Anaerobic digestion on a diary farm: Overview
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Energy in Agriculture, 4 (1985) 347--363 347 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
ANAEROBIC DIGESTION ON A DAIRY FARM: OVERVIEW
L.P. WALKER, R.A. PELLERIN, M.G. HEISLER, G.S. FARMER and L.A. HILLS
Department of Agricultural Engineering, College of Agriculture and Life Sciences, Cornell University, Riley Robb Hall, Ithaca, NY 14853 (U.S.A.)
(Accepted 24 July 1985)
Walker, L.P., PeUerin, R.A., Heisler, M.G., Farmer, G.S. and Hills, L.A., 1985. Anaerobic digestion on a dairy farm: overview. Energy Agric., 4: 347--363.
This paper explores the design, implementation and performance of an on-farm plug flow anaerobic digestion system. Capital costs for the digester and justifica- tions for certain design decisions are presented. Seasonal variation in the total and volatile solids concentrations, ammonia and organic nitrogen contents, and pH were documented. Biogas outflow of 400--495 m3/day was also documented for an 180-cow herd.
In the fall of 1980, Cornell University researchers embarked on an am- bitious undertaking: the design, implementation and demonstration of an integrated energy farm system. The primary goal was to reduce fossil fuels and fossil fuel-based inputs into the farm system by: (a) substituting energy efficient processes and practices for energy-intensive ones; and by (b) using solar-based energy sources -- wind, active solar and biomass. In addition, the sponsors of the project required that the system be designed for, and implemented on, a private farm. Although conducting this project on a private farm placed limits on the logistics of conducting research, it provided a unique opportunity for assessing the concept of an integrated farm system.
During the 1st year of the project, a variety of energy conservation practices and alternative energy sources were identified and evaluated (Walker et al., 1984). One solar-based energy source assessed was biogas produced from animal waste. This assessment suggested that a plug flow digester, working under mesophilic conditions, could be a reliable and low- cost energy source for the farm. As a result, a plug flow digester was de- signed and constructed on the Millbrook Farm.
This project was sponsored by the U.S. Department of Energy, New York State Energy Research and Development Authority, New York State Electric and Gas, Agway and Cornell University.
0167-5826/85/$03.30 1985 Elsevier Science Publishers B.V.
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The objectives of this paper are: (a) to identify the site characteristics and farm management constraints which influenced the initial digester design; (b) to discuss subsequent system modifications; and (c) to present some of the preliminary results on the digester performance.
Site description and constraints
At the start of the project, the farm consisted of approximately 200 head of dairy cattle -- 120 milking cows and associated youngstock. Total farm areage was 154 ha, with an additional 30 ha leased by the farm owners.
Figure 1 shows the layout of the farm. The dairy barn is a pole-clear span structure with a concrete block foundation. It is a freestall facility with drive-through feed bunkers and alleys. Manure was scraped from the alleyways of the dairy barn using a front-end loader. The youngstock barn is of similar construction, with feeding and manure handling in the same fashion as the dairy barn.
Figure 2 summarizes total farm fossil fuel and electricity usage at the time the project was started. Gasoline was the largest single fuel consumed on the farm. Planting, harvesting, waste management, feeding and miscel- laneous tasks consumed 800 GJ of gasoline.
Plug flow digester design variables and alternatives
Jewell et al. (1978, 1980) conducted the most comprehensive assessment of the plug flow digester concept. In this study, full-scale plug flow and completely mixed digesters were operated simultaneously. Under all full- scale tests, the plug flow digester out performed the completely mixed digester in terms of solids conversion efficiency and volumetric gas pro- duction rates (Jewell et al., 1980). The study found the plug flow digester achieved between 13 and 23% greater volatile solids destruction than the completely mixed unit, and produced net energy even during the coldest periods of the year.
Optimal retention time and insulation thicknesses for the walls, floor and surface of the digester were determined from an optimization study (Hills and Walker, 1982). The optimization involved the development of models of the digester energetics and economics. The variables optimized were hydraulic retention time, and insulation thickness for the walls, floor and surface of the digester. Other variables and parameters such as length- to-widt ratio, total and volatile solids concentration of the manure, daily averages of the soil and ambient temperatures, and a dollar value for the gas were also used in the model.
DISTRIBUT ION 9O0
OF- FUEL USE 1980
Fig. 2. Farm energy usage at the start of the project
Results from the optimization study indicated a hydraulic retention time of 25 days, and insulation thicknesses of 5 cm for the walls, floor and top of the digester would be the optimum assuming that biogas was worth $2.40/GJ. Further synthesis of the results from the optimization study and more engineering and economic analyses were done to assess the impact of an expanding herd on the digester design and operation. From this assessment, the dicision was made to design the digester with a retention time of 24 days for a herd with 180 milking cows and 120 asso- ciated dry cows and youngstock. The thicknesses of the polystrene insula- tion obtained from this assessment were 5 cm for the floor and walls, and 7.5 cm for the manure surface.
A perspective of the digester constructed is presented in Fig. 3. Digester dimensions are 22.6 m X 6.1 m X 2.4 m. Wall heights of 2.4 m were selected because this is the standard height of concrete walls fro basements. Manure entered the digester through two 0.8-m diameter corrugated steel pipes located in the northwest and northeast corners of the digester. The manure exits the digester through three PVC pipes located in the south end of the digester. These effluent pipes are connected to a manhole, which is con- nected to the manure storage facility (Fig. 3). To retard heat loss from the surface of the manure, insulation was placed in the digester above the manure surface (see Fig. 3).
The digester's flexible cover was initially a 0.9 mm thick hypalon material. A neoprene gasket was placed between the concrete and the flexible cover. A mechanical seal was selected because of the ease with which it could be installed and, more importantly, the ease with which it could be removed to make repairs on the digester. A silicone gel was applied between the flexible cover and neoprene gasket.
There are two heat exchangers in the digester, as shown in Fig. 4. An influent heat exchanger was designed to heat the incoming manure to 35C. The second or maintenance heat exchanger spans the length of the digester, as shown in Fig. 4. The two heat exchangers share a common return. Heat exchangers were also installed in each of the three hoppers to preheat incoming manure during winter.
Fig. 4. Digester heat exchange design.
Two categoreis of instrumentation were included in the system. The first is for the day-to-day management of the digester. In this system thermo- couples were interfaced with three temperature controllers which control the water circulators for the influent and maintenance heat exchangers. A similar arrangement was established for controlling the temperature of the hoppers. Gas production was measured with a positive displacement gas flow meter.
The second type of instrumentation monitored research variables and parameters. A microcomputer interfaced with an analog-to-digital converter, and a channel scanner read 27 thermocouples at different locations in the digester. In addition it read the gas flow meters, pressure transducer and weather station.
In addition to the on-line data acquisition system, manure samples were collected periodically and assessed for total and volatile solids, organic nitrogen and ammonia, and pH. Carbon dioxide was measured periodically using a fyrite tester. This method of measuring CO2 concentration in the gas was checked against a gas chromatograph for accuracy and found to be within 1 to 2 percentage points.
System cos ts
As the project proceeded, a detailed data base of the costs incurred for the various subsystems was compiled. Capital costs for the digester are summarized in Table 1. The total capital cost of $68 500 reflects the total investment for the digester system. Not reflected in the cost are the R & D costs associated with the design of the digester, nor the cost of contract and construction management.
The major operating costs for the digester system are propane for digester start-up, the cost of seeding the digester when performance is poor or during start-up, cost for maintaining the instrumentation and testing chem- icals, and labor cost. Annual propane usage will vary depending on the type of problems encountered with the digester over a given year. It is part of the cost of digester start-up and can be a very significant cost if problems with gas production occur during the winter months for extended periods. On the average, annual operating costs will be between $300 and $1000.
Using a herd size of 180 milking cows, the capital cost per cow was $380 per cow. The cost per volume of digester is $204/m 3.
Capital cost for digester
Category Cost (US$)
Digester and hoppers construction 44 300 Digester cover 2 400 Gas transport 400 Digester heat exchange system 4 100 Second digester heat exchange system 2 700 Hopper heat exchange 1 700 Back-up heating system 3 200 Digester instrumentation 2 200 Utility building 7 000
Total 68 500
SYSTEM OPERATION, PERFORMANCE AND MODIFICATIONS
All major construction was completed by December 1981. Several modifi- cations have been made to the systems. These modifications are discussed as the system performance is examined in this section.
Digester s ta rt-up
Heating of the digester was initiated in January 1982. Two 151-liter propane fueled hot water heaters, each rated at 11.7 kW, were installed, but not as the primary heating system. The waste heat from the cogeneration
system, fueled by biogas, is the primary heating system for the digester. The two heaters lacked the capacity to keep the digester at optimal tem- perature during this first Winter.
Digester cover failure
The first problem encountered with the digester was failure of the digester cover. In February 1982, two 5 cm long tears approximately 30 cm apart along one seam of the cover were discovered. One week after the cover was repaired additional tears along the adjacent seam occurred. A series of tests indicated the cover failure was caused by material failure. The cover was replaced with a 1.14-mm PVC-reinforced material with light gray cover and seams running perpendicular to the digester. Since its instal- lation, no problems have been encountered with the cover.
Quantity of manure and retention time
Since the herd was allowed to graze on pasture land during late spring, summer and early fall, the quantity of manure available for anaerobic digestion, and the retention time varied throughout the year. Volumetric measurements of the available manure were made several times. From these measurements hydraulic retention times were calculated. These re- sults are presented in Table 2.
Manure available for anaerobic digestion
Time frame Available manure (m3/day)
Hydraulic retention (days)
Summer 1983 7.1 35.5 Winter 1983/84 10.3 24.5 Fall 1984 10.4 24.2
Characteristics of substrate
Tabulated in Table 3 are the monthly averages for the total and volatile solids concentrations for the manure entering the digester from the dairy barn. A relatively high total solids (TS) concentration of 16.8% or higher was not unusual during the summer months. The volatile solids concentra- tion followed a similar pattern (see Table 3). Dally average volatile solids (VS) concentration for the last two years ranged from a high of 14.6% to a low of 11.2%. The total and volatile solids concentrations varied by as much as 3 percentage points between summer and winter. This variation
can be attributed to a number of factors, such as changes in feed ration and evaporation rate. Total and volatile solids concentrations for the young- stock barn are presented in Table 4. The youngstock barn exhibited the same seasonal variation observed in the main barn manure. Total and volatile solids from bothe the dairy barn and the youngstock barn are relatively high when compared to operating values reported by other investigators (Converse et al., 1977; Jewell et al., 1978, 1980; Bartlett et al., 1980; Schellenbach, 1982).
The ammonia and organic nitrogen concentrations for the manure from the dairy barn and the youngstock are also presented in Tables 3 and 4,
Monthly averages for dairy barn manure
Month Year pH TS VS (mg N per g manure) (%) (%)
NI-13--N Org--N TKN
May 82 6.97 14.00 11.15 1.22 3.29 4.51 June 82 7.35 15.63 12.39 1.31 4.20 5.50 July 82 August 82 15.02 12.96 1.31 3.73 5.04 September 82 7.44 14.05 11.89 1.27 3.54 4.80 October 82 8.09 13.30 11.46 1.63 3.88 5.51 November 82 8.05 12.81 11.09 1.38 3.60 4.99 December 82 8.15 13.13 11.44 1.38 3.57 4.95 January 83 8.16 12.72 11.10 1.37 4.14 5.51 February 83 8.37 13.60 12.14 1.59 4.46 6.05 March 83 8.37 13.71 12.28 2.22 3.28 5.50 April 83 8.48 13.58 12.22 2.17 3.13 5.30 May 83 June 83 July 83 7.34 15.65 13.81 0.56 2.95 3.51 August 83 6.86 16.54 14.48 1.55 3.70 5.25 September 83 7.21 16.78 14.61 1.76 3.55 5.31 October 83 7.83 15.31 13.32 1.78 3.43 5.21 November 83 8.22 14.01 12.52 2.40 3.90 5.66 December 83 8.41 13.41 11.95 2.26 3.59 5.85 January 84 8.39 13.60 12.20 2.20 3.56 5.76 February 84 8.21 13.24 11.80 2.15 3.42 5.58 March 84 8.29 13.79 12.37 2.34 3.53 5.87 April 84 8.10 13.50 12.12 2.39 3.50 5.88 May 84 8.09 13.06 11.39 1.99 2.97 4.95 June 84 7.18 14.69 11.80 1.69 3.31 5.00 July 84 7.21 15.03 12.91 1.27 3.19 4.46 August 84 7.10 14.26 12.18 1.33 1.99 4.32 September 84 7.24 14.60 12.53 1.66 3.41 5.07 October 84 8.17 13.42 11.65 2.10 3.31 5.32 November 84 8.54 13.14 11.41 2.09 3.51 5.60 December 84 8.52 12.39 10.83 2.16 3.43 5.59
respectively. Ammonia concentrations are at the lowest level during summer. Some of the seasonal variation in ammonia concentration can be attributed to the ammonia volatilization rate which is a function of ambient tem- perature. In studies conducted by Muck and Richards (1983), ammonia losses of 40--60% were observed in free stall barns at ambient temperatures of 20--25C. Variation in the amount of nitrogen present in the manure will also result from changes in feed ration. The nitrogen content of the main barn is consistently higher than that of the youngstock barn. This is due to the higher protein requirement of the milking herd.
The average daffy pH values for the manure from the main barn are also presented in Table 3. The pH range for the influent stream was 6.8--8.8.
Monthly averages for youngstock manure
Month Year pH TS VS (rag N per g manure) (%) (%)
Nt-I~--N Org--N TKN
May 82 7.62 14.35 12.25 1.27 2.49 3.76 June 82 7.66 15.98 13.68 1.15 4.05 4.95 July 82 August 82 13.11 11.45 0.69 3.23 3.92 September 82 7.96 12.43 10.83 0.80 2.92 3.71 October 82 8.17 13.08 11.27 1.15 3.21 4.35 November 82 8.00 12.67 10.71 1.10 3.07 4.27 December 82 7.76 12.74 11,47 0.89 2.78 3.67 January 83 8.28 12.77 11.30 1.24 3.20 4.44 February 83 8.55 12.46 10.89 1.58 3.36 4.94 March 83 8.46 12,87 11.56 1.67 2.93 4.60 April 83 8.31 13.28 11.93 1.70 2.70 4.40 May 83 June 83 July 83 7.36 11.86 10.65 0.67 2.82 3.50 August 83 7.45 12.41 10.95 1.21 2.83 4.05 September 83 7.61 12.21 10.55 1.24 2.86 3.74 October 83 7.87 13.34 11.60 1.22 3.03 4.25 November 83 8.27 13.58 11.72 1.43 3.20 4.63 December 83 8.21 13.78 11.83 1.84 3.19 5.03 January 84 8.29 13.31 11.57 1.69 3.29 4.98 February 84 8.31 14.51 12.33 2.20 3.78 5.98 March 84 8.28 15.00 13.08 2.24 4.07 6.31 April 84 8.27 14.89 12.46 2.01 3.59 5.60 May 84 8.28 13.29 11.05 2.05 3.18 5.24 June 84 8.07 12.33 10.20 1.84 3.04 4.88 July 84 8.19 11.82 9.61 1.22 2.55 3.82 August 84 8.03 11.75 9.61 1.59 2.76 4.36 September 84 8.03 11.78 9.60 1.75 2.99 4.74 October 84 8.16 13.07 11.22 1.40 3.18 4.58 November 84 8.58 12.82 10.81 1.94 3.47 5.41 December 84 8.48 12.85 11.12 2.17 3.11 5.28
The pH for the manure in the youngstock barn was in the same range. The pH range of the influent stream is higher than those reported by other investigators. This relatively high pH for the influent stream was not a particular concern because the pH ranges for growth of many bacterial species inherent to anaerobic fermentation are not known and many vary considerably (Zeikus, 1980). However, it has been documented that metha- nogenic species presently in culture do not grow below pH 6.0 and growth is significantly inhibited when the pH drops below 6.6 (Zeikus, 1980). There were periods during the summer of 1983 when the pH of the influent stream dropped below the normal manure pH for the farm. This problem will be explored later in this paper.
Characteristics of digested manure
Tabulated in Table 5 are the average daffy volatile and total solids con- centrations for the digester effluent. Volatile solids concentration varied throughout the year due to such factors as the influent volatile solids con- centration and temperature distribution in the digester. The reduction in volatile solids ranged between 10 and 30.
The pH of the digester effluent stream is also tabulated in Table 5. The sharp drop in pH during September and October of 1983 is symptomatic of a sick digester. During August, several problems encountered that will be discussed later in the paper.
Table 5 also presents the nitrogen content of the digester effluent. The ammonia concentration of the effluent is higher than that of the influent. During most of the 2.5 years of operation, the ammonia concentration of the effluent, and thus the entire digester, remained below 3.0 mg of ammonia per g of manure (3000 mg/1). However, during December of 1983, the effluent ammonia concentration began to exceed this level, indicating the potential for ammonia toxicity. This was also reflected in the daily gas output from the digester during the latter part of December 1983. This increase in the effluent ammonia concentration was also documented during November 1984.
Daily average gas output from the digester is presented in Fig. 5. In many respects, the peaks and valleys that occurred in the gas outflow are representative of periods of good and poor digester performance. The information presented in Fig. 5 is not adjusted for temperature. The tem- perature of the gas observed at the gas meter range from 26 to 38C over the year.
During most of August 1982, we observed an increase in the amount and quality of the gas with an average daily biogas production of 195 m 3. It is difficult to estimate gas production per cow-day because no accurate
Monthly averages for effluent
Month Year pH TS VS (mg N per g manure) (%) (%)
NI-F--N Org--N TKN
May 82 7.33 11.20 9.49 2.90 2.80 5.70 June 82 7.62 11.49 9.56 2.83 2.82 5.65 July 82 August 82 10.56 8.67 1.93 2.51 4.43 September 82 7.60 10.64 8.79 1.86 2.71 4.57 October 82 7.79 10.18 8.38 2.15 2.73 4.88 November 82 7.82 9.95 8.22 2.43 2.74 5.16 December 82 7.80 10.22 8.60 2.18 2.54 4.73 January 83 7.20 11.82 10.33 2.50 2.66 5.16 February 83 7.80 11.14 9.71 2.71 2.64 5.35 March 83 7.61 12.51 10.94 3.15 2.41 5.56 April 83 7.28 10.98 9.78 2.51 2.30 4.80 May 83 6.42 13.31 11.91 2.44 2.81 5.25 June 83 July 83 7.70 12.21 8.54 1.33 3.05 4.39 August 83 7.16 11.93 10.32 2.40 2.60 5.01 September 83 5.75 13.75 11.90 2.33 2.67 5.00 October 83 6.72 14.48 12.45 2.62 3.07 5.69 November 83 7.64 12.77 11.00 2.64 2.69 5.34 December 83 7.94 11.75 10.09 2.97 2.71 5.67 January 84 7.97 11.17 9.61 3.21 2.52 5.73 February 84 7.89 11.31 9.80 3.27 2.47 5.73 March 84 7.82 12.45 10.93 3.19 2.64 5.83 April 84 7.75 12.67 11.11 3.15 2.52 5.66 May 84 7.82 12.54 10.91 2.88 2.40 5.28 June 84 7.93 11.53 9.68 2.56 2.43 5.25 July 84 7.72 11.41 9.31 2.31 2.55 5.12 August 84 7.75 10.99 8.99 2.40 2.42 4.73 September 84 7.82 11.27 9.21 2.57 2.64 5.04 October 84 7.99 10.01 8.07 2.99 2.51 5.08 November 84 7.96 10.87 9.09 3.10 2.59 5.58 December 84 8.27 10.57 8.91 2.16 2.47 5.56
records o f the numbers and amount o f t ime an imals spent out to pasture were kept . Es t imates range f rom 1.6 to 2.0 m 3 o f b iogas per cow-day . Vo lu- met r i c p roduct iv i ty dur ing th is per iod was 0 .78 m 3 o f b iogas per m 3 o f work ing d igester vo lume.
In late August , the herd size grew f rom 120 mi lkers and assoc ia ted dry cows and youngstock to 180 mi lkers . Over the next 14 weeks , da i ly b iogas product ion increased to an average o f 298 m3/day for the month o f No- vember . Ear ly in December , gas product ion began to drop w i th the onset o f very co ld weather . Gas output deter io ra ted even fur ther a f te r the co- generat ion sys tem, the pr inc ipa l d igester heat ing sys tem, had to be shut down for repai rs .
GAS OUTPUT Dai ly Averages
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was inadvertently mixed with the manure of the main barn, eventually entering the digester. Samples removed from several locations within the digester showed copper levels four to five times those normally found in the manure of the farm. Given the high copper level, the dicision was made to drain the digester of its contents and start over again. The digester was drained early in September 1983.
In September 1983, the process of filling the digester was started. Heating of the digester and seeding occurred in late September. Biogas production remained below 100 m3/day until mid-October. Biogas production continued to increase, reaching an average of approximately 340 m3/day for the latter part of the month of November and most of December (see Fig. 5). Total biogas production for 1983 was 54 840 m 3. The average daffy biogas produc- tion for 1983 was 150.3 m3/day.
Gas production began to fall slowly during late December 1983 and through January and February 1984 (see Fig. 5). There were two possible causes for the drop in gas output during this period. Ammonia toxicity was one possible problem. In November, the ammonia concentration of the fresh manure jumped very dramatically (see Table 3). By December, the ammonia concentration of the digested manure was over 3 g of nitrogen per g of manure (3000 mg/1). Given the pH at which the digester is operated and the increase in ammonia concentration, ammonia toxicity was a realistic possibility. The other possibility was insufficient heating of the digester. On 16 January 1984, the cogeneration system was disassembled to evaluate the condition of the engine. Several problems were discovered with the engine and the unit had to be shut down for repairs. The cogeneration system was inoperative for the remainder of the winter. The back-up water system was used during the rest of the winter. There were periods during January and February 1984 when the back-up heating system could not meet the entire heating demand of the digester. Gas production began to increase in early March. By June 1984 biogas production was up to 450 m 3/day. Daffy biogas production continued to increase over the summer and early fall reaching a peak daily average gas production of 496 m3/day for the month of October. Total biogas production for 1984 was 133 880 m 3. The average daily biogas production for 1984 was 366.8 m3/day.
Gas quality as measured in percent COz is presented in Fig. 6. It has been the experience with this project that gas quantity and quality were strongly coupled to how uniform the temperatures are in the digester. Sudden changes in the feed ration for the dairy herd will also result in major changes in the gas quantity and quality. During periods when the digester has operated well, the CO2 concentration has generally ranged from 40 to 45%. For the last half of 1984 CO: concentration has held consistent- ly at 45%.
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GAS QUAL ITY Month ly Averagea
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removed it has been observed that the manure entering the northeast corner of the digester tends to flow along the east wall and the manure from the northwest along the west wall. Given that much of the heating of the manure is done in the first 15% of the digester volume, it is extremely important that the manure flow pattern is established so that manure flows out into the heating grid. A more complete analysis of the relationship between the flow pattern and the heating of the influent stream is needed to improve on the current design.
The gravity flow concept has worked extremely well in this design. Frozen manure tends to slow the movement of the digester from the hopper to the digester, however this was more of a benefit in that it allows the manure to be heated in the hopper for a longer period of time. We have encountered no plugging of the influent or effluent pipes.
There was much seasonal variation in the solids concentration, nitrogen content and pH of the influent stream. Although the pH of the manure from the farm was on the average higher than the 6.4--7.4 range quoted by other investigators (McCarty, 1964; Pfeffer, 1980; Jewell et al., 1980; Hashimoto et al., 1980), it has not had an adverse impact on gas production. In addition, the lower gas production observed by Hills (1983) in solid concentration range of 14--16% was not observed with our system.
An economic assessment of this system was not presented in this paper. From the preliminary economic assessment performed by the investigators (Walker et al., 1984), it was determined that cogeneration, the simultaneous production of electricity and hot water, was the most practical and eco- nomical use of the biogas.
Much insight has been gained into the performance of plug flow digesters as a result of this project. More importantly, much has been learned about integrating biomass energy systems into the physical and management structure of the farm. If this type of technology is truely to become a standard process in agricultural production systems, it mush be simple to operate, easy to maintain, and integrated into the management structure of the operation. The task of engineering a system such as a plug flow digester is a very sophisticated effort in design. Unlike completely mixed digesters where the substrate concentration and temperature distribution are forced to uniform values with mechanical mixing, the performance of the plug flow digester is strongly dependent on how well the fluid dynam- ics and fermentation kinetics are integrated.
Plug flow digesters are being constructed on a small number of livestock operations in the United States. As we develop better models and more experience, the designs will be more refined and the performance of these systems will be better. With rising energy costs and the need to utilize natural resources more effectively, this type of technology will play an important role toward making agricultural production systems energy self-sufficient.
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Converse, J.C., Graves, R.E. and Evans, G.W., 1977. Anaerobic degradation of dairy manure under mesophillic and thermophillic temperatures. Trans. ASAE, 20(2): 336--340.
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McCarty, P.L., 1964. Anaerobic waste treatment fundamentals. II, Environmental re- quirements and control. Public Works, 95: 123.
Muck, R.E. and Richards, B.K., 1983. Losses of manurial nitrogen in free-stall barns. Agric. Wastes, 7: 65--79.
Pfeffer, J.T., 1980. Anaerobic digestion process. In: D.A. Stafford, B.I. Wheatley and D.E. Hughes (Editors), Anaerobic Digestion. Applied Science, London, pp. 15--35.
Schellenbach, S., 1982. Case study of a farmer owned and operated 1000 head feedlot anaerobic digester. In: Syrup. Energy from Biomass and Wastes, 25--29 January 1982, VI. Institute of Gas Technology, Chicago, IL, pp. 454--566.
Walker, L.P., Ludington, D.C., Muck, R.E., Friday, R.E. and Heisler, M.G., 1984. The design and analysis of an energy integrated dairy system. Trans. ASAE, 27(1): 229-- 240.
Zeikus, J.G., 1980. Microbial populations in digesters. In: D.A. Stafford, B.I. Wheatley and D.E. Hughes (Editors), Anaerobic Digestion. Applied Science, London, pp. 61-- 89.