21 days which means that not all the substrate fed was completely digested during that TCL. However, in the sequence batch reactor (SBR) the solid retention time (SRT) is decoupled from the TCL; in this study the SRT maintained was 75.1 ± 7.7, 65.2 ± 1.6, and 61.5 ± 4.2 days at OLR of 3.0, 4.0, and 5.0 g TCOD kg 1 inoculum d 1 (Table 1), respectively, which allowed the complete degradation of the substrate fed. The soluble fraction of cow feces is composed of carbohydrates, amino acids, proteins, and fats while the dry matter is composed of fibers containing mainly cellulose (23.61%), hemicellulose (18.71%) and lignin (11.3%), and some forage proteins. Cellulose requires longer retention time than that required for the soluble carbohydrate, fats and proteins degradation by the anaerobic consortia of microorganisms. Cellulose hydrolysis has been shown to be the rate limiting step (Noike et al., 1985) particularly when the substrate is solid or in particulate form (Myint and Nirmalakhandan, 2006). Notice that an increase in the daily specific CH4 production rate can be observed (Table 3). The daily average specific CH4 production rate (NL CH4 kg 1 VS d 1) of the replicate bioreactors was 8.2 ± 1.5 (cycle 1), 8.5 ± 0.9 (cycle 2), 8.8 ± 0.8 (cycle 3), 10.7 ± 0.7 (cycle 4), 7.7 ± 0.5 (cycle 5), 8.0 ± 0.6 (cycle 6), 8.6 ± 0.8 (cycle 7), 10.5 ± 0.8 (cycle 8), 9.6 ± 0.6 (cycle 9), 9.8 ± 0.3 (cycle 10), 10.4 ± 0.5 (cycle 11), and 12.5 ± 1.0 (cycle 12). The overall average of methane production rate is 8.8 ± 1.1, 9.0 ± 1.3, and 10.6 ± 0.4 NL CH4 kg 1 VS d 1 at OLR of 3.0, 4.0, and 5.0 g TCOD kg 1 inoculum d 1, respectively. Notice that although the trend of methane production rate increased successively with the increase in the OLR no conclusion can be deducted because, in general, there were no significant statistical differences detected by Tukey’s multiple comparisons test (Table 3). No relevant data is available in the accessible literature on the performance of psychrophilic dry anaerobic digestion (PDAD); therefore, the results have been compared to the performance of mesophilic and thermophilic DAD of various substrates (Table 4).
Table 4 Methane yield (L CH4 kg Substrate
a b
1
3.2. Volatile fatty acids (VFAs) production Profiles of acetic, propionic, and butyric acids produced during the successive cycles of PDAD with increasing total solids percent
VS) of cow feces at different temperatures.
OLR (kg VS m 3 d 1) unless indicated otherwise
Dairy cow manure Dairy cow manure Dairy cow manure Cattle manure Cattle manure Cattle manure Dairy manure Cattle manure Cattle manure
3.0 g TCOD l 4.0 g TCOD l 5.0 g TCOD l 3.3 3.0 2.0
Dairy cattle manure Cow manure Cow manure
NRa
1 1 1
The test was carried out as per ISO 11734. With basal medium.
HRT (day)
Temperature (°C) 20
21 21 21 18 15 20 30 40 50 100 100 16 90
25
References 30
35
40
50
55
60
65
184.5 ± 24 195.2 ± 2.1 208.7 ± 8.7 260
270
250 260
242.7 135 164 230 148 ± 41 128 260b
280 202
270 230
240 165
This study This study This study Varel et al. (1980) Ahring et al. (2001) El-Mashad et al. (2004) Labatut et al. (2011) Somayaji and Khanna (1994) Shyam (2002) Shyam (2002) Møller et al. (2004) Preeti Rao and Seenayya (1994) Güngör-Demirci and Demirer (2004)
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15
3000 1
2
3
4
5
6
7
8
9
10
11
Cycle number
12
Total solids (%)
Acetate (mg L-1)
Cycle number
2500 2000 1500 1000 500
1
2
3
4
5
6
7
8
9
10 11 12
10
0 0
21
42
63
84 105 126 147 168 189 210 231 252 273
5 0
Time (days)
21
42
63
84 105 126 147 168 189 210 231 252 273
Time (days) Propionate (mg L-1)
3000
R1
Cycle number
2500
1
2
3
4
5
6
7
8
9
10
11
12
2000
1000 500 0
21
42
63
84 105 126 147 168 189 210 231 252 273
Time (days) 3000
Butyrate (mg L-1)
R3
Fig. 5. Total solid profile for the cow feces psychrophilic dry anaerobic digestion.
1500
0
Cycle number
2500
1
2
3
4
5
6
7
8
9
10
11
12
2000 1500 1000
The profile of propionic acid in the replicate bioreactots was similar to that of acetic acid. Butyric acid peaked also to levels between 1000 and 1500 mg L 1 after feedings and was consumed within a week to levels within 250 mg L 1. The concentrations of other volatile fatty acids (isobutyric-, iso-valeric-, and valeric-acid) were less than 100 mg L 1 immediately after feeding and less than 50 mg L1 during the remaining time of the cycles. The profiles of the VFAs concentration and the increase in methane yield during the successive cycles indicate that acetogenic and methanogenic reactions proceeded fairly well. The relative stability of the pH profile between 7.0 and 7.5 (Fig. 4) was due to the high alkalinity (7500–10,900 mg CaCO3 L 1) of the mixed liquor and the absence of VFAs accumulation.
500
3.3. Solids reduction
0 0
21
42 63
84 105 126 147 168 189 210 231 252 273
Time (days)
R1
R2
R3
Fig. 3. Volatile fatty acids profiles for the cow feces psychrophilic dry anaerobic digestion. A – acetic, B – propionic, C – butyric acids.
in the feed are shown in Fig. 3. The profiles of VFAs produced in the triplicate bioreactors were almost identical. Throughout the successive cycles, acetic acid concentration peaked immediately after feeding to levels between 1000 and 2000 mg L 1 but was consumed within a week in all bioreactors to about 500 mg L 1 and to within 100 ± 50 mg L 1 after two weeks indicating that methanogensis reaction from acetate was not a rate limiting step. Similarly, propionic acid peaked to levels between 500 and 600 mg L 1 after feedings and was consumed within a week to levels close to the detection limits of the instrument (25 ± 10 mg L 1).
9.0 8.5
pH
R2
Cycle number 1
8.0 7.5 7.0
2
3
4
5
6
7
8
9
10
11 12
6.5 6.0 5.5
The profiles of the total solids in the bioreactors during the successive cycles are shown in Fig. 5. The profiles of the TS were identical. Generally, the total solids contents of the inoculum increased slightly from cycle to cycle likely due to the increase in the microbial biomass. Notice that the percentage of total solids reduction during the individual cycles was about 3 ± 1.5. 3.4. Economic considerations Obviously no mixing and heating for the bioreactor increases the net energy yield per unit volume of the reactor. Huchel et al. (2006) reported that 564.4 mol CH4 m 3 substrate day 1 was required to maintain the temperature of an anaerobic digester at 37 °C when handling a substrate of 0.96% TS with an HRT of 20 days. Furthermore, increasing the total solids from 1% to 16% means around 94% reduction in the required volume of the bioreactor. Investment cost decreases by 70% when the reactor volume is reduced by 50% (Oosterkamp, 2011). Therefore, the PDAD offers the advantage of the combined saving in cost of construction and energy expenses of heating and mixing during operation. Ultimately, psychrophilic dry anaerobic digester could contribute to more energy output compared to mesophilic or thermophilic liquid or dry digester. 4. Conclusions
5.0 0
21
42 63
84
105 126 147 168 189 210 231 252 273
Time (days)
R1
R2
R3
Fig. 4. pH profile for the cow feces psychrophilic dry anaerobic digestion.
An average specific methane yield (SMY) of 222 ± 27.7 NL CH4 kg 1 VS fed has been achieved during the last four successive cycles at organic loading rate 5.0 g TCOD kg 1 inoculum d 1 and treatment cycle length (TCL) of 21 days. A maximum SMY of 262 ± 21.4 NL CH4 kg 1 VS fed (137 ± 8 NL CH4 kg 1 COD fed) with a maximum CH4 production rate of 10.7 ± 0.7 NL CH4 kg 1 VS d 1 have been accomplished at OLR of 5.0 g TCOD kg 1 inoculum d 1.
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The SMY and CH4 production rate increased gradually and stabilized during the long-term operation. References Abbassi-Guendouz, A., Brockmann, D., Trably, E., Dumas, C., Delgenès, J., Steyer, J., Escudié, R., 2012. Total solids content drives high solid anaerobic digestion via mass transfer limitation. Bioresour. Technol. 111, 55–61. Ahn, H.K., Smith, M.C., Kondrad, S.L., White, J.W., 2010. Evaluation of biogas production potential by dry anaerobic digestion of switchgrass-animal manure mixtures. Appl. Biochem. Biotechnol. 160, 965–975. Ahring, B.K., Ibrahim, A.A., Mladenovska, Z., 2001. Effect of temperature increase from 55 to 65 °C on performance and microbial population dynamics of an anaerobic reactor treating cattle manure. Water Res. 35, 2446–2452. APHA, 1992. Standard Methods for the Examination of Water and Wastewater, 18 ed. American Public Health Association, Washington, DC. Bauer, A., Bösch, P., Friedl, A., Amon, T., 2009. Analysis of methane potentials of steam-exploded wheat straw and estimation of energy yields of combined ethanol and methane production. J. Biotechnol. 142, 50–55. Baylis, K., Paulson, N.D., 2011. Potential for carbon offsets from anaerobic digesters in livestock production. Anim. Feed Sci. Technol. 166–167, 446–456. Challen Urbanic, J.M., VanOpstal, B., Parker, W., 2011. Anaerobic digestion of the organic fraction of municipal solid waste (OFMSW)—full scale vs laboratory results. J. Solid Waste Technol. Manage. 37, 33–39. Di Maria, F., Gigliotti, G., Sordi, A., Micale, C., Massaccesi, L., 2012. Start up of a preindustrial scale solid state anaerobic digestion cell for the co-treatment of animal and agricultural residues. In: ECOS 2012 – The 25th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, Perugia, Italy. El-Mashad, H.M., Zeeman, G., Van Loon, W.K.P., Bot, G.P.A., Lettinga, G., 2004. Effect of temperature and temperature fluctuation on thermophilic anaerobic digestion of cattle manure. Bioresour. Technol. 95, 191–201. Güngör-Demirci, G., Demirer, G.N., 2004. Effect of initial COD concentration, nutrient addition, temperature and microbial acclimation on anaerobic treatability of broiler and cattle manure. Bioresour. Technol. 93, 109–117. Hindrichsen, I.K., Wettstein, H.R., Machmuller, A., Jorg, B., Kreuzer, M., 2005. Effect of the carbohydrate composition of feed concentratates on methane emission from dairy cows and their slurry. Environ. Monit. Assess. 107, 329–350. Hofmann, N., Beaulieu, M.S., 2006. A Geographical Profile of Manure Production in Canada, 2001. Statistics Canada, Agriculture Division. Huchel, J., Van Dixhorn, L., Podwell, T., 2006. Anaerobic digesters heated by direct steam injection: experience and lessons learned. Proc. Water Environ. Fed. 2006, 407–414. IPCC, 1996. Revised IPCC guidelines for national greenhouse gas inventories. Reference Manual. BRACKNELL, UK. Kusch, S., Oechsner, H., Jungbluth, T., 2008. Biogas production with horse dung in solid-phase digestion systems. Bioresour. Technol. 99, 1280–1292. Labatut, R.A., Angenent, L.T., Scott, N.R., 2011. Biochemical methane potential and biodegradability of complex organic substrates. Bioresour. Technol. 102, 2255– 2264. http://dx.doi.org/10.1016/j.biortech.2010.10.035.
Li, D., Ma, L., Yuan, Z., Li, L., Sun, Y., 2006. Experimental study on dry anaerobic digestion of rice straw at ambient temperature in South China. Nongye Gongcheng Xuebao/Trans. Chinese Soc. Agric. Eng. 22, 176–179. Li, Y., Park, S.Y., Zhu, J., 2011. Solid-state anaerobic digestion for methane production from organic waste. Renew. Sustain. Energy Rev. 15, 821–826. Luning, L., Van Zundert, E.H.M., Brinkmann, A.J.F., 2003. Comparison of dry and wet digestion for solid waste. Water Sci. Technol. 48 (4), 5–20. Massé, D., Gilbert, Y., Topp, E., 2011. Pathogen removal in farm-scale psychrophilic anaerobic digesters processing swine manure. Bioresour. Technol. 102, 641– 646. Massé, D.I., Croteau, F., Masse, L., 2007. The fate of crop nutrients during digestion of swine manure in psychrophilic anaerobic sequencing batch reactors. Bioresour. Technol. 98, 2819–2823. Massé, D.I., Droste, R.L., Kennedy, K.J., Patni, N.K., Munroe, J.A., 1997. Potential for the psychrophilic anaerobic treatment of swine manure using a sequencing batch reactor. Can. Agric. Eng. 39, 25–33. Massé, D.I., Patni, N.K., Droste, R.L., Kennedy, K.J., 1996. Operation strategies for psychrophilic anaerobic digestion of swine manure slurry in sequencing batch reactors. Can. J. Civ. Eng. 23, 1285–1294. Møller, H.B., Sommer, S.G., Ahring, B.K., 2004. Methane productivity of manure, straw and solid fractions of manure. Biomass Bioenergy 26, 485–495. Myint, M., Nirmalakhandan, N., 2006. Evaluation of first-order, second-order, and surface-limiting reactions in anaerobic hydrolysis of cattle manure. Environ. Eng. Sci. 23, 970–980. Noike, T., Endo, G., Juu, E.C., Yaguchi, J.I., Matsumoto, J.I., 1985. Characteristics of carbohydrate degradation and the rate-limiting step in anaerobic digestion. Biotechnol. Bioeng. 27, 1482–1489. Oosterkamp, W.J., 2011. Enhancement of biogas from straw and manure – an annotated bibliography. . Preeti Rao, P., Seenayya, G., 1994. Improvement of methanogenesis from cow dung and poultry litter waste digesters by addition of iron. World J. of Microb. Biot. 10, 211–214. http://dx.doi.org/10.1007/bf00360890. Ramasamy, E.V., Abbasi, S.A., 2000. High-solids anaerobic digestion for the recovery of energy from municipal solid waste (MSW). Environ. Technol. 21, 345–349. Saady, N.M.C., Massé, D.I., 2013. Psychrophilic anaerobic digestion of lignocellulosic biomass: a characterization study. Bioresour. Technol. 142, 663–671. Schäfer, W., Lehto, M., Teye, F., 2006. Dry anaerobic digestion of organic residues onfarm – a feasibility study. Agrifood Research Reports 77. MTT Agrifood Research Finland. . Shyam, M., 2002. Agro-residue-based renewable energy technologies for rural development. Energy Sust. Dev. 6, 37–42. Somayaji, D., Khanna, S., 1994. Biomethanation of rice and wheat straw. World J. Microbiol. Biotechnol. 10, 521–523. Sun, G.C., Wu, Y.Z., Sha, S.J., Liu, K.X., 1987. Dry digestion of crop wastes: studies on dry anaerobic digestion with agricultural wastes. Biol. Wastes 20, 291–302. Varel, V.H., Hashimoto, A.G., Chen, Y.R., 1980. Effect of temperature and retention time on methane production from beef cattle waste. Appl. Environ. Microbiol. 40, 217–222. Wen, Z., Liao, W., Chen, S., 2004. Hydrolysis of animal manure lignocellulosics for reducing sugar production. Bioresour. Technol. 91, 31–39.
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Psychrophilic dry anaerobic digestion of dairy cow feces: Long-term operation Daniel I. Massé ⇑, Noori M. Cata Saady Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, 2000 College Street, Sherbrooke, Québec, Canada J1M 0C8
a r t i c l e
i n f o
Article history: Received 20 March 2014 Accepted 30 October 2014 Available online 27 November 2014 Keywords: Psychrophilic Dry Anaerobic Manure Straw
a b s t r a c t This paper reports experimental results which demonstrate psychrophilic dry anaerobic digestion of cow feces during long-term operation in sequence batch reactor. Cow feces (13–16% total solids) has been anaerobically digested in 12 successive cycles (252 days) at 21 days treatment cycle length (TCL) and temperature of 20 °C using psychrotrophic anaerobic mixed culture. An average specific methane yield (SMY) of 184.9 ± 24.0, 189.9 ± 27.3, and 222 ± 27.7 NL CH4 kg 1 of VS fed has been achieved at an organic loading rate of 3.0, 4.0, and 5.0 g TCOD kg 1 inoculum d 1 and TCL of 21 days, respectively. The corresponding substrate to inoculum ratio (SIR) was 0.39 ± 0.06, 0.48 ± .02, 0.53 ± 0.05, respectively. Average methane production rate of 10 ± 1.4 NL CH4 kg 1 VS fed d 1 has been obtained. The low concentration of volatile fatty acids indicated that hydrolysis was the reaction limiting step. Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
1. Introduction Current livestock production practices generate large amount of organic wastes in the form of manure, feed refusal, spoiled feeds and livestock mortalities. These wastes need to be stabilized prior valorisation. Livestock manure is responsible for about 6.6% and 1.1% of total greenhouse gas (GHG) emissions in the United States and Canada, respectively (Baylis and Paulson, 2011). Storage of dairy cow manure slurry in cool climate regions in the European countries and the U.S. contributes about 12% and 23% of the total CH4 emission, respectively (Hindrichsen et al., 2005; IPCC, 1996). Cattle alone produce between 86% and 75% of the manure generated by Canadian and USA livestock, respectively (Hofmann and Beaulieu, 2006; Wen et al., 2004). Management of animal manure translates into cost which could be compensated by converting the biodegradable components of manure into biofuel (methane) through anaerobic digestion with positive economic return. Currently, on-farm applications of manure anaerobic digestion are under intensive research to introduce technological advancement which decrease the environmental impact, increase bioenergy yield, and reduce cost. Fresh cow feces has a total solids content of about 12–16% and requires dilution before conventional wet anaerobic digestion to decrease the solids content in order to allow liquid handling and ⇑ Corresponding author. Tel.: +1 819 780 7128; fax: +1 819 564 5507. E-mail address: [email protected] (D.I. Massé). http://dx.doi.org/10.1016/j.wasman.2014.10.032 0956-053X/Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
processing (El-Mashad et al., 2004). However, dilution substantially increases the digester volume. Increasing the total solids of the substrate fed to anaerobic digestion (AD) is an important engineering design objective in order to decrease the bioreactor volume as well as the volume of the bioreactor effluent to store and to apply on the land (Luning et al., 2003), increase the energy yield per bioreactor unit volume, and reduce its construction and operation costs. Psychrophilic AD of cow feces has been developed recently at Agriculture and Agri-Food, Dairy and Swine Research and Development Centre (DSRDC) in Sherbrooke, Quebec–Canada (Massé et al., 1997, 1996). In comparison to mesophilic and thermophilic wet anaerobic digestion, psychrophilic dry AD of cow feces at its TS (13–16%) offers more advantages such less energy input for mixing and heating, increase the specific volumetric energy output of bioreactor. Cow feces is a complex substrate composed of soluble and particulate or fibrous components. Usually, it contains carbohydrates (cellulosic and hemicellulosic fibers and volatile fatty acids), lipids and fats, and proteins. Approximately 40–50% of the volatile solids (VS) in dairy manure is biodegradable lingocellulosics biomass containing reduced carbon which can be converted to CH4 (AbbassiGuendouz et al., 2012). Saady and Massé (2013) have shown that psychrophilic anaerobic digestion of particulate cellulose and hemicellulose (xylan) is feasible in static batch system. In addition, they demonstrated the feasibility of psychrophilic anaerobic digestion of dilute cow feces. Mesophilic and thermophilic dry anaerobic digestion (DAD) of the organic fraction of municipal solid wastes
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(OFMSW) have been demonstrated (Challen Urbanic et al., 2011; Li et al., 2011; Ramasamy and Abbasi, 2000). Generally, mesophilic or thermophilic DAD of high-solids agricultural wastes is relatively a new biotechnology (Ahn et al., 2010; Di Maria et al., 2012; Kusch et al., 2008). Farmers in cold-climate regions would be interested in psychrophilic DAD of livestock manure because of its obvious advantages over mesophilic and thermophilic operations. Unfortunately, lack of research and development of DAD of animal manure is behind its current low acceptance. Therefore, more work is needed to demonstrate its feasibility, to optimize its operation, to evaluate its economic benefits, develop, and adopt more suitable technologies. Ultimately, to make on-farm DAD acceptable it has to be an integral part of farm waste management systems (Schäfer et al., 2006). DAD of rice straw resulted in a greater specific biogas yield at a shorter treatment cycle compared to a wet process (Li et al., 2006); hence, DAD was considered more efficient than wet anaerobic digestion for rice straw (Sun et al., 1987). Luning et al. (2003) reported that dry and wet anaerobic digestion of the organic solid waste fraction were identical based on the specific gas yield and the organic loading rates. The psychrophilic anaerobic digestion (PAD) in sequential batch reactor (SBR), to digest agricultural wastes with TS contents lower than 12%, such as swine manure, has been developed at Agriculture and Agri-Food, Dairy and Swine Research and Development Centre (DSRDC) in Sherbrooke, Quebec–Canada. The SBR offers a simple operating system consisting of four consecutive steps for wet anaerobic digestion: fill, react, settle, and draw. For a dry anaerobic digestion where no liquid–solid phase separation occurs, the settle step is eliminated. Another modification is the draw step where digestate rather than decant is withdrawn as effluent. The SBR is charged with the feed (cow feces and wheat straw) during the fill step. The length of the react period should be sufficient to meet pre-determined treatment objectives (specific methane yield, VS and TCOD removal, etc.). During the draw period, some of the digestate is removed as effluent and some is used as inoculum for the next cycle. PAD offers several advantages such as it successfully reduces odors, decreases the organic pollution load by more than 70% (Massé et al., 1996), produces high quality biogas, significantly diminishes pathogens survival (Massé et al., 2011), and improves the agronomic value of digestate (Massé et al., 2007). The process offers the competitive advantages of great stability, robustness, maximum performance, and minimum supervision (Massé et al., 1997). Research is still needed to adapt this kind of process for agricultural wastes with high solids such as dairy manure with bedding or solid fraction of dairy manure without bedding. With the addition of bedding, the manure is usually under solid or semi-solid state (15–20% TS). To the best of the author’s knowledge, no previous reports are available in the accessible literature on psychrophilic DAD. This is the first report on successful psychrophilic (20 °C) dry anaerobic digestion of cow feces over long-term operation in sequence batch redactor. However, it still needs more development and optimization studies. The principal objective of this study was to assess the performance of psychrophilic dry anaerobic digestion of dairy cow manure (TS 13–16%) in long-term operation using sequence batch bioreactor.
reactor to the next feeding) in a temperature controlled room (20 °C). The reactors were fitted with two gas lines; one for purging with nitrogen gas immediately after feeding the substrate to maintain the anaerobic condition, and the second to release and measure the biogas produced into the bioreactor. A schematic showing the details of the reactor is shown in Fig. 1. A total of 12 cycles have been conducted at organic loading rate (OLR, g TCOD kg 1 inoculum d 1) of 3.0 (5 cycles), 4.0 (3 cycles), and 5.0 (4 cycles). The mass of inoculum and the cow feces fed to each bioreactor at the beginning of the successive cycles and the organic loading rate (OLR) are given in Table 1. Physico-chemical characteristics of the inoculum and substrates before feeding bioreactors were analyzed and are given in Table 2. The initial inoculum which has been used to initiate the reaction in the first cycle was obtained from a semi-industrial scale (11.4 m3; TS = 9%) psychrophilic (20 °C) anaerobic reactor fed with fresh dairy manure (12% TS), and operated as a SBR. Starting from the second cycle forwards, 6 kg of the digestate from the previous cycle has been used as inoculum for the next cycle in each reactor. Fresh feces from dairy cows was collected at the experimental farm of the DSRDC. Feces were collected on wood boards, before getting in contact with urine and bedding, transferred into a plastic drum, stored at 4 °C, before being fed to the reactors. At the TS content of the digesting matrix maintained during the entire operation no solid–liquid separation has been observed thus no decanting step was required in the operation of the SBR reactors. At the end of each cycle, the effluent has been withdrawn manually using a scoop since the digestate was in solid state. The remaining contents of the replicate reactors have been mixed, homogenized, and each empty reactor received 6 kg of the digestate as inoculum for the next cycle. The substrate and inoculum has been mixed manually for 5 min during feeding. Every week the content of the bioreactor is mixed manually for 5 min before sampling the content to ensure a homogenous and representative sample is taken. No mixing took place during other time of incubation; therefore, the process can be considered as a static dry anaerobic digestion. 2.2. Organic loading rate Organic loading rate (OLR) has been calculated based on the masses of TCOD and VS and the composition of the substrate fed (Table 2). OLR was expressed in g of VS fed per kg inoculum per day and g of TCOD fed per kg of inoculum per day. The substrate to inoculum ratio (SRI; based on mass) ranged between 0.36 and 0.48. 2.3. Biogas measurement Biogas production was measured daily with calibrated wet tip gas meters while the biogas components (CH4, H2S, CO2) were
7 6
1 1- 40 L plastic barrel 2- Barrel lid with metal lock
5
3- Gas release line 4- N2 purging line with valve
2. Materials and methods
5- Substrate-inoculum mixture
2.1. Experimental setup Triplicate 40-L cylindrical barrels bioreactors were set-up and operated as pseudo sequential batch reactors (PSBR) at a Treatment Cycle Length (TCL) of 21 days (measured from feeding the
2 4
3
6- Head space 7- Wet tip gas meter
Fig. 1. A schematic of the sequence batch reactor.
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Table 1 Organic loading rate and total solids of the feed. Cycle Inoculum (kg) Feces (kg) Substrate TS (%) Substrate to Solids retention Substrate TS (%) Organic loading rate time (SRT) (day) inoculum TCOD fed (g) VS fed (g) g TCOD kg ratio (SIR)
1
inoculum d
1
g VS fed kg inoculum d
1 2 3 4 5
6 6 6 6 6
2.35 2.8 2.554 1.94 2.185
12.7 12.8 13.3 13.2 13.2
0.39 0.47 0.43 0.32 0.36
74.6 66.0 70.3 85.9 78.7
12.7 12.8 13.3 13.2 13.2
389.4 413.3 378.2 378.7 379.1
267.9 319.2 303.9 230.9 257.8
3.1 3.3 3.0 3.0 3.0
2.1 2.5 2.4 1.8 2.0
6 7 8
6 6 6
2.913 2.913 2.739
13.2 13.2 12.5
0.49 0.49 0.46
64.3 64.3 67.0
13.2 13.2 12.5
505.4 533.7 504.5
343.7 343.7 306.8
4.0 4.2 4.0
2.7 2.7 2.4
9 10 11 12
6 6 6 6
3.419 3.419 2.857 2.857
12.5 12.5 16.5 16.5
0.57 0.57 0.48 0.48
57.9 57.9 65.1 65.1
12.5 12.5 16.5 16.5
629.8 629.8 630.0 630.0
382.9 382.9 408.6 408.6
5.0 5.0 5.0 5.0
3.0 3.0 3.2 3.2
1 1
Table 2 Physicochemical characteristics of the inoculum, cow feces and mixed liquor at the beginning of each digestion cycle. Cycle
Substrate
pH
TCOD (g kg
1
Inoculum Feces Mixed liquor
7.2 6.0 6.9 ± 0.0
2
Inoculum Feces Mixed liquor
3
1
)
1
VS (%)
Acetate (g kg
– 165.7 –
10.3 13.2 10.5 ± 0.2
8.8 11.8 9.0 ± 0.2
0.19 ± 0.02 3.4 1.95 ± 0.02
0.03 ± 0.0 0.88 0.61 ± 0.01
0.0 ± 0.0 1.39 0.57 ± 0.01
7.2 ± 0.0 5.9 6.9 ± 0.1
– 147.6 –
9.0 ± 0.2 12.8 10.2 ± 0.3
7.5 ± 0.2 11.4 8.7 ± 0.3
1.13 ± 0.09 2.8 1.61 ± 0.10
0.39 ± 0.03 1.3 0.50 ± 0.03
0.41 ± 0.01 0.8 1.10 ± 0.10
Inoculum Feces Mixed liquor
7.2 ± 0.1 5.9 7.0 ± 0.0
– 148.1 –
9.1 ± 0.3 13.3 10.2 ± 0.2
7.6 ± 0.3 11.9 8.7 ± 0.3
0.96 ± 0.05 3.4 1.50 ± 0.07
0.38 ± 0.02 0.88 0.50 ± 0.04
0.26 ± 0.03 0.11 0.26 ± 0.02
4
Inoculum Feces Mixed liquor
7.2 ± 0.1 6.0 7.0 ± 0.0
– 195.2 –
9.1 ± 0.1 13.24 10.4 ± 0.5
7.4 ± 0.3 11.9 8.9 ± 0.5
0.23 ± 0.02 3.82 1.47 ± 0.06
0.00 ± 0.00 1.15 0.38 ± 0.03
0.00 ± 0.00 0.80 0.13 ± 0.00
5
Inoculum Feces Mixed liquor
7.5 ± 0.0 6.6 7.3 ± 0.0
– 173.5 –
8.7 ± 0.1 13.2 9.9 ± 0.4
7.2 ± 0.2 11.8 8.4 ± 0.4
0.24 ± 0.03 4.0 0.99 ± 0.63
0.0 ± 0.0 1.05 0.28 ± 0.19
0.20 ± 0.05 0.65 0.10 ± 0.09
6
Inoculum Feces Mixed liquor
– 6.6 7.2 ± 0.2
– 173.5 –
9.3 ± 0.2 12.8 10.4 ± 0.1
7.6 ± 0.3 11.3 8.9 ± 0.1
0.13 ± 0.02 4.3 1.46 ± 0.09
0.02 ± 0.0 1.10 0.38 ± 0.05
0.0 ± 0.0 0.78 0.64 ± 0.09
7
Inoculum Feces Mixed liquor
7.2 ± 0.0 6.5 7.0 ± 1.0
– 183.2 –
9.3 ± 0.2 12.8 10.8 ± 0.5
7.7 ± 0.3 11.3 9.3 ± 0.4
0.14 ± 0.01 3.5 1.40 ± 0.16
0.02 ± 0.01 1.5 0.49 ± 0.07
0.00 ± 0.00 2.36 0.83 ± 0.10
8
Inoculum Feces Mixed liquor
7.3 ± 0.0 7.0 7.1 ± 0.0
– 184.2 –
9.1 ± 0.1 12.5 10.6 ± 0.7
7.6 ± 0.1 11.2 9.1 ± 0.7
0.18 ± 0.03 1.54 1.54 ± 0.12
0.05 ± 0.02 0.46 0.40 ± 0.03
0.05 ± 0.01 0.26 0.18 ± 0.02
9
Inoculum Feces Mixed liquor
7.3 ± 0.1 7.0 7.0 ± 0.0
– 184.2 –
9.0 ± 0.0 12.5 10.8 ± 0.2
7.5 ± 0.1 12.5 9.3 ± 0.2
0.16 ± 0.03 11.2 1.57 ± 0.12
0.03 ± 0.01 0.46 0.41 ± 0.03
0.16 ± 0.04 0.26 0.65 ± 0.06
10
Inoculum Feces Mixed liquor
7.3 ± 0.0 7.0 7.0 ± 0.0
– 184.2 –
9.2 ± 0.2 12.5 10.5 ± 0.1
7.8 ± 0.1 12.5 9.1 ± 0.1
0.19 ± 0.10 11.2 1.49 ± 0.15
0.05 ± 0.03 0.46 0.47 ± 0.02
0.18 ± 0.02 0.26 1.53 ± 0.08
11
Inoculum Feces Mixed liquor
7.1 ± 0.0 6.8 7.0 ± 0.0
– 220.5 –
9.2 ± 0.1 16.2 11.5 ± 0.0
7.8 ± 0.1 14.3 9.9 ± 0.0
0.18 ± 0.02 2.4 1.84 ± 0.19
0.03 ± 0.00 1.3 0.58 ± 0.04
0.11 ± 0.03 0.7 0.38 ± 0.03
12
Inoculum Feces Mixed liquor
7.2 ± 0.0 6.8 7.2 ± 0.0
– 220.5 –
10.1 ± 0.2 16.2 11.8 ± 0.1
8.4 ± 0.2 14.3 10.0 ± 0.1
0.11 ± 0.03 2.4 1.66 ± 0.38
0.01 ± 0.00 1.3 0.51 ± 0.12
0.01 ± 0.0 0.7 0.29 ± 0.07
determined weekly using a Hach Carle 400 AGC gas chromatograph (GC) (Chandler Engineering, Houston, TX) at 85 °C with a helium gas flow rate of 30 mL min 1. The GC calibration was performed weekly with a standard gas (27.3% CO2, 1.01% N2, 71.16% CH4, 0.53% H2S). Methane production is reported in normalized litres (NL CH4). Total cumulative CH4 yield was established at the end of each digestion cycle. Specific CH4 yield in each cycle was calculated as the ratio of CH4 produced over the mass of volatile solids (VS) fed to the reactor at the beginning of the cycle.
)
Propionate (g kg
1
TS (%)
)
Butyrate (g kg
1
)
2.4. Analytical methods Samples were collected from each bioreactor and analyzed weekly for volatile fatty acids (VFAs), total solids (TS), volatile solids (VS), pH, and alkalinity. Total chemical oxygen demand (TCOD) was determined before and after each treatment cycle. TCOD, TS, VS, alkalinity and pH were determined using standard methods (APHA, 1992). The cow feces samples taken for TCOD determination have been prepared using a blender and each sample has been
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taken under vigorous mixing to ensure a representative sample of the homogenized liquid and solid fractions is collected. VFAs concentration was measured with a Perkin Elmer gas chromatograph model 8310 (Perkin Elmer, Waltham, Mass.), equipped with a DB-FFAP high resolution column.
(11.3%). Similarly, wheat straw fibers composed of cellulose (38.61%), hemicellulose (25.14%) and lignin (7.3%). 3.1. Methane production The methane yield and stability of psychrophilic anaerobic bioreactors were evaluated in 12 successive cycles (252 days). The profiles of methane production expressed as specific methane yield (SMY; NL CH4 kg 1 VS fed) in the replicate bioreactors are shown in Fig. 2. The maximum SMYs calculated during the successive cycles are given in Table 3. Except for the SMY during cycle 5 (161.8 ± 11 NL CH4 kg 1 VS fed), the average SMY calculated increased gradually 171.9 ± 32 (cycle 1), 178.4 ± 19 (cycle 2), 185.5 ± 17 (cycle 3), and 224.4 ± 14 (cycle 4) NL CH4 kg 1 VS fed at OLR of 3.0 g TCOD kg 1 inoculum d 1 and TCL of 21 days (Fig 2 and Table 3). Tukey’s multiple comparison test at 95% confidence interval revealed that the SMY means during cycles 1–4 were not significantly different from each other likely because of the large standard deviations, however, the SMY during cycle 5 was significantly different from those obtained during the first four cycles is likely due to the variation in the quality of the cow feces fed (Table 2). Cow feces contains various types of material such as fibers (carbohydrate), protein, lipid, and amino acids. Notice that the theoretical methane potential per gram of VS is significantly greater for fat compared to proteins and carbohydrate (1014, 496, and 415 NL CH4 kg 1 VS, respectively). Although the physico-chemical characteristics shown in Table 2 (pH, TCOD, VS, TS, VFA were within the same range for cow feces used in the different cycles. The contents and biodegradability of carbohydrate, protein, lipid, amino acids, etc. might have been different and affected the measured specific methane yield from cycle to cycle. At OLR of 4.0 g TCOD kg 1 VS d 1, the SMY increased from 169 ± 13 (cycle 6), to 180 ± 17 (cycle 7), and to 221 ± 17 (cycle 8) with an overall average of 190 ± 27 NL CH4 kg 1 VS fed. At OLR of 5.0 g TCOD kg 1 inoculum d 1, significant increase in the SMY was observed with an overall average 222 ± 27.7 NL CH4 kg 1 VS fed. Tukey’s multiple comparison test at 95% confidence interval revealed that the averages of the SMY (based on total VS fed) during the cycles operated at OLR of 3.0, 4.0, and 5.0 g TCOD kg 1 inoculum d 1 were, in general, not significantly different from each other except for cycles 4, 8, and 12, respectively. Notice that the variation in the calculated SMY among the replicate bioreactors also decreased successively. The large value of the standard deviation of the SMY during cycle 1 is likely due to a gas leak which developed in one of the replicate bioreactor (R3, Fig 2.). Notice that after each increase in the OLR the SMY dropped slightly and then resumed its normal levels after two or three cycles; this indicates an adaptation phenomenon of the inoculum to the new OLR. The
2.5. Fiber analysis Cow feces is a complex substrate which contains fibers. The fiber content in cellulose, hemicellulose, and lignin was determined. Hemicellulose can be calculated as the difference between neutral detergent fiber (NDF) and acid detergent fiber (ADF), cellulose as the difference between acid detergent fiber and acid detergent lignin (ADL) (Bauer et al., 2009). 2.6. Statistical methods Data given in tables and shown in figures are the means and standard deviations. Statistical analysis was carried out using Minitab v. 14 statistical software (Minitab Inc., PA, USA). Data were analyzed by one-way ANOVA, followed by post hoc multiple comparisons (Tukey’s test) with a confidence level of 95% (i.e., p < 0.05). 3. Results and discussion
400
Specific CH 4 yield (NL CH 4 kg -1 VS fed )
350
Cycle number 1
2
3
4
5
6
7
8
9
10
11
12
5.0 4.0 3.0
300 250 200 150 100 50
Organic loading rate (g TCOD kg -1 inoculum d -1)
The percent of H2S in the biogas was less than 0.06% in all samples of gas analyzed during the successive cycles. The cow feces fed during the whole duration of the experiment contained fibers composed of cellulose (23.61%), hemicellulose (18.71%) and lignin
0 0 21 42 63 84 105 126 147168 189 210 231 252 273
Time (days) R1
R2
R3
Fig. 2. Specific methane yield profiles for the psychrophilic anaerobic digestion of cow feces.
Table 3 Rate and specific methane yield for the psychrophilic anaerobic digestion of cow feces. Retention time
Cycle
21 21 21 21 21 21 21 21 21 21 21 21
1 2 3 4 5 6 7 8 9 10 11 12
OLR (g TCOD kg 3 3 3 3 3 4 4 4 5 5 5 5
Feed TS (%) 1
inoculum d
1
) 12.7 12.8 13.3 13.2 13.2 13.2 13.2 12.5 12.5 12.5 16.5 16.5
SMY (NL CH4 kg
1
VS)
171.9 ± 32.2a 178.9 ± 19.4a 185.5 ± 17.0a 224.4 ± 13.7 b 161.8 ± 11.2a 168.8 ± 13.1a 180.1 ± 16.9a 220.7 ± 16.6 b 202.2 ± 13.0a,b 205.2 ± 7.1a,b 218.6 ± 9.6a,b 262.2 ± 21.4 c
SMY (NL CH4 kg
1
116.2 ± 21.8 135.8 ± 14.7 149.2 ± 13.7 136.8 ± 8.3 110.3 ± 7.6 115.1 ± 8.9 122.5 ± 11.5 134.3 ± 10.1 128.4 ± 8.3 130.3 ± 4.5 116.6 ± 5.1 154.6 ± 12.6
TCOD)
Rate of CH4 production (NL CH4 kg 1 VS d 1) 8.2 ± 1.5 8.5 ± 0.9 8.8 ± 0.8 10.7 ± 0.7 7.7 ± 0.5 8.0 ± 0.6 8.6 ± 0.8 10.5 ± 0.8 9.6 ± 0.6 9.8 ± 0.3 10.4 ± 0.5 12.5 ± 1.0
Note: SMY’s with different superscripts indicate that they are significantly different at 95% confidence level according to Tukey’s multiple comparison test.
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The average yield of 184.5 ± 24 NL CH4 kg 1 VS of cow feces (at OLR 3.0 g TCOD kg 1 inoculum d 1 or 2.12 g VSfed kg 1 inoculum d 1) obtained in this study after 21 days of psychrophilic (20 °C) incubation during 5 successive cycles is greater than the yield 161 NL CH4 kg 1 VS of dairy cattle feces (at OLR of 1.4 kg VS substrate kg 1 VS inoculum) reported by Møller et al. (2004) at 35 °C and a hydraulic retention time (HRT) of 40 days. For the same incubation time, Schäfer et al. (2006) reported a SMY of 85 L CH4 kg 1 VS from beef cattle manure in two-stage (hydrolysis–methanogensis) reactor with OLR of 6 and 3 kg VS m 3 d 1 for hydrolysis and methanogensis stages, respectively. The SMYs from cow feces at a TCL of 21 days in any of the PDAD 12 successive cycles obtained in this study are higher than the 135 and 164 L CH4 kg 1 VS obtained during mesophilic (30 °C) anaerobic digestion of dairy cattle manure reported by Somayaji and Khanna (1994) and Shyam (2002) at HRT of 40 and 50 days, respectively. The average SMY (184.5 ± 24 NL CH4 kg 1 VS) obtained at OLR 3.0 g TCOD kg 1 inoculum d 1 is greater than the 165 L CH4 kg 1 VS reported by Ahring et al. (2001) during thermophilic (65 °C) operation at OLR of 3.0 kg VS m 3 d 1 and HRT of 15 days. Interestingly, the average SMY (208.7 ± 8.7 NL CH4 kg 1 VS) obtained at OLR 5.0 g TCOD kg 1 inoculum d 1 is comparable to the 202 L CH4 kg 1 VS reported by Ahring et al. (2001) during thermophilic (55 °C) operation at the same OLR of 3.0 kg VS m 3 d 1 and HRT of 15 days. The average SMY obtained in this study (182.9 ± 16.9 NL CH4 kg 1 VS) at OLR of 3.0, 4.0, and 5.0 g TCOD kg 1 inoculum d 1 is less than those reported by Varel et al. (1980) [240–280 L CH4 kg 1 VS] at temperatures between 35 and 65 °C, HRT of 18 days, and OLR of 3.3 kg VS m 3 d 1 (Table 4). Compared to Varel’s et al. (1980) results for operations at 35–65 °C, the current study demonstrated a potential technology which minimises energy required for heating bioreactor. Achieving a stable anaerobic digestion of cow manure at psychrophilic condition over long-term operation is a significant improvement with potential market in cold climate area. The next objectives of this research would be shortening the retention time and increasing the organic loading rate. The specific methane yields obtained in this study provide evidence that PDAD of cow feces is practically feasible and as efficient as mesophilic and thermophilic anaerobic digestion given that a well-acclimatized inoculum is developed and maintained.
large values of the standard deviations of the SMY during cycle 2, 3, 6, 7, 8, and 9 are likely due to variation in the composition of the cow feces fed, slight gas leak which might have been undetected, or experimental error. Generally, the pattern of performance consistency of the replicate bioreactors indicates a stable reproducible process during the 252 days of operation because of the development of well-adapted inoculum to the dry psychrophilic conditions. Usually, two segments are recognized in the SMY curve during batch anaerobic digestion: an initial exponential phase followed by a slowdown phase (plateau) in CH4 production for the rest of the incubation period. The 12 cycles showed only the initial straight line part of the CH4 production curve because the TCL was limited to 21 days which means that not all the substrate fed was completely digested during that TCL. However, in the sequence batch reactor (SBR) the solid retention time (SRT) is decoupled from the TCL; in this study the SRT maintained was 75.1 ± 7.7, 65.2 ± 1.6, and 61.5 ± 4.2 days at OLR of 3.0, 4.0, and 5.0 g TCOD kg 1 inoculum d 1 (Table 1), respectively, which allowed the complete degradation of the substrate fed. The soluble fraction of cow feces is composed of carbohydrates, amino acids, proteins, and fats while the dry matter is composed of fibers containing mainly cellulose (23.61%), hemicellulose (18.71%) and lignin (11.3%), and some forage proteins. Cellulose requires longer retention time than that required for the soluble carbohydrate, fats and proteins degradation by the anaerobic consortia of microorganisms. Cellulose hydrolysis has been shown to be the rate limiting step (Noike et al., 1985) particularly when the substrate is solid or in particulate form (Myint and Nirmalakhandan, 2006). Notice that an increase in the daily specific CH4 production rate can be observed (Table 3). The daily average specific CH4 production rate (NL CH4 kg 1 VS d 1) of the replicate bioreactors was 8.2 ± 1.5 (cycle 1), 8.5 ± 0.9 (cycle 2), 8.8 ± 0.8 (cycle 3), 10.7 ± 0.7 (cycle 4), 7.7 ± 0.5 (cycle 5), 8.0 ± 0.6 (cycle 6), 8.6 ± 0.8 (cycle 7), 10.5 ± 0.8 (cycle 8), 9.6 ± 0.6 (cycle 9), 9.8 ± 0.3 (cycle 10), 10.4 ± 0.5 (cycle 11), and 12.5 ± 1.0 (cycle 12). The overall average of methane production rate is 8.8 ± 1.1, 9.0 ± 1.3, and 10.6 ± 0.4 NL CH4 kg 1 VS d 1 at OLR of 3.0, 4.0, and 5.0 g TCOD kg 1 inoculum d 1, respectively. Notice that although the trend of methane production rate increased successively with the increase in the OLR no conclusion can be deducted because, in general, there were no significant statistical differences detected by Tukey’s multiple comparisons test (Table 3). No relevant data is available in the accessible literature on the performance of psychrophilic dry anaerobic digestion (PDAD); therefore, the results have been compared to the performance of mesophilic and thermophilic DAD of various substrates (Table 4).
Table 4 Methane yield (L CH4 kg Substrate
a b
1
3.2. Volatile fatty acids (VFAs) production Profiles of acetic, propionic, and butyric acids produced during the successive cycles of PDAD with increasing total solids percent
VS) of cow feces at different temperatures.
OLR (kg VS m 3 d 1) unless indicated otherwise
Dairy cow manure Dairy cow manure Dairy cow manure Cattle manure Cattle manure Cattle manure Dairy manure Cattle manure Cattle manure
3.0 g TCOD l 4.0 g TCOD l 5.0 g TCOD l 3.3 3.0 2.0
Dairy cattle manure Cow manure Cow manure
NRa
1 1 1
The test was carried out as per ISO 11734. With basal medium.
HRT (day)
Temperature (°C) 20
21 21 21 18 15 20 30 40 50 100 100 16 90
25
References 30
35
40
50
55
60
65
184.5 ± 24 195.2 ± 2.1 208.7 ± 8.7 260
270
250 260
242.7 135 164 230 148 ± 41 128 260b
280 202
270 230
240 165
This study This study This study Varel et al. (1980) Ahring et al. (2001) El-Mashad et al. (2004) Labatut et al. (2011) Somayaji and Khanna (1994) Shyam (2002) Shyam (2002) Møller et al. (2004) Preeti Rao and Seenayya (1994) Güngör-Demirci and Demirer (2004)
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15
3000 1
2
3
4
5
6
7
8
9
10
11
Cycle number
12
Total solids (%)
Acetate (mg L-1)
Cycle number
2500 2000 1500 1000 500
1
2
3
4
5
6
7
8
9
10 11 12
10
0 0
21
42
63
84 105 126 147 168 189 210 231 252 273
5 0
Time (days)
21
42
63
84 105 126 147 168 189 210 231 252 273
Time (days) Propionate (mg L-1)
3000
R1
Cycle number
2500
1
2
3
4
5
6
7
8
9
10
11
12
2000
1000 500 0
21
42
63
84 105 126 147 168 189 210 231 252 273
Time (days) 3000
Butyrate (mg L-1)
R3
Fig. 5. Total solid profile for the cow feces psychrophilic dry anaerobic digestion.
1500
0
Cycle number
2500
1
2
3
4
5
6
7
8
9
10
11
12
2000 1500 1000
The profile of propionic acid in the replicate bioreactots was similar to that of acetic acid. Butyric acid peaked also to levels between 1000 and 1500 mg L 1 after feedings and was consumed within a week to levels within 250 mg L 1. The concentrations of other volatile fatty acids (isobutyric-, iso-valeric-, and valeric-acid) were less than 100 mg L 1 immediately after feeding and less than 50 mg L1 during the remaining time of the cycles. The profiles of the VFAs concentration and the increase in methane yield during the successive cycles indicate that acetogenic and methanogenic reactions proceeded fairly well. The relative stability of the pH profile between 7.0 and 7.5 (Fig. 4) was due to the high alkalinity (7500–10,900 mg CaCO3 L 1) of the mixed liquor and the absence of VFAs accumulation.
500
3.3. Solids reduction
0 0
21
42 63
84 105 126 147 168 189 210 231 252 273
Time (days)
R1
R2
R3
Fig. 3. Volatile fatty acids profiles for the cow feces psychrophilic dry anaerobic digestion. A – acetic, B – propionic, C – butyric acids.
in the feed are shown in Fig. 3. The profiles of VFAs produced in the triplicate bioreactors were almost identical. Throughout the successive cycles, acetic acid concentration peaked immediately after feeding to levels between 1000 and 2000 mg L 1 but was consumed within a week in all bioreactors to about 500 mg L 1 and to within 100 ± 50 mg L 1 after two weeks indicating that methanogensis reaction from acetate was not a rate limiting step. Similarly, propionic acid peaked to levels between 500 and 600 mg L 1 after feedings and was consumed within a week to levels close to the detection limits of the instrument (25 ± 10 mg L 1).
9.0 8.5
pH
R2
Cycle number 1
8.0 7.5 7.0
2
3
4
5
6
7
8
9
10
11 12
6.5 6.0 5.5
The profiles of the total solids in the bioreactors during the successive cycles are shown in Fig. 5. The profiles of the TS were identical. Generally, the total solids contents of the inoculum increased slightly from cycle to cycle likely due to the increase in the microbial biomass. Notice that the percentage of total solids reduction during the individual cycles was about 3 ± 1.5. 3.4. Economic considerations Obviously no mixing and heating for the bioreactor increases the net energy yield per unit volume of the reactor. Huchel et al. (2006) reported that 564.4 mol CH4 m 3 substrate day 1 was required to maintain the temperature of an anaerobic digester at 37 °C when handling a substrate of 0.96% TS with an HRT of 20 days. Furthermore, increasing the total solids from 1% to 16% means around 94% reduction in the required volume of the bioreactor. Investment cost decreases by 70% when the reactor volume is reduced by 50% (Oosterkamp, 2011). Therefore, the PDAD offers the advantage of the combined saving in cost of construction and energy expenses of heating and mixing during operation. Ultimately, psychrophilic dry anaerobic digester could contribute to more energy output compared to mesophilic or thermophilic liquid or dry digester. 4. Conclusions
5.0 0
21
42 63
84
105 126 147 168 189 210 231 252 273
Time (days)
R1
R2
R3
Fig. 4. pH profile for the cow feces psychrophilic dry anaerobic digestion.
An average specific methane yield (SMY) of 222 ± 27.7 NL CH4 kg 1 VS fed has been achieved during the last four successive cycles at organic loading rate 5.0 g TCOD kg 1 inoculum d 1 and treatment cycle length (TCL) of 21 days. A maximum SMY of 262 ± 21.4 NL CH4 kg 1 VS fed (137 ± 8 NL CH4 kg 1 COD fed) with a maximum CH4 production rate of 10.7 ± 0.7 NL CH4 kg 1 VS d 1 have been accomplished at OLR of 5.0 g TCOD kg 1 inoculum d 1.
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