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Anaerobic digestion of animal waste: Effect of mode of mixing
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Water Research 39 (2005) 3597–3606 www.elsevier.com/locate/watres
Anaerobic digestion of animal waste: Effect of mode of mixing Khursheed Karima,, Rebecca Hoffmanna, K. Thomas Klassonb, M.H. Al-Dahhana a
Chemical Reaction Engineering Laboratory (CREL), Department of Chemical Engineering, Washington University, St. Louis, MO 63130, USA b Southern Regional Research Center, USDA-ARS, New Orleans, LA 70124, USA Received 25 October 2004; received in revised form 11 June 2005; accepted 22 June 2005
Abstract Laboratory-scale digesters were operated to study the effect of mixing (via biogas recirculation, impeller mixing, and slurry recirculation) on biogas production. Three sets of experiments were performed using cow manure slurry feed with either 50, 100, or 150 g/L total solids (TS) concentrations (referred in the text as 5%, 10%, and 15% manure slurry). The experiments were conducted at a controlled temperature of 35 1C and a hydraulic retention time of 16.2 days, resulting in TS loadings of 3.1, 6.2, and 9.3 g/L d for 5%, 10%, and 15% manure slurry feeds, respectively. Results showed that the unmixed and mixed digesters performed quite similarly when fed with 5% manure slurry and produced biogas at a rate of 0.84–0.94 L/L d. The methane yield was found to be 0.26–0.28 L CH4/g volatile solids loaded. However, the effect of mixing and the mode of mixing became important when the digesters were fed thick manure slurry feeds (10% and 15%). Digesters fed with 10% and 15% manure slurry and equipped with external mixing produced about 10–30% more biogas than the unmixed digester. While the mixed digesters produced more biogas than unmixed digesters, digester mixing during start-up was not beneficial, as it resulted in lower pH, performance instability and prolonged start-up time. Mixing using biogas recirculation system was found not to be effective in the case of 15% manure slurry feed under the experimental conditions studied. r 2005 Elsevier Ltd. All rights reserved. Keywords: Anaerobic; Biogas; Digestion; Manure; Mixing
1. Introduction Expansion in livestock industries has brought in the requirement of safe disposal of the large quantities of animal manure generated at dairy, swine, poultry, and other animal farms. In the US, over 100 million tons of dry matter is produced every year (Fontenot and Ross, 1980). Different types of waste management options Corresponding author. Tel.: 314 9357187; fax: 314 9357211.
E-mail addresses: [email protected], [email protected] (K. Karim).
may include technologies based on physical, chemical, or biological conversions. Biological means of anaerobic decomposition of animal waste has the advantage of producing a fuel gas (methane) as well as generating odor-free residues rich in nutrients, which can be used as fertilizers. The anaerobic digestion process of animal waste is primarily affected by its retention time in the reactor and the degree of contact between incoming substrate (animal waste) and a viable bacterial population. These parameters are primarily a function of the hydraulic regime (mixing) in the reactors. The importance of mixing in achieving efficient substrate
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.06.019
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conversion has been noted by many researchers, although the optimum mixing pattern is a subject of much debate. An intermediate degree of mixing was found to be optimal for substrate conversion (Smith et al., 1996). According to Sawyer and Grumbling (1960) and Meynell (1976), mixing of the substrate in the digester helps to distribute organisms uniformly throughout the mixture and to transfer heat. Furthermore, agitation aids in particle size reduction as digestion progresses and in removal of gas from the mixture. Mixing can be accomplished through various methods, including mechanical mixers, recirculation of digester contents, or by recirculating the produced biogas to the bottom of the digester using pumps. Two other important aspects are the intensity and duration of mixing. The information available in the literature on the role of mixing in anaerobic digesters is contradictory. Most of the literature on anaerobic digestion, for both low and high solids applications, emphasizes the importance of adequate mixing to improve the distribution of enzymes and microorganisms throughout the digester (Chapman, 1989; Parkin and Owen, 1986; Lema et al., 1991; Stenstrom et al., 1983). Several studies indicated that a lack of sufficient mixing in low solids digesters dealing with municipal waste resulted in a floating layer of solids (Stenstrom et al., 1983; James et al., 1980; Diaz and Trezek, 1977). These literature sources reported that the mixing level was increased to prevent formation of the solids layer. Chen et al. (1990) observed the development of a floating layer of solids in a 4.5 m3 unmixed digester treating refuse derived fuel and primary sludge mixture. They compared the performance of an unmixed (downward flow) and a continuously impellermixed digester at mesophilic conditions (30–40 1C). The unmixed digester exhibited a higher methane yield than the continuously mixed digester. Ho and Tan (1985) reported greater gas production from palm oil mill effluents for a continuously mixed digester than for an unmixed digester. However, Dague et al. (1970) observed that shifting from continuous mixing to intermittent mixing (2 min of mixing/h) resulted in significantly higher gas production from a liquid municipal waste stream. Similar controversies and uncertainties have been reported in the case of livestock waste digestion. Intermittent mixing in the anaerobic digestion of livestock waste under mesophilic temperature conditions has been recommended by Mills (1979) and Smith et al. (1979). Hashimoto (1982) found higher biogas production from beef cattle wastes under both continuous mixing and vacuum than under intermittent mixing and normal pressure conditions. However, Ben-Hasson et al. (1985) observed 75% lower methane production rates for a continuously mixed reactor than for an unmixed reactor when treating dairy cattle manure. Whitmore et al. (1987) and Dolfing (1992) suggested that very rapid mixing disrupts the structure of flocks in completely mixed
reactors, thereby disturbing the syntrophic relationships between organisms. Stroot et al. (2001) suggested that vigorous continuous mixing may prevent good performance of high solids anaerobic digesters and that minimal mixing was sufficient to distribute the feed adequately, which may have allowed the formation of new spatial associations. The US EPA (1979) has recommended a power input of 0.2–0.3 HP/1000 cu ft (5.26–7.91 W/m3) for proper digester mixing. The questions raised in the literature about the role of mixing in anaerobic digesters suggest that more research is needed. Therefore, the present study was designed to digest cow manure slurry in laboratory-scale anaerobic digesters, with and without external mixing, at three different solids concentrations. The main focus was to evaluate the effect of mixing and modes of mixing (biogas recirculation, impeller mixing, or slurry recirculation) on the methane production rate at three different influent solids concentrations, while keeping same amount of energy applied per unit volume of the digester mixed.
2. Materials and methods The reported study was performed in three sets of experiments using four laboratory-scale digesters having a working volume of 3.73 L. Schematics of the digesters are shown in Fig. 1. Digesters were made of clear PVC and had hopper bottoms with a 251 slope angle. Biogas generated in the digesters was collected in tedlar bags. In the first set of experiments, Digester 1 was operated as unmixed (without an external mixing aid), while Digester 2 was mixed by biogas recirculation from the top of the digesters by an gas pump and a draft tube arrangement (Table 1, Fig. 1). The draft tube was located at 13 mm height from the hopper bottom. The biogas recirculation rate was kept as 1 L/min, as no significant change in the digester performance was observed with increased biogas recirculation rate up to 3 L/min rate (Karim et al., 2005). Digester 3 was mixed by a 62-mm-diameter axial-flowimpeller (Lightnin A-310, Rochester, New York, USA), and the impeller motor was a Model 5vb, EMI Inc. (Clinton, Connecticut, USA). Digester 4 was mixed by slurry recirculation. The pump used for slurry recirculation was, a Masterflex-pump from Cole Parmer Instrument Co. (Chicago, IL, USA). As mentioned above, Digesters 2, 3, and 4 were mixed continuously, while keeping a constant energy supply per unit volume of slurry (8 W/m3). In the case of digesters mixed by biogas recirculation, power per unit volume was calculated per Eq. (1) (Casey, 1986), P lG r P2 ¼ V ðl 1Þ
" # P1 ðl1Þ=l 1 , P2
(1)
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Gas bag for gas collection
Gas composition sampling
3599
Gas bag for gas collection
Gas composition sampling
Air pump for biogas recirculation
Wet test meter for periodic gas production measurements
Valve for feed addition 152 mm
Wet test meter for periodic gas production measurements
Valve for feed addition 152 mm
344 mm
344 mm 194 mm
194 mm
140 mm
Draft tube (38 mm dia) 26 mm
26 mm Valve for effluent and drain
(a)
Gas composition sampling
Valve for effluent and drain
(b)
Gas bag for gas collection
Gas bag for gas collection
Gas composition sampling
Impeller motor Wet test meter for periodic gas production measurements
Valve for feed addition
152 mm
Wet test meter for periodic gas production measurements
194 mm
Impeller (62 mm dia)
Valve for effluent and drain
Slurry recycling pump
26 mm
26 mm
(c)
152 mm
344 mm
344 mm 194 mm
Valve for feed addition
Valve for effluent and drain
(d)
Fig. 1. Schematic diagrams of the experimental set-ups. (a) Digesters 1, 5 and 9, (b) Digesters 2, 6 and 10, (c) Digesters 3, 7 and 11, and (d) Digesters 4 and 8.
where P is the power (W), V is the volume (m3) of the slurry mixed, Gr is the specific biogas recirculation rate (m3/d m3), P2 is the head space pressure equal to 101325 Pa (atmospheric pressure), P1 is the pressure at the injection point (i.e., P2+static head of slurry), and l is the polytrophic exponent. Under isothermal conditions the value of l approaches unity, while under adiabatic conditions its value for biogas is about 1.3. Since the digesters in this study were operated at a controlled temperature of 35 1C, the value of l was taken as 1.01, as suggested by Casey (1986). To keep the same power input per unit volume of the slurry, the impeller speed for Digester 3 was calculated as
275 rpm, using Eq. (2). The torque applied was determined by a rotating torque meter (Bex-O-Meter, Model 38, The Bex Company, San Francisco, CA, USA), P ¼ Torque ðN mÞ Angular velocity ðrpmÞ:
(2)
Similarly, the slurry recirculation flow rate of 0.82 L/ min for Digester 4 was selected based on, P ¼ rgHQ,
(3)
where Q is the discharge (m3/s), H is the head of the slurry (m), r is the density of the slurry pumped (kg/m3), and g ¼ 9:81 m=s2 .
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The digesters were inoculated with 373 mL (10% of the total working volume) anaerobic seed sludge collected from an active laboratory-scale digester containing cow manure. The seed sludge had total suspended solid (TSS) and volatile suspended solid (VSS) of 66.13 and 35.63 g/L, respectively. The remaining 90% of the working volume was filled with fresh prepared 5% manure slurry (i.e., having 50 g dry solids per liter of slurry). The solids concentrations selected for this study (50–150 g dry solids per liter) was based on the fact that dairy manure ‘‘as excreted’’ has approximately 12% total solids (TS) and 10.5% volatile solids (VS), while most of the treatment systems operate at a lower solids concentration than the ‘‘as excreted’’ values (Burke, 2001). The raw cow manure was collected fresh (less than 2 days old) from the University of Tennessee Institute of Agriculture, Knoxville, TN, and stored in a freezer. It was verified that the cows were not receiving any antibiotic treatment, as some of the antibiotic treatments limit the viability of methane generating microorganisms (Masse et al., 2002). The waste slurry was prepared from the collected raw manure by blending, screening, settling, and dilution. The blending of the manure was done at 10,500 rpm for 2 min in a household blender to break big pieces of straw and hay to create 5% and 10% manure slurries. Later on, an equal volume of water was added to the blended slurry Table 1 Operational conditions for the digesters Experiment set
Digester
Mode of mixing
Feed manure slurry (%)
1
1 2 3 4
Unmixed Biogas-mixed Impeller-mixed Slurry recirculation
5 5 5 5
2
5 6 7 8
Unmixed Biogas-mixed Impeller-mixed Slurry recirculation
10 10 10 10
3
9 10 11
Unmixed Biogas-mixed Impeller-mixed
15 15 15
and it was screened through a 2 mm sieve, followed by settling for 1 h to remove sand. Apart from blending all the above mentioned steps were followed in the preparation of 15% manure slurry. After TS for the prepared slurry was determined, it was diluted with tap water to achieve the required solid concentration (50, 100, or 150 g TS/L). The characteristics of the prepared manure feed slurry are given in Table 2. In the first set of experiments, digesters were operated at a hydraulic retention time (HRT) of 16.2 days with a 5% (i.e., 50 g TS/L) feed of cow manure slurry, resulting in TS and VS loading of 3.1 g/L d and 2 g/L d, respectively. About 460 mL of effluents were taken out from the bottom of the digesters on alternate days and then the same amount of freshly prepared cow manure slurry was added. Steady-state conditions were considered achieved when the variation in biogas production and total chemical oxygen demand (TCOD) concentration in the effluent was within 15% of the average value (Haghighi-Podeh et al., 1995). The second and third set of performance experiments were conducted to evaluate if mixing or modes of mixing becomes more important when the TS concentration in the feed is increased. In the second set of experiments, the digesters were fed with 10% (i.e., 100 g TS/L) manure slurry, resulting in TS and VS loading of 6.2 and 3.2 g/L d, respectively. Finally, in the third set of experiments, TS concentration in the feed slurry was increased to 15% (i.e., 150 g/L), resulting in TS and VS loading of 9.3 and 4.7 g/L d, respectively. In the third set of experiments, the digester mixed by slurry recirculation had to be discontinued as the recirculation of slurry having 15% TS using a peristaltic pump was not possible. All other conditions for the different sets of experiments, including HRT and feeding schedule, were maintained as described for the first set of experiments. The temperature in all experiments was kept at 35 1C by keeping the digesters in a temperature-controlled cabinet. Feed and effluent samples were analyzed for TS, VS, TSS, VSS, volatile fatty acids (VFA), TCOD, dissolved chemical oxygen demand (DCOD), and total nitrogen (TN). Total volume of the biogas generated was measured, and the composition of the biogas was analyzed three times a week. All analyses were performed per
Table 2 Characteristics of the prepared feed, 5%, 10% and 15% manure slurry Feed manure slurry (%)
TS (g/L)
VS (g/L)
TSS (g/L)
VSS (g/L)
TCOD (g/L)
DCOD (g/L)
5 10 15
5171 100 150
3472 5373 7576
3775 4078 NA
2573 3677 NA
5974 61710 NA
2071 2572 4277
TS ¼ total solids, VS ¼ volatile solids, TSS ¼ total suspended solids, VSS ¼ volatile suspended solids, TCOD ¼ total chemical oxygen demand, DCOD ¼ Dissolved chemical oxygen demand, NA ¼ not available, 7 shows the standard error.
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standard procedures (APHA, 1998), unless otherwise mentioned. Volatile fatty acids (formic, acetic, propionic, butyric, and valeric acids) were determined by centrifuging a small sample at greater than 10,000 rpm for 5 min, filtering the liquid through a 0.2-mm-pore-size filter, and injecting a 10-mL-sample into a high-pressure liquid chromatograph (HPLC). In the HPLC, the mobile phase (filtered 5 mM H2SO4) was pumped at 0.6 mL/ min through a 300 mm 7.8 mm (8 mm particle size) RHM Monosaccharide column (Phenomenex, Torrance, CA), held at a temperature of 65 1C, to a refractive index detector (Model 2410, Waters Corporation, Miltford, MA) held at a temperature of 40 1C. Biogas volume was measured using wet-test gas meters (GSA/Precision Scientific, Chicago, IL), and the samples (1 mL) for biogas composition were collected using a gas-tight syringe. The samples were injected in duplicate into a Gow-Mac (Model 69-350 Series, Lehigh Valley, PA) gas chromatograph (GC) equipped with a 6 ft 1/8 in stainless steel column packed with 80 100 Hayesep Q (Supelco, USA). The oven, injector, and thermal conductivity detector temperatures were kept as 45, 90, and 110 1C, respectively. The carrier gas (helium) flow rate through the column was maintained at 30 mL/min. Initially, the GC was calibrated with 99.9% pure methane (CH4) and nitrogen standards. Average steady-state data and the standard error presented in the paper have been calculated as a mean value over 20–30 days of observations. Statistical significance (P ¼ 0:05) of the experimental data was tested using one-way (or single factor) analysis of variance (ANOVA) statistical program (Microsoft Excel 2002). Least significant difference was used as for post hoc testing of data. Finally, the observed methane production rates for the three experimental sets were compared with the Hill’s model (Husain, 1998) predictions and discussed.
Digesters 1 through 4, respectively. The effluent DCOD concentration from the digesters was observed at 3.7–4.2 g/L, showing about a 79–81% reduction in the DCOD in the digesters under steady-state conditions. The TN content of the waste slurry remained unchanged in the anaerobic treatment. VFA concentrations in the effluents from the digesters were observed as less than 250 mg/L and the pH of the effluents was between 7 and 7.8. The daily biogas production for the four digesters has been shown in Fig. 2. The average biogas production rates for Digesters 1–4 were 0.8470.07, 0.9470.07, 0.8870.09, and 0.8570.09 L/L d with average methane content as 6473, 5673, 6173, and 6772%, respectively. The biogas production rates were calculated as volume of biogas produced per liter of digester volume per day and averaged over the last 30 days of operation. The biogas production rate data shows that Digester 2 produced slightly more biogas than the other digesters, but the corresponding methane content was found to be lower in comparison, probably due to infiltration of air, which was observed to be up to 18% in the case of Digester 2. It is worthwhile mentioning that biogas circulation in laboratory digesters increases the chances for ‘infiltration’ of air into the system (due to slight air permeability of tubing and leakage on the vacuum side of the air pump, etc.). Average steady-state performance data of the four digesters were found to be quite similar (Table 3). However, to evaluate the results further, the data were subjected to ANOVA. The ANOVA test showed that methane production data for the four digesters did not differ significantly at the 5% level (P ¼ 0:06; F ¼ 2:72; Fcrit ¼ 2.9; df ¼ 3, 32). Methane yield, calculated as the volume of methane produced per unit mass of VS added, was observed to be 0.26–0.28 L/g VS added (Table 3). It is important to note that the VS solids loading in the present study was 2 g/L d. The Unmixed (Digester 1) Biogas-mixed (Digester 2) Impeller-mixed (Digester 3) Impeller-mixed (Digester 3, duplicate run) Slurry recirculation (Digester 4)
3.1. First set of experiments (5% manure slurry feed) Four laboratory-scale anaerobic digesters were operated at 35 1C to produce biogas from 5% cow manure slurry feed. Initially there were variations in the daily biogas production for the four digesters, but the performance stabilized with time. After achieving steady-state conditions, all four digesters behaved quite similarly with the TS and VS reduction in the range of 37–40% and 50–63%, respectively. Total COD in the feed was about 58.7 g/L, about 33% of which was present in the form of dissolved COD. The reduction of TCOD was observed as 56%, 58%, 57%, and 56% for
Biogas production (L/d)
6
3. Results and discussion
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4
2
0 0
10
20
30 40 50 Days of operation
60
70
Fig. 2. Daily biogas production for Digesters 1–4 observed during the first experimental set using 5% feed slurry study.
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Table 3 Average biogas production rate and methane yield for the digesters over the last 30 days of operation Set of expt.
Digester
1
1 2 3 4
2
5 6 7 8
3
9 10 11
Mode of mixing
VS loading (g/L d)
a
Biogas production rate (L/L/d)
Methane yield (L CH4/g VS loaded)
b Statistical significance
Unmixed Biogas-mixed Impeller-mixed Slurry recirculation Unmixed Biogas-mixed Impeller-mixed Slurry recirculation Unmixed Biogas-mixed Impeller-mixed
2 2 2 2
0.8470.07 0.9470.07 0.8870.09 0.8570.09
0.27 0.26 0.27 0.28
A A A A
3.24 3.24 3.24 3.24
0.9270.1 1.0770.08 1.1470.13 1.2070.14
0.19 0.21 0.23 0.24
B C D D
3.24 3.24 3.24
1.1370.14 1.6470.32 1.2570.12
0.15 0.23 0.17
E F
7 shows the standard error Alphabetic symbols given in the last column depict statistically same/different performance of the digesters. Please note that the analysis of variance (ANOVA) was performed separately for each of the three sets of experiments. a
observed methane yield is comparable to the reported methane yield of 0.376 L/g VS added, observed at a loading of 2.86 g VS/L d (Linke, 1997). 3.2. Second set of experiments (10% manure slurry feed) The second set of experiments was also conducted with four digesters, unmixed (Digesters 5), biogas-mixed (Digester 6), impeller-mixed (Digester 7), and slurry recirculation (Digester 8). The goal was to study whether the role of mixing and mode of mixing became more important when the TS concentration in the animal waste slurry was increased. The total and VS concentration in the effluents from the four digesters are shown in Fig. 3. The digesters produced biogas consistently throughout the study period (Fig. 4). Digester 8, equipped with slurry recirculation, produced slightly more biogas than any of the other digesters, while the unmixed digester (Digester 5) produced the least. The average steady-state biogas production rate and methane yield data, calculated over a period of last 30 days of operation (from Day 41 to Day 71), are given in Table 3. Under steady-state conditions the biogas production rates for Digesters 5–8 were calculated as 0.9370.1, 1.0770.08, 1.1470.13, and 1.270.14 L/L d with 6673, 6574, 6573, and 6674% methane contents, respectively. The unmixed digester (Digester 5) produced biogas at a rate almost 22% less than the digester mixed by slurry recirculation (Digester 8), and the trend was very consistent as shown in Fig. 4. Digester 6 (mixed by biogas recirculation) produced biogas at a 10% lower rate than Digester 8. ANOVA of the daily biogas production data for the steady-state
Effluent TSand VS concentration (g/L)
b
100
Unmixed (TS) Biogas-mixed (TS) Impeller-mixed (TS) Slurry recirculation (TS)
90 80
Unmixed (VS) Biogas-mixed (VS) Impeller-mixed (VS) Slurry recirculation (VS)
70 60 50 40 30 20 10 0
10
20
30 40 50 Days of operation
60
70
80
Fig. 3. Total and volatile solids (TS and VS) concentrations in the effluents collected from Digesters 5–8 during the 10% feed slurry study.
period at the 5% level showed significant difference among the four digesters (P ¼ 4:41 107 ; F ¼ 14:4; Fcrit ¼ 2.76; df ¼ 3, 56). However, there was no significant difference (P ¼ 0:26; F ¼ 1:31; Fcrit ¼ 4.22; df ¼ 1, 26) between the daily biogas production of Digester 7 (impeller-mixed) and Digester 8 (slurry recirculation).
3.3. Third set of experiments (15% manure slurry feed) The goal of the third set of experiments was to study the role of mixing in the case of digesters fed with extremely concentrated cow manure slurry (150 g TS/L).
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Biogas (Biogas-mixed) Biogas (Slurry recirculation) VS (Feed)
120
12 80
10 8
40
6 4
0
2 0 0
10
20
30 40 50 60 Days of operation
70
-40 80
Fig. 4. Daily biogas production from Digesters 5–8 along with the TS and VS concentrations in the 10% feed slurry study.
Since it was very difficult to recirculate 15% waste slurry using a peristaltic pump, the digester mixed by slurry recirculation was not included in this study. The daily biogas productions for unmixed, biogas-mixed and impeller-mixed digesters are shown in Fig. 5. It is clear from the figure that unmixed digester (Digester 9) showed a quicker start-up than the mixed digesters (Digesters 10 and 11). The unmixed digester took less than 20 days to get stabilized, while both of the mixed digesters were unstable for more than 30 days. Another important observation was the lower pH inside the mixed digesters during the start-up period (Fig. 6). Low pH inside the mixed digesters can be attributed to improved hydrolysis due to external mixing (via biogas recirculation or impeller), which might have resulted in high VFA concentration inside the digesters. Unfortunately, VFA data could not be obtained for this set of experiments due to the complications caused by high solids concentration in the effluent. However, observation of low biogas production in mixed digesters during start-up also supports the possibility of VFA build-up inside the mixed digesters, which is known to inhibit the biogas production activity. After 1 month of operation, the biogas-mixed digester showed higher biogas production than the other two (Fig. 5). Initially, the reason behind this was unexplained. However, when the digesters were disassembled after completion of the study, the biogas-mixed digester was found to be almost completely clogged. This made it clear that the higher amount of biogas production from the biogas-mixed digester was due to the accumulation of biodegradable solids inside the digester. Solids accumulated at the bottom of all three digesters, but the biogasmixed digester (Digester 10) was severely clogged from top to bottom with some channels in the annular section (in between the draft tube and the digester side wall). The severe clogging of the biogas-mixed digester was due to the presence of the static internals in the form of draft tube, injector tube and hangers.
3603
10 Unmixed
9 Biogas production (L/d)
Biogas (Unmixed) Biogas (Impeller-mixed) TS (Feed)
14
Biogas mixed
Impeller mixed
8 7 6 5 4 3 2 1 0 0
10
20
30 40 50 Days of operation
60
70
Fig. 5. Daily biogas production for Digesters 9–11 during the 15% feed slurry study.
9 Unmixed
Biogas-mixed
Impeller-mixed
8 7 6 pH
Daily biogas production (L/d)
16
Feed TSand VS concentration (g/L)
K. Karim et al. / Water Research 39 (2005) 3597–3606
5 4 3 2 0
10
20 30 40 Days of operation
50
60
Fig. 6. pH in the effluents from Digesters 9–11 collected during the 15% feed slurry study.
The results for TS and VS in the effluents are shown in Fig. 7. During the first 20 days, the TS and VS concentration in the effluent from the biogas-mixed digester was rather consistent; later, the variation in TS and VS data is noted. This supports the fact that the biogas-mixed digester (Digester 10) was unstable. From Fig. 7, it is also clear that the impeller-mixed digester was adequately mixed with less variation in TS and VS in the effluent throughout the 2 months period. As shown in Table 3, the average biogas production rates (from day 36 to day 60) for the unmixed (Digester 9), biogas-mixed (Digester 10) and impellermixed (Digester 11) digesters were calculated to be 1.1370.14, 1.6470.32, and 1.2570.12 L/L d, with the methane contents of 64%, 66%, and 63%, respectively. Digester 10 (biogas-mixed) had a significantly higher rate of biogas generation. This is not surprising since this digester was found clogged. Consequently, the biodegradable solids would have been retained in the biogas-mixed digester longer than other two digesters, resulting in more biogas generation. Statistically, at the
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Effluent TSand VS concentration (g/L)
180 160
Unmixed TS
Biogas mixed TS
Impeller mixed TS
Unmixed VS
Biogas mixed VS
Inpeller mixed VS
140 120 100 80 60 40 20 0
10
20
30 40 Days of operation
50
60
Fig. 7. Plot showing total solids and volatile solids (TS and VS) concentrations in the effluents collected from Digesters 9–11 during the 15% feed slurry study.
5% level there was significant difference (P ¼ 0:02; F ¼ 5:42; and Fcrit ¼ 4.26; df ¼ 1, 24) in the biogas production rates of the unmixed and impeller-mixed digesters. Because of clogging Digester 10 might have had longer HRT than the other two digesters, thus its data was not included in the statistical analysis.
4. Discussion In the investigation of unmixed and mixed digesters fed with 5% feed slurry (loading ¼ 2 g VS/L d), the mixing or the three different modes of mixing did not significantly affect the digesters performance. Within the group of mixed digesters, mechanical mixers are reported to be the most efficient in terms of power consumed per gallon mixed (Brade and Noone, 1981). Thus, the digester mixed by an impeller had ‘‘better’’ mixing than the others; however, performance was similar to the other digesters. We have shown, using ANOVA, that mixing or mode of mixing had a statistically negligible effect on the digesters performance in the case of 5% manure slurry. The above findings raised questions of whether the 16.2 days HRT was long enough for the microbes to assimilate all the readily available organics or if the mixing intensity was not high enough to play a role. To answer this question, second and third sets of experiments were performed to evaluate whether the role of mixing becomes more significant with an increase in TS concentration in the feed slurry. Statistical analysis of the steady-state biogas production rate (volume of biogas/volume of digester/day) data for the second set of experiments showed significant difference among the digesters (at the 5% level, P ¼ 1:26 107 ; F ¼ 15:8;
Fcrit ¼ 2.76; df ¼ 3, 58), with 0.08 as the least significant difference value. Therefore, unmixed digester biogas production rate was significantly different from all other digesters. The data further show that the slurry recirculation digester (Digester 8) had the highest biogas production rate (Table 3). The slurry recirculation digester produced about 29% more biogas than the unmixed digester (Digester 5), and the impeller-mixed digester (Digester 7) produced about 22% more biogas than the unmixed digester. The biogas-mixed digester produced about 15% more biogas than the unmixed digester. Therefore, the results show that mixing improved the biogas production when a thicker manure slurry (10%) was fed. Interestingly, the biogas production data from the third set of experiments, when the digesters were fed with 15% manure slurry, also shows that the impeller-mixed digester (Digester 11) produced more biogas than unmixed digesters (Table 3). ANOVA analysis showed that at the 5% level there was significant difference in the biogas production rates of the unmixed and impeller-mixed digesters. Please note that the biogas-mixed digester (Digester 10) was found clogged, which would result in longer HRT for the solids than in the other two digesters, and its data was not included in the statistical analysis. Based on the results obtained, we conclude that the role of mixing becomes more important with an increase in TS concentration in the feed slurry. However, observations of lower pH and less biogas production in the case of mixed digesters fed with 15% manure slurry suggest that mixing of digester during start-up is not beneficial, as it lengthen the start-up time. So far as the mode of mixing is concerned, in the studies using 5% manure slurry, there was no significant difference in the performance of the digesters being mixed by three different modes of mixing. However, when the digesters were fed with 10% manure slurry, the influence by the mode of mixing was evident (P ¼ 0:03; F ¼ 3:77; Fcrit ¼ 3.22; df ¼ 2, 41 at the 5% level). There was no significant difference in the biogas production for the slurry recirculation digester and the impeller-mixed digester, as mentioned earlier. The high biogas production in the case of the slurry recirculation digester may be attributed to the fact that the particles, chunks and flocks were exposed to higher shear and were crushed while passing through the hub of the recycling pump used. One of the roles that mixing plays inside a digester is to minimize stratification and accumulation of inert solids, especially if the feed manure has a high concentration of inert solids, such as sand from bedding material. Solids accumulation was not observed during first set of experiments (with 5% manure slurry). However, during second and third sets of experiments significant amount of solids accumulation was observed in both unmixed and mixed digesters. Digesters 5–8
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RM ¼ gB0 sI,
(4)
where g is the amount of methane produced per VS consumed (methane yield), B0 is the biodegrability constant (equal to the VS destroyed per VS added), s is the loading rate (equal to the concentration of VS in the feed divided by HRT), and I is a ‘productivity index’ defined by I ¼ 0:5 þ ð1=2:95Þ Arctan½ðt sÞ=0:211, where t is the ‘‘stress index.’’
(5)
1.40 Methane production rate (L/L-d)
were found to have about 337, 205, 260, and 190 g (dry weight) solids accumulated inside, which had about 23%, 9%, 5%, and 6% VS, respectively. From the results of the third set of experiments it is clear that digester mixing by biogas recirculation is not a promising option when a digester is being fed with thick manure slurry, as the biogas-mixed digester (Digester 10) was found to be severely clogged from top to bottom with some channels in the annular section. The severe clogging of the biogasmixed digester was due to the presence of the static internals in the form of draft tube, injector tube and hangers. This finding, however, may not translate to larger-scale digesters. Stratification and clogging problems are crucial as it ultimately reduces the effective volume of the digester and lead to its failure. This suggests that digester mixing and mode of mixing become critical with an increase in the TS concentration in the feed slurry. The digester configuration and external mixing aid should be properly designed and selected based on hydrodynamics as explained elsewhere (Karim et al., 2004). To summarize: Among the three modes of mixing used, biogas recirculation seems to be a promising option for digesters fed with dilute manure slurry. However, biogas recirculation or slurry recirculation is not an effective option in the case of digesters being fed with thick manure slurry. Mechanical impellers are reported to be most efficient in terms of power consumed per gallon mixed (Brade and Noone, 1981), but the energy requirement for impeller mixing increases in orders of magnitude with the size of the digester. The biogas production rates and the methane yields observed during the studies reported in this paper are summarized in Table 3. Methane yield is defined as the volume of methane produced per unit weight of VS loaded. It is obvious from the tabulated data that the biogas production rate increased when there was an increase in the TS concentration in the feed slurry. However, there was a significant decrease in the methane yield. In other words, at higher solids loading, VS were not being exhausted up to their full potential. Similar observations were reported by Linke (1997). The methane production rates observed during the present study was compared with the predictions of the empirical kinetic model proposed by Hill and described by Husain (1998). As per the model, for a continuously fed anaerobic digester methane production rate (RM in L CH4/L d),
3605
Hill's model Unmixed Biogas mixed Impeller mixed Slurry recirculation
1.20 1.00 0.80 0.60 0.40 0.20 1
1.5
2 2.5 3.5 4 4.5 3 Volatile solidsloading rate (g/L-d)
5
Fig. 8. Comparison of experimental methane production rate with the prediction of Hill’s model (Note: Experimental data corresponding to 1.54 g VS/L d loading was taken from another study conducted by our group reported elsewhere (Karim et al., 2005)).
Hill (see Husain, 1998) recommended that the values of g, B0, and t are set to 0.5 (L CH4/L d), 0.483 (g TVS/ g TVS), and 9.21 (g TVS/L d), respectively, for confined beef cattle manure. These values were substituted in the above equations to calculate the methane production rates for the experimental VS loading rates. A comparison of the experimental data with the model predictions are shown in Fig. 8. Note that some of the experimental data plotted in Fig. 8, corresponding to the 1.54 g VS/L d loading, was taken from another study conducted by our group using the same reactors and manure from the same source (Karim et al., 2005). It is clear from the figure that Hill’s model slightly under-predicted the performance at loading rates of 1.54 and 2 g VS/L d. However, in the case of higher loadings (3.24 g VS/L d) the model significantly over-predicted the methane production rates. In fact, the empirical model proposed by Hill is based on the assumption that methane production rate increases linearly with increase in VS loading before reaching the highest possible loading, at which time the digester fails (Husain, 1998). However, from Fig. 8, it is clear that the relation between VS loading and methane production rate in not linear.
5. Conclusions We found no effect of mixing on digesters performance when fed with 5% manure slurry. However, the effect of mixing and the mode of mixing became prominent when digesters were fed with thicker manure slurry (10% and 15%). In the case of 10% and 15% manure slurry, the mixed digesters produced more biogas than unmixed digesters, but digester mixing during start-up was not beneficial as it resulted in lower
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pH, performance instability, and prolonged start-up time. Mixing using biogas recirculation system was not found effective in the case of digester fed with 15% manure slurry under the experimental conditions studied. The observed methane production rates for smaller VS loadings (1.54 and 2 g VS/L d), were comparable with the prediction of Hill’s model. However, at higher loading rates (3.24 and 4.7 g VS/L d) the model significantly over-predicted the methane production rates.
Acknowledgements The authors thank the US Department of Energy and its Office of Energy Efficiency and Renewable Energy for sponsoring the research project (Identification No. DE-FC36-01GO11054).
References APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington, DC. Ben-Hasson, R.M., Ghaly, A.E., Singh, R.K., 1985. Design and evaluation of no-mix energy efficient anaerobic digester. Proceedings, Annual Meeting, Canadian Society of Agricultural Engineering, June 23–27, Charlottetown, P.E.I. Brade, C.E., Noone, G.P., 1981. Anaerobic digestion—need it be expensive? Water Pollut. Control 80, 70–76. Burke, D.A., 2001. Dairy Waste Anaerobic Digestion Handbook. Environmental Energy Company, 6007 Hill Street, Olympia, WA 98516. Casey, T.J., 1986. Requirements and methods for mixing in anaerobic digesters. In: Bruce, A.M., Kouzeli-Kastri, A., Newman, P.J. (Eds.), Anaerobic Digestion of Sewage Sludge and Organic Agricultural Wastes. Elsevier Applied Science Publishers, Barking England, pp. 90–103. Chapman, D., 1989. Mixing in anaerobic digesters: state of the art. In: Cheremisinoff, P. (Ed.), Encyclopedia of Environmental Control Technology, vol. 3. Gulf Publishing Company, Houston, pp. 325–354. Chen, T., Chynoweth, D.P., Biljetina, R., 1990. Anaerobic digestion of municipal solid waste in a nonmixed solids concentrating digester. Appl. Biochem. Biotechnol. 24/25, 533–544. Dague, R.R., McKinney, R.E., Pfeffer, J.T., 1970. Solids retention in anaerobic waste treatment systems. J. Water Pollut. Control Fed. 42 (2 Part 2), R29–R46. Diaz, L., Trezek, G., 1977. Biogasification of a selected fraction of municipal solid wastes. Compost Sci. 8–13. Dolfing, J., 1992. The energetic consequences of hydrogen gradients in methanogenic ecosystems. FEMS Microbiol. Ecol. 101, 183–187. Fontenot, J.P., Ross, I.J., 1980. Animal waste utilization. In: Livestock Waste: A Renewable Resource. Proceedings of the Fourth International Symposium on Livestock Wastes, ASAE, St. Joseph, MI.
Haghighi-Podeh, M.R., Bhattacharya, S.K., Mingbo, Q., 1995. Effects of nitrophenols on acetate utilizing methanogenic systems. Water Res. 29, 391–397. Hashimoto, A.G., 1982. Effect of mixing duration and vacuum on methane production rate from beef cattle waste. Biotechnol. Bioeng. 24, 9–23. Ho, C.C., Tan, Y.K., 1985. Anaerobic treatment of palm oil mill effluent by tank digesters. J. Chem. Technol. Biotechnol. 35b, 155–164. Husain, A., 1998. Mathematical models of the kinetics of anaerobic digestion—a selected review. Biomass Bioenergy 14 (5/6), 561–571. James, S., Wiles, C., Swartzbaugh, J., Smith, R., 1980. Mixing in large-scale municipal solid waste-sewage sludge anaerobic digesters. Biotechnol. Bioeng. Symp. 14. Karim, K., Varma, R., Vesvikar, M., Al-Dahhan, M.H., 2004. Flow pattern visualization of a simulated digester. Water Res. 38, 3659–3670. Karim, K., Klasson, K.T., Hoffmann, R., Drescher, S.R., DePaoli, D.W., Al-dahhan, M.H., 2005. Anaerobic digestion of animal waste: Effect of mixing. Bioresource Technology 96 (14), 1607–1612. Lema, J.M., Mendez, R., Iza, J., Garcia, P., FernandezPolanco, F., 1991. Chemical reactor engineering concepts in design and operation of anaerobic treatment processes. Water Sci. Technol. 24, 79–86. Linke, B., 1997. A model for anaerobic digestion of animal waste slurries. Environ. Technol. 18, 849–854. Masse, D.I., Lu, D., Masse, L., Droste, R.L., 2002. Effect of antibiotics on psychrophilic anaerobic digestion of swine manure slurry in sequencing batch reactors. Bioresource Technol. 75, 205–211. Meynell, P.-J., 1976. Methane: Planning a Digester. Prism Press, London, pp. 55–57. Mills, P.J., 1979. Minimization of energy input requirements of an anaerobic digester. Agric. Wastes 1, 57–59. Parkin, G.F., Owen, W.F., 1986. Fundamentals of anaerobic digestion of wastewater sludges. J. Environ. Eng. 112, 867–920. Sawyer, C.N., Grumbling, A.M., 1960. Fundamental consideration in high-rate digestion. J. Sew. Eng. Div. ASCE 86, 49–63. Smith, R.J., Hein, M.J., Greinier, T.H., 1979. Experimental methane production from animal excreta in pilot-scale and farm-size units. J. Animal Sci. 48, 202–217. Smith, L.C., Elliot, D.J., James, A., 1996. Mixing in upflow anaerobic filters and its influence on performance and scaleup. Water Res. 30, 3061–3073. Stenstrom, M., Ng, A., Bhunia, P.K., Abramson, S., 1983. Anaerobic digestion of municipal solid waste. J. Environ. Eng.—ASCE 109, 1148–1158. Stroot, P.G., McMahon, K.D., Mackie, R.I., Raskin, L., 2001. Anaerobic codigestion of municipal solid waste and biosolids under various mixing conditions––I. Digester performance. Water Res. 35, 1804–1816. US EPA, 1979. Process Design Manual for Sludge Treatment and Disposal. EPA 625/1-79-011. Cincinnati, Ohio. Whitmore, T.N., Lloyd, D., Jones, G., Williams, T.N., 1987. Hydrogen-dependent control of the continuous anaerobic digestion process. Appl. Microbiol. Biotechnol. 26, 383–388.
Water Research 39 (2005) 3597–3606 www.elsevier.com/locate/watres
Anaerobic digestion of animal waste: Effect of mode of mixing Khursheed Karima,, Rebecca Hoffmanna, K. Thomas Klassonb, M.H. Al-Dahhana a
Chemical Reaction Engineering Laboratory (CREL), Department of Chemical Engineering, Washington University, St. Louis, MO 63130, USA b Southern Regional Research Center, USDA-ARS, New Orleans, LA 70124, USA Received 25 October 2004; received in revised form 11 June 2005; accepted 22 June 2005
Abstract Laboratory-scale digesters were operated to study the effect of mixing (via biogas recirculation, impeller mixing, and slurry recirculation) on biogas production. Three sets of experiments were performed using cow manure slurry feed with either 50, 100, or 150 g/L total solids (TS) concentrations (referred in the text as 5%, 10%, and 15% manure slurry). The experiments were conducted at a controlled temperature of 35 1C and a hydraulic retention time of 16.2 days, resulting in TS loadings of 3.1, 6.2, and 9.3 g/L d for 5%, 10%, and 15% manure slurry feeds, respectively. Results showed that the unmixed and mixed digesters performed quite similarly when fed with 5% manure slurry and produced biogas at a rate of 0.84–0.94 L/L d. The methane yield was found to be 0.26–0.28 L CH4/g volatile solids loaded. However, the effect of mixing and the mode of mixing became important when the digesters were fed thick manure slurry feeds (10% and 15%). Digesters fed with 10% and 15% manure slurry and equipped with external mixing produced about 10–30% more biogas than the unmixed digester. While the mixed digesters produced more biogas than unmixed digesters, digester mixing during start-up was not beneficial, as it resulted in lower pH, performance instability and prolonged start-up time. Mixing using biogas recirculation system was found not to be effective in the case of 15% manure slurry feed under the experimental conditions studied. r 2005 Elsevier Ltd. All rights reserved. Keywords: Anaerobic; Biogas; Digestion; Manure; Mixing
1. Introduction Expansion in livestock industries has brought in the requirement of safe disposal of the large quantities of animal manure generated at dairy, swine, poultry, and other animal farms. In the US, over 100 million tons of dry matter is produced every year (Fontenot and Ross, 1980). Different types of waste management options Corresponding author. Tel.: 314 9357187; fax: 314 9357211.
E-mail addresses: [email protected], [email protected] (K. Karim).
may include technologies based on physical, chemical, or biological conversions. Biological means of anaerobic decomposition of animal waste has the advantage of producing a fuel gas (methane) as well as generating odor-free residues rich in nutrients, which can be used as fertilizers. The anaerobic digestion process of animal waste is primarily affected by its retention time in the reactor and the degree of contact between incoming substrate (animal waste) and a viable bacterial population. These parameters are primarily a function of the hydraulic regime (mixing) in the reactors. The importance of mixing in achieving efficient substrate
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.06.019
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conversion has been noted by many researchers, although the optimum mixing pattern is a subject of much debate. An intermediate degree of mixing was found to be optimal for substrate conversion (Smith et al., 1996). According to Sawyer and Grumbling (1960) and Meynell (1976), mixing of the substrate in the digester helps to distribute organisms uniformly throughout the mixture and to transfer heat. Furthermore, agitation aids in particle size reduction as digestion progresses and in removal of gas from the mixture. Mixing can be accomplished through various methods, including mechanical mixers, recirculation of digester contents, or by recirculating the produced biogas to the bottom of the digester using pumps. Two other important aspects are the intensity and duration of mixing. The information available in the literature on the role of mixing in anaerobic digesters is contradictory. Most of the literature on anaerobic digestion, for both low and high solids applications, emphasizes the importance of adequate mixing to improve the distribution of enzymes and microorganisms throughout the digester (Chapman, 1989; Parkin and Owen, 1986; Lema et al., 1991; Stenstrom et al., 1983). Several studies indicated that a lack of sufficient mixing in low solids digesters dealing with municipal waste resulted in a floating layer of solids (Stenstrom et al., 1983; James et al., 1980; Diaz and Trezek, 1977). These literature sources reported that the mixing level was increased to prevent formation of the solids layer. Chen et al. (1990) observed the development of a floating layer of solids in a 4.5 m3 unmixed digester treating refuse derived fuel and primary sludge mixture. They compared the performance of an unmixed (downward flow) and a continuously impellermixed digester at mesophilic conditions (30–40 1C). The unmixed digester exhibited a higher methane yield than the continuously mixed digester. Ho and Tan (1985) reported greater gas production from palm oil mill effluents for a continuously mixed digester than for an unmixed digester. However, Dague et al. (1970) observed that shifting from continuous mixing to intermittent mixing (2 min of mixing/h) resulted in significantly higher gas production from a liquid municipal waste stream. Similar controversies and uncertainties have been reported in the case of livestock waste digestion. Intermittent mixing in the anaerobic digestion of livestock waste under mesophilic temperature conditions has been recommended by Mills (1979) and Smith et al. (1979). Hashimoto (1982) found higher biogas production from beef cattle wastes under both continuous mixing and vacuum than under intermittent mixing and normal pressure conditions. However, Ben-Hasson et al. (1985) observed 75% lower methane production rates for a continuously mixed reactor than for an unmixed reactor when treating dairy cattle manure. Whitmore et al. (1987) and Dolfing (1992) suggested that very rapid mixing disrupts the structure of flocks in completely mixed
reactors, thereby disturbing the syntrophic relationships between organisms. Stroot et al. (2001) suggested that vigorous continuous mixing may prevent good performance of high solids anaerobic digesters and that minimal mixing was sufficient to distribute the feed adequately, which may have allowed the formation of new spatial associations. The US EPA (1979) has recommended a power input of 0.2–0.3 HP/1000 cu ft (5.26–7.91 W/m3) for proper digester mixing. The questions raised in the literature about the role of mixing in anaerobic digesters suggest that more research is needed. Therefore, the present study was designed to digest cow manure slurry in laboratory-scale anaerobic digesters, with and without external mixing, at three different solids concentrations. The main focus was to evaluate the effect of mixing and modes of mixing (biogas recirculation, impeller mixing, or slurry recirculation) on the methane production rate at three different influent solids concentrations, while keeping same amount of energy applied per unit volume of the digester mixed.
2. Materials and methods The reported study was performed in three sets of experiments using four laboratory-scale digesters having a working volume of 3.73 L. Schematics of the digesters are shown in Fig. 1. Digesters were made of clear PVC and had hopper bottoms with a 251 slope angle. Biogas generated in the digesters was collected in tedlar bags. In the first set of experiments, Digester 1 was operated as unmixed (without an external mixing aid), while Digester 2 was mixed by biogas recirculation from the top of the digesters by an gas pump and a draft tube arrangement (Table 1, Fig. 1). The draft tube was located at 13 mm height from the hopper bottom. The biogas recirculation rate was kept as 1 L/min, as no significant change in the digester performance was observed with increased biogas recirculation rate up to 3 L/min rate (Karim et al., 2005). Digester 3 was mixed by a 62-mm-diameter axial-flowimpeller (Lightnin A-310, Rochester, New York, USA), and the impeller motor was a Model 5vb, EMI Inc. (Clinton, Connecticut, USA). Digester 4 was mixed by slurry recirculation. The pump used for slurry recirculation was, a Masterflex-pump from Cole Parmer Instrument Co. (Chicago, IL, USA). As mentioned above, Digesters 2, 3, and 4 were mixed continuously, while keeping a constant energy supply per unit volume of slurry (8 W/m3). In the case of digesters mixed by biogas recirculation, power per unit volume was calculated per Eq. (1) (Casey, 1986), P lG r P2 ¼ V ðl 1Þ
" # P1 ðl1Þ=l 1 , P2
(1)
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Gas bag for gas collection
Gas composition sampling
3599
Gas bag for gas collection
Gas composition sampling
Air pump for biogas recirculation
Wet test meter for periodic gas production measurements
Valve for feed addition 152 mm
Wet test meter for periodic gas production measurements
Valve for feed addition 152 mm
344 mm
344 mm 194 mm
194 mm
140 mm
Draft tube (38 mm dia) 26 mm
26 mm Valve for effluent and drain
(a)
Gas composition sampling
Valve for effluent and drain
(b)
Gas bag for gas collection
Gas bag for gas collection
Gas composition sampling
Impeller motor Wet test meter for periodic gas production measurements
Valve for feed addition
152 mm
Wet test meter for periodic gas production measurements
194 mm
Impeller (62 mm dia)
Valve for effluent and drain
Slurry recycling pump
26 mm
26 mm
(c)
152 mm
344 mm
344 mm 194 mm
Valve for feed addition
Valve for effluent and drain
(d)
Fig. 1. Schematic diagrams of the experimental set-ups. (a) Digesters 1, 5 and 9, (b) Digesters 2, 6 and 10, (c) Digesters 3, 7 and 11, and (d) Digesters 4 and 8.
where P is the power (W), V is the volume (m3) of the slurry mixed, Gr is the specific biogas recirculation rate (m3/d m3), P2 is the head space pressure equal to 101325 Pa (atmospheric pressure), P1 is the pressure at the injection point (i.e., P2+static head of slurry), and l is the polytrophic exponent. Under isothermal conditions the value of l approaches unity, while under adiabatic conditions its value for biogas is about 1.3. Since the digesters in this study were operated at a controlled temperature of 35 1C, the value of l was taken as 1.01, as suggested by Casey (1986). To keep the same power input per unit volume of the slurry, the impeller speed for Digester 3 was calculated as
275 rpm, using Eq. (2). The torque applied was determined by a rotating torque meter (Bex-O-Meter, Model 38, The Bex Company, San Francisco, CA, USA), P ¼ Torque ðN mÞ Angular velocity ðrpmÞ:
(2)
Similarly, the slurry recirculation flow rate of 0.82 L/ min for Digester 4 was selected based on, P ¼ rgHQ,
(3)
where Q is the discharge (m3/s), H is the head of the slurry (m), r is the density of the slurry pumped (kg/m3), and g ¼ 9:81 m=s2 .
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3600
The digesters were inoculated with 373 mL (10% of the total working volume) anaerobic seed sludge collected from an active laboratory-scale digester containing cow manure. The seed sludge had total suspended solid (TSS) and volatile suspended solid (VSS) of 66.13 and 35.63 g/L, respectively. The remaining 90% of the working volume was filled with fresh prepared 5% manure slurry (i.e., having 50 g dry solids per liter of slurry). The solids concentrations selected for this study (50–150 g dry solids per liter) was based on the fact that dairy manure ‘‘as excreted’’ has approximately 12% total solids (TS) and 10.5% volatile solids (VS), while most of the treatment systems operate at a lower solids concentration than the ‘‘as excreted’’ values (Burke, 2001). The raw cow manure was collected fresh (less than 2 days old) from the University of Tennessee Institute of Agriculture, Knoxville, TN, and stored in a freezer. It was verified that the cows were not receiving any antibiotic treatment, as some of the antibiotic treatments limit the viability of methane generating microorganisms (Masse et al., 2002). The waste slurry was prepared from the collected raw manure by blending, screening, settling, and dilution. The blending of the manure was done at 10,500 rpm for 2 min in a household blender to break big pieces of straw and hay to create 5% and 10% manure slurries. Later on, an equal volume of water was added to the blended slurry Table 1 Operational conditions for the digesters Experiment set
Digester
Mode of mixing
Feed manure slurry (%)
1
1 2 3 4
Unmixed Biogas-mixed Impeller-mixed Slurry recirculation
5 5 5 5
2
5 6 7 8
Unmixed Biogas-mixed Impeller-mixed Slurry recirculation
10 10 10 10
3
9 10 11
Unmixed Biogas-mixed Impeller-mixed
15 15 15
and it was screened through a 2 mm sieve, followed by settling for 1 h to remove sand. Apart from blending all the above mentioned steps were followed in the preparation of 15% manure slurry. After TS for the prepared slurry was determined, it was diluted with tap water to achieve the required solid concentration (50, 100, or 150 g TS/L). The characteristics of the prepared manure feed slurry are given in Table 2. In the first set of experiments, digesters were operated at a hydraulic retention time (HRT) of 16.2 days with a 5% (i.e., 50 g TS/L) feed of cow manure slurry, resulting in TS and VS loading of 3.1 g/L d and 2 g/L d, respectively. About 460 mL of effluents were taken out from the bottom of the digesters on alternate days and then the same amount of freshly prepared cow manure slurry was added. Steady-state conditions were considered achieved when the variation in biogas production and total chemical oxygen demand (TCOD) concentration in the effluent was within 15% of the average value (Haghighi-Podeh et al., 1995). The second and third set of performance experiments were conducted to evaluate if mixing or modes of mixing becomes more important when the TS concentration in the feed is increased. In the second set of experiments, the digesters were fed with 10% (i.e., 100 g TS/L) manure slurry, resulting in TS and VS loading of 6.2 and 3.2 g/L d, respectively. Finally, in the third set of experiments, TS concentration in the feed slurry was increased to 15% (i.e., 150 g/L), resulting in TS and VS loading of 9.3 and 4.7 g/L d, respectively. In the third set of experiments, the digester mixed by slurry recirculation had to be discontinued as the recirculation of slurry having 15% TS using a peristaltic pump was not possible. All other conditions for the different sets of experiments, including HRT and feeding schedule, were maintained as described for the first set of experiments. The temperature in all experiments was kept at 35 1C by keeping the digesters in a temperature-controlled cabinet. Feed and effluent samples were analyzed for TS, VS, TSS, VSS, volatile fatty acids (VFA), TCOD, dissolved chemical oxygen demand (DCOD), and total nitrogen (TN). Total volume of the biogas generated was measured, and the composition of the biogas was analyzed three times a week. All analyses were performed per
Table 2 Characteristics of the prepared feed, 5%, 10% and 15% manure slurry Feed manure slurry (%)
TS (g/L)
VS (g/L)
TSS (g/L)
VSS (g/L)
TCOD (g/L)
DCOD (g/L)
5 10 15
5171 100 150
3472 5373 7576
3775 4078 NA
2573 3677 NA
5974 61710 NA
2071 2572 4277
TS ¼ total solids, VS ¼ volatile solids, TSS ¼ total suspended solids, VSS ¼ volatile suspended solids, TCOD ¼ total chemical oxygen demand, DCOD ¼ Dissolved chemical oxygen demand, NA ¼ not available, 7 shows the standard error.
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standard procedures (APHA, 1998), unless otherwise mentioned. Volatile fatty acids (formic, acetic, propionic, butyric, and valeric acids) were determined by centrifuging a small sample at greater than 10,000 rpm for 5 min, filtering the liquid through a 0.2-mm-pore-size filter, and injecting a 10-mL-sample into a high-pressure liquid chromatograph (HPLC). In the HPLC, the mobile phase (filtered 5 mM H2SO4) was pumped at 0.6 mL/ min through a 300 mm 7.8 mm (8 mm particle size) RHM Monosaccharide column (Phenomenex, Torrance, CA), held at a temperature of 65 1C, to a refractive index detector (Model 2410, Waters Corporation, Miltford, MA) held at a temperature of 40 1C. Biogas volume was measured using wet-test gas meters (GSA/Precision Scientific, Chicago, IL), and the samples (1 mL) for biogas composition were collected using a gas-tight syringe. The samples were injected in duplicate into a Gow-Mac (Model 69-350 Series, Lehigh Valley, PA) gas chromatograph (GC) equipped with a 6 ft 1/8 in stainless steel column packed with 80 100 Hayesep Q (Supelco, USA). The oven, injector, and thermal conductivity detector temperatures were kept as 45, 90, and 110 1C, respectively. The carrier gas (helium) flow rate through the column was maintained at 30 mL/min. Initially, the GC was calibrated with 99.9% pure methane (CH4) and nitrogen standards. Average steady-state data and the standard error presented in the paper have been calculated as a mean value over 20–30 days of observations. Statistical significance (P ¼ 0:05) of the experimental data was tested using one-way (or single factor) analysis of variance (ANOVA) statistical program (Microsoft Excel 2002). Least significant difference was used as for post hoc testing of data. Finally, the observed methane production rates for the three experimental sets were compared with the Hill’s model (Husain, 1998) predictions and discussed.
Digesters 1 through 4, respectively. The effluent DCOD concentration from the digesters was observed at 3.7–4.2 g/L, showing about a 79–81% reduction in the DCOD in the digesters under steady-state conditions. The TN content of the waste slurry remained unchanged in the anaerobic treatment. VFA concentrations in the effluents from the digesters were observed as less than 250 mg/L and the pH of the effluents was between 7 and 7.8. The daily biogas production for the four digesters has been shown in Fig. 2. The average biogas production rates for Digesters 1–4 were 0.8470.07, 0.9470.07, 0.8870.09, and 0.8570.09 L/L d with average methane content as 6473, 5673, 6173, and 6772%, respectively. The biogas production rates were calculated as volume of biogas produced per liter of digester volume per day and averaged over the last 30 days of operation. The biogas production rate data shows that Digester 2 produced slightly more biogas than the other digesters, but the corresponding methane content was found to be lower in comparison, probably due to infiltration of air, which was observed to be up to 18% in the case of Digester 2. It is worthwhile mentioning that biogas circulation in laboratory digesters increases the chances for ‘infiltration’ of air into the system (due to slight air permeability of tubing and leakage on the vacuum side of the air pump, etc.). Average steady-state performance data of the four digesters were found to be quite similar (Table 3). However, to evaluate the results further, the data were subjected to ANOVA. The ANOVA test showed that methane production data for the four digesters did not differ significantly at the 5% level (P ¼ 0:06; F ¼ 2:72; Fcrit ¼ 2.9; df ¼ 3, 32). Methane yield, calculated as the volume of methane produced per unit mass of VS added, was observed to be 0.26–0.28 L/g VS added (Table 3). It is important to note that the VS solids loading in the present study was 2 g/L d. The Unmixed (Digester 1) Biogas-mixed (Digester 2) Impeller-mixed (Digester 3) Impeller-mixed (Digester 3, duplicate run) Slurry recirculation (Digester 4)
3.1. First set of experiments (5% manure slurry feed) Four laboratory-scale anaerobic digesters were operated at 35 1C to produce biogas from 5% cow manure slurry feed. Initially there were variations in the daily biogas production for the four digesters, but the performance stabilized with time. After achieving steady-state conditions, all four digesters behaved quite similarly with the TS and VS reduction in the range of 37–40% and 50–63%, respectively. Total COD in the feed was about 58.7 g/L, about 33% of which was present in the form of dissolved COD. The reduction of TCOD was observed as 56%, 58%, 57%, and 56% for
Biogas production (L/d)
6
3. Results and discussion
3601
4
2
0 0
10
20
30 40 50 Days of operation
60
70
Fig. 2. Daily biogas production for Digesters 1–4 observed during the first experimental set using 5% feed slurry study.
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Table 3 Average biogas production rate and methane yield for the digesters over the last 30 days of operation Set of expt.
Digester
1
1 2 3 4
2
5 6 7 8
3
9 10 11
Mode of mixing
VS loading (g/L d)
a
Biogas production rate (L/L/d)
Methane yield (L CH4/g VS loaded)
b Statistical significance
Unmixed Biogas-mixed Impeller-mixed Slurry recirculation Unmixed Biogas-mixed Impeller-mixed Slurry recirculation Unmixed Biogas-mixed Impeller-mixed
2 2 2 2
0.8470.07 0.9470.07 0.8870.09 0.8570.09
0.27 0.26 0.27 0.28
A A A A
3.24 3.24 3.24 3.24
0.9270.1 1.0770.08 1.1470.13 1.2070.14
0.19 0.21 0.23 0.24
B C D D
3.24 3.24 3.24
1.1370.14 1.6470.32 1.2570.12
0.15 0.23 0.17
E F
7 shows the standard error Alphabetic symbols given in the last column depict statistically same/different performance of the digesters. Please note that the analysis of variance (ANOVA) was performed separately for each of the three sets of experiments. a
observed methane yield is comparable to the reported methane yield of 0.376 L/g VS added, observed at a loading of 2.86 g VS/L d (Linke, 1997). 3.2. Second set of experiments (10% manure slurry feed) The second set of experiments was also conducted with four digesters, unmixed (Digesters 5), biogas-mixed (Digester 6), impeller-mixed (Digester 7), and slurry recirculation (Digester 8). The goal was to study whether the role of mixing and mode of mixing became more important when the TS concentration in the animal waste slurry was increased. The total and VS concentration in the effluents from the four digesters are shown in Fig. 3. The digesters produced biogas consistently throughout the study period (Fig. 4). Digester 8, equipped with slurry recirculation, produced slightly more biogas than any of the other digesters, while the unmixed digester (Digester 5) produced the least. The average steady-state biogas production rate and methane yield data, calculated over a period of last 30 days of operation (from Day 41 to Day 71), are given in Table 3. Under steady-state conditions the biogas production rates for Digesters 5–8 were calculated as 0.9370.1, 1.0770.08, 1.1470.13, and 1.270.14 L/L d with 6673, 6574, 6573, and 6674% methane contents, respectively. The unmixed digester (Digester 5) produced biogas at a rate almost 22% less than the digester mixed by slurry recirculation (Digester 8), and the trend was very consistent as shown in Fig. 4. Digester 6 (mixed by biogas recirculation) produced biogas at a 10% lower rate than Digester 8. ANOVA of the daily biogas production data for the steady-state
Effluent TSand VS concentration (g/L)
b
100
Unmixed (TS) Biogas-mixed (TS) Impeller-mixed (TS) Slurry recirculation (TS)
90 80
Unmixed (VS) Biogas-mixed (VS) Impeller-mixed (VS) Slurry recirculation (VS)
70 60 50 40 30 20 10 0
10
20
30 40 50 Days of operation
60
70
80
Fig. 3. Total and volatile solids (TS and VS) concentrations in the effluents collected from Digesters 5–8 during the 10% feed slurry study.
period at the 5% level showed significant difference among the four digesters (P ¼ 4:41 107 ; F ¼ 14:4; Fcrit ¼ 2.76; df ¼ 3, 56). However, there was no significant difference (P ¼ 0:26; F ¼ 1:31; Fcrit ¼ 4.22; df ¼ 1, 26) between the daily biogas production of Digester 7 (impeller-mixed) and Digester 8 (slurry recirculation).
3.3. Third set of experiments (15% manure slurry feed) The goal of the third set of experiments was to study the role of mixing in the case of digesters fed with extremely concentrated cow manure slurry (150 g TS/L).
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Biogas (Biogas-mixed) Biogas (Slurry recirculation) VS (Feed)
120
12 80
10 8
40
6 4
0
2 0 0
10
20
30 40 50 60 Days of operation
70
-40 80
Fig. 4. Daily biogas production from Digesters 5–8 along with the TS and VS concentrations in the 10% feed slurry study.
Since it was very difficult to recirculate 15% waste slurry using a peristaltic pump, the digester mixed by slurry recirculation was not included in this study. The daily biogas productions for unmixed, biogas-mixed and impeller-mixed digesters are shown in Fig. 5. It is clear from the figure that unmixed digester (Digester 9) showed a quicker start-up than the mixed digesters (Digesters 10 and 11). The unmixed digester took less than 20 days to get stabilized, while both of the mixed digesters were unstable for more than 30 days. Another important observation was the lower pH inside the mixed digesters during the start-up period (Fig. 6). Low pH inside the mixed digesters can be attributed to improved hydrolysis due to external mixing (via biogas recirculation or impeller), which might have resulted in high VFA concentration inside the digesters. Unfortunately, VFA data could not be obtained for this set of experiments due to the complications caused by high solids concentration in the effluent. However, observation of low biogas production in mixed digesters during start-up also supports the possibility of VFA build-up inside the mixed digesters, which is known to inhibit the biogas production activity. After 1 month of operation, the biogas-mixed digester showed higher biogas production than the other two (Fig. 5). Initially, the reason behind this was unexplained. However, when the digesters were disassembled after completion of the study, the biogas-mixed digester was found to be almost completely clogged. This made it clear that the higher amount of biogas production from the biogas-mixed digester was due to the accumulation of biodegradable solids inside the digester. Solids accumulated at the bottom of all three digesters, but the biogasmixed digester (Digester 10) was severely clogged from top to bottom with some channels in the annular section (in between the draft tube and the digester side wall). The severe clogging of the biogas-mixed digester was due to the presence of the static internals in the form of draft tube, injector tube and hangers.
3603
10 Unmixed
9 Biogas production (L/d)
Biogas (Unmixed) Biogas (Impeller-mixed) TS (Feed)
14
Biogas mixed
Impeller mixed
8 7 6 5 4 3 2 1 0 0
10
20
30 40 50 Days of operation
60
70
Fig. 5. Daily biogas production for Digesters 9–11 during the 15% feed slurry study.
9 Unmixed
Biogas-mixed
Impeller-mixed
8 7 6 pH
Daily biogas production (L/d)
16
Feed TSand VS concentration (g/L)
K. Karim et al. / Water Research 39 (2005) 3597–3606
5 4 3 2 0
10
20 30 40 Days of operation
50
60
Fig. 6. pH in the effluents from Digesters 9–11 collected during the 15% feed slurry study.
The results for TS and VS in the effluents are shown in Fig. 7. During the first 20 days, the TS and VS concentration in the effluent from the biogas-mixed digester was rather consistent; later, the variation in TS and VS data is noted. This supports the fact that the biogas-mixed digester (Digester 10) was unstable. From Fig. 7, it is also clear that the impeller-mixed digester was adequately mixed with less variation in TS and VS in the effluent throughout the 2 months period. As shown in Table 3, the average biogas production rates (from day 36 to day 60) for the unmixed (Digester 9), biogas-mixed (Digester 10) and impellermixed (Digester 11) digesters were calculated to be 1.1370.14, 1.6470.32, and 1.2570.12 L/L d, with the methane contents of 64%, 66%, and 63%, respectively. Digester 10 (biogas-mixed) had a significantly higher rate of biogas generation. This is not surprising since this digester was found clogged. Consequently, the biodegradable solids would have been retained in the biogas-mixed digester longer than other two digesters, resulting in more biogas generation. Statistically, at the
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Effluent TSand VS concentration (g/L)
180 160
Unmixed TS
Biogas mixed TS
Impeller mixed TS
Unmixed VS
Biogas mixed VS
Inpeller mixed VS
140 120 100 80 60 40 20 0
10
20
30 40 Days of operation
50
60
Fig. 7. Plot showing total solids and volatile solids (TS and VS) concentrations in the effluents collected from Digesters 9–11 during the 15% feed slurry study.
5% level there was significant difference (P ¼ 0:02; F ¼ 5:42; and Fcrit ¼ 4.26; df ¼ 1, 24) in the biogas production rates of the unmixed and impeller-mixed digesters. Because of clogging Digester 10 might have had longer HRT than the other two digesters, thus its data was not included in the statistical analysis.
4. Discussion In the investigation of unmixed and mixed digesters fed with 5% feed slurry (loading ¼ 2 g VS/L d), the mixing or the three different modes of mixing did not significantly affect the digesters performance. Within the group of mixed digesters, mechanical mixers are reported to be the most efficient in terms of power consumed per gallon mixed (Brade and Noone, 1981). Thus, the digester mixed by an impeller had ‘‘better’’ mixing than the others; however, performance was similar to the other digesters. We have shown, using ANOVA, that mixing or mode of mixing had a statistically negligible effect on the digesters performance in the case of 5% manure slurry. The above findings raised questions of whether the 16.2 days HRT was long enough for the microbes to assimilate all the readily available organics or if the mixing intensity was not high enough to play a role. To answer this question, second and third sets of experiments were performed to evaluate whether the role of mixing becomes more significant with an increase in TS concentration in the feed slurry. Statistical analysis of the steady-state biogas production rate (volume of biogas/volume of digester/day) data for the second set of experiments showed significant difference among the digesters (at the 5% level, P ¼ 1:26 107 ; F ¼ 15:8;
Fcrit ¼ 2.76; df ¼ 3, 58), with 0.08 as the least significant difference value. Therefore, unmixed digester biogas production rate was significantly different from all other digesters. The data further show that the slurry recirculation digester (Digester 8) had the highest biogas production rate (Table 3). The slurry recirculation digester produced about 29% more biogas than the unmixed digester (Digester 5), and the impeller-mixed digester (Digester 7) produced about 22% more biogas than the unmixed digester. The biogas-mixed digester produced about 15% more biogas than the unmixed digester. Therefore, the results show that mixing improved the biogas production when a thicker manure slurry (10%) was fed. Interestingly, the biogas production data from the third set of experiments, when the digesters were fed with 15% manure slurry, also shows that the impeller-mixed digester (Digester 11) produced more biogas than unmixed digesters (Table 3). ANOVA analysis showed that at the 5% level there was significant difference in the biogas production rates of the unmixed and impeller-mixed digesters. Please note that the biogas-mixed digester (Digester 10) was found clogged, which would result in longer HRT for the solids than in the other two digesters, and its data was not included in the statistical analysis. Based on the results obtained, we conclude that the role of mixing becomes more important with an increase in TS concentration in the feed slurry. However, observations of lower pH and less biogas production in the case of mixed digesters fed with 15% manure slurry suggest that mixing of digester during start-up is not beneficial, as it lengthen the start-up time. So far as the mode of mixing is concerned, in the studies using 5% manure slurry, there was no significant difference in the performance of the digesters being mixed by three different modes of mixing. However, when the digesters were fed with 10% manure slurry, the influence by the mode of mixing was evident (P ¼ 0:03; F ¼ 3:77; Fcrit ¼ 3.22; df ¼ 2, 41 at the 5% level). There was no significant difference in the biogas production for the slurry recirculation digester and the impeller-mixed digester, as mentioned earlier. The high biogas production in the case of the slurry recirculation digester may be attributed to the fact that the particles, chunks and flocks were exposed to higher shear and were crushed while passing through the hub of the recycling pump used. One of the roles that mixing plays inside a digester is to minimize stratification and accumulation of inert solids, especially if the feed manure has a high concentration of inert solids, such as sand from bedding material. Solids accumulation was not observed during first set of experiments (with 5% manure slurry). However, during second and third sets of experiments significant amount of solids accumulation was observed in both unmixed and mixed digesters. Digesters 5–8
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RM ¼ gB0 sI,
(4)
where g is the amount of methane produced per VS consumed (methane yield), B0 is the biodegrability constant (equal to the VS destroyed per VS added), s is the loading rate (equal to the concentration of VS in the feed divided by HRT), and I is a ‘productivity index’ defined by I ¼ 0:5 þ ð1=2:95Þ Arctan½ðt sÞ=0:211, where t is the ‘‘stress index.’’
(5)
1.40 Methane production rate (L/L-d)
were found to have about 337, 205, 260, and 190 g (dry weight) solids accumulated inside, which had about 23%, 9%, 5%, and 6% VS, respectively. From the results of the third set of experiments it is clear that digester mixing by biogas recirculation is not a promising option when a digester is being fed with thick manure slurry, as the biogas-mixed digester (Digester 10) was found to be severely clogged from top to bottom with some channels in the annular section. The severe clogging of the biogasmixed digester was due to the presence of the static internals in the form of draft tube, injector tube and hangers. This finding, however, may not translate to larger-scale digesters. Stratification and clogging problems are crucial as it ultimately reduces the effective volume of the digester and lead to its failure. This suggests that digester mixing and mode of mixing become critical with an increase in the TS concentration in the feed slurry. The digester configuration and external mixing aid should be properly designed and selected based on hydrodynamics as explained elsewhere (Karim et al., 2004). To summarize: Among the three modes of mixing used, biogas recirculation seems to be a promising option for digesters fed with dilute manure slurry. However, biogas recirculation or slurry recirculation is not an effective option in the case of digesters being fed with thick manure slurry. Mechanical impellers are reported to be most efficient in terms of power consumed per gallon mixed (Brade and Noone, 1981), but the energy requirement for impeller mixing increases in orders of magnitude with the size of the digester. The biogas production rates and the methane yields observed during the studies reported in this paper are summarized in Table 3. Methane yield is defined as the volume of methane produced per unit weight of VS loaded. It is obvious from the tabulated data that the biogas production rate increased when there was an increase in the TS concentration in the feed slurry. However, there was a significant decrease in the methane yield. In other words, at higher solids loading, VS were not being exhausted up to their full potential. Similar observations were reported by Linke (1997). The methane production rates observed during the present study was compared with the predictions of the empirical kinetic model proposed by Hill and described by Husain (1998). As per the model, for a continuously fed anaerobic digester methane production rate (RM in L CH4/L d),
3605
Hill's model Unmixed Biogas mixed Impeller mixed Slurry recirculation
1.20 1.00 0.80 0.60 0.40 0.20 1
1.5
2 2.5 3.5 4 4.5 3 Volatile solidsloading rate (g/L-d)
5
Fig. 8. Comparison of experimental methane production rate with the prediction of Hill’s model (Note: Experimental data corresponding to 1.54 g VS/L d loading was taken from another study conducted by our group reported elsewhere (Karim et al., 2005)).
Hill (see Husain, 1998) recommended that the values of g, B0, and t are set to 0.5 (L CH4/L d), 0.483 (g TVS/ g TVS), and 9.21 (g TVS/L d), respectively, for confined beef cattle manure. These values were substituted in the above equations to calculate the methane production rates for the experimental VS loading rates. A comparison of the experimental data with the model predictions are shown in Fig. 8. Note that some of the experimental data plotted in Fig. 8, corresponding to the 1.54 g VS/L d loading, was taken from another study conducted by our group using the same reactors and manure from the same source (Karim et al., 2005). It is clear from the figure that Hill’s model slightly under-predicted the performance at loading rates of 1.54 and 2 g VS/L d. However, in the case of higher loadings (3.24 g VS/L d) the model significantly over-predicted the methane production rates. In fact, the empirical model proposed by Hill is based on the assumption that methane production rate increases linearly with increase in VS loading before reaching the highest possible loading, at which time the digester fails (Husain, 1998). However, from Fig. 8, it is clear that the relation between VS loading and methane production rate in not linear.
5. Conclusions We found no effect of mixing on digesters performance when fed with 5% manure slurry. However, the effect of mixing and the mode of mixing became prominent when digesters were fed with thicker manure slurry (10% and 15%). In the case of 10% and 15% manure slurry, the mixed digesters produced more biogas than unmixed digesters, but digester mixing during start-up was not beneficial as it resulted in lower
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pH, performance instability, and prolonged start-up time. Mixing using biogas recirculation system was not found effective in the case of digester fed with 15% manure slurry under the experimental conditions studied. The observed methane production rates for smaller VS loadings (1.54 and 2 g VS/L d), were comparable with the prediction of Hill’s model. However, at higher loading rates (3.24 and 4.7 g VS/L d) the model significantly over-predicted the methane production rates.
Acknowledgements The authors thank the US Department of Energy and its Office of Energy Efficiency and Renewable Energy for sponsoring the research project (Identification No. DE-FC36-01GO11054).
References APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington, DC. Ben-Hasson, R.M., Ghaly, A.E., Singh, R.K., 1985. Design and evaluation of no-mix energy efficient anaerobic digester. Proceedings, Annual Meeting, Canadian Society of Agricultural Engineering, June 23–27, Charlottetown, P.E.I. Brade, C.E., Noone, G.P., 1981. Anaerobic digestion—need it be expensive? Water Pollut. Control 80, 70–76. Burke, D.A., 2001. Dairy Waste Anaerobic Digestion Handbook. Environmental Energy Company, 6007 Hill Street, Olympia, WA 98516. Casey, T.J., 1986. Requirements and methods for mixing in anaerobic digesters. In: Bruce, A.M., Kouzeli-Kastri, A., Newman, P.J. (Eds.), Anaerobic Digestion of Sewage Sludge and Organic Agricultural Wastes. Elsevier Applied Science Publishers, Barking England, pp. 90–103. Chapman, D., 1989. Mixing in anaerobic digesters: state of the art. In: Cheremisinoff, P. (Ed.), Encyclopedia of Environmental Control Technology, vol. 3. Gulf Publishing Company, Houston, pp. 325–354. Chen, T., Chynoweth, D.P., Biljetina, R., 1990. Anaerobic digestion of municipal solid waste in a nonmixed solids concentrating digester. Appl. Biochem. Biotechnol. 24/25, 533–544. Dague, R.R., McKinney, R.E., Pfeffer, J.T., 1970. Solids retention in anaerobic waste treatment systems. J. Water Pollut. Control Fed. 42 (2 Part 2), R29–R46. Diaz, L., Trezek, G., 1977. Biogasification of a selected fraction of municipal solid wastes. Compost Sci. 8–13. Dolfing, J., 1992. The energetic consequences of hydrogen gradients in methanogenic ecosystems. FEMS Microbiol. Ecol. 101, 183–187. Fontenot, J.P., Ross, I.J., 1980. Animal waste utilization. In: Livestock Waste: A Renewable Resource. Proceedings of the Fourth International Symposium on Livestock Wastes, ASAE, St. Joseph, MI.
Haghighi-Podeh, M.R., Bhattacharya, S.K., Mingbo, Q., 1995. Effects of nitrophenols on acetate utilizing methanogenic systems. Water Res. 29, 391–397. Hashimoto, A.G., 1982. Effect of mixing duration and vacuum on methane production rate from beef cattle waste. Biotechnol. Bioeng. 24, 9–23. Ho, C.C., Tan, Y.K., 1985. Anaerobic treatment of palm oil mill effluent by tank digesters. J. Chem. Technol. Biotechnol. 35b, 155–164. Husain, A., 1998. Mathematical models of the kinetics of anaerobic digestion—a selected review. Biomass Bioenergy 14 (5/6), 561–571. James, S., Wiles, C., Swartzbaugh, J., Smith, R., 1980. Mixing in large-scale municipal solid waste-sewage sludge anaerobic digesters. Biotechnol. Bioeng. Symp. 14. Karim, K., Varma, R., Vesvikar, M., Al-Dahhan, M.H., 2004. Flow pattern visualization of a simulated digester. Water Res. 38, 3659–3670. Karim, K., Klasson, K.T., Hoffmann, R., Drescher, S.R., DePaoli, D.W., Al-dahhan, M.H., 2005. Anaerobic digestion of animal waste: Effect of mixing. Bioresource Technology 96 (14), 1607–1612. Lema, J.M., Mendez, R., Iza, J., Garcia, P., FernandezPolanco, F., 1991. Chemical reactor engineering concepts in design and operation of anaerobic treatment processes. Water Sci. Technol. 24, 79–86. Linke, B., 1997. A model for anaerobic digestion of animal waste slurries. Environ. Technol. 18, 849–854. Masse, D.I., Lu, D., Masse, L., Droste, R.L., 2002. Effect of antibiotics on psychrophilic anaerobic digestion of swine manure slurry in sequencing batch reactors. Bioresource Technol. 75, 205–211. Meynell, P.-J., 1976. Methane: Planning a Digester. Prism Press, London, pp. 55–57. Mills, P.J., 1979. Minimization of energy input requirements of an anaerobic digester. Agric. Wastes 1, 57–59. Parkin, G.F., Owen, W.F., 1986. Fundamentals of anaerobic digestion of wastewater sludges. J. Environ. Eng. 112, 867–920. Sawyer, C.N., Grumbling, A.M., 1960. Fundamental consideration in high-rate digestion. J. Sew. Eng. Div. ASCE 86, 49–63. Smith, R.J., Hein, M.J., Greinier, T.H., 1979. Experimental methane production from animal excreta in pilot-scale and farm-size units. J. Animal Sci. 48, 202–217. Smith, L.C., Elliot, D.J., James, A., 1996. Mixing in upflow anaerobic filters and its influence on performance and scaleup. Water Res. 30, 3061–3073. Stenstrom, M., Ng, A., Bhunia, P.K., Abramson, S., 1983. Anaerobic digestion of municipal solid waste. J. Environ. Eng.—ASCE 109, 1148–1158. Stroot, P.G., McMahon, K.D., Mackie, R.I., Raskin, L., 2001. Anaerobic codigestion of municipal solid waste and biosolids under various mixing conditions––I. Digester performance. Water Res. 35, 1804–1816. US EPA, 1979. Process Design Manual for Sludge Treatment and Disposal. EPA 625/1-79-011. Cincinnati, Ohio. Whitmore, T.N., Lloyd, D., Jones, G., Williams, T.N., 1987. Hydrogen-dependent control of the continuous anaerobic digestion process. Appl. Microbiol. Biotechnol. 26, 383–388.