Agricultural Anaerobic Digesters
The need to develop alternative energy sources has become increasingly apparent with incidents of fuel shortages and escalating prices in recent years. Producing methane (valued for its use as a fuel) by anaerobic digestion of agricultural organic residues, especially animal manures, is technically feasible and has been practiced in India and Germany. Renewed attention is now being directed to biogas generation as a source of fuel in the U.S.A.
Since most previous research was directed primarily to the waste-treatment potential of anaerobic fermentation, rather than to energy recovery, information on biogas-generation-system designs for livestock-management practices and systems is limited. Therefore, the research project "Research on Methane Gas Production from Farm Wastes" was initiated in 1974 by the Agricultural Engineering Department of The Pennsylvania State University in cooperation with the departments of Sanitary Engineering (Raymond W. Reagan) and Dairy and Animal Science (August E. Branding). Financial support was received from the Pennsylvania Department of Agriculture.
Since the Penn State project began, studies of farm digesters have been undertaken by research workers at several other locations. Results of some of these studies are now available and are incorporated into this Bulletin.
Advantages and disadvantages of anaerobic digestion of manure and other farm residues are listed as follows (Lapp 1974, Jewell1974, Jewell et al. 1974):
Advantages of anaerobic digestion
- Organic content of the residue is reduced and stabilized so that final disposal presents a reduced pollution problem.
- Digested residue is an almost-odorless free-flowing liquid.
- The fertilizing nutrients are conserved.
- Methane, a constituent of the gases produced by the process, has significant value as fuel.
- Weed seeds and some pathogens may be destroyed during digestion.
- Rodents and flies are not attracted to the digested residue.
- The sludge can be easily dewatered.
- Particulate matter in sludge may have refeeding potential.
- Organic nitrogen will be hydrolized to ammonia nitrogen.
Disadvantages of anaerobic digestion
- Equipment is complex and involves high initial investment.
- Daily feeding of digesters at controlled loading rates is desirable.
- Energy input is required.
- High standards of maintenance and management are required.
- Strict explosion-proof standards must be maintained.
- Temperature (and perhaps volatile acid concentration and pH) must be controlled to optimize gas production.
- Some chemicals, if present in excessive quantities, can inhibit the digestion process.
- Digestion systems will reduce, but not eliminate, the solids content; digested liquid slurry remains a pollutant unless subjected to further treatment.
Earlier anaerobic digesters for biogas production
The fact that a combustible methane gas is produced from anaerobic digestion of biological material was recognized by Von Helmont in 1630, Shirley in 1667, Volta in 1776, Priestly in 1790, and Dalton in 1804. Laboratory experiments to produce methane from manure were made in 1806 by Humphrey Davy.
The history of municipal sewage sludge digestion can be traced back to the 1850s with the development of tanks to treat settled waste-water solids. A treatment scheme known as the Mouras automatic scavenger was developed in Vesoul, France, about 1860. Donald Camerson, who in 1895 built the first septic tank for the English city of Exeter, used its methane gas for lighting streets near the treatment plant. A full-scale plant existed as early as 1857 in Bombay, India. The continuous digestion process, as developed by Imhoff around 1900, became generally accepted for the treatment of municipal wastewater. One of the earliest U.S. installations using separate digestion tanks was at Baltimore, Maryland, in 1911. From 1920 to 1935 the anaerobic digestion process for stabilization of municipal sludge was studied and improved extensively.
During World War II approximately 30 farm-scale biogas plants for the anaerobic treatment of manure were developed in Germany, using the Schmidt-Eggersglüss and the Darmstadt methods. Only one of these appears to be still in use. Ducellier and Ismann in Algeria also worked on the problem at this time. A large number of small, inexpensive digesters, based largely on research work by R. B. Singh (1971, 1972), have been in operation in India since around 1960. Reports on many such digesters have also come from Taiwan and other parts of the world. The gas from these small digesters is used for cooking and for driving electric generators.
Studies at Penn State
The primary objective of the Penn State research was to determine the technical requirements and economic feasibility of producing methane gas by anaerobic digestion of dairy-cow manure. A review of literature revealed a substantial amount of laboratory experience with methane digestion from farm wastes, mainly dairy manure. However, information on experiences with full-scale digesters in operation on commercial-size farms in the United States was not found. As a consequence, it was decided to build a digester of sufficient size to study the engineering problems related to the use of digesters on dairy farms. The University dairy barns have facilities for 50 cows each, which is representative of the size of an average Pennsylvania dairy herd. Therefore, the prototype digester was built to process manure from one of the 50-cow barns at the University Dairy Center. Later, it was used to process manure from 100 cows and was found to perform satisfactorily, producing a proportionally increased rate of gas.
The digester was designed and constructed during 1974 and 1975. Operation began in the fall of 1975 and continued until the cows were turned out to pasture in the spring. After shutdown, the digester was cleaned and examined to determine condition of the components. Based on the first year's experiences, a number of components were rebuilt or replaced. The digester was restarted in late 1976 and operated until spring of 1977, when it was again shut down for a brief period because of gas leakage through the roof. An improved gas seal was installed, and the digester placed back in operation from July until December 1977. Considerably higher loading rates were used during parts of this latter test period.
Operation of this full-size digester will continue for further studies relating to anaerobic digestion of other agricultural residues, use of the processed material, and the role of digesters in the overall energy picture on animal farms. This publication is based mainly on experiences with the Penn State digester up to December 1977, and is intended to give the reader suggestions on the construction, operation, and costs of an anaerobic digester for agricultural wastes.
Modern farm-size digesters
Design characteristics and operating experiences have been reported for a number of farm-size digesters currently or formerly in operation in a number of locations (Table 1). Some are pictured in Figures 1 to 5.
Table 1. Modern farm-size digesters described in the literature (as of fall, 1978).
|Type and location||Digester volume|
|Pennsylvania (Penn State)||3,500||100||Persson et al. 1977|
Persson, Bartlett 1978
|New York (Cornell Univ.)||1,350|
|Jewell et al. 1977 a, b|
|Wisconsin (Jefferson)||5,000||140||Pigg 1977|
|Wisconsin (Rice Lake)||5,500||154||Abeles 1978|
Martin, Dale 1978
|Denmark (Stenderup)||2 x 1,750||2 x 50||Persson 1977|
|Germany (Darmstadt system)||700-1,050||20-30||Stauss 1952|
|Sweden (Gustafs)||3,500||100||Thyselius 1977|
|Dairy and swine|
|Germany (Schmidt-Eggersguss system)||3,500-35,000||100-1,000||Persson 1952|
|Dairy and other|
|Florida (Bartow)||85,000||2,400||Davenport 1978|
|Iowa (IA State Univ.)||2,800||80||Lapp 1978|
|Michigan (Custer)||5,300||150||Abeles 1978|
Martin, Dale 1978
|Oklahoma (Guymon)||530,000||15,000||Lapp 19788|
|Washington (Monroe)||a||a||Clark 1976|
|Beef and swine|
|Great Britain (Winkfield)||16,000||455||Lovelidge 1976|
|Iowa (Mount Pleasant)||7,400||208||Persson 1978|
|Minnesota (Wyoming)||1,750||50||Persson 1978|
|Missouri (Univ. of MO)||5,000||140||Fischer et al. 1978|
|New Jersey||2,700||76||Brodie 1974|
|Denmark (Skarrild)||3,400||90||Persson 1977|
|Swine and poultry|
|Georgia (Canton)||8,500||240||Booram et al. 1975|
|Iowa (West Union)||67,000||1,900||Anon 1977|
|Maine (Univ. of ME)||2,000||56||Hassan et al. 1975|
|Wisconsin (Ripon)||3,500||100||Converse 1977|
aThe digester in Monroe, WA, is reported to have a capacity of "350 head."
Figure 1. The Penn State experimental anaerobic digester.
Figure 2. Anaerobic digester for swine manure at the University of Missouri (courtesy of D.M. Sievers).
Figure 3. Two digesters built into a barn at Stenderup, Denmark.
Figure 4. Trench-type plug-flow digester with plastic fabric cover at the Cornell University Research Farm.
Figure 5. Biogas plant for 30 dairy cows, using the Schmidt-Eggersgüss system, in Germany, 1952. Tractor is carrying equipment with compressed gas cylinders; vehicle at left is a slurry-transport wagon.
Section 1. Microbial and Chemical Processes
The anaerobic digester is a system for biological conversion of biodegradable organic materials into methane (CH4), carbon dioxide (C02), water, and other gases. The microbes that produce methane gas cannot live in the presence of oxygen, so the digester must be sealed from the atmosphere. Such an environment (anaerobic) cannot sustain respiration, and thus is a hazard to all oxygen-breathing organisms.
The principal parts of a high-rate anaerobic digester are shown in Figure 6.
Figure 6. Schematic of a high-rate anaerobic digester.
Fundamental Biochemistry and Microbiology
Several groups of methane bacteria, each fermenting a limited number of organic compounds, work together to stabilize complex mixtures of organic materials typical of waste-water sludge and agricultural residue.
The chemistry and microbiology of anaerobic treatment of complex organic matter is thought to occur in two phases, as outlined in Figure 7, In the first phase, acid-forming bacteria convert, by hydrolysis and fermentation, the complex materials (such as fats, proteins, and carbohydrates) in the slurry to simple organic fatty acids, primarily acetic and propionic acids. In the second phase, the organic acids are converted by "methane-forming" bacteria to gaseous end products (methane and carbon dioxide), Some of the methane-formers utilize formic acid and methanol and grow rapidly, requiring less than a 2-day slurry-retention time. However, the most important methane bacteria are slow-growing and require acetic and propionic acids. These bacteria require slurry-retention times of 4 days or more, and for this reason may be the growth-limiting step of an anaerobic digester operation.
Figure 7. Chemistry and microbiology of anaerobic treatment of organic materials (from McCarty, 1B64).
Attempts to isolate the most efficient methane-producing bacteria are under way (Nanson 1973; Zeikus 1973; Bryant 1973), but present digesters operate with whatever types of bacteria develop naturally in the slurry. The most suitable bacterial fauna is determined by the ingredients of the slurry. A change in the input slurry, such as might result from a change in the animal diet, may result in a temporary reduction in gas production, reflecting a readjustment in the bacteria population (Fischer et al. 1978).
Acetic acid is one of the most important intermediate acids produced by the decomposition of complex organics and is the source of most of the methane produced. Reduction of carbon dioxide accounts for most of the remaining methane generation.
A breakdown of protein and other nitrogen-containing substances occurs parallel to the methane-forming process. This process provides the necessary raw material for growth of the bacteria. Small amounts of ammonia and hydrogen sulfide are also produced in the digester.
The biological pathway for methane formation from a complex organic material is further illustrated in Figure 8. The process is similar, in principle, to the process in the digestive tract of ruminant animals.
Figure 8. Methane formation from a complex organic material (from McCarty, 1964). Percentages refer to the chemical oxygen demand (COD).
A relatively small fraction of the raw organic material is used by the bacteria to produce new cells. The major part, 90 percent of the original energy, is converted to methane. The process does not create or consume a measurable amount of heat.
Environmental Requirements for the Process
The methane-producing microbial process is affected by numerous interdependent environmental factors, such as temperature, retention time, loading rate, and agitation. An effective production system must control all these factors through provisions within its structural and equipment components.
The rate of solids conversion and resultant gas production is closely related to temperature (Fig 9, 10). Under optimum environmental temperature, gas production will begin to occur in about 4 days and will continue for several weeks.
Figure 9. Effect of temperature and mixing on solids-reduction rate (from Regan 1975; Ljunggren and Petre 1976; Roediger 1967).
Figure 10. Effect of temperature on gas-production rate (from Roediger 1967).
One maximum in gas production, caused by bacteria that thrive best in the so-called mesophilic temperature range, occurs at 95°F (35°C), with rapid decrease as the temperature goes below 90°F (32°C) or above 100°F (38°C). A second maximum of somewhat higher rates of gas output is produced by bacteria in the thermophilic range, 120 to 140°F (49-60°C). However, most materials digest as well at the lower range, which requires less heat to bring them up to temperature and to offset heat losses to the environment. Thermophilic bacteria are very sensitive to changes within the digester, whereas mesophilic bacteria are more stable.
Substantial time is required for microbial activity to break down organic matter and convert it to gas. The time that the substrate must remain in the digester environment thus determines the size of the digester chamber required for a given daily amount of organic matter input.
With the continuous-type digester, new substrate is added at frequent intervals (one or more times daily) in amounts related to the retention time. As an example, one-tenth of the digester liquid volume should be added daily for a 10-day retention time. Addition of small amounts at a time avoids the danger of loading shock, such as occurs when cold input reduces digester temperature or when the composition of the input material changes. The calculated retention time will normally not be the same as the actual treatment time for all individual particles in the slurry, but will represent the average treatment time.
In Penn State laboratory tests, a retention time of 7 to 10 days yielded the maximum rate of gas production per unit of digester chamber volume (Fig 11, curve c) when the slurry concentration was 7 percent volatile solids. Longer retention times yielded a slightly greater total amount of gas for a given amount of manure processed (Fig 11, curve a). The longer slurry retention time also permitted a greater reduction of digestible solids in the slurry (Fig 12).
Figure 11. Effect of retention time on gas production from dairy manure with bedding, feeding twice daily. Curves a, c: Laboratory test in a single-chamber digester, input concentration 6.8 percent volatile solids (from Patelunas 1976; Patelunas and Regan 1977). Curves b, d: Full-size two-chamber digester, input concentration of 11.8 percent volatile solids (from Persson and Bartlett 1978).
Figure 12. Remaining solids as a function of retention time (from Lawrence 1971). Slightly slower breakdown rate was observed by Morris et al. (1977), and by Patelunas and Regan (1977).
Experience at Penn State showed that a 7- to 14-day retention time was also sufficient to stabilize the substrate satisfactorily with regard to preventing malodor problems with the digester effluent.
Thus for maximum heat-energy recovery in relation to capital outlay and for adequate stabilization of the slurry, a 7- to 14-day retention time is recommended for digesters of dairy-barn manure with approximately 8 percent solids. Longer retention times may be needed for slurries with higher solids concentration, for other types of agricultural residue, and for lower operating temperatures (Table 2).
|Daily VSa loading rate (per unit digester volume)||Daily biogas production||Digester volume per animal|
|(lb/ft3)||(kg/m3)||per unit digester volume||per animal||(ft3)||(m3)|
Source: References listed in Table 1
aVS = volatile solids (total solids less ash)
bTS =total solids
cValue suggested for design of modern high-rate digesters
dValues reported by various workers with farm-size digesters
To assure that all oxygen has been excluded from the material entering the digester, it must be fully saturated with water. Furthermore, solids concentration affects flowability, which is essential for ease of mixing the slurry within the digester and for flow out of the digester, as well as for handling by pumps. Increased dilution may also be required for handling agricultural residue in order to avoid pump clogging. Additional water may, therefore, be needed to dilute the manure, possibly in as much volume as the original manure if large amounts of bedding or litter are used. Excessive dilution should be avoided, however, in order to maximize the amount of gas-producing organic material in a given digester volume and to reduce the total amount of material to be handled.
A wide range in recommendations for the solids concentration of the digester substrate appears in the literature (Table 2). Some authors indicate that anaerobic bacteria function most efficiently when the volatile solids content of the slurry is around 8 percent However, other studies have shown effective digester operation at considerably higher concentrations of volatile solids. The Penn State digester operated satisfactorily at an input slurry concentration of 13 percent solids (wet basis). The resulting effluent concentration was approximately 8 percent solids for a 20-day retention time.
Since the solids content of liquid manure affects its flowability and related handling properties, the maximum concentration for dependable mixing inside the digester depends on the type of agitation used. Large centrifugal manure pumps can satisfactorily handle liquid manure with a solids content of up to 12 percent. Smaller pumps require that the solids content be between 5 and 8 percent in order to avoid high power requirements and risk of blockage in pumps and pipe lines. Other types of material movers can handle material with considerably higher solids contents. As an example, in preliminary tests the ram-type pump used in the Penn State system handled a mixture of cow manure and sawdust bedding with solids content up to 22 percent.
The capacity of a digester to convert volatile solids into methane is related to its loading rate, which is defined as the amount of volatile solids fed to the digester per day per unit volume of the digester. A high loading rate is desirable because it means that a digester of a given size can handle a larger amount of manure in relation to its size, which also relates to its capital cost. Expressed in another way, a smaller digester can be used for the same given amount of manure if the loading rate is higher. A high loading rate will result in a high daily gas production and a high daily rate of volatile solids reduction, but it will give a smaller percentage conversion of the volatile solids to gas. A combination of high solids concentration in the input slurry and short retention time results in a high loading rate.
Published values of loading rates used in high-rate agricultural digesters are listed in Table 2, indicating that dairy-cow manure and beef-cattle manure permit higher loading rates than do poultry and swine manure.
Thorough agitation of the substrate in the digester is required to distribute organisms uniformly throughout the mixture and to transfer heat from the heating surface to the incoming slurry so as to maintain uniform substrate temperature. Furthermore, agitation aids in particle-size reduction as digestion progresses and in removal of the gas from the mixture.
The effect of agitation in reducing digestion time is plotted in Figure 9.
Input Slurry Composition
The functioning of an anaerobic digester on a farm benefits from the fact that the input material from animal production is reasonably uniform when compared to the input material at municipal sewage-treatment plants. However, proper care must be exercised by farm operators to prevent materials that would disturb the functioning of the digester from being discarded into the manure system. The material classified as manure in this bulletin includes feces, urine, bedding material, wasted feed, antislip material (barn snow), and grit tracked in by animals and barn workers.
The amount and composition of manure produced by different animals is reported in literature such as Midwest Plan Service's Structures and Environment Handbook (1975) and the 1978-1979 Agricultural Engineers Yearbook. Large variations from these values may be expected from farm to farm and season to season, depending on type of feed, amount of pasturing, and types and amounts of bedding material used. The major component of dairy-cow manure is water (about 85%). The remainder is solid matter, which consists of organic (volatile solids) and inorganic (ash) materials.
Among the organic compounds in the manure are protein, starch, fat, cellulose, and lignin. Dairy wastes have been determined to contain as as much as 30 percent cellulose and 20 percent lignin. The major chemical element in manure is carbon; other elements include nitrogen, oxygen, hydrogen, and minerals. The ash originates partly from the excrement and waste feed, but to a greater extent from the grit and anti-slip material on the barn floor.
Besides manure, the farm may have available several other kinds of production residue that could be used in the digester for methane-gas production. Such materials as milk-room wastes, straw, corn husks, grass, leaves, and waste from agricultural-processing industries can be readily converted to methane in anaerobic digesters.
Carbon (carbohydrates) and nitrogen (protein} are the principal elementary nutrients for anaerobic bacteria. The carbon component of the incoming organic material is the ingredient converted to methane in the process. Nitrogen is necessary as food for the bacteria and as a catalyst for the process to proceed most efficiently. However, if the nitrogen content is too high, the process is retarded or stopped. According to Singh (1971), digestion proceeds at an optimum rate when the ratio of carbon to nitrogen is approximately 30. On the other hand Sievers and Brune (1978) recommend ratios between 16 and 19. The availability of these elements varies widely in manures from different animal species, with age and diet of the animals, and with manure management.
The nitrogen content of animal excrement exists in many forms (ammonia, NH3; nitrates, NO3; proteins, etc.). Whereas anaerobic bacteria can utilize most forms of nitrogen, the total nitrogen content as determined by the Kjeldahl test (TKN) can be considered "available" nitrogen. The greatest amount of nitrogen in manure is in the urine; feces and bedding material have little nitrogen.
The carbon content in dairy manure is slightly higher than required for an efficient balance, and swine and poultry manures have excess nitrogen. Consequently, adding swine or poultry manure to the dairy manure will increase the gas production and the efficiency of the process. However, this is not practical unless the two livestock species are housed on the same farm. Similarly, digesters for swine and chicken manure become more stable when material that contains an excess of carbon in relation to nitrogen (e.g., bedding or litter) is added.
Only a fraction of the volatile solids (VS) in the manure can be converted to gas by the bacteria. Lignin is practically unaffected by the bacteria in the digester, and cellulose is broken down only very slowly.
The biological oxygen demand (BOD) value may be used as a measure of the biodegradability of the slurry. A BOD/VS ratio close to 1 indicates that most of the volatile solids can be converted. Dairy manure has low BOD/VS values (around 25%), whereas swine and poultry manure show higher values. On the basis of volatile-solids percentage and the BOD/VS values of dairy manure, as little as 20 percent of the total solids may be available for conversion in the digester.
Based on some analyses for typical incoming solids, the expected production of biogas (with 60% methane) has been calculated to be approximately 11 ft3/lb (0.7 m3/kg) of converted volatile solids. Conversion rates are often given relating gas output to the amount of volatile solids fed to the digester, as in Table 2. Naturally, these figures should be less than 11 ft3/lb (0.7 m3/kg) because not all volatile solids are biodegradable and because not all biodegradable solids are converted during the limited time (retention time) that the material is in the digester.
Methane bacteria are sensitive to extreme values of alkalinity and acidity (pH). The optimum pH for these bacteria is in the range of 6.6 to 7.6. Beyond these limits, methane fermentation will be greatly retarded, and with continued operation will stop completely. Acidity and alkalinity of the digester slurry are mainly a function of the properties of the raw material fed to the digester. If the pH of material fed into the digester is nearly neutral (pH = 7.0 ± 0.5), satisfactory conditions will normally result. Highly acid or alkaline material should not be used as input to a digester.
Properly operated, cattle-manure digesters will usually stay well within safe pH limits. If the pH in a continuous-feed digester becomes too low, it can be brought up to normal again by recycling fresh effluent to the inlet or by reducing the amount of raw slurry that is fed to the digester. If the slurry becomes too alkaline, carbon dioxide will increase, which will have the effect of making the mixture more acidic, thus correcting itself. Small concentrations of sodium, potassium, calcium, and magnesium (up to 200 ppm or 200 mg/liter), have been found to stimulate the anaerobic process. Concentrations above 5,000 ppm (5,000 mg/liter) may inhibit methane production.
Ammonia concentrations in the range of 1,500 to 3,000 ppm (1,500-3,000 mg/liter) are assumed to be toxic to methane bacteria in the pH range of 7.2 to 7.4. However, concentrations in excess of 3,000 ppm (3,000 mg/liter) have been found to be toxic regardless of pH. High concentrations of ammonia have been assumed to be the main factor limiting success with anaerobic digesters for swine and poultry wastes. This toxicity can be controlled by reducing the pH to 7.2 to 7.0 through dilution with water.
Materials that may be toxic to microbial life must be prevented from entering the digester. Common among materials associated with livestock-production operations are health-related drugs and disinfectant compounds. Generally, dosages normal for disease-control purposes will not be excreted in quantities sufficient to affect microbial activity adversely. However, the common practice of disposing of unused materials in the manure-distribution system must be avoided.
The volume of the effluent is slightly less (2 to 5%) than the volume of the input, due to biodegradation of some of the volatile solids. The remaining solids are mainly those that are difficult to break down by bacterial activity:The relative solids reduction, expressed in percent of initial amount of volatile solids, is higher for low loading rates than for higher loading rates. Conversely, a smaller percentage of the volatile solids component is reduced with higher loading rates. However, a high loading rate results in a higher rate of solids conversion per day. Approximately one-half of the volatile solids in fresh manure will be converted in a l0-day treatment period.
Most of the nitrogen in the input manure will remain in the processed slurry. Some is transformed in the digester from organic nitrogen to ammonia nitrogen, making it more readily available for plants. Phosphorus and potassium contents remain unchanged.
The processed manure does not have a foul odor and does not attract flies. Homogenized by lengthy agitation in the digester, it can be handled easily by liquid-manure pumps and tank spreaders for distribution on land.
Biogas Amounts and Composition
The daily rate of gas production is commonly estimated on the basis of amount per animal or amount per unit of digester volume. The rate is influenced by such factors as digester temperature, slurry composition and concentration, and digester loading rate. Gas output reported from digesters for various livestock manures is listed in Table 2.
The gas produced by an anaerobic digester has been shown to be mainly methane (CH4) and carbon dioxide (CO2), with small amounts of other gases, including nitrogen and hydrogen sulphide. Analyses of the gas from the Penn State digester showed an average of 60 percent methane and 30 to 35 percent carbon dioxide (the remainder was nitrogen). Other sources have reported methane contents of 55 to 70 percent. With a methane content of 60 percent, biogas has a net heat content of 545 Btu/ft3 (20.5 Ml/m3).
During its first season of operation, the Penn State digester was held at 82°F (28°C) for start-up. At this temperature, daily gas production in a typical period was only 0.5 ft3 per cubic foot of digester capacity (0.5 m3/m3), which was not sufficient for maintaining even this low temperature by use of its own gas production. After increasing the temperature to 95°F (35°C), gas production was in excess of needs for slurry-temperature maintenance. Workers with other farm-size digesters emphasize the importance of maintaining slurry temperature at 95°F for satisfactory gas production. A low gas-production rate results in inefficient operation of the digester.
The biogas did not have an unpleasant odor. Only traces of ammonia or hydrogen sulphide could be detected (presence of hydrogen sulphide signifies an acid process, which is undesirable in a digester operation). Inspection of the gas pipes after several months of digester operation revealed a thin coating of sulphur, indicating the presence of sublimated sulphur in the gas.
Section 2. Design and Function of the Digester System
Main Components of the System
The principal components of an anaerobic digester system for biogas production on a farm include one or more digester chambers, a facility for slurry preparation, storage for the processed slurry, a gas-collecting space, and an area for the mechanical equipment needed for slurry heating and agitation. The flow-chart (Fig 13) shows how these functions relate to each other in the digester system. Components should be arranged to permit a simple flow path for the material through the system and to minimize heat losses. Ease of accessibility to the various components should be emphasized, since these and the related equipment will require inspection, adjustment, and maintenance. Safety considerations must be considered in planning a digester system.
Figure 13. Functions and processes in an agricultural anaerobic digester. Main material flow is from left to right; not all functions listed may be incorporated in all digesters. Shaded area shows processes occurring within the main digester chamber.
Designs of some existing and proposed anaerobic digesters for farm use, illustrating alternatives for chamber construction, gas storage, and slurry handling, appear in Figure 14.
Figure 14. Typical designs for agricultural digesters. A is a rigid-chambered digester with fixed roof; B is a rigid-chambered digester with floating roof; and C has a flexible digester chamber.
A digester design utilizing a bag of plastic or rubber in a trench (Fig 14c) has attracted considerable attention, and testing is under way (Jewell et al. 1978a, 1978b). Heat insulation, agitation, material flow, and sludge removal may present special problems with this design.
The following discussion will emphasize mainly rigid-wall digester designs and mechanical-system components as used in the Penn State digester (Fig 15). It will include recommended improvements based on operating experiences with Penn State's design and descriptions of alternative designs used in other digesters. Design parameters and examples are presented for each major component to aid in calculating dimensions for a contemplated digester. The best design choice will be determined by the situation on the individual farm. Many good variations in digester design are possible, and improvements occur constantly. The descriptions presented here should be considered as background examples and guidelines for design, construction, and operation of digesters.
Figure 15. The experimental Penn State digester, showing components in use 1976-1978.
Types of Digesters
Digesters may be operated as a batch process or as a continuous-feed process. In the batch system the digester is filled with slurry that is allowed to remain in the digester until the desired treatment is finished. The biodegraded slurry is then removed and replaced with a new batch of material. Batch digesters have advantages where the availability of raw materials is sporadic or limited to coarse plant material requiring handling by special equipment that may need to be scheduled at a specific time. Batch-type digesters require little daily attention. However, the rate of gas production is uneven; it increases slowly after start-up and decreases gradually after a brief period of peak production. The disadvantage of uneven gas production can be reduced by using additional batch digesters charged at specific intervals. Due to their lower gas output, batch systems that would necessitate the expense of more than one digester will likely be limited to small farms.
In the continuous-feed process, small amounts of slurry are fed into the digester at frequent intervals (one or more times daily) and a corresponding amount of substrate is displaced and discharged from the digester each time new material is added. One type of continuous-flow digester is the tunnel type (plug flow) wherein the material is fed in at one end of a long digester and displaces material along its length until it overflows at the other end. The tunnel length assures that material remains in the digester for a sufficient length of time for digestion to take place.
With continuous-feed digesters, the rate of production of both gas and sludge is more uniform. Such digesters are especially well adapted to systems where raw materials consist of a regular supply of easily digestible waste from nearby sources, such as livestock manure (Stoner 1974).
In continuous-feed digesters, a small amount of the fresh manure may flow directly to the overflow without being digested. With good mixing, this amount will be negligible. However, baffles or other features that minimize the chance of the incoming material going directly out the overflow should be considered.
Due to the microbial process occurring in two definite steps (the acid-forming and the methane-forming stages), it has been suggested by Jewell et al. (1974a) that a two-stage digester should be more efficient than a single-stage unit. In this way, each stage could be controlled for maximum efficiency of each group of bacteria. A two-stage arrangement could be either two separate digesters following each other, or one digester chamber divided into two sections with a flow passage through the dividing wall. Two-stage digesters are common in municipal sewage-digestion systems where the main interest is to have low solids content in the effluent rather than to produce the maximum amount of methane gas. However, work by Zajic et al. (1974) did not show that the two-stage method was necessarily superior to the single-stage method.
The Penn State digester was designed as a two-stage unit using a divider wall across the digester chamber (Fig 16, 17). Silo-type single-stage and two-stage digesters and the trench-type digester are sketched in Figure 18.
Figure 16. Primary chamber of the Penn State digester. Slurry-inlet pipe is labeled "A"
Figure 17. Secondary chamber of the Penn State digester. Effluent-outlet pipe is labeled "A."
Figure 18. Single-stage, two-stage, and tunnel-type digester designs.
Digester Size Calculations
The digester should normally be designed and constructed as small and as compact as possible, as it is the most expensive component of the system. A procedure for calculating chamber volume of a continuous-feed digester for a dairy operation of 100 cows follows:
A. Calculate weight and volume of manure to be handled. One cow will produce:
Weighta 110 lb/day (50 kg/day)
Volumea 1.8 ft3/day (0.05 m3/day)
Total daily weight produced by 100 cows:
100 X 110 = 11,000 lb (4,990 kg)
Total daily volume produced by 100 cows:
100 x 1.8 = 180 ft3 (5.1 m3)
B. Calculate dry matter to be handled. Solids content of manure is expected to be about 15 percenta
Dry matter: 11,000 x 0.15 = 1,650 lb (750 kg)
C. Calculate weight and volume of slurry to be moved into the digester each day.
Dry matter concentration of slurry: 13 percent
Total volume: excreted manure + added water
Total weight of slurry: dry matter/0.13 1,650/0.13 = 12,700 lb (5,760 kg)
Total volumec of slurry: total weight/62
12,700/62 = 205 ft3 (5.8 m3)
D. Calculate volume of digester chamber for specific retention time.
Assume retention time of 14 daysb
Chamber volume: daily slurry production x retention time
205 ft3 x 14 = 2,870 ft3 (81 m3)
E. Check daily loading rate of volatile solids (VS).
Volatile solids are 85 percent of total solids (dry matter)a
VS: 1,650 x 0.85 = 1,400 lb (64 kg)
Daily loading rate:
daily total VS/volume of digester chamber 1,400/2,870 = 0.5 lb/ft3 (7.7 kg/m3)
a Midwest Plan Service 1975; ASAE Yearbook 1978
b From Table 2, page 16
c The Penn State Dairy Reference Manual lists the weight of dairy manure to be approximately 62 lb/ft3
The daily loading rate falls midrange within the limits of 0.13 to 0.7 lb per ft3 of digester volume expressed for dairy cattle manure in Table 2.
In calculating dimensions for a digester, include a freeboard of a foot or more to provide temporary slurry storage in case of stoppage in the slurry outlet.
In order to keep heat losses from the digester surfaces to a minimum, the height and diameter of the digester should be approximately equal. To provide the volume required for 100 dairy cows, and a 12-in freeboard, a chamber approximately 16 ft (4.9 m) in diameter and 15 ft (4.6 m) in depth is needed.
Gas generated during digestion collects in the top of the digester, and the space provided for collection and temporary storage should be about 10 to 20 percent of the total digester volume. This minimum size of 10 to 20 percent is frequently inadequate for intermittent gas use. However, it is usually less expensive to provide other forms of gas storage than it is to construct the digester oversize.
Construction material properties
Proper selection and protection of construction material against corrosion is important because many digester components function in contact with gases and liquids that are very corrosive. Materials that are exposed to the biogas and condensed moisture from the biogas are subjected to severe corrosion attack (Fig 19, 20).
Figure 19. Corroded ladder. The upper rung, located above the slurry surface and exposed to the biogas atmosphere, shows much more corrosion than do rungs that were constantly submerged.
Figure 20. Corroded aluminum outlet pipe.
Mild steel may be left unprotected only if it is constantly submerged in the slurry, though additional material thickness is recommended for satisfactory service life. Regular or enamel paints and galvanized coatings have a short life in the biogas atmosphere. Epoxy paint coatings seem effective if the steel surface has been cleaned and prepared according to instructions, which usually specify sandblasting. Stainless steel, as used in hose clamps, etc., may suffer rapid deterioration in the biogas environment. Aluminum (Fig 20), copper, and brass corrode rapidly due to the sulphur in the gas.
Plastic pipes and pipe fittings, as well as glassfiber-reinforced plastic sheets, seem resistant to deterioration inside the digester. However, at the elevated temperatures, certain kinds of plastic lose some of their creep strength and will sag if not sufficiently supported. Some types of plastic films and foams will deteriorate rapidly when exposed to sunlight. Wood, especially when pressure-treated with creosote, appears to stand up well in the digester environment.
Pipes for the gas system should be corrosion-resistant and of liberal size. Plastic and stainless steel may be used as described earlier. According to some gas-pipeline codes, only polyethylene-type plastic is permitted (Milo 1974).
Wall construction materials
Materials used for the main digester construction should be strong, liquid tight, and durable against corrosion and weathering. Materials considered by Sievers et al. (1975) to be suitable for digester construction include concrete, glass-lined steel, and fiberglass. Although wood has been suggested as a suitable material for digester construction, no reports of its use were found. Unprotected steel can be expected to corrode rapidly. Glass-reinforced plastics (fiberglass) offer high corrosion-resistance properties, but may require custom fabrication for suitable size and strength requirements.
Concrete digesters can be readily made in monolithic form or with precast concrete elements (staves). The precast concrete elements offer flexibility in size selection (silo diameter and height) and can be erected on site without the need for special equipment.
The spacing of steel reinforcing hoops should be based on the hydraulic pressure, which will be far greater than that normally encountered with silage. Inadequate reinforcement will result in expansion or possible bursting of the silo. Even a small amount of expansion can result in cracks or joint separation that may permit slurry and gas leakage.
The Penn State digester was constructed of 4- by 5-ft (1.2- by 1.5-m) precast concrete interlocking panels (Fig 21), with reinforcing hoops spaced according to variations in fluid pressure. These pressures are greatest at the bottom and progressively decrease as the fluid depth decreases toward the top of the wall.
Figure 21. Assembling the Penn State digester.
Foundation and floor
Due to the extreme weight of the concrete silo construction, a reinforced concrete foundation and floor are required to avoid the possibility of differential settlement that could cause the wall to crack and leak, unless the foundation is on bedrock or extremely hard and well-compacted soil. Generally, the foundation and floor are integrally cast as a flat concrete slab. Several existing digester designs show a poured sloping bottom (Fig 14A, 14B) for easy removal of sludge accumulation. With proper agitation in the digester, sludge accumulation will be minimal, making it questionable that the resultant reduction of the digester capacity will be sufficient to warrant the added cost of constructing a sloping floor. However, a means of removing the heavy mineral materials that settle to the bottom of the digester is necessary to prevent their accumulation to the extent of substantially reducing the active volume of the digester. Methods that have been used for sludge removal include installation of a pipe extending to the bottom of the digester to be used as a suction line, or a channel for an auger that will convey the sludge out of the digester. The favorable topography at the Penn State digester site made it possible to use a permanently installed horizontal sludge-removal auger in the floor (Fig 15) that empties into a line surfacing down-slope from the digester.
Placement of the digester partly below ground can aid in reducing the lift required to pump manure into it and, under certain topography conditions, the slurry can flow by gravity.
A critical component of the digester is the roof assembly, which must both seal out atmospheric gases and prevent escape of the methane that is produced. Thus, the roof must consist of materials that are weather-resistant and impermeable to methane. Since the amount of material inside the digester varies during its operation, it has become common to provide a floating or flexible roof that moves up and down according to the amount of liquid and gas inside. When a fixed roof is used, some reliable means for expansion must be provided (Fig 22). Under certain regulations, a fixed roof is not permitted in a gas-producing structure such as the digester. Also, if the digester is to be used for gas storage, a floating or flexible roof is needed to adjust the gas-storage volume in relation to the rate that gas is being produced.
Figure 22. Variations in roof designs for anaerobic digesters.
Floating roof: The floating roof assembly for the Penn State digester was fabricated from the roof and upper wall section of a standard circular galvanized-steel grain-storage bin. The top was 18 ft (5.5 m) in diameter and the side wall was 40 in (1 m) in height. The interior surfaces were sprayed with 3 in (80 mm) of polyurethane foam to provide heat insulation. The final gas seal for the seams in the roof was provided by an inside membrane. The gas seal between the roof and the main digester walls was achieved by submerging the lower edge of the roof in the slurry (Fig 22A). A more elaborate water seal (similar to Fig 22B) was included in the original Penn State design, but it became filled with slurry (creating operational problems) and was removed. The simpler design, however, permits a small amount of gas to escape through the slurry between the roof and the digester wall.
During operation of the digester, it was noted that the lower sidewall surfaces of the roof assembly not protected by polyurethane insulation and exposed alternately to slurry and atmosphere became severely corroded; the galvanized coating was nearly corroded away by the end of the first year's operation. The surface was then cleaned thoroughly and coated with the epoxy paint, which seemed to provide better protection against rapid corrosion. It is suggested that sidewalls of a digester roof assembly be made of fiberglass.
Galvanized sheet metal was satisfactory for the overhead outer surface of the roof assembly; this area was exposed only to the exterior atmosphere. Its inner surface was protected by the coating of polyurethane insulation.
Access to the interior of the digester should be provided; the Penn State design included a manhole in the roof assembly (Fig 23). The cover must be fitted with a gas-tight seal.
Figure 23. Roof hatch assembly of the Penn State digester.
As it is necessary to hoist the roof assembly into place with a crane (Fig 24), the construction of the roof assembly must include strong connecting points for attaching the hoisting cables.
Figure 24. Installing the digester roof at Penn State.
Flexible roof: Due to the difficulties of making a floating roof move up and down without catching and jamming on the digester walls, other roof designs have been tried.
One alternative is a flexible roof material, such as sheet plastic or rubber. However, not all plastic or rubber materials are impermeable to gas or durable in a biogas atmosphere. Manufacturers should be consulted for specifications and warranties of their products for this purpose.
A flexible sheet roof may require mechanical protection and structural reinforcement to withstand wind and gas pressures and to avoid deterioration by sunlight. Attainment of a seal at the point where the sheet joins the digester wall can best be obtained by extending the sheet a foot (300 mm) or so below the surface of the liquid.
Fixed roof: This roof can be made as an integral part of the digester chamber. However, if such a design is used, some means of pressure release must be provided, such as a fail-safe relief valve or expandable gas-storage compartment Caution is required in using a fixed roof, because of the possible danger of explosion due to excessive pressure buildup. The Missouri digester (Sievers et al. 1975) was constructed with a thick concrete roof and uses a large plastic bag for gas storage. Even with a thick concrete roof, the interior of the gas chamber must be sealed with a paint or mastic compound or a membrane that is not permeable to gas. The roof must have sufficient weight or must be loaded to withstand the gas pressure.
Insulating and sealing the digester
Due to the 95°F (35°C) temperature necessary for effective biological activity inside the digester, heat insulation is required for all digesters in geographical areas where the outside temperature goes considerably below the inside temperature. The amount of insulation required is determined by the minimum temperature expected at the digester site. It has been suggested that sufficient insulation be provided to keep the digester, without heat being added, from cooling more than 2°F (1°C) during the 24-hour period when outside temperatures are at their coldest.
Suggested insulation values ("R") range from 8 to 40; this is equivalent to a thickness of 2 to 10 in (50 to 250 mm) of expanded polystyrene. The Penn State digester wall consists of precast ribbed concrete silo staves lined with a 1/2-in (13-mm) coating of Gunite (concrete plaster) to seal the joints. Over this was placed two layers of 2-in (50- mm) expanded polystyrene sheets (total thickness of 4 inches), sealed in turn by a %-in (20-mm) thickness of Gunite as the interior coating of the chamber. This composite construction represents an approximate "R" value of 18. Experience has shown the desirability of placing the insulation on the inner walls of the digester; because of its warmth, rodents are attracted to the digester during winter and their burrowing would damage exterior insulation.
In the interest of heat conservation, the digester roof and floor should also be well-insulated. The Penn State digester roof was insulated on the inside by a 3-in (80-mm) thickness of sprayed-on polyurethane foam. Since the roof must be gas-tight, the spray-on urethane was selected to serve the dual purpose of insulating the walls and ceiling and sealing the joints in the panels. However, the foam application was not a satisfactory seal, due either to improper application or because of excessive flexing when the assembly was lifted into place. If a sprayed-on gas seal is attempted, the work should be done by an experienced contractor who has the necessary high-capacity equipment and can guarantee his work.
To overcome gas-leakage problems, a sheet of 30-mil (0.8-mm) Hypalon, with nylon-fiber reinforcement, was finally applied to the inside surface of the Penn State digester roof assembly. This formed a satisfactory seal, as indicated by gas pressures as high as 7-in (1.7 kPa) water gage.
Insulation was applied to the gas agitation lines and the heating-water pipes wherever they were exposed to outside air-conditions or were in unheated building areas. Temperatures of both the gas and the water in their respective systems were equal to or above digester temperatures. Without good insulation, the large temperature differences between the pipes and the outside air will result in large heat losses.
The digestion process is most efficient if the manure is fed into the digester as soon as possible after it leaves the animal. Consequently, delays in moving the manure from the animal housing to the digester should be avoided. However, to reduce sensitivity to temporary breakdowns of equipment, a storage space sufficient to hold the manure produced for a brief period may be advantageous. Normally, this storage space should be reserved for emergency use. Because the amount of manure coming from the gutter varies, a hopper should be provided at the inlet so manure is always available for the pump to work constantly at full capacity without running the risk of pumping air into the digester.
The slurry-preparation area should be kept as warm as possible - at least above freezing - to avoid the possibility of equipment damage. It is suggested that the slurry be prepared inside or directly adjacent to the animal shelter to utilize animal heat. The slurry preparation area is one of the likely places for trouble in the operation and should consequently have adequate working space for normal operation, maintenance, and possible repair work.
An adequate water suppiy is needed in the slurry-preparation area. This may be provided by an overhead storage tank of sufficient size to hold the dilution water needed each time the digester is fed. Schematic sketches of the slurry-preparation area with the ram-type-pump feeding system used for the Penn State digester are shown in Figures 25 and 26. A photograph of the system appears in Figure 27.
Figure 25. Section view of ram-pump installation used in the Penn State digester, showing slurry-preparation area.
Figure 26. Top view of slurry-preparation area.
Figure 27. Slurry-preparation area at the Penn State digester. Gutter cleaner discharges directly into ram-pump hopper; water can be added from storage tank at upper left. Power unit and control panel is located at upper right.
Storage for processed manure
The size of the storage area for the digester effluent depends on how the manure is to be managed after processing. If it is to be spread daily on land, the storage space need be only large enough to hold a few days output, which will allow for delays because of equipment breakdown or extremely severe weather. On the other hand, if manure is to be stored until the best time for land application, larger storage will be necessary. With long-term storage, dewatering might be advantageous. In this way the liquid could be distributed by sprinkler irrigation, and the solids stockpiled and distributed whenever convenient. Solids do not require a special storage facility. Storage of the liquid will require a water-tight pit or an approved earthen impoundment. It may be possible to recycle some of the water removed from the process slurry for use as dilution water for fresh manure input. This procedure will reduce the volume of processed slurry that must be stored, as well as the amount of processed manure that must be hauled to the field. Storage of processed manure in the digester its.elf is not economical, due to the much higher cost of digester space compared with storage space.
Housing for mechanical components
The components of the heating, gas circulation, and control systems should be housed in a structure with good heat insulation. Water and condensate will be present in pipelines, which must be protected against freezing. Consequently, the structure should be attached or closely adjacent to the digester so that lines can be short and insulation and heat loss minimized. There should be room inside the structure for the operator to supervise the operation during low outside temperatures. Controlled ventilation should be provided to eliminate the risk of the operator inhaling toxic gases. Construction material should be specified with consideration for the potential fire hazards in this structure. Doors to the facility should be large enough to facilitate installation of preassembled equipment (Fig 28).
Figure 28. Prefabicated equipment shelter used at the Penn State digester.
The mechanical components of a biogas generator system include equipment for handling manure slurry and biogas and for heating the digester.
The slurry-handling system consists of equipment for barn cleaning, slurry preparation, feeding the digester, agitating the digester substrate, slurry removal, and effluent storage and/or distribution. With the continuous-feed digester, slurry handling occurs once or twice daily when the barn is cleaned. Daily handling procedures should be managed so as to require a minimum of time and labor for manually operating the mechanical components. With frequent feeding, relatively small amounts of material need to be handled each time the system is operated. This reduces the demand for high-capacity equipment. Because of the strong possibility of malfunction, alternative systems should be provided for handling the manure from the barn to the digester and for removing slurry from the digester.
Figure 29. A slurry-handling system employing gravity feed and mechanical agitation.
Gravity feed: Gravity flow should be used wherever possible, such as when the manure comes out of the animal shelter at a sufficient elevation to flow unaided down into the digester. Generally, this can be done only with small digesters or with tunnel-type digesters, which have low height compared to their length and width. An example of a gravity-flow design appears in Figure 14.
In a modification of the gravity-feed system, the slurry (after preparation) is pumped into a tank that is at sufficient elevation above the digester so that it will flow from there by gravity. The Penn State digester was originally designed and operated in this way. An electrically driven centrifugal manure pump mixed the manure and water to form a slurry, which was then pumped into an elevated tank that discharged by gravity to near the bottom of the digester (Fig 30). This digester-feeding system required dilution to about 8 percent solids for satisfactory handling by the pump against a 10-ft (3-m) head. The electrically driven centrifugal pump was frequently clogged by large wood chips in the sawdust bedding and by slugs of rejected feed, particularly long hay.
Figure 30. A slurry-handling system in which slurry is pumped from the sump into an elevated tank, and then feeds by gravity into the digester. This system was used in early experiments at Penn State.
Pressure feed: Because of the clogging problems, the Penn State digester was modified to a forced-feed system using a ram pump located directly beneath the gutter cleaner (Fig 31). The ram forces the manure into the digester against an 8-ft (2.4-m) head of slurry. This pump has a 12- by 18-inch (300- by 450-mm) opening and a discharge line 12 in (300 mm) in diameter, making it suitable for handling manure that contains long fibers.
Figure 31. Ram-type manure pump employed at the Penn State digester.
Dilution water was added to the manure in the gutter by gravity flow from an overhead storage tank near the ram-pump hopper (Fig 25). The discharge line from the storage tank was a 1.5-in (38-mm) hose equipped with a PVC stopcock-type valve. This size of line allowed water flow at a sufficient rate to produce a slurry mix with 13 percent solids concentration, which proved satisfactory for hydraulic flow through the digester.
The reciprocating action of the plunger through the manure and water in the hopper provided mixing of the slurry before it entered the digester. Experience showed that better mixing resulted by adding the water at the end of the gutter cleaner instead of directly into the pump hopper.
The input pipe discharged the slurry in close proximity to the heated wall panel so that the slurry was heated as quickly as possible (Fig 15, 16). Figures 14A and 14C illustrate other pressure-fed systems using a ram pump and a centrifugal (Monoye) pump.
Slurry removal: Continuous-feed digesters are designed to discharge processed slurry through an overflow pipe or over an edge each time the digester is fed. Dependable outflow is necessary to avoid the need to open the digester to remove stoppage. Opening the digester will require shutdown and may lead to malfunction due to admission of oxygen. For dependable flow through the outlet pipe, the slurry must be kept well mixed. Examples of outlet tubes used in the Penn State digester are shown in Figures 15 and 17. In addition to the normal outflow, emergency overflow devices should be provided against overfilling the digester. These outlets should be located where an overflow will do the least damage. Protection against overfill can be provided by the equivalent of a surge tank in the feed pipe.
Dewatering the effluent is a possible means of making the manure-dilution water available for reuse. If mechanical dewatering of effluent is included in the system, the dewaterer should be located directly following the digester slurry outlet. In this way the dewaterer can operate with fairly low capacity and some of the heat content of the liquid can be saved by return of the warm liquid to the manure-pump hopper.
Sludge removal: Provision must be made for removal of the heavy sludge that settles in the bottom of the digester, since large accumulations of sludge will reduce the active volume of the digester available for gas production. The Penn State digester had outlets in its floor, opened and closed by sliding valves operated by hydraulic cylinders controlled from a remote location (Figs 15, 32). When the valves were opened, the fluid pressure forced the sludge down into a horizontal channel in which an auger helped to convey the sludge to the storage pit.
Figure 32. Hydraulically operated gate to the sludge-removal auger.
The sludge-removal mechanisms were operated weekly to prevent buildup. With this arrangement, the operator could draw off sludge from either chamber of the digester or from the storage tank by selectively actuating the gate-control valve for the respective chamber. The sludge auger was 6 in (150 mm) in diameter by 28 ft (8.5 m) long. It was powered by a direct-coupled submerged hydraulic motor operated at 60 rpm, which required approximately 0.5 hp (0.35 kW).
The hydraulic motor that powered the sludge-removal auger and hydraulic cylinders were operated by an electrically driven hydraulic pump. The same hydraulic pump that powered the ram pump can be used. A tractor hydraulic system may serve as a standby hydraulic-power source.
Alternative methods that have been used for removing sludge are an inclined auger conveyor driven by an external power source or the suction line from a vacuum-type tank spreader wagon extended to the bottom of the digester through the feed and/or over-flow pipe.
Slurry pumps and pipelines: Equipment to feed the digester at a dairy barn must satisfy the following requirements:
- It should be electrically powered for convenience due to frequent use and the need for operation from a remote location. This puts the limit on the permissible power at approximately 7.5 hp (5 kW) in normal farm installations.
- The pump must be capable of handling a semisolid mixture that might occasionally contain large wood chips and long hay particles, as well as handling pure liquid.
- It must be capable of providing a sufficient head to force material to the upper level of the liquid in the digester and also to overcome the occasional high flow resistance when a slug of solid material occurs in the manure.
The above power limitation prevents the use of the large centrifugal pumps normally used for handling liquid manure. Possible alternatives are the hydraulically operated ram-type manure pump, conveyors of the type used for gutter cleaners, and augers. All mechanical components should be located so that they can be serviced without interrupting the functioning of the digester.
Because manure coming from the barn should be exposed as little as possible to oxygen, which can kill the anaerobic bacteria present in the manure, it should not be agitated unnecessarily. The ram-type manure pump and screw conveyors provide some desirable mixing and homogenizing action before the manure enters the digester. Slurries containing long fibrous materials such as long hay may require a pump equipped with shredding or cutting components. This equipment will have a high power requirement.
In a gravity-flow system, the forces moving the slurry are very small. Consequently, large-diameter pipes and ducts (Fig 33) must be used. Smaller pipes may be used with pump-type handling systems, which develop considerable pressure to move the manure. However, in selecting pipe size, consideration must be given not only to its capacity to handle manure during the pumping operation, but also to prevention of stoppage or plugging of the pipes when the pumps are not in operation. The slurry behaves as an easily pumpable liquid when it is well agitated, but after being stationary in the pipes for a few hours the solids stratify and congeal. Consequently, all pipes should be much larger than required for liquid-flow considerations.
Figure 33. Slurry inlet (feeding) and outlet (overflow) pipes. This straight-line arrangement permits mechanical cleaning from the outside without interrupting digester operation. Sampling of digester contents is also a simple operation.
Inaccessible elbows in flow pipes must be eliminated (Fig 34). If a turn must be included, a "T" or "Y" fitting should be used so one branch is accessible from the outside. If the "T" is below slurry level in the digester, a plug or valve arrangement (as illustrated in Fig 34) that can be removed or opened without emptying the digester should be installed. Cleanout and shutoff gates should be installed where necessary for cleaning and maintaining the pipe system.
Figure 34. Bent inlet (feeding) and outlet (overflow) pipes installed in a digester. A system of plugs and valves needs to be provided, so that extender pipes can be installed for cleaning without shutting down the digester.
Installation of the main pipes in the digester should be so that a small steel pipe can be inserted from the outside for flushing and cleaning with water. Plastic pipes are preferable for slurry handling. They are resistant to corrosion from the slurry and require fewer joints than concrete or vitreous-clay pipes.
Some digesters have been built with little or no provision for agitation of the slurry in the digester. However, most operators have recognized the need for agitation to improve heat transfer and distribution of the microorganisms, as well as to provide good fluid consistency for reliable outflow. Thorough mixing also prevents stagnation of material in sections of the digester and formation of a scum layer that may retard gas release from the surface of the slurry. Agitation permits a reduction in retention time (Fig 9) and thereby increases the digester working capacity.
Three basic systems for slurry agitation have been used in various digesters: mechanical agitation inside the digester, slurry recirculation through an outside pump, and gas recirculation (Fig 35). Slurry can be agitated either intermittently or continuously. Intermittent agitation requires relatively high power input for a brief period of time to accomplish complete mixing each time the system is operated, whereas continuous agitation can normally be considerably less intensive because the material is not permitted to come to rest and settle.
Figure 35. Systems for slurry agitation. Agitation is necessary to assure even distribution of the micro-organisms throughout the slurry, and to provide an even substrate temperature.
With mechanical agitation, a paddle must be driven by an externally powered shaft extending through the digester roof or wall. Maintaining a tight seal around the rotating shaft is difficult. Also, the fluid drag on the blade is high, necessitating large power input for operation at sufficient speed to accomplish effective mixing. Start-up resistance in settled slurry may be very high and is known to have caused several agitator failures.
Hydraulic mixing with a solids-handling pump requires large-capacity equipment, and the pump and lines are subject to clogging, especially when long hay fibers or large particles of wood from the bedding are present.
The gas agitation system consists of a gas pump that draws biogas from the digester gas chamber and returns it through the recirculation pipe to the bottom of the digester. The bubbles rise to the surface, recirculating the slurry and breaking up any surface layer that may be formed. The pump must develop sufficient pressure to overcome the head (dependent upon the depth of the liquid) of the slurry in the digester (Fig 36, 38).
Figure 36. The Penn State gas agitation system. Biogas produced by the microorganisms in digesting the slurry is pumped back into the digester to provide bubbling action, which provides mixing of incoming slurry and prevention of solid scum formation on the slurry surface. Two diffusers (detailed in B) are used in the primary chamber; a third operates in the secondary chamber.
Figure 37. Arrangement of the gas pump and accessories in the Penn State digester system.
Figure 38. Arrangement of the diffuser-pipe-manifold in the Penn State system. It is important that the manifold be located higher than the surface of the slurry in the digester.
Gas-recirculation systems are very reliable since the equipment moves gas only, avoiding fouling problems usually associated with movement of liquids that may contain fibrous materials or other solids.
Penn State gas agitation system: In the Penn State digester, the gas was taken from a high point under the floating roof to a gas pump in the equipment shelter. It was then pumped back into the digester through three diffusers (two in the primary and one in the secondary digester chamber) located close to the bottom of the digester and near its middle wall. Components of the gas system are illustrated in Figures 36 through 41.
Figure 39. Diffusers, detailed at right, in the primary chamber of the Penn State system. Note the slurry inlet pipe and the hydraulically opened sludge cleaning gate (floor of digester).
Figure 40. Biogas pump with electric drive motor, condensate traps, and outlet valves.
Figure 41. Diffuser gas manifold (note Fig 38). The three diffuser pipes are pictured near the bottom right of the photo; the white horizontal fitting is the tube through which biogas is collected from the digester.
The gas pump was a positive-displacement vane-type rotary pump normally used as a vacuum pump for milking machines. The pressure required at the pump to provide active bubbling through 15 ft (3.8 m) of slurry was determined to be 13 in of mercury (44 kPa). The pump was operated with a gas-circulation rate of 12 ft3/min (0.34 m3/min), which corresponded to approximately 0.003 ft3/min per cubic foot of digester volume (0.003 m3/ min per m3). Higher gas-circulation rates were used initially but did not seem to provide an advantage.
The lubrication system for the pump was modified by installing the lubricating oil inlet on the gas intake side of the pump to make it operate satisfactorily in the pressure mode. The standard lubricator for pump operation in vacuum mode was used.
Distribution of the recirculated gas to the respective diffusers can be adjusted by throttling values in the manifold outlet lines (Fig 38, 41). Because of its corrosive properties, the gas was carried in PVC pipes and rubber hoses wherever possible. However, the high temperatures of the gas at the pump outlet caused the pipes of regular PVC to soften and change size, especially where connected to steel pipes. Thus leaks developed fairly soon in threaded joints and under hose-clamp connections. These gas-leakage problems were overcome by replacing the PVC schedule-40 pipes with CPVC schedule-80 plumbing and by inserting stainless-steel sleeves inside the ends of the plastic pipes.
Because of high moisture and temperature in the digester, the gas contains a considerable amount of moisture. As the gas cools in the pipes outside the digester, condensation occurs, requiring installation of condensate traps at all low points in the gas lines. Also, a trap is required for the spent oil from the gas pump.
Those portions of the gas agitation pipes submerged in the slurry will be filled with slurry when the gas system is not operating. If an extended shutdown occurs, the solids may accumulate and form a mat that will plug the pipes. Consequently, these pipes should always be installed so they can be cleaned mechanically from the outside, which means that elbows or fittings must be avoided in the gas lines inside the digester.
Check valves (Fig 38) were installed in the diffuser pipe to prevent backflow of the digester liquid into the gas lines. Throttling valves installed in each diffuser line made it possible to shut off one diffuser at a time so as to check if all diffusers were operating properly. The valves were located well above the digester level of the slurry surface to avoid risk of contamination by slurry backflow. Care was taken to prevent these components from freezing, since the gas lines contained condensate. A flame trap (combined with a condensate trap) was installed in the gas lines just outside the digester.
Gas agitation during the 1975-76 season, when the incoming slurry was homogenized in the centrifugal manure pump, was sufficient to prevent formation of a surface crust. During the 1976-77 winter/spring operation, which relied on the ram-pump feed system for mixing, a thick mat or crust formed on the slurry surface, except directly above the diffusers. The gas bubble velocity was high enough to keep the surface open at these points. The mat was composed mainly of the long hay fibers that were abundant in the gutter cleaner discharge. Apparently most of the slurry was well mixed as indicated by uniform temperatures measured during digester operation. During the summer/fall 1977 operation, when care was taken to prevent long hay from entering the digester, there was no substantial crust formation.
Biogas output system
In the Penn State system, gas in excess of that needed for digester agitation was taken from the pressure side of the gas pump for use in the boiler and elsewhere (Fig 42). Because of high moisture conditions in the digester, the gas was saturated with moisture. This moisture condensed in the boiler manifold, causing restriction of gas flow and creating problems due to the liquid's chemical aggressiveness. Therefore, gas taken for heating the boiler or other gas-burning appliances was cooled down sufficiently to condense most of the moisture. After cooling, it was re-heated (Fig 42) to reduce the chance of further condensation in the distribution lines. The condensate traps at the Penn State digester were drained manually, but at some locations an automatic type of condensate trap could be installed.
Figure 42. The gas-usage system at the Penn State anaerobic digester. A portion of the digester output is utilized in firing the boiler that provides hot water for maintaining slurry temperature at 95°F
Following cooling and reheating, an automatic pressure-reduction valve reduced the gas pressure in the line to about 12 in (30 em) water gage. Further treatment of the gas was not necessary for its use in the boiler. When several gas outlets are used, a pressure regulator for the digester-heating boiler and another regulator for other gas outlets may be desirable to prevent interference in pressure supply to the boiler. Pressure on the high-pressure side of the pressure regulators does not normally fluctuate very much, due to the high capacity of the gas pump.
Gas-metering devices should be installed to measure gas production, which is an important indicator of whether the digester is functioning properly. Gas meters used in home installations were found to malfunction after a brief period of use. A suitable method of measuring gas flow is through an orifice calibrated with a differential manometer. A simple design of such an orifice and a schematic of the apparatus is shown in Figure 43.
Figure 43. Flow-meter arrangement for measuring volume of biogas going for external use. Lower sketch represents detail of the orifice. Commercial household-type gas meters were not satisfactory for this purpose.
Both the boiler and the vent for excess gas in the Penn State system were controlled by temperature and pressure switches that actuated solenoid valves. To determine the amount of gas used, hour meters were installed to record the time the solenoid valves were open.
The temperature for efficient digester operation is 95°F (35°C), which is generally warmer than outside temperatures, so heat must be supplied to raise incoming manure slurry up to this temperature and to offset heat losses through walls, roof, and floor of the digester. Normally, this heat input can be supplied by burning a portion of the gas produced by the digestion process. An alternative heating method is to use the biogas as fuel for a stationary combustion engine and to utilize the engine-cooling water and hot exhaust gases to heat the digester.
Digester-heating systems that have been used to heat the slurry include the heated wall or floor (Penn State, India), hot-water coil in digester (Missouri, Cornell, Denmark, and others), steam injection into digester (Darmstadt), hot-water injection in digester (Cornell and others), hot-water injection at slurry preparation, outside heat exchanger using hot water and/or hot effluent (Sweden, England, Washington, Georgia, and many municipal systems). and solar heat on the digester roof. The more common types are illustrated in Figures 44 through 47.
Figure 44. Digester with heated inner partition and gas agitation of slurry.
Figure 45. Digester with heating coils in the digestion chamber. Slurry agitated mechanically.
Figure 46. System for maintaining slurry temperature through addition of hot water during slurry preparation. Such a system has been tested in work at Cornell University.
Figure 47. This digester-heating system uses an outside heat exchanger and a slurry pump to move the slurry through the exchanger. Source of heat is hot water.
In the Penn State digester (Fig 48). the slurry was heated by passing it along the heat-exchange surface in the middle wall (Fig 49), which contained heating pipes cast into concrete panels. Hot water from a gas-fired standard cast-iron boiler (Fig 50, 51) was forced through the pipes by a circulation pump. The boiler was fired with LP gas for start-up and switched to biogas when production permitted. Thereafter, the boiler was operated on biogas entirely, except for its pilot light, which used LP gas continuously. Converting the boiler to biogas required larger burner nozzles than for LP gas, and almost complete closure of the air intake to the burner manifold.
Figure 48. Schematic of the heated middle wall in the Penn State digester. Four wall sections are connected in two separate loops. Hot water flows through the pipes at a constant rate.
Figure 49. The heated middle wall. The wall has darkened where pipes are embedded in the concrete.
Figure 50. The Penn State boiler and heat-circulation system.
Figure 51. Boiler (lower center), with expansion tank (above) and cylinder (left) of LP fuel for the pilot light.
The biogas fuel supply to the boiler was adjusted according to digester-heating needs by regulating gas pressure at the burner with a throttling valve. The pressure was manually adjusted between 0.8 and 5 in (20 and 125 mm) water gage, which corresponded to 35 to 95 ft3/hr (1 to 2.7 m3/hr) of biogas, depending on outside temperatures and loading rate. The relationship of gas pressure to gas volume was predetermined by nozzle-calibration tests. The corresponding heat input to the boiler was calculated to be 19,200 to 52,000 Btu/br (5.7 to 15.4 kW)· from the known low heat value of methane (910 Btu/ft3, or 34 MJ/m3) and the methane content of the gas (60%).
Figure 52. The fuel and burner system of the Penn State digester heating boiler.
Digester heat losses: Digester heat losses through floor, walls, and roof can be calculated from the following equation:
Q =U A (t1 - t0)
where: Q = heat flow
U = thermal conductivity coefficient
A = area
t1 = inside temperature
t0 = outside temperature
An example of the heat-loss calculation for the Penn State digester, using values for heat transfer coefficients and expected outside temperatures, is presented in Table 3.
Table 3. Penn State digester heat-loss calculations, based on 95°F. digester-operating temperature and lowest outside temperatures recorded during the 1976-1977 season.
|Component||Material||Area (A)||Outside surface temperature||Heat transfer coefficient (U)||Heat loss (Q)|
|(ft2)||(m2)||(°F)||(°C)||(Btu/hr per ft2 per °F)||(W/m2 per °C)||(Btu/hr)||(kW)|
|Wall below ground||4-inch styrofoam + 4-inch concrete||630||58||42||6.5||0.068||0.39||2,270||0.65|
|Wall above ground||4-inch styrofoam + 4-inch concrete||375||35||10||-12||0.069||0.39||2,200||0.64|
|Roof||3-inch urethane + 1/16-inch galv. steel||500||47||10||-12||0.53||0.30||2,250||0.65|
a Inside temperature = 61°F (16°C) due to insulating layer of settled solids.
Guidelines suggested earlier -- that insulation should be sufficient to prevent the digester temperature from dropping more than 2°F/24 hr (1°C/d) -- would correspond to an hourly average heat loss equal to 8 Btu/hr per ft2 (115 W/m2) of digester surface area. The amount of insulation used in the Penn State design would permit approximately one-half of this temperature drop.
Heating requirements for slurry input: The amount of heat needed to heat the incoming slurry can be calculated from the following equation:
Q = m Cp(t1-t2)
where: Q = heat
m = feeding rate
Cp = specific heat
t1 = initial temperature
t2 = final temperature.
The specific heat (Cp) of manure was assumed to be equal to the specific heat of water. This estimate is high, since the solids content of the slurry is between 5 and 15 percent, and the solids can be expected to have a lower specific heat than water. A sample calculation, based on data for the Penn State digester operation, is presented in Table 4.
Table 4. Heat required to raise temperature of input slurry to operating temperature (95°F, 35°C) in the Penn State digester.
|Total manure input (per day)||11,000||5,000||46||8||539,000||570|
|Dilution water (per day)||1,700||770||42||6||90,000||95|
|Total daily requirement||629,000||665|
Boiler size selection: The total heat input needed for the lowest outside temperature expected, as shown in Table 5, was used as the basis for calculating the required boiler size.
Table 5. Heat requirements of the Penn State digester system.
|Area of loss or consumption||Heat|
|Heat losses through floor, wall, and roof||8,000||2.3|
|Heat input to warm the slurry||26,200||7.7|
|Total heat for the digester chamber||34,200||10.0|
|Heat losses from lines (estimated at 10%)||3,400||1.0|
|Total heat needed for the digester||37,600||11.0|
|Energy input to the boiler (58% efficiency)||64,800||19.0|
The manufacturer's rating for boilers indicated a boiler efficiency of 0. 7 with LP gas and 0.78 with natural gas. The value of 0.58 was used in calculating the needed boiler capacity due to intermittent and low-load operation. A boiler with a rated input value of 66,000 Btu/hr (19.5 kW) was selected for the Penn State digester installation. Experience in operating the digester has shown this size boiler is adequate.
Biogas consumption by the boiler versus average outside temperature for a period when the feeding rate was constant is plotted in Figure 53.
Figure 53. Boiler biogas consumption versus outside temperature by the Penn State digester during spring 1977. Feeding rate was 158 ft3 (4.46 m3) of slurry per day. Each point (X) represents actual average daily use during a 3- to 10-day period of uniform outside temperature. Digester operating temperature was 95°F.
Temperature distribution in the Penn State digester: Thermocouples located just above the floor, close to the outside walls, and in the center of the slurry recorded practically the same temperature -- a difference of only 2°F (1°C) -- indicating that the slurry was well mixed. However, lower temperatures were found on the floor surface, indicating the desirability of installing floor insulation of approximately the same value as in the walls.
Temperature measurements on the outside of the digester showed an almost constant year-round soil temperature beneath the digester floor. The average temperature outside the buried parts of the walls varied slightly with the outside temperature. Due to the large masses involved, the heating needs of the digester did not follow variations in the outside air temperature. This experience seems to justify calculating heating needs on monthly air-temperature averages.
Digester heat-exchange areas: Temperature of the heating surface in contact with the slurry cannot go much above 110°F (43°C) without risk of damage to microbial action and subsequent low gas production. Also, elevated temperatures may cause the slurry to cake onto the surface, which will considerably reduce the heat-transfer rate. For this reason, temperature of the water entering the digester heat exchanger must be adjusted with regard to the type of heat-transfer surface. If metallic tubes in direct contact with the slurry are used, the water temperature must be lower than if plastic tubes are used. The water temperature can be higher if the heater tubes are embedded in concrete so the heat is dissipated over a larger surface area (as in the Penn State digester, which used the central wall separating the primary and secondary chambers as the heating area).
The heat-exchange surface in the Penn State digester consisted of four precast concrete divider-wall panels, each providing 18 fF (1.7 m2) of heat-exchange area to the primary chamber (the side of the panel facing the secondary chamber was insulated). Each panel contained 47 linear ft (14m) of three-fourths-inch (20-mm) steel pipe embedded one-half inch (13 mm) from the surface facing the primary chamber.
In order to attain sufficient heat-transfer capacity, temperature of the water going to the heat panels had to be maintained at between 130 and 150°F (54 and 65°C), which was achieved by setting an automatic water-mixing valve. The boiler temperature was normally held at 180°F (82°C), The water returning from the digester to the boiler was normally 6 to 10°F (3 to 5.5°C) colder than the outgoing water. Attempts to measure the flow rate of circulating hot water indicated approximately 5 gal/min (18 liter/min). This flow rate and a temperature difference of 10°F (5.5°C) corresponds to a heat-transfer rate of approximately 25,000 Btu/hr (7 kW).
Thermocouples attached on the surface of the concrete panels indicated a temperature drop of 30°f (17°C) from the hot water to the concrete surface, reflecting the advantage of embedding the heating coils in concrete rather than having them in direct contact with the slurry. The hot water can be kept at a higher temper.ature without risk of overheating the slurry. After the first 18 months of operation, the heat panels showed no deposits of caked-on manure, as would occur with exposed pipes. A darker shade directly over where the heat pipes were embedded indicated some temperature variation over the concrete surface.
Under extreme cold-weather conditions, return water temperature was as much as 11°F (6°C) less than outgoing temperature, which resulted in an estimated 27,500 Btu/hr (8 kW) of heat through 72ft2 (6.7 m2) of concrete panel surface area. The inside area of the heating-water tubes was 40 ft2 (3.7 m2), the slurry temperature was 95°F (35°C) and the water temperature was 140°F (60°C). This gives a heat-transfer rate of 380 Btu/hr per ft2 (1.2kW/m2) of panel area and 690 Btu/hr per ft2 (2.2 kW/m2) of heating-tube area.
From experience in operating the digester, it appears that the heat-transfer area in the digester was on the lower limit and that a somewhat larger heat-transfer area should be used. Suggested design values for a digester system of similar size are 30,000 Btu/hr (9 kW). This requires a panel area of approximately 90 sq ft, or 0.003 ft2/Btu per hr (1 m2/kW) and a heating coil area of 0.0017 ft2/Btu per hr (0.55 m2/kW). Thus, to transfer 30,000 Btu/hr (9 kW) with a panel surface temperature of 108°F (42°C), the following heat-transfer coefficients can be calculated for the various media:
Hot water to heat panel surface = 29 Btu/hr per ft2 per °F (167 W/m2 per °C)
Heat panel surface to slurry = 22Btu/hr per ft2 per °F (120 W/m2 per °C)
These data assume that slurry agitation is at least equivalent to that used in the Penn State digester.
Controls and safety devices
Since the anaerobic digester for biogas production from manure is a complex biochemical-processing system, it requires accurate control of many components for high efficiency and safety. It should be assumed that any component of the system can fail at a critical moment, requiring the availability of a substitute method of maintaining proper operation until the malfunctioning component can be repaired.
In order to limit the need for manual labor and supervision, manual control of operations should be confined to the initial starting of the process and overseeing that the system functions properly. Proper controls and alarms should be installed to keep equipment and process functioning within design limits and to call the operator's attention to malfunctions.
Heating system: For effective regulation of the digester temperature at close to 95°F (35°C), a remote-sensor type of thermostat should be used to control the operation of the heating unit (boiler). The sensor should be located in an area where the slurry is actively agitated in order to assure quick response to changes in temperature of the digester substrate. The thermostat should be installed to control a solenoid valve in the gas-supply line to the boiler, according to heat needs. Fuel for the pilot flame should be supplied by LP gas (or other independent source) to assure that low-quality biogas or an interruption in the biogas supply would not cause the pilot flame to be extinguished, resulting in the burner not igniting when the thermostat calls for heat.
In the early Penn State digester design, the upper-limit automatic temperature controller installed on the purchased boiler was used to prevent it from overheating; the boiler was run continuously, with the boiler temperature regulated by manually adjusting biogas pressure according to heat need. Fuel for the pilot flame was supplied by LP gas. The hot-water circulator operated continuously to insure that the water temperature in outside pipes did not drop below freezing.
In the later design, the boiler was moved closer to the digester so that hot-water pipes did not go outdoors. A thermostat placed in the center of the digester turned the biogas flow to the boiler on and off by a solenoid valve in the biogas-supply line.
Gas system: The rate of gas use for outside purposes does not normally correspond exactly to its rate of production. Therefore, control devices are needed to avoid excessively high or low pressures within the digester. A floating roof automatically eliminates the danger of excessive pressure in the digester; if more gas is produced than is being used, the roof will rise and allow excess gas to escape under the edge of the roof. As an alternative, the upper limit of gas pressure in the system can be controlled by a pressure sensor coupled to a solenoid valve in a line extending into the atmosphere.
On the other hand, drawing off more gas than is being produced would eventually lead to a partial vacuum in the digester, which could collapse the roof. For this reason, the digester must have a vacuum-actuated switch that will close the gas outlet and avoid development of negative pressures within the digester. The gas pump can develop damaging high pressures, so safety devices (Fig 37) should be installed to shut it off if excessive pressures or temperatures should develop.
Flame traps should be installed in gas lines to prevent the danger of a backfire reaching any component of the system containing substantial amounts of gas (Fig 54).
Figure 54. Flame traps suitable for use in an anaerobic digester system.
Slurry system: Daily feeding of the anaerobic digester is a routinely repeated operation and should consequently be automated to the fullest extent. Slurry-handling conveyors and pumps should be equipped with pressure-relief and flow-limit control devices to guard against slugs of material entering the slurry system and causing overflow, stoppage, or breakage of the equipment. The slurry level in the digester can best be controlled by fixing the elevation of the overflow passages. These passages should be adequate to minimize the possibility of blockage. However, they should be checked periodically for accumulations of fibrous material that might restrict flow. An alarm device that will give an early warning against overfilling is desirable.
Hazards: Materials used in constructing a digester system should be fireproof to prevent secondary fires from being started by small gas flames that might occur in normal operation of a gas-fired heating system. Sensors should be installed to activate an audible signal if the boiler or gas system temperature and/or pressure goes outside of set limits, or if a fire should occur in the digester equipment room. Signs warning of danger should be prominently displayed around the digester (Fig 55), and the system should be fenced in and locked to keep animals and unauthorized persons away.
Figure 55. The Penn State digester in operation. Note "Warning" sign.
Indicators of malfunction
There are several biological indicators available to an operator for monitoring the need for routine process adjustment:
Temperatures should be measured at different locations in the system. Any change from normal temperature values may indicate blockage, uneven mixing of the slurry, or malfunction of the heating system.
Biogas production will usually be reduced by any biological upset. Normal variability in gas generation would be expected if the loading rate were changed or the digester temperature fluctuated. Therefore, gas production alone cannot be relied upon as an indicator of biological problems.
The volatile acids level is an indicator of the acid-forming bacteria activity; high levels indicate that these bacteria are producing intermediate acids faster than the methane formers can utilize these compounds. From an operational standpoint, volatile acid variability would reflect irregularity in numerous factors that affect total gas production. Measurement of the volatile acids concentration requires precise chemical tests.
The primary form of alkalinity is ammonium bicarbonate. It alone is not a responsive indicator. However, alkalinity and volatile-acid measurements can be used together as an effective indicator. Alkalinity should always exceed volatile acids.
The pH level is an indicator of the balance in acid and methane bacteria activity, but is not as responsive as volatile acids and alkalinity, due to the buffering effect of alkalinity in the digester. Proper operation can normally be expected in the pH range of 6.8 to 7.4. Determination of pH concentration can be done on site with readily available instruments or methods.
The odor of digester gases is distinct, but it is generally not considered to be unpleasant when the digester is operating properly. A foul odor is a qualitative indicator of biological upset. It is not an early indicator of the problem because of the long solids-retention time, but it may be interpreted as an indication that adjustment is needed.
Warning: Operators should be alert to the presence of distinct biogas odor. as it is evidence of leakage in the gas system. Personnel should never enter the digester chamber until the chamber has been completely emptied and thoroughly ventilated. Remaining sludge may continue to produce methane. CO2, and H2S, all of which are extremely dangerous to workers in confined spaces.
Section 3: Operational Procedure
To start the operation, the Penn State digester was filled (Fig 56) with warm active slurry obtained from a municipal anaerobic digester. The amount pumped in was sufficient to cover the lower edge of the roof assembly, thereby sealing the chamber against oxygen from the atmosphere. The municipal sludge also provided seeding with methane bacteria.
Figure 56. Filling the digester with processed sludge from a municipal anaerobic digester. Methane-producing bacteria in the sludge provided rapid start-up for the system.
After filling, the material was allowed to stand for approximately 24 hours so that oxygen entrained during handling and oxygen remaining in the gas-collection chamber could be consumed in the surface layer. The heating and mixing systems were then started to bring the temperature back up to about 95°F (35°C).
After the digester reached normal operating temperature, feeding of manure began slowly. The loading schedule was about one-tenth normal loading the first day, followed by increased increments of about one-tenth each successive day, until the full loading rate was reached on the l0th day.
Daily Operational Procedure
The digester was fed twice daily, in conjunction with cleaning the barn. At the time of each feeding, condensate traps in the gas lines were drained and equipment monitoring devices checked to verify that the boiler, hot-water circulator, gas pressures, and digester temperatures were normal. Adjustments were made as necessary to compensate for abnormalities or changes in climatic conditions.
With the original slurry-feeding system (using the centrifugal pump), dilution water was first added to the gutter for a set length of time to give the proper slurry concentration. The slurry pump was set on mixing mode and the gutter cleaner was operated to fill the mixing hopper. After thorough mixing, the slurry pump was switched to pumping mode, and the gravity-feed hopper (Fig 30) was filled and then emptied into the digester. These mixing and feed-hopper-filling procedures were repeated for successive batches. The respective hopper sizes required processing three batches each time the digester was fed.
With the ram-pump slurry-feeding system, dilution water was added to the manure in the gutter by gravity flow from an overhead storage tank near the ram-pump hopper (Fig 57). The attendant manually started the gutter cleaner and ram pump, and manually adjusted the dilution water supply in accordance with the amount of manure in the gutter cleaner. It was considered necessary to have an attendant present during the digester-feeding process to make adjustments for extremes in manure-feed rate, to remove materials that might interfere with pump operation, or to correct equipment malfunctions. At times the gutter discharge was in excess of the ram-pump capacity, requiring the gutter cleaner to be stopped momentarily to prevent the pump hopper from overflowing. At other times the manure supplied was less than the pump capacity, requiring that the pump be stopped momentarily to avoid pumping air into the manure pipeline. Occasionally the check valve in the ram pump became fouled, causing backflow from the digester, which required correction by the operator (Fig 58).
Figure 57. Slurry flow characteristics are most satisfactory when dry-matter concentration is about 13 percent of the total. Water is added in the gutter cleaner to achieve this consistency.
Figure 58. Handles on the ram-pump check valves provide for manual closing of the valves if they should stick in the open position.
Digester effluent flowed automatically from the digester to the storage tank, which overflowed to the large storage pit. Occasionally the effluent-discharge opening clogged, requiring operator attention to flush out the overflow pipe. The total daily time needed to feed and monitor the digester, in conjunction with gutter cleaning, amounted to approximately 1.5 hour.
Additional Periodic Service
Operations required at approximately weekly intervals included filling the lubricating-oil reservoir of the gas pump and operating the sludge-removal augers in the bottom of the digester. This required an operator's attention for approximately one-half hour each week.
In addition to the operator's daily and weekly attention, periodic maintenance was needed for such items as lubricating electric motors and adjusting drive belts and chains. The gutter-cleaner flights were checked frequently to reduce the chance of a flight becoming detached and fouling the manure pump.
The volume of effluent is slightly Jess than the volume of slurry fed to the digester. Effluent is usually applied to agricultural land for utilization by crops. The volume of processed slurry can be used as the basis for estimating storage needs or the number of loads to be hauled daily. Unlike unprocessed manure, the processed manure requires very little agitation in the storage pit prior to hauling. It can be readily pumped from storage with suction equipment.
On occasions the regular digester-feeding schedule at the Dairy Center was interrupted because of equipment malfunction. If the interruption was brief (1 to 2 days), no problems were experienced upon resumption of full-scale operations. In one instance, the entire roof was removed for 48 hours for repair of gas leaks. During this period, the gas agitation system was shut off to avoid exposing the slurry, other than its surface layer, to oxygen. Also, feeding was stopped to prevent chilling the digester contents in the inlet area because of lack of heating and mixing.
After the roof was replaced, the system was allowed to stand for a few hours in order for the oxygen within the gas chamber to be consumed by organisms in the surface layer. Upon resumption of gas recirculation, 4 hours after the roof was replaced, the biogas supply was adequate to resume operation of the boiler, and the regular feeding schedule was resumed immediately.
Occasionally the digester effluent outlet stopped up or the ram-pump check valves became jammed in a partially opened position. Stoppage of the effluent line caused the digester to overflow; clogging of the ram-pump check valves caused the digester contents to flow back and overflow the pump hopper.
A method to prevent stoppage of manure slurry lines is to fill the critical components with water or a very thin slurry. Effluent stoppage in the Penn State digester was most effectively removed by inserting a long three-quarter-in (20-mm) steel pipe coupled to a pressurized water hose. Attempts to use the pipe as a ramrod without water pressure were generally not successful; the material was only compacted by the ramming actions. Problems' with the check valve in the slurry-feed line were alleviated by attaching a "T" handle to the check-valve pivot shafts so the attendant could force the valves wide open when the ram-pump piston was in its compression stroke.
The chance of blockage in the submerged portion of the gas agitation lines increases if the gas-pump operation must be interrupted. If a long interruption occurs, it may be desirable to fill the lines with water or to operate them occasionally to prevent blockage.
Standby devices should be located on site and tested before they are needed to assure that they are ready to be used when vital system components fail or malfunction.
Section 4: Storage and Use of Biogas and Effluent
The digester produces gas continuously, making it well-suited for use where fuel is needed continuously, such as in space-heating systems. However, fuel needs for most other heating purposes tend to be intermittent, making storage provisions desirable. For uses such as heating water, a minimal amount of storage may suffice just to even out the fluctuations during the day. Often, the collector space at the top of the digester, which is necessary to compensate for variations in liquid level and gas production, may be sufficient to compensate also for daily variations in gas consumption. If utilization is less than the gas produced only on certain days, for instance during weekends, it is normally not practical to save the surplus gas during these days.
Variable-volume gas storage was provided at the Penn State digester by the floating roof which held the pressure in the chamber constant at approximately 1 inch (0.25 kPa) water gage above atmospheric pressure. This pressure was determined by the weight and area of the floating roof. Pressures as high as 6 inch (1.5 kPa) water gage were developed when the roof was anchored down. Even higher gas pressures have been reported for other digesters where the structure could withstand the load or was sufficiently ballasted. However, the storage volume under a floating roof is normally considerably less than the daily gas production from the digester. Therefore, larger storage volumes are desirable for the best possible use of the gas. Several types of gas storage systems are illustrated in Figure 59. Figures 60 and 61 show installations of the flexible-bag and pressure-tank storages in use.
Figure 59. Biogas storage systems. Sizable gas-storage systems require a considerable investment.
Figure 60. Flexible-bag storage of biogas at low pressure (courtesy of D.M. Sievers).
Figure 61. A used propane tank was utilized for medium-pressure biogas storage near Ripon, Wisconsin.
Size of the gas storage needed for daily, but intermittent, gas use can be shown by the following example. If the total gas production is 5,300 ft3/day (150m3/d) and the heating need is 1,600 ft3/day (45m3/d), the necessary gas storage will be 3,500 ft3 (100m3) if cooling water from an internal combustion engine driving an electric generator provides the heat for the digester, and 2,000 ft3 (55 m3) if a separate heating boiler is used. The motor-generator is assumed to be running 8 hours daily.
Compression of the gas concentrates energy and thereby reduces storage-space requirements. However, high-pressure storage requires expensive compressors and tanks. The relationship of storage pressure to energy concentration is shown in Table 6. Attaining very high concentration of energy by liquefying the gas, as is done with propane, cannot be done with methane at normal temperatures; it requires a temperature of -117°F (-83°C) at a pressure of 5,000 psi (35 MPa). The very low temperature of -260°F (-162°C) is required to liquefy methane at atmospheric pressure.
Table 6. Effect of pressure on energy concentration of stored biogas.
|Pressure (gauge)||Concentration (volume)a||Energy contentb||Storage|
|Biogas (60% methane)|
|20||0.14||2.4||2.4||1,310||48||Floating roof or flexible bag|
|100||0.7||7.8||7.8||4,600||161||Low-pressure steel tank|
|300||2||21.4||21.4||11,450||422||Medium-pressure steel tank|
|1,000||7||72||72||39,240||1,460||High-pressure steel tank|
|3,000||20||250||250||136,250||5,030||High-pressure steel tank|
|Refined Biogas (100% methane)|
|1,000||7||72||72||66,000||2,460||High-pressure steel tank|
|3,000||20||250||250||228,000||8,500||High-pressure steel tank|
|LP Gas (propane and butane)|
|No. 2 Fuel Oil|
aGas volume, at standard atmospheric pressure and temperature, per unit of storage
bLow heat value
Scrubbing the biogas to remove the inert carbon dioxide should be considered as a means of minimizing the cost per unit of energy stored. Although gas-scrubbing systems are not extremely complex, they will require substantial additional capital outlay and management, making their feasibility questionable except for very large digester systems or where the gas is to be sold commercially. An alternative to a large or high-pressure storage is to use the digester-produced gas for only part of the energy needs and supplement it with other fuels as needed. Because of storage costs, large seasonal energy needs (such as for drying grain or hay) cannot be met economically by digester gas.
Converting gas energy to other forms
The energy of the produced gas can also be saved by converting it to other forms of energy. Electricity seems to be the best alternative and has been used in several systems.
Another approach is to store the energy as heated water. This can be as hot water for domestic use, for space heating, or for producing a temperature increase in the digester itself. The heat energy contained in 1 ft3 (0.03 m3) of biogas is sufficient to increase the temperature of 1 gal (3.8 liters) of water approximately 46°F or 25°C. Converting the methane into methanol is not feasible on the average farm, as it involves a catalytic process requiring high pressure and high temperature.
Systems for Gas Usage
Gas for external use may be taken from an outlet in the line on the high-pressure side of the gas agitation pump. After passing it through a cooling coil to condense the moisture (Fig 42) and a pressure reducer, it can be distributed for outside use. A separate pressure regulator should be installed in each supply line for outside uses. Also, condensate traps must be installed at low positions in the distribution lines. Shutoff valves are suggested at several points in the system to permit disassembly for cleaning. A comprehensive summary on installation of equipment for biogas use has been published by the Gobar Research Station in India (Singh 1971).
Use in boilers and water heaters
Throughout the test periods at Penn State, biogas successfully fueled the boiler that heated the digester chamber. Preliminary tests were made of its usefulness as fuel for a household-type water heater and for operating spark-ignition and compression-ignition internal-combustion engines.
The gas-fired cast-iron boiler was of commercial manufacture, of a type that would normally be fueled with LP gas. Several changes were required to prepare the unit for fueling with biogas:
- The diameter of the nozzle orifice was increased. The heating value of biogas is only 30 percent that of LP gas and only 60 percent that of natural gas, and line pressures were less with biogas than with LP gas. Consequently, the diameter of the burner nozzle orifices was enlarged approximately 70 percent for biogas utilization.
- Air supply to the burner was decreased. The air-inlet ports had to be almost completely closed, which required special covers for the air inlets.
- Moisture was removed. A system of cooling and heating the gas in combination with condensate traps facilitated delivery of the gas to valves and orifices without risk of condensation in these narrow channels. Pressure-regulator valves were kept warm to prevent condensation.
- An independent fuel source (LP gas) was used for the pilot light. This was because the quality and supply of biogas may not be totally reliable. The pilot light required approximately 1 lb (0.45 kg) LP gas per day.
A 40-gal (150-liter) water heater was operated experimentally, using biogas as fuel. This test indicated that a gas consumption of 1.4 ft3/min (40 liter/min) would heat 0.6 gal/min (2.3 liter/min) of water from 50 to 150°F (10 to 65°C). This corresponds to a thermal efficiency of 65 percent, compared to a rated efficiency of 70 percent. Further adjustment of the burner would probably improve its performance when fueled with biogas.
Since well-adjusted and fully loaded boilers and other water-heating devices may be assumed to have 70 percent efficiency on biogas, as well as on other gaseous fuels, the fuel consumption for a water heater can be calculated from the relationship that 110 ft3 (3.2 m3) of biogas (59,000 Btu, or 62 MJ) will heat 54 gal (190 liter) of water from 50 to 150°F (10 to 65°C). In a home-heating system, 185 ft3/hr (5.2 m3/h) of biogas would give a rated (input) heating capacity of 100,000 Btu/hr (29 kW).
Use in internal combustion engines
Biogas-fueled engines are common in municipal sewage-treatment plants. Many experimental digesters have furnished gas for engines, tractors, trucks, or automobiles (Fig 62). However, a fuel tank that will store sufficient biogas to operate a mobile vehicle will be quite large, so use of biogas as a motor fuel will likely be confined to stationary engines.
Figure 62. A motor-generator powered by biogas. Only large digesters can provide sufficient gas to fuel equipment of this size.
Biogas has a high (100 to 110) octane rating and consequently can be used in high-compression engines. However, the high octane rating also means that the fuel mixture must be ignited by a spark or by some other fuel. In spark-ignition engines, biogas alone can be used as fuel. In diesel engines, however, a small amount of regular diesel fuel must be injected in order to achieve ignition of the biogas. In this case, the engine may run on 20 percent liquid fuel and 80 percent biogas.
The heat value per unit volume of an appropriate biogas-air mixture is only 60 percent of the heat value of a gasoline-air mixture, and only 75 percent of the heat value of the fuel mixture used in a diesel engine. Consequently, the maximum power output from an engine operated on biogas will be 20 to 40 percent less than that of the engine operating on liquid fuels.
Since the combustion velocity of the biogas-air mixture is considerably lower than the combustion velocity of a gasoline-air mixture, ignition has to occur considerably earlier in the cycle than for normal gasoline operation. Consequently, engine timing on spark-ignition engines must be advanced. Experiments with biogas at Penn State revealed that the engine timing must be advanced approximately 25 crankshaft degrees in order to achieve efficient combustion.
Conversion of a compression-ignition (diesel) engine from liquid fuel to biogas is more complicated than conversion of a spark-ignition engine. Advancing the ignition (injection) timing cannot be done easily from outside the diesel engine; it requires an internal adjustment of the fuel pump. The fuel pump may also have to be modified so that it will be capable of furnishing only the very small exact amounts of liquid fuel required for a mixed biogas-diesel operation. Energy conversion should, however, be more efficient with a diesel engine than with a spark-ignition engine. As a fuel, biogas seems to be better suited for low-speed diesel engines than for high-speed engines.
Special carburetors or diesel-conversion kits for biogas are available. For the Penn State studies, a simple manually adjusted fuel-mixing device worked well under fairly constant loads. The engine was started on liquid fuel.
For engines that are properly modified and adjusted, the heat-energy consumption when fueled with biogas is approximately equal to that when burning liquid fuels. A biogas containing 60 percent methane has a heat value of 545 Btu/ft3 (20.5 Mj/m3), which could be expected to be 22.5 ft3/hphr (0.85 m3/kWh) for a spark-ignition engine at 75 percent load. Therefore, daily gas production of 3,700 ft3 (105 m3) would permit an engine to develop 20.5 hp (15.5 kW) for 8 hours per day.
Penn State tests, comparing fuel consumption and power output for a tractor engine operating on biogas and on gasoline, showed that 100 ft3 (2.85 m3) of biogas corresponded to approximately 0.5 gal (1.9 liter) of gasoline in effective fuel value. Tests of a converted diesel tractor in Germany reported biogas consumption of 16 ft3/hphr (0.6 m3/kWh) at full load and 22 ft3/hphr (0.85 m3/kWH) at 40 percent load (Seifert 1955). Biogas consumption in a pickup truck was 11 ft3/mile (0.2 m3/km) in studies reported by Lapp (1977).
Concern regarding the harmful possible effect of hydrogen sulphide and other gases on the engine has been expressed. Tests of sufficient duration to study this effect have not been conducted at Penn State, but the gas agitation pump showed no visible wear after 4 months' continuous use. The housing of the pump has a cast-iron inner surface, and fiber-like vanes operate against it producing conditions somewhat similar to an engine. The used oil coming from the pump ranged between pH 7.3 and 7.6, indicating that it was not acidic in nature.
Reports of experience in the petroleum industry seem to confirm this observation (Milo 1974). Petroleum workers have noticed that high-strength steel, such as in valve springs, is more susceptible to ill effects of H2S than are cast iron and the lower-grade steels. German tractors have operated more than 5,000 hours on biogas without problems (Seifert 1955). In these machines, the lubricating oil was changed more frequently -- at 120-hour rather than 240-hour intervals.
Cleaning the gas
Analysis of biogas shows its composition to be approximately 60 percent methane, which represents a low heat content of 545 Btu/ft3 (20.5 MJ/m3). Other components include water vapor, carbon dioxide, traces of hydrogen sulphide, ammonia, and oil from the pump.
Biogas burns readily as produced, However, the water vapor in the gas should be removed in order to prevent its condensation prior to use as fuel in a furnace or water heater; this will aid in preventing fouling of the burners and control devices. Biogas for open-flame combustion in enclosed areas, such as for cooking, should be cleaned.
Water vapor: In the digester chamber, biogas becomes saturated with water; at a temperature of 95°F and RH of 100%, one-thousand cubic feet of gas will contain 2.5 lb of water (40 g/m3). But at an ambient temperature of 60°F (15.5°C), biogas will contain only 0.8 lb of water per 1,000 ft3 (13 g/m3), and at 32°F (O°C) it will contain only 0.3 lb H20 per 1,000 cubic feet (5 g/m3). As the gas cools upon leaving the digester chamber, the water vapor cools and condenses to liquid and may interfere with the functioning of pressure reducers, boiler orifices, and other devices. Water also increases the rate of corrosive attack on metals. For these reasons, water vapor should be reduced at an early stage, either by cooling the gas and trapping the moisture in condensate traps or by passing it through a water-absorption medium such as calcium chloride or an organic absorbent.
Carbon dioxide: Carbon dioxide scrubbing can be done by cooling and compressing the gas. If the gas is to be used for fueling an engine or a boiler, it is doubtful if the improvement in performance will be sufficient to justify the additional complications and cost of scrubbing. C02 must be removed from gas that is to be sold commercially.
Hydrogen sulphide: Removal of hydrogen sulphide may be desirable if the gas is to be used in engines or if piped for long distances. A reduction to approximately 1 mg/ liter of gas has been recommended. H2S can be removed by passing biogas through ferrous oxide, iron filings, or steel wool.
Oil: The major part of the oil from the gas pump can be removed in a condensate trap. Further cleaning of the gas is not needed for use in boilers or other gas burners.
Use of the Digester Effluent
Nitrogen content of the digester effluent is essentially the same as that of the input material; its fertilizer value is quite comparable to that of unprocessed manure. However, part of the organic nitrogen in the manure has been converted to ammonia forms that are more readily available to plants but also more susceptible to loss through volatization. Loss of nitrogen from the digester effluent may be reduced by addition of small amounts of phosphoric acid to the stored effluent (Bartlett et al. 1978). Digester effluent is partly stabilized so that it does not have an offensive odor and does not attract pests, such as flies. However, it remains high in biological oxygen demand, and thus is a pollutant if discharged into surface waters. Samples of the processed manure stored at room temperature at Penn State were still inactive after 12 months.
Storage and handling facilities comparable to ordinary manure-management systems will be required for the effluent. Effluent from a digester with agitation will be well homogenized and will require minimum agitation in the storage pit prior to loading.
Dewatering the effluent and using sprinkler-irrigation equipment to distribute the liquid on land may be feasible for large operations. Dewatered stabilized solids do not require confined storage facilities and can be easily handled by conventional tractor front-end loaders and manure spreaders.
Section 5: Cost of Anaerobic Digestion and Biogas Production from Dairy-barn Manure
Many of the components of the Penn State digester are commercially available; prices can be obtained from their manufacturers. However, several parts for the Penn State project were constructed according to specifications. Table 7 lists the major components of the system with approximate costs for the Penn State digester construction and equipment components. The costs reflect agricultural contractor construction methods and 1975 price levels. Costs of the respective items can be expected to vary, depending on the price situation for a given area. The major part of the work should be done by a contractor, but some labor input could be supplied by the farmer.
Table 7. Approximate cost, based on 1975-1976 prices, for the principal components of an anaerobic digester for a milking herd of 100 cows (about 3,500 ft3 or 100 m3 capacity).
|Foundation (including sludge-auger housing)||$2,500|
|Digester (including insulation)||4,700|
|Digester roof (including insulation)||1,500|
|Effluent disposal facilitya||2,000|
|Manure slurry handling system (Hydra-ram pump, sludge auger)||6,000|
|Heating system (boiler, circulator, heat panels)||1,000|
|Gas recirculating system (gas pump, manifold, diffusers)||1,000|
|Supplies and labor||1,300|
|Total initial cost||20,000|
aEffluent from the Penn State digester was stored in a small steel tank suitable for the experimental installation.
Based on experiences in operating the Penn State digester, operating costs for a normal farming operation are estimated to be as follows. As discussed under "Daily Operating Procedure", when feeding the digester in conjunction with cleaning the gutter twice daily, the operator time required for tending the digester was approximately 45 minutes per cleaning. In addition, weekly labor expended in operating the bottom augers and servicing the equipment was about 30 minutes. Labor for spreading material from an anaerobic-digester system will be about the same as with a regular liquid-manure-handling system.
Electricity was used for lighting the equipment area, operating the gas recirculation pump, hot water-circulating pump, manure pump, and hydraulic pump that in turn operated the sludge-removal gates and the hydraulic motor that drove the sludge auger. Electric power used for operating the system, measured by meters, averaged 25 kWh/day.
The principal expendable supplies for operating the system were lubricating oil for the gas pump and LP gas for the pilot light in the boiler. Consumption of these items were, respectively:
- Oil = 1 gal/month
- LP gas = 30 lb/month
Maintenance and depreciation
During the useful life of the equipment, it is expected that certain mechanical components, especially wearing surfaces and vibrating members, will need periodic repairs. Some components will have a relatively short useful life. Substantial corrosion may occur on some components, especially those in contact with the biogas. The structural components,! f built of durable materials, can be expected to require minimum repair and have a relatively long useful life.
Estimated annual costs for the Penn State digester system, based on established values for standard types of manure-handling equipment, are presented in Tables 8 and 9.
Table 8. Estimated annual maintenance and depreciation costs for an anaerobic digester.
|Gas recirculating system||1,000||8||75|
|Total annual maintenance costs:a||645|
|Total depreciation costs:||1,435|
aASAE Data: ASAE D230.2, based on the assumption that repair costs over the life of the equipment will be equivalent to 60 percent of initial cost.
Table 9. Summary of annual costs for an anaerobic digester.
|Interest (based on capital costs)||1,200|
|Electricity (25 kWh/day; $0.04/kWh)||365|
|Supplies (vacuum pump oil, LP gas for boiler start-up and pilot light)||125|
aDoes not include the cost of labor, estimated to be about 1.5 man hour per day, for slurry preparation and checking the operation of the system.
Effects of Larger Herd
Some of the tests with the Penn State digester indicated that it could handle up to 0.65 lb/ft3 (10.3 kg/m3) of volatile solids daily. This loading rate corresponds to the manure production of about 130 cows, as indicated by Midwest Plan Service's Structures and Environment Handbook and the 1978-79 Agricultural Engineer's Yearbook. Thus, attainable net gas production for the digester designed for 100 cows could be much higher with this higher loading rate, but operating costs would increase very little.
Value of Digester Output Products
If the net output of biogas from the digester is used for heating purposes in a furnace, space heater, water heater, etc., its value should be compared to the heating value of natural gas, LP gas, or fuel oil. In a 100-cow manure digester operating under winter conditions of 25°F (-4°C), net biogas output (60% methane) content has equivalent fuel values for uses other than heating the digester as shown in Table 10. For a digester operating on a year-round basis, the average daily net output will be increased substantially (approximately 30%) because the digester heating load will be greatly reduced, due to the much higher temperature of input slurry in warm weather.
Table 10. Approximate equivalent fuel values for net biogas production from a 100-cow anaerobic digester.
|Type of Fuel||Daily (Winter)||Annuala|
|Biogas (60% methane)||3,700 ft3|
|1.35 X 106 ft3|
|Natural gas||1,980 ft3|
|72.3 X 104 ft3|
(2.04 X 104 m3)
|LP gas||102 lb|
|3.72 X 104 lb|
(1.68 X 104 kg)
|Fuel oil||14.7 gal|
(2.03 x 104 liters)
aBased on the assumption that, on a year-round basis, about 70 percent of the digester energy output is in excess of the amount needed to maintain slurry temperature.
If the biogas is used for generating electricity and the waste heat from the engine is used to heat the digester, the calculated output of electric energy is 150 kWh/day, or 55,000 kWh/12-month season. In such a system, there would be some excess heat from the engine.
Processed manure as fertilizer
No substantial reduction in nitrogen occurs during the anaerobic-digestion process. Effluent analysis showed a nitrogen content of 1.6 percent. This amounts to a daily output of 55 lb (25 kg) of nitrogen from a 100-cow dairy herd.
Additional Benefits of Anaerobic Digester
In special cases, farmers may have to take costly steps for manure management in order to satisfy environmental requirements with regard to odor and/or other pollution problems. In such cases, an anaerobic digester could be used to overcome these problems satisfactorily (Fig 63). The economic value of this benefit is difficult to assess. However, it may be of substantial economic importance in some cases (Capener and Braithwaite 1978).
Figure 63. Anaerobic digestion of swine manure was practiced on this farm near Mt. Pleasant, Iowa, in order to overcome malodor problems in this densely populated area.
Comprehensive lists of additional references on anaerobic digestion of agricultural and similar residue are available in Abeles et al. (1977?), Kirsch and Sykes 1971, Ljunggren and Petre 1976, Noren 1976, Regan 1975, Stoner 1974, and Wheatstone et al. 1974.
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Prepared by S.P.E. Persson, H.D. Bartlett, A.E. Branding, and R.W. Regan