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High rate psychrophilic anaerobic digestion of high solids (35%) dairy manure in sequence batch reactor
Bioresource Technology 186 (2015) 74–80
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High rate psychrophilic anaerobic digestion of high solids (35%) dairy manure in sequence batch reactor Noori M. Cata Saady, Daniel I. Massé ⇑ Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, Sherbrooke, Quebec J1M 0C8, Canada
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Psychrophilic dry anaerobic digestion
(PDAD) of cow feces and wheat straw (CFWS). 1 1 At OLR 6.0 g TCOD kg d and 21 days the yield is 163.3 ± 5.7 N L CH4 kg1 VS. PDAD of CFWS is feasible at 35% TS in feed and OLR of 6.0 g TCOD kg1 d1. VS-based inoculum to substrate ratio of 2.13 ± 0.2. PDAD of CFWS (TS 35%) is as efficient as mesophilic DAD.
a r t i c l e
i n f o
Article history: Received 22 January 2015 Received in revised form 4 March 2015 Accepted 7 March 2015 Available online 13 March 2015 Keywords: Anaerobic digestion Cow manure Dry Psychrophilic Wheat straw
a b s t r a c t Zero liquid discharge is increasingly adopted as an objective for waste treatment process. The objective of this study was to increase the feed total solids (TS) and the organic loading rate (OLR) fed to a novel psychrophilic (20 °C) dry anaerobic digestion (PDAD). Duplicate laboratory-scale bioreactors were fed cow feces and wheat straw (35% TS in feed) at OLR of 6.0 g TCOD kg1 inoculum d1 during long-term operation (147 days consisting of 7 successive cycles). An overall average specific methane yield (SMY) of 151.8 ± 7.9 N L CH4 kg1 VS fed with an averaged volatile solids removal of 42.4 ± 4.3% were obtained at a volatile solids-based inoculum-to-substrate ratio (ISR) of 2.13 ± 0.2. The operation was stable as indicated by biogas and VFAs profiles and the results were reproducible in successive cycles; a maximum SMY of 163.3 ± 5.7 N L CH4 kg1 VS fed was obtained. Hydrolysis was the reaction limiting step. High rate PDAD of 35% TS dairy manure is possible in sequential batch reactor within 21 days treatment cycle length. Crown Copyright Ó 2015 Published by Elsevier Ltd. All rights reserved.
1. Introduction The main waste by-product from livestock operations is manure containing biodegradable organic matter. The amount of manure has been increasing worldwide as a result of the livestock industry growth. Manure handling and disposal represent a substantial part of farm operation cost and also represent environmental, economic, and social challenges to livestock operations. ⇑ 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.biortech.2015.03.038 0960-8524/Crown Copyright Ó 2015 Published by Elsevier Ltd. All rights reserved.
Therefore, developing robust and cost effective technologies for on-farm manure-to-energy conversion can solve some of its environmental problems and improve the social acceptability of livestock operations. The generation of revenue will help to offset manure management cost. Currently, anaerobic digestion is well-established and accepted on-farm manure treatment which generates combustible biogas containing methane. Dairy manure on livestock operations that use bedding is characterized by its high solids; depending on the amount of bedding used the total solids (TS) of manure may be as high as 40%. Wet anaerobic digestion (WAD; TS < 15%) has been
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used to treat manure; however, currently developing dry anaerobic digestion (DAD; TS > 15%) is gaining interest because it offers several advantages when compared with WAD. For example, DAD requires a digester of smaller size compared to wet anaerobic digestion (WAD); this translates in reduced capital and operational costs, and higher volumetric bioenergy yield (Luning et al., 2003). The advantages of DAD have been demonstrated at mesophilic and thermophilic conditions for agricultural wastes and livestock manure (15–20% TS) (Ahn et al., 2010; Böske et al., 2015; Di Maria et al., 2012; Jha et al., 2013; Kusch et al., 2008; Wei et al., 2014; Zhu et al., 2014). However, most manure producing regions such as Canada, Europe, northern part of the USA and China are classified as cold regions. Therefore, psychrophilic (15–25 °C) operation would be more suitable for cold-climate regions if its feasibility is demonstrated because maintaining mesophilic and thermophilic conditions by heating the bioreactor reduces the net energy output. Kashyap et al. (2003) stated that developing a psychrophilic anaerobic digestion process to convert cattle manure to biogas is still a technological challenge. Since then, some technological advancement have been achieved; several on-farm psychrophilic WAD processes have been developed and deployed (Massé et al., 2010). Accommodating manure with high solids content (>15%) in psychrophilic operation is still a challenge that needs a technical solution. Presently, developing and optimizing a psychrophilic dry anaerobic digestion (PDAD) process is under extensive research worldwide. Recently, a PDAD process of cow feces with and without wheat straw in sequential batch reactor (SBR) at 20 °C, has been developed at the Dairy and Swine Research and Development Centre (DSRDC) of Agriculture and Agri-Food Canada, in Sherbrooke, Quebec-Canada (Massé et al., 2015). The process has been demonstrated in long term operation (> 450 days) at TS of 27% and organic loading rate of 3 g TCOD kg1 inoculum d1 (Massé and Saady, 2015). The upper threshold limit for the total solids at which successful dry anaerobic digestion can be conducted is not well determined. Recently, Motte et al. (2013) concluded that wheat straw mesophilic conversion to volatile fatty acids (VFAs) decreased with the increase in its TS from 10% to 33% with no changes in the volatile fatty acids profiles until a clear limit at 28% TS supposedly because of the decrease of free water in the media. No information exists regarding the threshold limit of the TS content for a substrate composed of cow feces and wheat straw digested at psychrophilic condition. This study is a step on developing PDAD by increasing the total solids content of the feed by 30% (from TS of 27% to 35%) while doubling the organic loading rate (from 3.0 to 6.0 g TCOD kg1 inoculum d1) fed to sequence batch reactor. The principal objectives of this study were to answer the question: is it possible to have a high rate PDAD of high total solids (35%) dairy manure at relatively short treatment cycle length (21 days). To the authors’ best knowledge, this is the first reports on successful long-term operation (147 days consisting of 7 successive treatment cycles) of PDAD at 35% TS in feed and OLR of 6.0 g TCOD kg1 inoculum d1.
the following steps: day 1: loading the reactor with inoculum, feeding the substrate, and mixing the inoculum and substrate; day 1–21: reaction; and day 21: unloading the digestate and starting the next cycle. 2.2. Bioreactor Duplicate 40-L cylindrical (0.312 m in diameter 0.520 m in height) plastic barrels bioreactors have been operated as sequential batch reactors (SBR) at a treatment cycle length (TCL) of 21 days in a temperature-controlled room (20 °C). The average mass of combined feed and inoculum in the reactor was about 7.74 ± 0.03 kg based on the masses of the cow feces, wheat straw and inoculum used. The reactors were fitted with two gas lines; one for purging the nitrogen gas immediately after feeding/ loading the substrate to expel O2 and initiate an anaerobic environment inside reactor; and the second to release and measure the biogas produced. The biogas was sampled and its composition was analyzed once a week (Fig. 1). The barrel was kept upside down after it has been filled so that the wet content works as a water seal in addition to the seal of the barrel’s lid to ensure gas tightness. 2.3. Wet tip tank gas meter The wet tip tank gas meter (Beal, 1998) was manufactured at the workshop of the DSRDC. It is a transparent box made of acrylic contains a pivoting plastic tipping bucket (two equal-size compartments) and faced downwards just vertically above to a port that releases biogas under the pivoting point. The tipping container was placed submerged inside the water-filled acrylic box. When biogas released into one compartment reaches a certain known volume, the container tips (the compartment filled with gas pivots upward) due to the buoyancy of the gas and releases the biogas which bubbles upwards to escape from water and leaves the box. Upon tipping (pivoting) the second compartment on the other side of the container moves downward and biogas released from the port starts to accumulate inside it, and so on. Each tip was counted using a reed switch which sends an electrical pulse to an electronic digital counter (manufacturer: OmRon; model number: H7EC-NVB). The tip counts were monitored daily and used to calculate the volume of the biogas produced. The wet tip tank gas meter calibration was checked weekly. After feeding, the reactor was held upside down so that the inoculum–substrate mixture provides sealing in addition to the locked lid of the barrel. 2.4. Inoculum and substrate The initial inoculum was obtained from a laboratory scale (40 L) psychrophilic (20 °C) anaerobic sequence batch reactor fed with fresh dairy manure and wheat straw (35% TS); the performance of the seeding inoculum has been reported previously (Saady and
7 6
1 1- 40 L plastic barrel
2. Methods
The experiments assessed the effect of the organic loading rate (6.0 g TCOD kg1 inoculum day1) on CH4 production and process stability of psychrophilic anaerobic digestion of cow feces and wheat straw. The substrate total solids (TS) have been kept at 35%. The 147 days of operation comprised of seven successive cycles. The treatment cycle length (TCL) was 21 days measured between successive feedings of the reactor. The cycle includes
2- Barrel lid with metal lock
5
2.1. Experimental setup
3- Gas release line 4- N 2 purging line with valve 5- Substrate-inoculum mixture
2 4
3
6- Head space 7- Wet tip tank gas meter
Fig. 1. Schematic diagram of the dry anaerobic digester.
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Massé, 2013). 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 was collected on wood boards, before getting in contact with urine and bedding, transferred into a plastic drum, stored at 4 °C (for a maximum of 30 days before feeding), and triplicate representative samples have been taken from each batch of cow feces fed to the reactors in each treatment cycle after thorough mixing and homogenization and before feeding the reactors. Three different batches of cow feces from the same experimental farm at DSRDC, Sherbrooke, Quebec were used during the 7 cycles as indicated in Table 1 (the first batch has been used for cycles 1 and 2; the second batch has been used for cycles 3, 4 and 5, and the third batch has been used for cycles 6 and 7). Wheat straw was harvested at the DSRDC’s experimental farm during fall 2012 and fall 2013 and chopped (3 mm) using a laboratory mill (Thomas Wiley Laboratory Mill Model 4, Arthur H. Thomas Company, Philadelphia, PA). The substrate and inoculum have been mixed manually for 5 min during feeding. Every week the content of the bioreactor was mixed manually for 5 min before sampling the content to ensure a homogenous and representative sample was taken. No mixing took place during other time of the treatment cycle; therefore, the process can be considered as a static dry anaerobic digestion. Physicochemical characteristics of the cow feces fed in each treatment cycle are given in Table 1. Physico-chemical characteristics of the inoculum and inoculum-substrate mixture (ISM) before feeding bioreactors are given in Table 2.
2.5. Organic loading rate (OLR) Wheat straw and cow feces were mixed manually to obtain the desired substrate TS content (35%) while maintaining the design organic loading rate (OLR) of 6.0 g TCOD kg1 inoculum day1 (equivalent to 4.47 ± 0.02 g VS kg1 inoculum d1). The mass of
inoculum, feces, and/or straw fed to each bioreactor at the beginning of the successive cycles, the OLR and inoculum-to-substrate ratio (ISR) are given in Table 3. The ISR was expressed in kg of total VS fed per kg VS of inoculum averaged at 2.13 ± 0.2. 2.6. Sampling The inoculum-substrate mixture (ISM) was sampled on day (0) immediately after feeding. Samples were also taken on day 7 by opening the reactor, mixing its content manually for 5 min to ensure that the sample is representative of the reactor content. The reactor content was also sampled at the end of the treatment cycle (on day 21); this sample represented the digestate (effluent) of which 6 kg were used as inoculum in the next cycle. The reactor has been flushed with nitrogen to maintain anaerobic digestion. The gas samples were taken through gas sampling port sealed with septa and installed on the gas line mid-way between the barrel and the wet tip gas meter using a 10 mL plastic syringe. 2.7. Biogas measurement Biogas volume produced was measured daily using calibrated wet tip gas meters while the biogas components (CH4, CO2 and H2S) were determined weekly using a Hach Carle 400 AGC gas chromatograph (Model 04131-C, Chandler Engineering, Houston, TX) configured for the application 131-C. The application uses a packed 3.175 mm diameter column Chromosorb W, high performance (HP) grade composed of 1.8 m (805 porapak N + 205 Porapak Q), 2.1 m (80% molecular sieve 13X + 20% molecular Sieve 5A), and 1.8 m (80% OV-101 on chromosorb). The column and thermal conductivity detector were operated at 85 °C with a helium gas flow rate of 30 mL min1. The GC calibration was performed weekly with a standard gas (27.3% CO2, 1.01% N2, 71.16% CH4, 0.53% H2S); several standard gas samples were injected at the beginning and end of gas analysis as well as after every 10 injections of experimental gas samples.
Table 1 Physicochemical characteristics of the cow feces fed in each treatment cycle. Cycle
Substrate
pH
TCOD (g kg1)
TS (%)
VS (%)
Acetate (g kg1)
Propionate (g kg1)
Butyrate (g kg1)
1–2 3–5 6–7 1–4
Cow feces Cow feces Cow feces Wheat straw
6.71 6.74 7.02 NA
155.2 169.0 180.0 1097
11.5 12.6 13.3 89
10.1 11.5 11.8 84.9
3.9 4.01 2.93 NA
0.96 1.41 0.86 NA
0.50 2.35 0.55 NA
Fiber components (% of dry matter) Cellulose
Hemicellulose
Lignin
24.39 26.33 23.50 38.61
19.28 16.35 16.72 25.14
12.74 10.67 13.45 7.3
Table 2 Physico-chemical characteristics of the inoculum and inoculum-substrate mixture at the beginning of each digestion cycle. Cycle
TYPE
pH
Acetate (g kg1)
Propionate (g kg1)
Butyrate (g kg1)
1
Inoculum ISM
7.88 ± 0.02 7.60 ± 0.03
0.13 ± 0.01 1.23 ± 0.07
0.02 ± .00 0.55 ± 0.01
Inoculum ISM
8.27 ± 0.07 8.03 ± 0.11
0.18 ± 0.01 1.18 ± 0.37
Inoculum ISM
8.04 ± 0.18 7.75 ± 0.11
Inoculum ISM
2 3 4 5 6 7
Alkalinity (g kg1 as CaCO3)
TS (%)
VS (%)
0.09 ± 0.00 0.40 ± 0.02
10.104 ± 0.81
19.5 ± 0.4 20.9 ± 0.7
16.7 ± 0.5 18.2 ± 0.2
0.03 ± 0.01 0.39 ± 0.09
0.22 ± 0.00 0.37 ± 0.02
10.168 ± 1.3
20.8 ± 0.4 23.0 ± 0.3
18.1 ± 0.1 20.5 ± 0.1
0.14 ± 0.01 1.26 ± 0.45
0.03 ± 0.01 0.32 ± 0.10
0.02 ± 0.00 0.12 ± 0.21
9.314 ± 0.13
23.7 ± 1.9 24.8 ± 1.9
21.3 ± 1.8 22.4 ± 1.8
8.02 ± 0.08 7.91 ± 0.14
0.13 ± 0.03 0.82 ± 0.47
0.03 ± 0.01 0.23 ± 0.15
0.02 ± 0.00 0.07 ± 0.05
9.321 ± 0.67
22.9 ± 1.0 23.9 ± 0.7
20.4 ± 0.9 21.6 ± 0.5
Inoculum ISM
7.89 ± 0.04 7.63 ± 0.14
0.14 ± 0.02 1.04 ± 0.49
0.03 ± 0.01 0.36 ± 0.02
0.01 ± 0.00 0.26 ± 0.16
10.00 ± 0.45
22.8 ± 0.8 24.7 ± 1.4
20.2 ± 0.7 21.9 ± 1.4
Inoculum ISM
8.15 ± 0.19 7.75 ± 0.08
0.240.05 1.06 ± 0.64
0.04 ± 0.01 0.39 ± 0.32
0.02 ± 0.00 0.03 ± 0.01
9.715 ± 0.54
23.8 ± 0.4 25.6 ± 1.2
21.3 ± 0.3 23.2 ± 1.1
Inoculum ISM
8.05 ± 0.11 7.78 ± 0.13
0.27 ± 0.02 0.85 ± 0.41
0.05 ± 0.02 0.27 ± 0.10
0.03 ± 0.01 0.04 ± 0.02
9.16 ± 0.11
24.4 ± 0.4 25.5 ± 0.5
21.8 ± 0.4 23.0 ± 0.5
Note: ISM = Inoculum-substrate mixture immediately after feeding.
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N.M.C. Saady, D.I. Massé / Bioresource Technology 186 (2015) 74–80 Table 3 Details of the organic load fed to each bioreactor at the beginning of the successive cycles. Cycle
1 2 3 4 5 6 7
Feces (kg)
1.268 1.268 1.227 1.227 1.227 1.218 1.218
Straw (kg)
0.510 0.510 0.500 0.500 0.500 0.489 0.489
ISR (VS-based ratio)
1.78 1.93 2.26 2.16 2.14 2.28 2.34
Organic loading TCOD fed (g)
VS fed (g)
g TCOD kg1 inoculum d1
g VS kg1 inoculum d1
756.3 756.3 755.9 755.9 755.9 756.0 756.0
561.6 561.6 566.1 566.1 566.1 559.4 559.4
6.0 6.0 6.0 6.0 6.0 6.0 6.0
4.46 4.46 4.49 4.49 4.49 4.44 4.44
Note: In all cycles the reactors have been inoculated with 6 kg of inoculum which has been transferred from the previous cycle. Feed TS has been kept at 35% in all cycles.
The linearity of the quantification regression equation covered the range from 0.001% to 100% (pure gas injection) with R2 value of 0.9991, 0.9988, and 0.9968 for CH4, CO2, and H2S. The quantification regression equation has been generated using certified standards containing different proportions of the gases measured. Methane (CH4) production is reported in normalized litres (N L CH4), i.e., the CH4 volume produced was corrected to standard temperature and pressure (STP) (273oK; 1 atm) using Eq. (1).
V CH4STP ¼ g V m
T s Pm T m Ps
ð1Þ
where: Vm is the measured volume of biogas (L), g is the percentage of CH4 in biogas, Tm and Pm which are the actual temperature (K) and atmospheric pressure (kPa) at the time of measurement, and Ts and Ps are the standard temperature (K) and pressure (kPa). V CH4STP is the volume (L) of methane at the standard temperature and atmospheric pressure (273 K and 101.3 kPa, respectively). The specific methane yield (SMY) obtained during an individual cycle (TCL = 21 days) has been calculated by divining the cumulative methane volume produced during that cycle by the mass of volatile solids (VS) contained in the manure fed at the beginning of that cycle as shown in Eq. (2).
SMY ¼
Cumulative methane volume VS fed
ð2Þ
2.8. Analytical methods Samples were collected from each bioreactor and analyzed weekly for volatile fatty acids (VFAs), total solids (TS), volatile solids (VS), and pH. 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). Alkalinity: method number 2320B used Potentiometric titration to preselected pH 4.38, Total solids: method number 2540B, Volatile solids method number: 2540E. VFA: method number 5560D. The concentrations of individual volatile fatty acids (VFAs) including acetic, propionic, butyric, isobutyric, butyric, valeric and isovaleric acids have been measured using Perkin Elmer gas chromatograph (GC) model 8310 (Perkin Elmer, Waltham, Mass.) equipped with an autosampler to facilitate the analysis, fitted with FID, and equipped with a J&W Scientific DB-FFAP high resolution column (30 m 0.53 mm 1.00 lm; Chromatographic Specialties Inc., Ontario) (Massé et al., 2003). Helium, flowing at 9.5 mL min1, was employed as the carrier. The injector temperature was maintained at 200 °C, while the detector temperature was set at 250 °C. A 10 gram of sample was diluted in 20 mL of deionized water, mixed thoroughly and the liquid part was separated and transferred into a centrifuge tube. After centrifugation for 30 min at 46,300 g, 2 mL of the supernatant was transferred
into 3 mL centrifuge tube, acidified with phosphoric acid, and centrifuged for 15 min at 16,300 g. A 0.5 mL of the supernatant was filtered through a 0.45 lm nylon syringe filter into 1.5 mL GC tube. The detection limit of acetic acid was 3.5 mg L1 while the detection lime of all other acids was 1.5 mg L1. 2.9. Fiber analysis The complex substrates (cow feces and wheat straw) were subjected to fiber analysis to determine their content of cellulose, hemicellulose, and lignin. 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; Saady and Massé, 2013). 3. Results and discussion Feasibility of PDAD-SBR has been demonstrated for digesting cow feces (13–16% TS) during long-term operation (252 days) of operation with an average specific methane yield (SMY) of 222 ± 27.2 N L CH4 kg1 VS fed at OLR of 5.0 g TCOD kg1 inoculum d1 (Massé and Saady, 2015). Furthermore, its feasibility for digesting dairy manure (cow feces and wheat straw) at 27% total solids has successfully been demonstrated at OLR of 3 g TCOD kg1 inoculum d1 in long term operation (273 days) with an average SMY of 182.9 ± 16.9 N L CH4 kg1 VS fed (Massé et al., 2015). The results of this study answer the question: is it possible to have a high rate (OLR of 6.0 g TCOD kg1 inoculum d1) anaerobic digestion of high total solids (35%) dairy manure in sequence batch reactor at relatively short treatment cycle length (21 days)? These microorganisms-challenging conditions have been applied and the performance has been evaluated based on specific methane yield, volatile fatty acid levels, and VS removal for long-term (147 days) operation comprising 7 successive cycles. Quality of the cow feces as described by TCOD, TS, and VS content varied significantly. During the first 2 cycles, the cow feces had lower value of TCOD, TS, and VS compared to that used during the rest of cycles. 3.1. Methane production The performance of psychrophilic SBR has been evaluated in 7 successive cycles at OLR of 6.0 g TCOD kg1 inoculum d1 (equivalent to 4.47 ± 0.02 g VS kg1 inoculum d1). Cumulative methane production profiles expressed as average specific methane yield (SMY) of the quadruplicate bioreactors are shown in Fig. 2. The maximum SMYs calculated during the successive cycles are given in Table 4. Based on the total VS fed (cow feces plus wheat straw), the average SMY calculated ranged between 141.8 ± 5.4 and 163.3 ± 5.7 N L CH4 kg1 VS fed in cycle 1 and 3, respectively.
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batch reactor with an average solid retention time (SRT) of about 94.4 ± 0.5 days allowed the degradation of the fiber component in the substrate fed. The average specific CH4 production rate (N L CH4 kg1 VS d1) of the duplicate bioreactors ranged between 6.8 ± 0.3 (cycle 1) and 7.8 ± 0.3 (cycle 3); the overall average of the specific CH4 production rate for the 7 successive cycles was 7.2 ± 0.4 N L CH4 kg1 VS d1. The results have been compared to the performance of mesophilic and thermophilic DAD of various substrates (Table 6) because data on performance of psychrophilic dry anaerobic digestion of cow feces and wheat straw is not available in the accessible literature. The average specific methane yield (151.8 ± 7.9 N L CH4 kg1 VS of substrate fed (35% TS at OLR 6.0 g TCOD kg1 inoculum or 4.47 ± 0.02 kg VS fed kg1 inoculum d1) achieved in this study after 21 days of psychrophilic (20 °C) incubation is comparable to the yield of 160 N L CH4 kg1 VS of dairy manure, straw, and oat husk (TS 17% at OLR of 3.4 g VS kg1 d1) reported by Schäfer et al., 2006 for Jarna biogas plant in Sweden which operates at 38 °C and retention time of 22 days. Notice that the data from Jarna plant was for inoculum which has been adapted to the substrate and the steady state operation condition for three years at the time of the study reported by Schäfer et al. (2006); a longer adaptation of psychrophilic inoculum is expected to increase the rate of CH4 production and shorten the TCL for a fixed design OLR. The SMYs from cow feces and wheat straw at an TCL of 21 days in any of the PDAD seven successive cycles (TS 35%) obtained in this study were higher than 28 L CH4 kg1 VS of dairy manure and switch grass (15% TS) obtained by Ahn et al. (2010) during 62 days of thermophilic (55 °C) incubation. Ahn et al. (2010) study was single batch (62 days) experiment with low ISR (0.2); therefore, the results of the current study demonstrate the importance of prolonged solid retention time provided by the sequence batch reactor and the importance of ISR. Inoculum adaptation to the high solids content condition is another necessary step; this study used an inoculum which has been adapted
Fig. 2. Specific methane yield profiles for psychrophilic dry anaerobic digestion of cow feces and wheat straw (35% TS).
The overall average SMY of the 7 successive cycles was 151.8 ± 7.9 N L CH4 kg1 VS fed (113.1 ± 6.1 N L CH4 kg1 TCOD fed. Although the organic loading rates (6.0 g TCOD kg1 inoculum day1), the feed total solids (35%), and the TCL (21 days) have been maintained the same during the successive cycles, the SMY fluctuated from cycle to cycle. This fluctuation could have partially been caused by variation in composition of the cow feces fed (Table 1). Notice that the overall average of the SMY (150.4 ± 12.1 N L CH4 kg1 VS fed) during cycle 1 and 2 was not significantly different from the overall average of the SMY (156.7 ± 6.2 N L CH4 kg1 VS fed) during the cycles 3, 4 and 5 or the overall average of the SMY (145.9 ± 0.7 N L CH4 kg1 VS fed) during cycle 6 and 7. Notice that wheat straw provided on average 75.5 ± 1.2% of the VS fed, 72.5 ± 1.2% of the TCOD fed, and 79.1 ± 0.7% of the fiber fed, and 28.8 ± 0.2% of the feed wet mass (Table 5). The dry matter of wheat straw is composed of cellulose (38.61%), hemicellulose (25.14%) and lignin (7.3%). Lignocellulose fibers in wheat straw and cow feces require a relatively longer treatment cycle length during anaerobic digestion than that required for conversion of the soluble biodegradable components of cow feces. The sequence
Table 4 Rate and specific methane yield for the psychrophilic dry anaerobic digestion of cow feces and wheat straw (35% TS). Cycle number
Cow feces TCOD/VS ratio
1 2 3 4 5 6 7
SMY (N L CH4 kg1 VS) a
1.54 1.54 1.47 1.47 1.47 1.53 1.53
141.8 ± 5.4 158.9 ± 6.2b 163.3 ± 5.7b 156.1 ± 0.0c 150.9 ± 9.3a,b,c 145.4 ± 5.7a 146.4 ± 1.6a
SMY (N L CH4 kg1 TCOD)
Rate of CH4 production (N L CH4 kg1 VS d1)
105.6 ± 4.0 118.3 ± 4.6 121.6 ± 4.2 116.9 ± 0.0 112.9 ± 7.0 107.7 ± 4.2 108.5 ± 1.2
6.8 ± 0.3 7.6 ± 0.3 7.8 ± 0.3 7.4 ± 0.0 7.2 ± 0.4 6.9 ± 0.3 7.0 ± 0.1
Note: 1 – The treatment cycle length (TCL) in all cycles was 21 days. 2 – The numbers given are averages and standard deviations. 3 – SMY with the same superscript are statistically not significantly different from each other (based on Tukey’s test of multiple comparisons at 95% significance level).
Table 5 Contribution of wheat straw to the feed fiber, TCOD, VS and mass. Cycle
CF (kg)
WS (kg)
Feces COD (g kg1)
Feces TS%
Feces VS%
Feces fiber in feed (kg)
WS fiber in feed (kg)
WS/CF fiber ratio in feed
WS fiber in feed (%)
WS/CF COD ratio in feed
WS COD in feed (%)
WS/CF VSratio in feed
WS VS in feed (%)
WS mass in feed %
1 2 3 4 5 6 7
1.218 1.218 1.227 1.227 1.227 1.218 1.218
0.489 0.489 0.500 0.500 0.500 0.489 0.489
155.2 155.2 169.0 169.0 169.0 180.3 180.3
11.5 11.5 12.6 12.6 12.6 13.3 13.3
10.1 10.1 11.5 11.5 11.5 11.8 11.8
0.082 0.082 0.082 0.082 0.082 0.087 0.087
0.322 0.322 0.316 0.316 0.316 0.309 0.309
3.92 3.92 3.83 3.83 3.83 3.56 3.56
79.7 79.7 79.3 79.3 79.3 78.1 78.1
2.84 2.84 2.65 2.65 2.65 2.44 2.44
74.0 74.0 72.6 72.6 72.6 71.0 71.0
3.38 3.38 3.01 3.01 3.01 2.89 2.89
77.2 77.2 75.1 75.1 75.1 74.3 74.3
28.7 28.7 29.0 29.0 29.0 28.6 28.6
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N.M.C. Saady, D.I. Massé / Bioresource Technology 186 (2015) 74–80 Table 6 Comparative performance of dry anaerobic digestion of cow manure and wheat straw. Substrate and inoculum
Temperature (°C)
TS (%)
ISR
OLR (g TCOD kg1 inoculum day1)
Cow feces and wheat straw Cow feces and wheat straw
20 20
35 27
2.13 ± 0.20e 5.55 ± 0.01e
6.0 3.0
CM:WWS (2:3 mass ratio) Aerobically pre-treated SM, agricultural residues
35 35 35 55
16 28 28 15
0.2 NR NR 0.2
0.35a 0.28b
DM and SG Rice straw and corn stover inoculated with (1:1) sewage sludge: pig manure
26–28
85% beef manure plus 15% grass silage
35
15 20 25 30 35 28
0.2
NR
Retention time (days) 21 21
SMY (N L CH4 kg1 total VS)
References
151.8 ± 7.9 182.9 ± 16.9
This study Massé et al. (2015) Li et al. (2011) Di Maria et al. (2012) Ahn et al. (2010) Sun et al. (1987)
NR
63 130 65 62
328 55 22 28c
NR NR NR NR NR 0.9
156 156 168 198 198 100
346 339 382 423 34 227c c
Beef manure plus straw
32
18
NR
3.2
28
181
Pig manure with turnip rape straw and wheat straw DM, straw, and oat husk
35
16
NR
NR
120
122c
38
17
NR
3.4
22
160c
DM, straw, and oat husk
4.1
Fresh HM and straw 37
20
0.2
NR NR NR
84 28 42 72
146 175 208
Schäfer et al. (2006) Schäfer et al. (2006) Schäfer et al. (2006) Schäfer et al. (2006) Schäfer et al. (2006) Kusch et al. (2008)
CM = cow manure; DM = dairy manure; HM = horse manure; PM = poultry manure; SG = swithchgrass; SM = swine manure; WWS = waste water sludge; NR = not reported. Notes: a OLR units are kg TCOD kg1 VS. b OLR units are kg VS substrate kg1 VS inoculum. c SMY units are L CH4 kg1 VS d- SMY is per kg COD destroyed. e VS-based.
over a 36 months period to stepwise increase in the feed’s total solids (data not shown). In addition, sufficient quantity of inoculum has been used (average VS-based ISR of 2.13 ± 0.20). Despite that the average SMY (151.8 ± 7.9 N L CH4 kg1 VS fed) is lower than the 181 L CH4 kg1 VS of beef manure and straw (TS 18% and OLR of 3.2 g VS kg1 inoculum d1) at 32 °C and retention time of 28 days reported by Schäfer et al. (2006) (Table 6); notice that Schäfer et al. (2006) reported their results (181 L CH4 kg1 VS) at room temperature and atmospheric pressure and it would be around 163 N L CH4 kg1 VS if corrected to standard conditions. Compared to Schäfer et al. (2006) result, the current study demonstrated an increase of 94.4% in the feed total solids which represent a 51.5% reduction in the required volume of the bioreactor. Moreover, the current study decreased the treatment cycle length by 25% compared to Schäfer et al. (2006) and this represents an additional reduction of 25% in the volume of the bioreactor required. Furthermore, the current study saved the energy consumed in heating the bioreactor (to increase the temperature from 20 to 37 °C). Notice that the high yields (> 250 N L CH4 kg1 VS fed) reported by Li et al. (2011) was for mesophilic anaerobic digestion of cow manure and wastewater sludge (16% TS) in 63 days incubation. Similarly, the yields (339–423 N L CH4 kg1 total VS) reported by Sun et al. (1987) in Table 6 have been obtained for long retention times (156–198 days) and low OLR (0.35 kg TCOD kg1 inoculum day1) in mesophilic anaerobic digestion of rice straw and corn stover (TS 15–30%). These researchers reported a very low SMY (34 N L CH4 kg1 total VS) for the same substrate and experimental conditions at TS 35%. Achieving a stable dry anaerobic digestion of cow manure and wheat straw at psychrophilic condition and feed TS of 35% and OLR of 6.0 g TCOD kg1 inoculum d1 over long-term operation is a significant improvement given that 30% TS has been recently identified as a threshold above which methanogenesis was strongly inhibited for batch anaerobic digestion of cardboard at 35 °C (Abbassi-Guendouz et al., 2012).
Fig. 3. Typical volatile fatty acids profile during psychrophilic anaerobic digestion of 35% total solids cow feces and wheat straw.
Although the OLR used in this study (4.47 ± 0.02 g VS kg1 inoculum d1) is greater than the OLR of 3.0 g TVS L1 reactor d1 which is used in farm anaerobic digestion plants in Europe (Bolzonella et al., 2011) the TCL in this study is less by 50%. The specific methane yields obtained in this study provide evidence that PDAD of cow manure and straw is technically feasible at feed TS 35% and high OLR and is as efficient as mesophilic DAD given that a sufficient quantity of well-acclimatized inoculum is used. 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 in the feed were almost similar but at different concentration levels (data not shown). A typical profile of acetic, propionic, and butyric acids during a complete cycle is shown in Fig. 3. Throughout the successive cycles, acetic acid concentration peaked immediately after feeding to levels between 850 ± 350 mg L1 depending on its level in the cow feces fed, but it was consumed within a week and its concentrations were maintained within 190 ± 70 mg L1 indicating
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production rate of 7.8 N L CH4 kg1 VS day1 have been obtained. PDAD of dairy cow manure and wheat straw (total solids of 35%) at OLR of 6.0 g TCOD kg1 inoculum d1 and treatment cycle length of 21 in sequential batch reactor is feasible. References
Fig. 4. Typical pH and volatile solids profile during psychrophilic anaerobic digestion of 35% total solids cow feces and wheat straw.
that methanogenesis reaction from acetate was not a rate limiting step. Similarly, propionic and butyric acids peaked to levels less than 280 ± 110 and 170 ± 110 mg L1, respectively, after feedings and were consumed within a week to levels close to the detection limits of the instrument (25 ± 10 mg L1). Concentrations of other volatile fatty acids (isobutyric-, iso-valeric-, and valeric-acid) were less than 25 mg L1 throughout the entire successive cycles. The profiles of the VFAs concentration and the progress of methane production during the successive cycles suggest a pseudo steady-state condition and indicate that acetogenic and methanogenic reactions proceeded fairly well. Inhibitory effects of VFAs depend among other factors on the concentrations, pH, and alkalinity (Angelidaki et al., 1993). The VFAs toxicity increases with drop in pH. Acetate and propionate concentrations of 8976 and 2358 mg L1 caused 50% inhibition in methane production at pH 7.5 (Salces, 2010). The pH profile (Fig. 4) ranged between 7.4 at the beginning of the cycle (day 0) and 8.1 at the beginning of the cycle (day 21) with an overall average of 7.9 ± 0.2. The stability of pH was due to the sufficient buffering capacity of reactor content (9.65 ± 0.41 g CaCO3 kg1). The low rate of hydrolysis and the continuous consumption of VFAs by acetogens and methanogens prevented VFAs accumulation and maintained their concentrations below the inhibitory levels during the entire operation. Since no fluctuation has been observed in the rate of biogas production (Fig. 2) then there was no cause of inhibition whether due to VFAs accumulation, pH drop, ammonia inhibition or other causes. The profiles of the VFAs concentrations, the stable methane production and yield during the successive cycles indicate that acetogenic and methanogenic reactions proceeded fairly well. 3.3. Solids reduction Profiles of the total and volatile solids in the bioreactors during the successive cycles were identical in the duplicate bioreactors (data not shown). Volatile solids (VS) removal was 42.4 ± 4.3%. The overall average TS and VS of the digesting mixture (inoculum and feed) at the beginning of each cycle was around 24.2 ± 1.8% and 21.7 ± 1.7%. Similarly, the overall average of TS and VS of the inoculum at the end of each cycle was 23.1 ± 0.8% and 20.5 ± 0.9%, respectively. 4. Conclusions Average specific methane yield from a novel psychrophilic (20 °C) dry anaerobic digestion (PDAD) was 151.8 ± 7.9 N L CH4 kg1 VS. The results were reproducible and the operation was stable in 147 days of successive cycles (21 days each); a maximum SMY of 163.3 ± 5.7 N L CH4 kg1 VS fed with a maximum CH4
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 (4), 965–975. Angelidaki, I., Ellegaard, L., Ahring, B.K., 1993. A mathematical model for dynamic simulation of anaerobic digestion of complex substrates: focusing on ammonia inhibition. Biotechnol. Bioeng. 42 (2), 159–166. APHA, 1992. Standard Methods for the Examination of Water and Wastewater,, 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 (1), 50–55. Beal, L.J., 1998. Laboratory scale testing of an anaerobic waste treatment system for a confectionery wastewater. The University of Tennessee, Knoxville, Tennessee. Bolzonella, D., Cavinato, C., Fatone, F., Frison, N., Pavan, P., Cecchi, F., 2011. Anaerobic digestion of livestock effluents, energy crops, and agro-waste: renewable energy potential and nutrients management. In: International Symposium on Anaerobic Digestion of Solid Waste and Energy Crops (ADSW&EC), August 28–September 01, 2011, Vienna, Austria. Böske, J., Wirth, B., Garlipp, F., Mumme, J., Van den Weghe, H., 2015. Upflow anaerobic solid-state (UASS) digestion of horse manure: thermophilic vs. mesophilic performance. Bioresour. Technol. 175, 8–16. 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, June 26–29, 2012, Perugia, Italy. Jha, A.K., Li, J., Zhang, L., Ban, Q., Jin, Y., 2013. Comparison between wet and dry anaerobic digestions of cow dung under mesophilic and thermophilic conditions. Adv. Water Resour. Protect. (AWRP) 1 (2), 28–38. Kashyap, D.R., Dadhich, K.S., Sharma, S.K., 2003. Biomethanation under psychrophilic conditions: a review. Bioresour. Technol. 87 (2), 147–153. Kusch, S., Oechsner, H., Jungbluth, T., 2008. Biogas production with horse dung in solid-phase digestion systems. Bioresour. Technol. 99 (5), 1280–1292. Li, J., Jha, A.K., He, J., Ban, Q., Chang, S., Wang, P., 2011. Assessment of the effects of dry anaerobic codigestion of cow dung with waste water sludge on biogas yield and biodegradability. Int. J. Phys. Sci. 6, 3679–3688. 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, 15–20. Massé, D.I., Masse, L., Croteau, F., 2003. The effect of temperature fluctuations on psychrophilic anaerobic sequencing batch reactors treating swine manure. Bioresour. Technol. 89 (1), 57–62. Massé, D.I., Masse, L., Xia, Y., Gilbert, Y., 2010. Potential of low-temperature anaerobic digestion to address current environmental concerns on swine production. J. Anim. Sci. 88 (13 Suppl), E112–120. Massé, D.I., Saady, N.M.C., 2015. Psychrophilic dry anaerobic digestion of dairy cow feces: long-term operation. Waste Manage 36, 86–92,. Massé, D.I., Saady, N.M.C., Rajagopal, R., 2015. Psychrophilic dry anaerobic digestion of high solids content dairy manure: long-term operation. Biol. Eng. Trans. ASABE 7 (3), 99–112. Motte, J.C., Escudié, R., Bernet, N., Delgenes, J.P., Steyer, J.P., Dumas, C., 2013. Dynamic effect of total solid content, low substrate/inoculum ratio and particle size on solid-state anaerobic digestion. Bioresour. Technol. 144, 141–148. Saady, N.M.C., Massé, D.I., 2013. Psychrophilic anaerobic digestion of lignocellulosic biomass: a characterization study. Bioresour. Technol. 142, 663–671. Salces, B.M., 2010. Anaerobic Digestion of Livestock Wastes: Vegetable Residues as Co-Substrate and Digestate Post-Treatment. University de Valladolid, . Schäfer, W., Lehto, M., Teye, F. 2006. Dry Anaerobic Digestion of Organic Residues On-farm – A Feasibility Study. Agrifood Research Reports 77. MTT Agrifood Research Finland. URL: . 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 (4), 291–302. Wei, S., Zhang, H., Cai, X., Xu, J., Fang, J., Liu, H., 2014. Psychrophilic anaerobic codigestion of highland barley straw with two animal manures at high altitude for enhancing biogas production. Energy Convers. Manage. 88, 40–48. Zhu, J., Zheng, Y., Xu, F., Li, Y., 2014. Solid-state anaerobic co-digestion of hay and soybean processing waste for biogas production. Bioresour. Technol. 154, 240– 247.
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High rate psychrophilic anaerobic digestion of high solids (35%) dairy manure in sequence batch reactor Noori M. Cata Saady, Daniel I. Massé ⇑ Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, Sherbrooke, Quebec J1M 0C8, Canada
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Psychrophilic dry anaerobic digestion
(PDAD) of cow feces and wheat straw (CFWS). 1 1 At OLR 6.0 g TCOD kg d and 21 days the yield is 163.3 ± 5.7 N L CH4 kg1 VS. PDAD of CFWS is feasible at 35% TS in feed and OLR of 6.0 g TCOD kg1 d1. VS-based inoculum to substrate ratio of 2.13 ± 0.2. PDAD of CFWS (TS 35%) is as efficient as mesophilic DAD.
a r t i c l e
i n f o
Article history: Received 22 January 2015 Received in revised form 4 March 2015 Accepted 7 March 2015 Available online 13 March 2015 Keywords: Anaerobic digestion Cow manure Dry Psychrophilic Wheat straw
a b s t r a c t Zero liquid discharge is increasingly adopted as an objective for waste treatment process. The objective of this study was to increase the feed total solids (TS) and the organic loading rate (OLR) fed to a novel psychrophilic (20 °C) dry anaerobic digestion (PDAD). Duplicate laboratory-scale bioreactors were fed cow feces and wheat straw (35% TS in feed) at OLR of 6.0 g TCOD kg1 inoculum d1 during long-term operation (147 days consisting of 7 successive cycles). An overall average specific methane yield (SMY) of 151.8 ± 7.9 N L CH4 kg1 VS fed with an averaged volatile solids removal of 42.4 ± 4.3% were obtained at a volatile solids-based inoculum-to-substrate ratio (ISR) of 2.13 ± 0.2. The operation was stable as indicated by biogas and VFAs profiles and the results were reproducible in successive cycles; a maximum SMY of 163.3 ± 5.7 N L CH4 kg1 VS fed was obtained. Hydrolysis was the reaction limiting step. High rate PDAD of 35% TS dairy manure is possible in sequential batch reactor within 21 days treatment cycle length. Crown Copyright Ó 2015 Published by Elsevier Ltd. All rights reserved.
1. Introduction The main waste by-product from livestock operations is manure containing biodegradable organic matter. The amount of manure has been increasing worldwide as a result of the livestock industry growth. Manure handling and disposal represent a substantial part of farm operation cost and also represent environmental, economic, and social challenges to livestock operations. ⇑ 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.biortech.2015.03.038 0960-8524/Crown Copyright Ó 2015 Published by Elsevier Ltd. All rights reserved.
Therefore, developing robust and cost effective technologies for on-farm manure-to-energy conversion can solve some of its environmental problems and improve the social acceptability of livestock operations. The generation of revenue will help to offset manure management cost. Currently, anaerobic digestion is well-established and accepted on-farm manure treatment which generates combustible biogas containing methane. Dairy manure on livestock operations that use bedding is characterized by its high solids; depending on the amount of bedding used the total solids (TS) of manure may be as high as 40%. Wet anaerobic digestion (WAD; TS < 15%) has been
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used to treat manure; however, currently developing dry anaerobic digestion (DAD; TS > 15%) is gaining interest because it offers several advantages when compared with WAD. For example, DAD requires a digester of smaller size compared to wet anaerobic digestion (WAD); this translates in reduced capital and operational costs, and higher volumetric bioenergy yield (Luning et al., 2003). The advantages of DAD have been demonstrated at mesophilic and thermophilic conditions for agricultural wastes and livestock manure (15–20% TS) (Ahn et al., 2010; Böske et al., 2015; Di Maria et al., 2012; Jha et al., 2013; Kusch et al., 2008; Wei et al., 2014; Zhu et al., 2014). However, most manure producing regions such as Canada, Europe, northern part of the USA and China are classified as cold regions. Therefore, psychrophilic (15–25 °C) operation would be more suitable for cold-climate regions if its feasibility is demonstrated because maintaining mesophilic and thermophilic conditions by heating the bioreactor reduces the net energy output. Kashyap et al. (2003) stated that developing a psychrophilic anaerobic digestion process to convert cattle manure to biogas is still a technological challenge. Since then, some technological advancement have been achieved; several on-farm psychrophilic WAD processes have been developed and deployed (Massé et al., 2010). Accommodating manure with high solids content (>15%) in psychrophilic operation is still a challenge that needs a technical solution. Presently, developing and optimizing a psychrophilic dry anaerobic digestion (PDAD) process is under extensive research worldwide. Recently, a PDAD process of cow feces with and without wheat straw in sequential batch reactor (SBR) at 20 °C, has been developed at the Dairy and Swine Research and Development Centre (DSRDC) of Agriculture and Agri-Food Canada, in Sherbrooke, Quebec-Canada (Massé et al., 2015). The process has been demonstrated in long term operation (> 450 days) at TS of 27% and organic loading rate of 3 g TCOD kg1 inoculum d1 (Massé and Saady, 2015). The upper threshold limit for the total solids at which successful dry anaerobic digestion can be conducted is not well determined. Recently, Motte et al. (2013) concluded that wheat straw mesophilic conversion to volatile fatty acids (VFAs) decreased with the increase in its TS from 10% to 33% with no changes in the volatile fatty acids profiles until a clear limit at 28% TS supposedly because of the decrease of free water in the media. No information exists regarding the threshold limit of the TS content for a substrate composed of cow feces and wheat straw digested at psychrophilic condition. This study is a step on developing PDAD by increasing the total solids content of the feed by 30% (from TS of 27% to 35%) while doubling the organic loading rate (from 3.0 to 6.0 g TCOD kg1 inoculum d1) fed to sequence batch reactor. The principal objectives of this study were to answer the question: is it possible to have a high rate PDAD of high total solids (35%) dairy manure at relatively short treatment cycle length (21 days). To the authors’ best knowledge, this is the first reports on successful long-term operation (147 days consisting of 7 successive treatment cycles) of PDAD at 35% TS in feed and OLR of 6.0 g TCOD kg1 inoculum d1.
the following steps: day 1: loading the reactor with inoculum, feeding the substrate, and mixing the inoculum and substrate; day 1–21: reaction; and day 21: unloading the digestate and starting the next cycle. 2.2. Bioreactor Duplicate 40-L cylindrical (0.312 m in diameter 0.520 m in height) plastic barrels bioreactors have been operated as sequential batch reactors (SBR) at a treatment cycle length (TCL) of 21 days in a temperature-controlled room (20 °C). The average mass of combined feed and inoculum in the reactor was about 7.74 ± 0.03 kg based on the masses of the cow feces, wheat straw and inoculum used. The reactors were fitted with two gas lines; one for purging the nitrogen gas immediately after feeding/ loading the substrate to expel O2 and initiate an anaerobic environment inside reactor; and the second to release and measure the biogas produced. The biogas was sampled and its composition was analyzed once a week (Fig. 1). The barrel was kept upside down after it has been filled so that the wet content works as a water seal in addition to the seal of the barrel’s lid to ensure gas tightness. 2.3. Wet tip tank gas meter The wet tip tank gas meter (Beal, 1998) was manufactured at the workshop of the DSRDC. It is a transparent box made of acrylic contains a pivoting plastic tipping bucket (two equal-size compartments) and faced downwards just vertically above to a port that releases biogas under the pivoting point. The tipping container was placed submerged inside the water-filled acrylic box. When biogas released into one compartment reaches a certain known volume, the container tips (the compartment filled with gas pivots upward) due to the buoyancy of the gas and releases the biogas which bubbles upwards to escape from water and leaves the box. Upon tipping (pivoting) the second compartment on the other side of the container moves downward and biogas released from the port starts to accumulate inside it, and so on. Each tip was counted using a reed switch which sends an electrical pulse to an electronic digital counter (manufacturer: OmRon; model number: H7EC-NVB). The tip counts were monitored daily and used to calculate the volume of the biogas produced. The wet tip tank gas meter calibration was checked weekly. After feeding, the reactor was held upside down so that the inoculum–substrate mixture provides sealing in addition to the locked lid of the barrel. 2.4. Inoculum and substrate The initial inoculum was obtained from a laboratory scale (40 L) psychrophilic (20 °C) anaerobic sequence batch reactor fed with fresh dairy manure and wheat straw (35% TS); the performance of the seeding inoculum has been reported previously (Saady and
7 6
1 1- 40 L plastic barrel
2. Methods
The experiments assessed the effect of the organic loading rate (6.0 g TCOD kg1 inoculum day1) on CH4 production and process stability of psychrophilic anaerobic digestion of cow feces and wheat straw. The substrate total solids (TS) have been kept at 35%. The 147 days of operation comprised of seven successive cycles. The treatment cycle length (TCL) was 21 days measured between successive feedings of the reactor. The cycle includes
2- Barrel lid with metal lock
5
2.1. Experimental setup
3- Gas release line 4- N 2 purging line with valve 5- Substrate-inoculum mixture
2 4
3
6- Head space 7- Wet tip tank gas meter
Fig. 1. Schematic diagram of the dry anaerobic digester.
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Massé, 2013). 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 was collected on wood boards, before getting in contact with urine and bedding, transferred into a plastic drum, stored at 4 °C (for a maximum of 30 days before feeding), and triplicate representative samples have been taken from each batch of cow feces fed to the reactors in each treatment cycle after thorough mixing and homogenization and before feeding the reactors. Three different batches of cow feces from the same experimental farm at DSRDC, Sherbrooke, Quebec were used during the 7 cycles as indicated in Table 1 (the first batch has been used for cycles 1 and 2; the second batch has been used for cycles 3, 4 and 5, and the third batch has been used for cycles 6 and 7). Wheat straw was harvested at the DSRDC’s experimental farm during fall 2012 and fall 2013 and chopped (3 mm) using a laboratory mill (Thomas Wiley Laboratory Mill Model 4, Arthur H. Thomas Company, Philadelphia, PA). The substrate and inoculum have been mixed manually for 5 min during feeding. Every week the content of the bioreactor was mixed manually for 5 min before sampling the content to ensure a homogenous and representative sample was taken. No mixing took place during other time of the treatment cycle; therefore, the process can be considered as a static dry anaerobic digestion. Physicochemical characteristics of the cow feces fed in each treatment cycle are given in Table 1. Physico-chemical characteristics of the inoculum and inoculum-substrate mixture (ISM) before feeding bioreactors are given in Table 2.
2.5. Organic loading rate (OLR) Wheat straw and cow feces were mixed manually to obtain the desired substrate TS content (35%) while maintaining the design organic loading rate (OLR) of 6.0 g TCOD kg1 inoculum day1 (equivalent to 4.47 ± 0.02 g VS kg1 inoculum d1). The mass of
inoculum, feces, and/or straw fed to each bioreactor at the beginning of the successive cycles, the OLR and inoculum-to-substrate ratio (ISR) are given in Table 3. The ISR was expressed in kg of total VS fed per kg VS of inoculum averaged at 2.13 ± 0.2. 2.6. Sampling The inoculum-substrate mixture (ISM) was sampled on day (0) immediately after feeding. Samples were also taken on day 7 by opening the reactor, mixing its content manually for 5 min to ensure that the sample is representative of the reactor content. The reactor content was also sampled at the end of the treatment cycle (on day 21); this sample represented the digestate (effluent) of which 6 kg were used as inoculum in the next cycle. The reactor has been flushed with nitrogen to maintain anaerobic digestion. The gas samples were taken through gas sampling port sealed with septa and installed on the gas line mid-way between the barrel and the wet tip gas meter using a 10 mL plastic syringe. 2.7. Biogas measurement Biogas volume produced was measured daily using calibrated wet tip gas meters while the biogas components (CH4, CO2 and H2S) were determined weekly using a Hach Carle 400 AGC gas chromatograph (Model 04131-C, Chandler Engineering, Houston, TX) configured for the application 131-C. The application uses a packed 3.175 mm diameter column Chromosorb W, high performance (HP) grade composed of 1.8 m (805 porapak N + 205 Porapak Q), 2.1 m (80% molecular sieve 13X + 20% molecular Sieve 5A), and 1.8 m (80% OV-101 on chromosorb). The column and thermal conductivity detector were operated at 85 °C with a helium gas flow rate of 30 mL min1. The GC calibration was performed weekly with a standard gas (27.3% CO2, 1.01% N2, 71.16% CH4, 0.53% H2S); several standard gas samples were injected at the beginning and end of gas analysis as well as after every 10 injections of experimental gas samples.
Table 1 Physicochemical characteristics of the cow feces fed in each treatment cycle. Cycle
Substrate
pH
TCOD (g kg1)
TS (%)
VS (%)
Acetate (g kg1)
Propionate (g kg1)
Butyrate (g kg1)
1–2 3–5 6–7 1–4
Cow feces Cow feces Cow feces Wheat straw
6.71 6.74 7.02 NA
155.2 169.0 180.0 1097
11.5 12.6 13.3 89
10.1 11.5 11.8 84.9
3.9 4.01 2.93 NA
0.96 1.41 0.86 NA
0.50 2.35 0.55 NA
Fiber components (% of dry matter) Cellulose
Hemicellulose
Lignin
24.39 26.33 23.50 38.61
19.28 16.35 16.72 25.14
12.74 10.67 13.45 7.3
Table 2 Physico-chemical characteristics of the inoculum and inoculum-substrate mixture at the beginning of each digestion cycle. Cycle
TYPE
pH
Acetate (g kg1)
Propionate (g kg1)
Butyrate (g kg1)
1
Inoculum ISM
7.88 ± 0.02 7.60 ± 0.03
0.13 ± 0.01 1.23 ± 0.07
0.02 ± .00 0.55 ± 0.01
Inoculum ISM
8.27 ± 0.07 8.03 ± 0.11
0.18 ± 0.01 1.18 ± 0.37
Inoculum ISM
8.04 ± 0.18 7.75 ± 0.11
Inoculum ISM
2 3 4 5 6 7
Alkalinity (g kg1 as CaCO3)
TS (%)
VS (%)
0.09 ± 0.00 0.40 ± 0.02
10.104 ± 0.81
19.5 ± 0.4 20.9 ± 0.7
16.7 ± 0.5 18.2 ± 0.2
0.03 ± 0.01 0.39 ± 0.09
0.22 ± 0.00 0.37 ± 0.02
10.168 ± 1.3
20.8 ± 0.4 23.0 ± 0.3
18.1 ± 0.1 20.5 ± 0.1
0.14 ± 0.01 1.26 ± 0.45
0.03 ± 0.01 0.32 ± 0.10
0.02 ± 0.00 0.12 ± 0.21
9.314 ± 0.13
23.7 ± 1.9 24.8 ± 1.9
21.3 ± 1.8 22.4 ± 1.8
8.02 ± 0.08 7.91 ± 0.14
0.13 ± 0.03 0.82 ± 0.47
0.03 ± 0.01 0.23 ± 0.15
0.02 ± 0.00 0.07 ± 0.05
9.321 ± 0.67
22.9 ± 1.0 23.9 ± 0.7
20.4 ± 0.9 21.6 ± 0.5
Inoculum ISM
7.89 ± 0.04 7.63 ± 0.14
0.14 ± 0.02 1.04 ± 0.49
0.03 ± 0.01 0.36 ± 0.02
0.01 ± 0.00 0.26 ± 0.16
10.00 ± 0.45
22.8 ± 0.8 24.7 ± 1.4
20.2 ± 0.7 21.9 ± 1.4
Inoculum ISM
8.15 ± 0.19 7.75 ± 0.08
0.240.05 1.06 ± 0.64
0.04 ± 0.01 0.39 ± 0.32
0.02 ± 0.00 0.03 ± 0.01
9.715 ± 0.54
23.8 ± 0.4 25.6 ± 1.2
21.3 ± 0.3 23.2 ± 1.1
Inoculum ISM
8.05 ± 0.11 7.78 ± 0.13
0.27 ± 0.02 0.85 ± 0.41
0.05 ± 0.02 0.27 ± 0.10
0.03 ± 0.01 0.04 ± 0.02
9.16 ± 0.11
24.4 ± 0.4 25.5 ± 0.5
21.8 ± 0.4 23.0 ± 0.5
Note: ISM = Inoculum-substrate mixture immediately after feeding.
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N.M.C. Saady, D.I. Massé / Bioresource Technology 186 (2015) 74–80 Table 3 Details of the organic load fed to each bioreactor at the beginning of the successive cycles. Cycle
1 2 3 4 5 6 7
Feces (kg)
1.268 1.268 1.227 1.227 1.227 1.218 1.218
Straw (kg)
0.510 0.510 0.500 0.500 0.500 0.489 0.489
ISR (VS-based ratio)
1.78 1.93 2.26 2.16 2.14 2.28 2.34
Organic loading TCOD fed (g)
VS fed (g)
g TCOD kg1 inoculum d1
g VS kg1 inoculum d1
756.3 756.3 755.9 755.9 755.9 756.0 756.0
561.6 561.6 566.1 566.1 566.1 559.4 559.4
6.0 6.0 6.0 6.0 6.0 6.0 6.0
4.46 4.46 4.49 4.49 4.49 4.44 4.44
Note: In all cycles the reactors have been inoculated with 6 kg of inoculum which has been transferred from the previous cycle. Feed TS has been kept at 35% in all cycles.
The linearity of the quantification regression equation covered the range from 0.001% to 100% (pure gas injection) with R2 value of 0.9991, 0.9988, and 0.9968 for CH4, CO2, and H2S. The quantification regression equation has been generated using certified standards containing different proportions of the gases measured. Methane (CH4) production is reported in normalized litres (N L CH4), i.e., the CH4 volume produced was corrected to standard temperature and pressure (STP) (273oK; 1 atm) using Eq. (1).
V CH4STP ¼ g V m
T s Pm T m Ps
ð1Þ
where: Vm is the measured volume of biogas (L), g is the percentage of CH4 in biogas, Tm and Pm which are the actual temperature (K) and atmospheric pressure (kPa) at the time of measurement, and Ts and Ps are the standard temperature (K) and pressure (kPa). V CH4STP is the volume (L) of methane at the standard temperature and atmospheric pressure (273 K and 101.3 kPa, respectively). The specific methane yield (SMY) obtained during an individual cycle (TCL = 21 days) has been calculated by divining the cumulative methane volume produced during that cycle by the mass of volatile solids (VS) contained in the manure fed at the beginning of that cycle as shown in Eq. (2).
SMY ¼
Cumulative methane volume VS fed
ð2Þ
2.8. Analytical methods Samples were collected from each bioreactor and analyzed weekly for volatile fatty acids (VFAs), total solids (TS), volatile solids (VS), and pH. 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). Alkalinity: method number 2320B used Potentiometric titration to preselected pH 4.38, Total solids: method number 2540B, Volatile solids method number: 2540E. VFA: method number 5560D. The concentrations of individual volatile fatty acids (VFAs) including acetic, propionic, butyric, isobutyric, butyric, valeric and isovaleric acids have been measured using Perkin Elmer gas chromatograph (GC) model 8310 (Perkin Elmer, Waltham, Mass.) equipped with an autosampler to facilitate the analysis, fitted with FID, and equipped with a J&W Scientific DB-FFAP high resolution column (30 m 0.53 mm 1.00 lm; Chromatographic Specialties Inc., Ontario) (Massé et al., 2003). Helium, flowing at 9.5 mL min1, was employed as the carrier. The injector temperature was maintained at 200 °C, while the detector temperature was set at 250 °C. A 10 gram of sample was diluted in 20 mL of deionized water, mixed thoroughly and the liquid part was separated and transferred into a centrifuge tube. After centrifugation for 30 min at 46,300 g, 2 mL of the supernatant was transferred
into 3 mL centrifuge tube, acidified with phosphoric acid, and centrifuged for 15 min at 16,300 g. A 0.5 mL of the supernatant was filtered through a 0.45 lm nylon syringe filter into 1.5 mL GC tube. The detection limit of acetic acid was 3.5 mg L1 while the detection lime of all other acids was 1.5 mg L1. 2.9. Fiber analysis The complex substrates (cow feces and wheat straw) were subjected to fiber analysis to determine their content of cellulose, hemicellulose, and lignin. 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; Saady and Massé, 2013). 3. Results and discussion Feasibility of PDAD-SBR has been demonstrated for digesting cow feces (13–16% TS) during long-term operation (252 days) of operation with an average specific methane yield (SMY) of 222 ± 27.2 N L CH4 kg1 VS fed at OLR of 5.0 g TCOD kg1 inoculum d1 (Massé and Saady, 2015). Furthermore, its feasibility for digesting dairy manure (cow feces and wheat straw) at 27% total solids has successfully been demonstrated at OLR of 3 g TCOD kg1 inoculum d1 in long term operation (273 days) with an average SMY of 182.9 ± 16.9 N L CH4 kg1 VS fed (Massé et al., 2015). The results of this study answer the question: is it possible to have a high rate (OLR of 6.0 g TCOD kg1 inoculum d1) anaerobic digestion of high total solids (35%) dairy manure in sequence batch reactor at relatively short treatment cycle length (21 days)? These microorganisms-challenging conditions have been applied and the performance has been evaluated based on specific methane yield, volatile fatty acid levels, and VS removal for long-term (147 days) operation comprising 7 successive cycles. Quality of the cow feces as described by TCOD, TS, and VS content varied significantly. During the first 2 cycles, the cow feces had lower value of TCOD, TS, and VS compared to that used during the rest of cycles. 3.1. Methane production The performance of psychrophilic SBR has been evaluated in 7 successive cycles at OLR of 6.0 g TCOD kg1 inoculum d1 (equivalent to 4.47 ± 0.02 g VS kg1 inoculum d1). Cumulative methane production profiles expressed as average specific methane yield (SMY) of the quadruplicate bioreactors are shown in Fig. 2. The maximum SMYs calculated during the successive cycles are given in Table 4. Based on the total VS fed (cow feces plus wheat straw), the average SMY calculated ranged between 141.8 ± 5.4 and 163.3 ± 5.7 N L CH4 kg1 VS fed in cycle 1 and 3, respectively.
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N.M.C. Saady, D.I. Massé / Bioresource Technology 186 (2015) 74–80
batch reactor with an average solid retention time (SRT) of about 94.4 ± 0.5 days allowed the degradation of the fiber component in the substrate fed. The average specific CH4 production rate (N L CH4 kg1 VS d1) of the duplicate bioreactors ranged between 6.8 ± 0.3 (cycle 1) and 7.8 ± 0.3 (cycle 3); the overall average of the specific CH4 production rate for the 7 successive cycles was 7.2 ± 0.4 N L CH4 kg1 VS d1. The results have been compared to the performance of mesophilic and thermophilic DAD of various substrates (Table 6) because data on performance of psychrophilic dry anaerobic digestion of cow feces and wheat straw is not available in the accessible literature. The average specific methane yield (151.8 ± 7.9 N L CH4 kg1 VS of substrate fed (35% TS at OLR 6.0 g TCOD kg1 inoculum or 4.47 ± 0.02 kg VS fed kg1 inoculum d1) achieved in this study after 21 days of psychrophilic (20 °C) incubation is comparable to the yield of 160 N L CH4 kg1 VS of dairy manure, straw, and oat husk (TS 17% at OLR of 3.4 g VS kg1 d1) reported by Schäfer et al., 2006 for Jarna biogas plant in Sweden which operates at 38 °C and retention time of 22 days. Notice that the data from Jarna plant was for inoculum which has been adapted to the substrate and the steady state operation condition for three years at the time of the study reported by Schäfer et al. (2006); a longer adaptation of psychrophilic inoculum is expected to increase the rate of CH4 production and shorten the TCL for a fixed design OLR. The SMYs from cow feces and wheat straw at an TCL of 21 days in any of the PDAD seven successive cycles (TS 35%) obtained in this study were higher than 28 L CH4 kg1 VS of dairy manure and switch grass (15% TS) obtained by Ahn et al. (2010) during 62 days of thermophilic (55 °C) incubation. Ahn et al. (2010) study was single batch (62 days) experiment with low ISR (0.2); therefore, the results of the current study demonstrate the importance of prolonged solid retention time provided by the sequence batch reactor and the importance of ISR. Inoculum adaptation to the high solids content condition is another necessary step; this study used an inoculum which has been adapted
Fig. 2. Specific methane yield profiles for psychrophilic dry anaerobic digestion of cow feces and wheat straw (35% TS).
The overall average SMY of the 7 successive cycles was 151.8 ± 7.9 N L CH4 kg1 VS fed (113.1 ± 6.1 N L CH4 kg1 TCOD fed. Although the organic loading rates (6.0 g TCOD kg1 inoculum day1), the feed total solids (35%), and the TCL (21 days) have been maintained the same during the successive cycles, the SMY fluctuated from cycle to cycle. This fluctuation could have partially been caused by variation in composition of the cow feces fed (Table 1). Notice that the overall average of the SMY (150.4 ± 12.1 N L CH4 kg1 VS fed) during cycle 1 and 2 was not significantly different from the overall average of the SMY (156.7 ± 6.2 N L CH4 kg1 VS fed) during the cycles 3, 4 and 5 or the overall average of the SMY (145.9 ± 0.7 N L CH4 kg1 VS fed) during cycle 6 and 7. Notice that wheat straw provided on average 75.5 ± 1.2% of the VS fed, 72.5 ± 1.2% of the TCOD fed, and 79.1 ± 0.7% of the fiber fed, and 28.8 ± 0.2% of the feed wet mass (Table 5). The dry matter of wheat straw is composed of cellulose (38.61%), hemicellulose (25.14%) and lignin (7.3%). Lignocellulose fibers in wheat straw and cow feces require a relatively longer treatment cycle length during anaerobic digestion than that required for conversion of the soluble biodegradable components of cow feces. The sequence
Table 4 Rate and specific methane yield for the psychrophilic dry anaerobic digestion of cow feces and wheat straw (35% TS). Cycle number
Cow feces TCOD/VS ratio
1 2 3 4 5 6 7
SMY (N L CH4 kg1 VS) a
1.54 1.54 1.47 1.47 1.47 1.53 1.53
141.8 ± 5.4 158.9 ± 6.2b 163.3 ± 5.7b 156.1 ± 0.0c 150.9 ± 9.3a,b,c 145.4 ± 5.7a 146.4 ± 1.6a
SMY (N L CH4 kg1 TCOD)
Rate of CH4 production (N L CH4 kg1 VS d1)
105.6 ± 4.0 118.3 ± 4.6 121.6 ± 4.2 116.9 ± 0.0 112.9 ± 7.0 107.7 ± 4.2 108.5 ± 1.2
6.8 ± 0.3 7.6 ± 0.3 7.8 ± 0.3 7.4 ± 0.0 7.2 ± 0.4 6.9 ± 0.3 7.0 ± 0.1
Note: 1 – The treatment cycle length (TCL) in all cycles was 21 days. 2 – The numbers given are averages and standard deviations. 3 – SMY with the same superscript are statistically not significantly different from each other (based on Tukey’s test of multiple comparisons at 95% significance level).
Table 5 Contribution of wheat straw to the feed fiber, TCOD, VS and mass. Cycle
CF (kg)
WS (kg)
Feces COD (g kg1)
Feces TS%
Feces VS%
Feces fiber in feed (kg)
WS fiber in feed (kg)
WS/CF fiber ratio in feed
WS fiber in feed (%)
WS/CF COD ratio in feed
WS COD in feed (%)
WS/CF VSratio in feed
WS VS in feed (%)
WS mass in feed %
1 2 3 4 5 6 7
1.218 1.218 1.227 1.227 1.227 1.218 1.218
0.489 0.489 0.500 0.500 0.500 0.489 0.489
155.2 155.2 169.0 169.0 169.0 180.3 180.3
11.5 11.5 12.6 12.6 12.6 13.3 13.3
10.1 10.1 11.5 11.5 11.5 11.8 11.8
0.082 0.082 0.082 0.082 0.082 0.087 0.087
0.322 0.322 0.316 0.316 0.316 0.309 0.309
3.92 3.92 3.83 3.83 3.83 3.56 3.56
79.7 79.7 79.3 79.3 79.3 78.1 78.1
2.84 2.84 2.65 2.65 2.65 2.44 2.44
74.0 74.0 72.6 72.6 72.6 71.0 71.0
3.38 3.38 3.01 3.01 3.01 2.89 2.89
77.2 77.2 75.1 75.1 75.1 74.3 74.3
28.7 28.7 29.0 29.0 29.0 28.6 28.6
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N.M.C. Saady, D.I. Massé / Bioresource Technology 186 (2015) 74–80 Table 6 Comparative performance of dry anaerobic digestion of cow manure and wheat straw. Substrate and inoculum
Temperature (°C)
TS (%)
ISR
OLR (g TCOD kg1 inoculum day1)
Cow feces and wheat straw Cow feces and wheat straw
20 20
35 27
2.13 ± 0.20e 5.55 ± 0.01e
6.0 3.0
CM:WWS (2:3 mass ratio) Aerobically pre-treated SM, agricultural residues
35 35 35 55
16 28 28 15
0.2 NR NR 0.2
0.35a 0.28b
DM and SG Rice straw and corn stover inoculated with (1:1) sewage sludge: pig manure
26–28
85% beef manure plus 15% grass silage
35
15 20 25 30 35 28
0.2
NR
Retention time (days) 21 21
SMY (N L CH4 kg1 total VS)
References
151.8 ± 7.9 182.9 ± 16.9
This study Massé et al. (2015) Li et al. (2011) Di Maria et al. (2012) Ahn et al. (2010) Sun et al. (1987)
NR
63 130 65 62
328 55 22 28c
NR NR NR NR NR 0.9
156 156 168 198 198 100
346 339 382 423 34 227c c
Beef manure plus straw
32
18
NR
3.2
28
181
Pig manure with turnip rape straw and wheat straw DM, straw, and oat husk
35
16
NR
NR
120
122c
38
17
NR
3.4
22
160c
DM, straw, and oat husk
4.1
Fresh HM and straw 37
20
0.2
NR NR NR
84 28 42 72
146 175 208
Schäfer et al. (2006) Schäfer et al. (2006) Schäfer et al. (2006) Schäfer et al. (2006) Schäfer et al. (2006) Kusch et al. (2008)
CM = cow manure; DM = dairy manure; HM = horse manure; PM = poultry manure; SG = swithchgrass; SM = swine manure; WWS = waste water sludge; NR = not reported. Notes: a OLR units are kg TCOD kg1 VS. b OLR units are kg VS substrate kg1 VS inoculum. c SMY units are L CH4 kg1 VS d- SMY is per kg COD destroyed. e VS-based.
over a 36 months period to stepwise increase in the feed’s total solids (data not shown). In addition, sufficient quantity of inoculum has been used (average VS-based ISR of 2.13 ± 0.20). Despite that the average SMY (151.8 ± 7.9 N L CH4 kg1 VS fed) is lower than the 181 L CH4 kg1 VS of beef manure and straw (TS 18% and OLR of 3.2 g VS kg1 inoculum d1) at 32 °C and retention time of 28 days reported by Schäfer et al. (2006) (Table 6); notice that Schäfer et al. (2006) reported their results (181 L CH4 kg1 VS) at room temperature and atmospheric pressure and it would be around 163 N L CH4 kg1 VS if corrected to standard conditions. Compared to Schäfer et al. (2006) result, the current study demonstrated an increase of 94.4% in the feed total solids which represent a 51.5% reduction in the required volume of the bioreactor. Moreover, the current study decreased the treatment cycle length by 25% compared to Schäfer et al. (2006) and this represents an additional reduction of 25% in the volume of the bioreactor required. Furthermore, the current study saved the energy consumed in heating the bioreactor (to increase the temperature from 20 to 37 °C). Notice that the high yields (> 250 N L CH4 kg1 VS fed) reported by Li et al. (2011) was for mesophilic anaerobic digestion of cow manure and wastewater sludge (16% TS) in 63 days incubation. Similarly, the yields (339–423 N L CH4 kg1 total VS) reported by Sun et al. (1987) in Table 6 have been obtained for long retention times (156–198 days) and low OLR (0.35 kg TCOD kg1 inoculum day1) in mesophilic anaerobic digestion of rice straw and corn stover (TS 15–30%). These researchers reported a very low SMY (34 N L CH4 kg1 total VS) for the same substrate and experimental conditions at TS 35%. Achieving a stable dry anaerobic digestion of cow manure and wheat straw at psychrophilic condition and feed TS of 35% and OLR of 6.0 g TCOD kg1 inoculum d1 over long-term operation is a significant improvement given that 30% TS has been recently identified as a threshold above which methanogenesis was strongly inhibited for batch anaerobic digestion of cardboard at 35 °C (Abbassi-Guendouz et al., 2012).
Fig. 3. Typical volatile fatty acids profile during psychrophilic anaerobic digestion of 35% total solids cow feces and wheat straw.
Although the OLR used in this study (4.47 ± 0.02 g VS kg1 inoculum d1) is greater than the OLR of 3.0 g TVS L1 reactor d1 which is used in farm anaerobic digestion plants in Europe (Bolzonella et al., 2011) the TCL in this study is less by 50%. The specific methane yields obtained in this study provide evidence that PDAD of cow manure and straw is technically feasible at feed TS 35% and high OLR and is as efficient as mesophilic DAD given that a sufficient quantity of well-acclimatized inoculum is used. 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 in the feed were almost similar but at different concentration levels (data not shown). A typical profile of acetic, propionic, and butyric acids during a complete cycle is shown in Fig. 3. Throughout the successive cycles, acetic acid concentration peaked immediately after feeding to levels between 850 ± 350 mg L1 depending on its level in the cow feces fed, but it was consumed within a week and its concentrations were maintained within 190 ± 70 mg L1 indicating
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production rate of 7.8 N L CH4 kg1 VS day1 have been obtained. PDAD of dairy cow manure and wheat straw (total solids of 35%) at OLR of 6.0 g TCOD kg1 inoculum d1 and treatment cycle length of 21 in sequential batch reactor is feasible. References
Fig. 4. Typical pH and volatile solids profile during psychrophilic anaerobic digestion of 35% total solids cow feces and wheat straw.
that methanogenesis reaction from acetate was not a rate limiting step. Similarly, propionic and butyric acids peaked to levels less than 280 ± 110 and 170 ± 110 mg L1, respectively, after feedings and were consumed within a week to levels close to the detection limits of the instrument (25 ± 10 mg L1). Concentrations of other volatile fatty acids (isobutyric-, iso-valeric-, and valeric-acid) were less than 25 mg L1 throughout the entire successive cycles. The profiles of the VFAs concentration and the progress of methane production during the successive cycles suggest a pseudo steady-state condition and indicate that acetogenic and methanogenic reactions proceeded fairly well. Inhibitory effects of VFAs depend among other factors on the concentrations, pH, and alkalinity (Angelidaki et al., 1993). The VFAs toxicity increases with drop in pH. Acetate and propionate concentrations of 8976 and 2358 mg L1 caused 50% inhibition in methane production at pH 7.5 (Salces, 2010). The pH profile (Fig. 4) ranged between 7.4 at the beginning of the cycle (day 0) and 8.1 at the beginning of the cycle (day 21) with an overall average of 7.9 ± 0.2. The stability of pH was due to the sufficient buffering capacity of reactor content (9.65 ± 0.41 g CaCO3 kg1). The low rate of hydrolysis and the continuous consumption of VFAs by acetogens and methanogens prevented VFAs accumulation and maintained their concentrations below the inhibitory levels during the entire operation. Since no fluctuation has been observed in the rate of biogas production (Fig. 2) then there was no cause of inhibition whether due to VFAs accumulation, pH drop, ammonia inhibition or other causes. The profiles of the VFAs concentrations, the stable methane production and yield during the successive cycles indicate that acetogenic and methanogenic reactions proceeded fairly well. 3.3. Solids reduction Profiles of the total and volatile solids in the bioreactors during the successive cycles were identical in the duplicate bioreactors (data not shown). Volatile solids (VS) removal was 42.4 ± 4.3%. The overall average TS and VS of the digesting mixture (inoculum and feed) at the beginning of each cycle was around 24.2 ± 1.8% and 21.7 ± 1.7%. Similarly, the overall average of TS and VS of the inoculum at the end of each cycle was 23.1 ± 0.8% and 20.5 ± 0.9%, respectively. 4. Conclusions Average specific methane yield from a novel psychrophilic (20 °C) dry anaerobic digestion (PDAD) was 151.8 ± 7.9 N L CH4 kg1 VS. The results were reproducible and the operation was stable in 147 days of successive cycles (21 days each); a maximum SMY of 163.3 ± 5.7 N L CH4 kg1 VS fed with a maximum CH4
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