Methane Generation From Livestock Waste (2024)

AE-105

Purdue University
Cooperative Extension Service
West Lafayette, IN 47907

Contents

Advantages and Disadvantages of Methane The Methane Generation Process Methane from Animal Waste--Potential and Problems Digester Gas Value and Uses Energy value of the gas Uses for the gasDigester Design and Construction Digester size and environmental requirements Digester construction requirementsDigester Gas Collection Storage, Corrosion and Safety Collecting the gas Storing the gas Minimizing odor and corrosion problems Safety considerationsMonitoring the Digester Recent Digester Innovations Corn cob digesters Thermophillic digesters Liquid manure-pit digestersDetermining the Feasibility of Methane Production Summarizing the Example Results Additional Information on Methane Generation 

Methane, which is the principal component of natural gas (95-98percent), is removed commercially from deposits deep within theearth. This methane was formed millions of years ago in swampy areas(hence it is sometimes called "swamp gas") by the biologicalconversion of organic matter.

The technology needed to generate methane from livestock waste andother farm refuse has been known for about 100 years. But due toinexpensive and abundant petroleum energy, its use has been limited inthe U.S. Today, however, high energy costs and low profit marginsexperienced in some livestock enterprises as well as recent digesterimprovements through research are causing many farmers to re-evaluatethe feasibility of on-farm production of methane gas from livestockwaste.

This publication describes the methane generation process, discussesthe design of on-farm systems and their problems, and provides aprocedure for determining the potential of developing the technologyon your farm.

ADVANTAGES AND DISADVANTAGES OF METHANE GENERATION

Advantages. The main one is that a useful end-product, methane gas,is produced. Also, the odor potential of a well digested livestockwaste is considerably reduced. Although digested waste has slightlyless fertilizer value than nondigested waste, it is more readilyavailable to plants. It is simply converted to a more useful form.

Disadvantages. There are several that must be carefullyconsidered in assessing the potential of on fore-arm methane generation.

* A methane digester is large and expensive. The expense stems fromthe fact that it must be well-insulated, air-tight and supplied asource of heat. The size of a conventional digester is equal to 15-20times the daily waste volume produced, or more if the waste is dilutedbefore digestion. The volume of waste that must be disposed ofincreases accordingly if dilution water is used.

* A very high level of management is required. A methane digestercan be extremely sensitive to environmental changes, and abiological upset may take months to correct. Methane generation ceasesor is very low during an upset.

* Start-up--usually the most critical phase of methanegeneration-is difficult. Methane-producing bacteria are veryslow-growing, and several weeks are required to establish a largebacterial population.

* Methane is difficult to store, since at normal temperatures thegas can be compressed but not liquefied without special, veryexpensive equipment.

* Finally, methane can form an explosive mixture if exposed to air.

THE METHANE GENERATION PROCESS

Methane generation is accomplished by anaerobic digestion(biological oxidation in the absence of oxygen) of organic substancessuch as livestock waste and plant refuse. The gas produced in anon-farm digester is only about 65 percent methane, the rest beingcarbon dioxide and trace organic gases.

Methane generation requires two main groups of anaerobicbacteria-the `acid formers', which convert waste to organic acids; andthe `methane formers', which then convert these organic acids tomethane and carbon dioxide (Figure 1). Also, there are two distincttemperature ranges in which these bacteria can produce significantamounts of methane gas-the mesophillic range (90-110F)and thethermophillic range (120-140F). Recent research using thermophillicbacteria shows some promise and will be discussed briefly later;however, this publication deals mainly with conventional digestionunits operated in the mesophillic range.

Figure 1. The methane generation process.

Methane generation is much like the controlled combustion(incomplete burning) of wood to produce charcoal-i.e., burning asubstance in an air-limited environment to produce a more readilyusable, yet high-energy end-product. The combustion of charcoalrequires oxygen to be completed and produces heat, ash, water vapor andcarbon dioxide. The combustion of methane also requires oxygen to burnproducing heat, water vapor and carbon dioxide.

METHANE FROM LIVESTOCK WASTE--POTENTIAL AND PROBLEMS

Production potential. Methane production is usually expressedin terms of cubic feet of gas generated per pound of volatile solidsdestroyed. Volatile solids are the organic portion of livestock waste;about 80 percent of the manure solids are volatile. A gallon of liquidmanure containing 8 percent solids potentially can provide about 3 3/4cubic feet of digester gas, or 2 1/2 cubic feet of methane (Roughly10-13 cubic feet of gas can be produced per pound of volatile solidsdestroyed in a properly-operating digester. Since about half of thevolatile solids added can be destroyed and half to three-fourths ofthe gas produced will be methane, about 5 cubic feet of digester gas(3 cubic feet of methane) can be produced per pound of total manuresolids added).

In terms of digester size, it is possible to produce 3/4 to 2 1/2cubic feet of gas (1/2 to 1 1/2 cubic feet of methane) per cubic footof digester volume. The gas production expected from various livestockspecies is shown in Table 1.

Table 1. Daily Waste and Methane Production by Dairy, Beef and Swineper 1000 Pounds of Animal Weight.

 Item Dairy Beef Swine----------------------------------------------------------- Raw manure (lb.) 82.0 60.0 65.0 Total solids (lb.) 10.4 6.9 6.0 Volatile solids (lb.) 8.6 5.9 4.8 Methane potential (cu.ft.)* 28.4 19.4 18.6----------------------------------------------------------- * Based on 65 percent of gas being methane.

Toxic components in waste. Several substances commonly foundin livestock waste can inhibit methane production if present in largeenough concentrations. The most common is ammonia because it ispresent in large quantities in animal urine. An ammonia concentrationof 1500 parts per million (ppm) is considered the maximum allowablefor good methane production (Table 2). Above that level, the wasteshould be diluted with water.

Table 2. Effect of Ammonia Concentration on Methane Production.

 Concentration (mg/l of Ammonia-N) Effect---------------------------------------------- 5 - 200 Beneficial 200 - 1000 No adverse effect 1500 - 3000 Possible inhibition at higher pH Values Above 3000 Toxic------------------------------------------------

Certainly, large quantities of antibiotics and cleaningdisinfectants should be kept out of the digester. For this reason,consider excluding farrowing building waste from the digester. Theantibiotic rumensin is also toxic to methane bacteria and should notbe fed to cattle whose waste is to be used for methane generation.

Value of adding crop residues. The primary limitation onlivestock waste loading rates is the high nitrogen (N) contentcompared to its carbon (C) content. The ratio of carbon to nitrogen inthat waste added to the digester should be 20 parts C to one part Nfor optimum methane production.

Crop residues and leaves, which are usually low in nitrogen contentbut high in carbon, could be useful in improving digesterperformance. Mixing crop residue with high nitrogen livestock wasteprovides a more favorable C:N ratio; and gas production shouldincrease accordingly.

DIGESTER GAS VALUE AND USES

Energy Value of the Gas

If we know the potential for methane production from variouslivestock species and the cost of comparable amounts of commercialfuel, we can then determine the value of digester gas. Assumingdigester gas has an energy value of 650 British Thermal Units (BTUs)per cubic foot and a gallon of propane fuel, with an energy value of91,700 BTUs, costs 60 cents (1980 price), it takes about 235 cubicfeet of digester gas to equal one dollar's worth of propane. Table 3estimates the value of the potential gas production from eachlivestock type.

Unfortunately, up to 1% of this gas must be used to heat the manurethat is put into the digester. In addition, some heat is needed tokeep the digester warm during winter months.

Table 3. Value of Dairy, Beef and Swine Waste for Methane Generation.*

 Average Animals needed animal Value per to equal Species weight animal $1.00 per day---------------------------------------------------- lb. cents/day no. Dairy cattle 1300 17 6 Beef cattle 900 9 11 Swine 150 1.3 77---------------------------------------------------- *These calculated values are based on the methane productionassumptions outlined in the text, using 1% of the methane to maintaindigester temperature and a value of 60 cents/gal. for propane (1980price).

Uses for the Gas

Digester gas can be used wherever natural gas isapplicable. Digester gas requirements for household activities werecomputed at Pennsylvania State University (Table 4). On the farm, itcan be used for grain dryers or to operate gas water heaters, whichprovide heat back to the digester and floor heat for nearby livestockbuildings. It can also be burned in a commercial space heater.

Table 4. Digester Gas Requirements for VariousHousehold Uses.

 Household use Gas volume needed------------------------------------------- Cooking 20-25 cu.ft./hr. per burner, or 150-300 cu.ft/day Heating 165 cu.ft./hr. per 100,000 BTU/hr. input Lighting 2-3 cu.ft./hr. per mantle-------------------------------------------

Its greatest potential is as a heating fuel. The equipment neededfor heating is nearly identical to that required by natural gas,except that the gas ports and air supply must be modified to ensureproper combustion. Natural gas burners need some modifications,since the flame from burning digester gas tends to `lift' from theburner. Therefore, a certain amount of trial and error is required,and the holes in the burner mantle will probably have to be enlargedsomewhat.

Engine-driven devices are not very efficient when operated directlyfrom a methane generator. For example, an electric generator (drivenby a gas engine) operating at variable torque has a poor efficiency,because the hourly gas production and consumption are nearly the same,regardless of the amount of loading on the system. About 16-18 cubicfeet of digester gas are required per horsepower-hour, assuming anenergy value of 650 BTU's per cubic foot.

Engine efficiency can be improved by removing carbon dioxide fromthe digester gas before combustion, then burning the remainingmethane. Digester gas can also be injected into the air stream in astationary diesel engine. Up to 90 percent of the fuel entering anengine by this technique can be methane gas.

One potential use for digester gas just now receiving attention isas a heat source to operate an on fore-arm alcohol production plant. Someproducers are experimenting with a system of fermenting corn toalcohol that includes feeding the `stillage grain' byproduct tolivestock, using livestock manure to generate methane, then using themethane directly to fuel the alcohol production process.

With current alcohol production technology, an energy equivalent ofabout 30 cubic feet of digester gas is required to liquefy and fermentthe grain to produce one gallon of ethanol, and another 60 cubicfeet of digester gas per gallon of alcohol for distillation to 160-180proof quality. This equals about one gallon of alcohol for every fivebeef feeders, whereas the stillage from a gallon of alcohol canprovide feed for about three beef feeders.

DIGESTER DESIGN AND CONSTRUCTION

Digester Size and Environmental Requirements

Digester size depends on the amount of waste to be digested and theamount of dilution required. Recommended loading rates vary withanimal species and with how the waste is handled. For example, if theurine (which contains about two-thirds of the ammonia) is excludedfrom the waste, loading can be correspondingly greater. Table 5shows the digester volume needed and other digester design values foreach livestock type.

Table 5. Methane Digester Design Values for Dairy, Beef and Swine.*

 Item Dairy Beef Swine--------------------------------------------------------------------------------------------------Manure: water dilution ratio 1:0 (undil.) 1:0.92 1:2.2Amount of dilution water per 1000 lb. of animal weight 0 gal. 7.0 gal. 18.0 gal.Detention time 15 days 15 days 12.5 daysDigester volume per 1000 lb. of animal weight 20.6 cu.ft. 28.8 cu.ft. 43.4 cu.ft.Estimated gas per 1000 lb. of animal weight 43.7 cu.ft. 29.8 cu.ft. 28.6 cu.ft.Estimated methane production per 1000 lb. of animal weight 28.4 cu.ft. 19.4 cu.ft. 18.6 cu.ft.Estimated daily methane production per cu.ft. of digester volume 1.4 cu.ft. 0.67 cu.ft. 0.43 cu.ft.--------------------------------------------------------------------------------------------------- * From MWPS-19, "Livestock waste Management with Pollution Control."

Start-up can be speeded by providing a source of methanebacteria. One way of doing this is to initially fill 20-25 percent ofthe digester volume with active waste digester sludge from a municipalsewage plant, then to gradually increase the amount of livestock wasteadded at each loading over a 6-8 week period until the system is fullyoperational.

Optimum conditions for digester operation are: uniformloading (preferably daily), neutral acidity, 95F temperature, a20:1 carbon/nitrogen ratio and levels of harmful substancesbelow their inhibiting limits. A near-neutral acidity (pH=7.0) isa good indicator of proper operation. This means that the bacterialpopulations are in balance, with the `acid formers' producing onlyas much organic acids as the `methane formers' can use.

Less-than-optimum environmental conditions can cause a digesterupset, usually resulting in acid conditions. This is becauseacid-forming bacteria will thrive under a much wider range ofenvironmental conditions than the slower-growing methane-formingbacteria.

Acid conditions can be temporarily controlled by adding an alkalinesubstance such as lime. However, the original cause of the imbalancemust be found and corrected if gas production is to be maintained.

Digester Construction Requirements

Digesters must be airtight and must be constructed so that they canbe insulated, heated and the contents stirred. The digesters shown inFigures 2-5 are not necessarily the best possible designs, but aresome that have been successfully used or proposed for use byknowledgeable individuals.

Insulating. Because temperature is critical to methane generation,heat conservation in the digester is essential. To utilize theinsulating properties of the soil, consider mounding the soil uparound the tank or burying the tank in a well-drained site so that thesoil's full insulating potential can be realized (Figure 2).

Figure 2. Below-ground concrete silo digester with floating cover andinterim effluent storage tank (to prevent oxygen from enteringdigester).

Insulate the digester surface to a level of at least R=10 where itis in contact with the ground and to at least R=20 where it is incontact with air (R is a measure of a material's ability to resist theflow of heat. The higher the R value, the better the insulationvalue). See Purdue Extension Publication AE-95, "Insulating Livestockand Other Farm Buildings," for information on selecting and installinginsulation.

Heating. The system most commonly used to provide ayear-round 95F temperature for methane generation is a heat exchangerwhere hot water pipes are placed within the digester. The water can beheated outside the digester, possibly using a methane-fired waterheater.

For best results, waste should be preheated before adding it to thedigester. As much as five times more heat may be needed for thepreheating process as for maintaining digester temperature.

Stirring. Mixing is important to ensure adequate contactbetween the bacteria and the waste and also to help strip gas out ofthe liquid. Mixing can be done using either: (1) a mechanical mixer,(2) a compressor to bubble collected gas back through the digesterliquid or (3) a closed-circuit manure pump.

A mechanical stirrer works well as long as a good air seal ismaintained. Atmospheric oxygen must be excluded from the digester, toeliminate the threat of explosion. One method of doing this is with afloating cover, as illustrated in Figures 2 and 3.

Figure 3. Schematic of a digester with floating cover.

If a compressor is used for mixing, piping can be insertedinto the digester, and recirculated gas from a storage unit injectedby means of an open pipe or diffuser at the bottom of the tank. Thiscreates turbulence and keeps the solids in suspension.

To facilitate the manure pump method of stirring, set pipingwhen the digester is constructed. Either a diaphragm or trash pumplocated outside the digester should work well for this purpose.

For the mechanical or pump-type methods, to determine the horsepower(hp) needed to mix the digester contents, use the equation:

 hp=0.185 x % total solids x liquid capacity (in 1000 cu.ft. units).

For example, a 10,000-cubic foot digester containing waste at 6percent solids would require a 11.1 hp mixer (0.185 X 6% X 10). As tofrequency of stirring, some small-scale studies indicate thatintermittent stirring (3-4 times per day) is about as effective ascontinuous stirring.

DIGESTER GAS COLLECTION, STORAGE, CORROSION AND SAFETY CONCERNS

Collecting the Gas

As stated earlier, digester gas is usually 60-70 percent methane,with the rest being carbon dioxide, some hydrogen sulfide and othertrace gases. To insure against contamination with atmospheric oxygen apositive static pressure of at least 3 inches water should bemaintained over the liquid in the digester and gas collection systems.

This can be done by collecting gas using either (1) a floating coveron top of the digester or (2) a pressure regulator to release gas fromthe digester after a certain pressure level has been reached. In thecase of the former, the cover `floats' on the compressed gas above theliquid. Some gas storage occurs under the cover, and the weight of thecover provides a positive pressure for gas distribution.

Figure 4. Digester consisting of a liquid manure tank constructedinside a grain bin. The area between the two tank walls is filled withinsulation.

Figure 5. Schematic of a small, two-stage digester similar inprinciple to those used in municipal sewage treatment plants.

Any gas piping used should either slope back to the digester or havecondensate traps to prevent water vapor from condensing and blockingthe lines when the gas cools. Also, it is important that a gas meterbe installed on the gas collection line in order to monitor digesteroperation; a high, stable gas production level usually indicates goodoperation.

Storing the Gas

Gas storage vessels should be designed with variable volumes,because they must adjust for differences in the rate of gas productionand consumption while maintaining uniform pressure. Of course, themost practical way of minimizing costly gas storage is to find usesfor the gas which parallel its production rate.

A floating digester cover can be used for gas storage as wellas for gas collection. This is simply a pontoon cover which floats onthe liquid surface and has skirt plates extending down into the liquidto provide a seal (see Figure 3). The weight of the floating coverprovides a pressure head and allows the gas to be withdrawn as it isneeded.

A high-pressure gas storage unit is possible but quiteexpensive for on-farm use. It can be either cylindrical or sphericalin shape and of welded-steel construction. Since there is a danger ofexplosion or leakage with pressurized storage, consult a knowledgeableengineer and metal working shop for help. Medium-pressure storage(less than 100 psi) is more feasible for on-farm use thanhigh-pressure storage.

Some pressurization increases the amount of energy that can bestored (Table 6). But it is impractical to liquefy methane on thefarm, since 700 pounds per square inch (psi) pressure and -150F arerequired to do it.

Table 6. Relationship of Pressure to Heat Content of Stored Digester Gas.

 BTUs per BTUs per Pressure cubic foot gallon------------------------------------ 15 psi 650 87 30 psi 1300 170 45 psi 1950 260 60 psi 2600 350 75 psi 3250 435 90 psi 3900 520------------------------------------

Minimizing Odor and Corrosion Problems

Hydrogen sulfide, which has a rotten egg smell, could be produced ifthe waste contains large amounts of sulfates. In general, however, thegas from a properly-operating digester should have only a slightodor, because both major components--methane and carbon dioxide--areodorless. In any case, the gas produced is stored in an airtightcontainer and burned, thus eliminating any odor problems.

Corrosion is another matter. It can be severe. Therefore, the gasshould probably be passed through a filter containing lead filings ora mixture of woodchips and iron oxide to remove any hydrogensulfide. "Sorb beads" manufactured by Mobil Oil can also be used toremove hydrogen sulfide and water vapor.

For removal of water vapor alone, consider using a condenser. And toremove carbon dioxide, use a molecular sieve.

Safety Considerations

Methane is extremely explosive when mixed with air at the proportionsof 6-15 percent methane. Digester gas is heavier than air andsettles to the ground, displacing oxygen. If hydrogen sulfide ispresent, the digester gas can be a deadly poison.

Always vent the open side of manometers (static pressure gauges) andpressure relief valves to the outside, and provide plenty ofventilation if the digester is located indoors. Be careful whencompressing and storing digester gas. Special equipment and specialtanks must be used if the gas is stored at high pressures.

MONITORING THE DIGESTER

Fortunately, unlike that added to municipal sewage digesters, livestock waste is fairly uniform in composition. Once the process has started and reached steady state, upsets are not too common if the digester is properly managed. Monitoring digester operation, nevertheless, is a good idea and can be accomplished fairly easily, using gas production or pH of the digester liquid as an indicator.

Gas production. This is the simplest and most reliable indicator. In a batch-loaded digester (one in which waste is added every month or so), if gas production drops off gradually, the food supply available to the bacteria is probably depleted, indicating it's time to add more waste to the digester. If gas production drops off rapidly (within 1 or 2 days), the reason is probably an upset digester. Among the potential causes, the major ones are: too high a level of toxic compounds in the waste feed, too high a feed rate or too cold a temperature in the digester.

A low digester temperature could be the result of a failure in theheating system. If a large amount of waste is added at one time, itshould be preheated to 95F to prevent thermal shock to the methanebacteria. Better performance is usually obtained with continuousloading-i.e., where the digester is loaded with smaller amounts ofwaste on a daily basis.

pH level. The pH (level of acidity or alkalinity) can be measured easily by inserting pH paper into the digester liquid and comparing the resulting color intensity that develops with a color chart. The pH should be as close to 7.0 (neutral) as possible. A pH below 6.0 indicates a digester upset. You can purchase pH paper from most drug stores, aquarium shops or wine supply stores.

RECENT DIGESTER INNOVATIONS

Corn Cob Digesters

A laboratory study at Purdue University found that an anaerobic digester containing corn cobs can be used to treat swine waste and produce methane at temperatures as low as 65F (Figure 6). The study used a detention time of 5 days and a loading rate of 7.5 pounds of volatile solids per cubic foot per day. This system holds a great deal of promise for on farm use, with daily gas production as high as 1.5 volumes of gas per volume of digester.

Figure 6. Below-ground corn cob digester with plastic cover.

Since the cobs are high in carbon but low in nitrogen, theyimprove the C:N ratio by supplying additional organic carbon. Theyalso provide a support medium onto which bacteria can attach and beretained within the digester instead of being removed with thedigester effluent.

Thermophillic Digesters

Thermophillic (high-temperature) digesters have been designed that operate satisfactorily at a 5-day detention time and a solids level of 10-20 percent. Digester gas production has been around 11 cubic feet per pound of volatile solids destroyed. Operation is normally started by bringing the digester up to a temperature of 130F at a rate of about 3F per week.

In many ways, thermophillic digestion is better than digestion at950F. Gas production is about 20 percent higher and solids breakdownabout 10 percent higher. In addition, the higher temperature killsmore pathogenic bacteria, thus allowing the digested waste to beused as a feed supplement without further sterilization.

But thermophillic bacteria digestion also has its disadvantages. Themethane content of the gas is somewhat lower (55 percent), anddigester operation is not quite as stable as conventional digesters.

The main drawback, however, is the high temperaturerequired. Roughly twice as much heat is needed as for conventionaldigesters. Thus, the digester must be well-insulated (R=20 fordigester surfaces in contact with the ground and R=30+ it exposedto the atmosphere). In addition, the digester should be stirred toinsure good contact between bacteria and organic matter and tomaximize gas production.

With the low detention time and high temperatures, some means ofreclaiming the heat lost in the digester effluent is necessary to makethe system economical. A considerable amount of laboratory and pilotplant testing is currently being done to determine the feasibilityof thermophillic digesters.

Liquid Manure-Pit Digesters

Researchers at the University of Florida studied an above-ground liquid manure storage structure which was modified for use as a methane digester. The study employed a large, covered storage tank, with waste additions made daily. Gas production was about 60 percent of that in a conventional digester. From this and related work, the following suggestions are offered concerning manure-pit digesters:

* The tank must be seeded initially with bacteria from an active anaerobic digester at a level of 10-20 percent of the tank volume to provide sufficient methane bacteria.

* A seed-to-waste feed ratio on a dry weight basis of at least 20:1 is needed for dairy manure. In other words, it 1000 gallons of liquid dairy manure are normally added each day, the storage tank should initially be filled with about 20,000 gallons of digester sludge. After the tank becomes full, it is pumped down to the 20,000-gallon level and operation begun again.

If municipal sludge is used as seed, a volume ratio of 40:1 isrequired because the solids content of digester sludge from amunicipal sewage treatment plant is about half that of dairy manure.

* Temperature control in this type of digester is not critical aslong as the temperature is between 70F and 95F. Digestion ispossible at 70F because bacteria are not removed from the effluentand the large numbers of bacteria offset the reduced biologicalactivity at the low temperature.

* The manure storage tank should have a capacity for 180 daysstorage, because it takes about 100 days for gas production to achievea steady rate. A floating, gas-tight cover should be used to capturethe gas as it is released from the liquid manure.

* When the temperature of the digester liquid goes below 70F, gasproduction is very low and operation is likely to beunstable. Therefore, heating and insulation are needed in the winterif the digester is to be a reliable source of energy.

Because of the very large volume of heated liquid required comparedto a conventional digester, it seems at this point that the manure-pitdigester will only be practical in the southern parts of theU.S. Certainly anyone considering converting a manure tank to thissystem should check with their county Extension office for the latestresearch information and management recommendations.

DETERMINING THE FEASIBILITY OF METHANE PRODUCTION

The economics of methane production are generally consideredquestionable, even at today's escalating fuel prices. But energy costsand availability tomorrow could change that feasibilitydrastically. The following example, while by no means a completeeconomic analysis, should provide the farmer with a rough idea of thevalue of methane generation on his farm.

Example: A farmer with a 100-cow dairy herd wants to know if he can economically justify a methane digester. Average cow weight is 1300 pounds. Manure from calves, heifers and dry cows will not be available for use in the digester.

 Our Your Items and calculations example value------------------------------------------------------------------------------------------------A. Determine potential volume of gas produced per day. 43.7 cu.ft/ 1. Gas produced per 1000 lb. of animal weight (from Table 5). = 1000lb. __________ 2. Gas produced per animal per day. Avg. wt./hd. x Step A.1 (1300 lb. x 43.7 cu.ft./1000 lb.) = 56.8 cu.ft./hd __________ 3. Total gas produced per day. No. animals x Step A.2 (100 hd. x 56.8 cu.ft./hd.) = 5680 cu.ft __________ 4. Methane produced per 1000 lb. of animal weight (from Table 5). = 28.4 cu.ft/ __________ 1000 lb. 5. Methane produced per animal per day. Avg. wt./hd. x Step A.4 = 36.9 cu.ft __________ (1300 lb. x 28.4 cu.ft./1000 lb.) 6. Total methane produced per day. = 3690 cu.ft. __________ No. animals x Step A.5 (100 hd. x 36.9 cu.ft./hd.) B. Determine amount and value of energy produced. 1. Energy value per day. (Assumes 1/4 of the methane must be recycled to provide heat for the digester, energy value of the remaining 75% is 950 BTU/cu.ft., and a gallon of propane has an energy value of 91,700 BTU and costs 60 cents.) Energy value of methane x usable methane x Step A.6 2,630,000 (950 BTU/cu.ft. x 75% x 3690 cu.ft./day) = BTU/day __________ 2. Propane equivalent of heat produced. Step B.1 / BTU/gal. propane (2.63 mil. BTU/day / 91,700 BTU/gal.) = 28.7 gal. __________ 3. Dollar value of energy produced per day. Price of propane x Step B.2 ($0.60/gal. x 28.7 gal.) = $17.22/day __________ 4. Dollar value of energy produced per year. Days/yr. x Step B.3 (365 days x $17.22/day) = $6285.30 __________C.Determine digester tank volume and dimensions 1. Design liquid volume in the digester (from Table 5) Digester volume/1000 lb. animal wt. x wt./hd. x no. hd. (20.6 cu.ft./1000 lb. x 1300 lb. x 100 hd.) = 2678 cu.ft _________ 2. Total digester volume (including 1/2 day's storage for gas produced) in cubic feet. Step C.1 + (1/2 day x Step A.3) (2678 cu.ft. + (1/2 day x 5680 cu. ft/day)) = 5518 cu.ft _________ 3. Total digester volume in gallons. Gals./cu.ft. x Step 0.2 (7.5 gal./cu. ft. x 5518 cu.ft.) = 41,385 gal. _________ 4. Tank height. (Use 14 foot high digester for this example.) = 14 ft _________ 5. Diameter of circular tank for height chosen. ((Step C.2 / Step C.4) x 1.27)1/2 ((5518 cu.ft x 14 ft) x 1.27)1/2 = 24 ft _________D. Determine digester cost, including insulation, heater and mixer. 1. Cost of digester, including tank cover and pump. (Assume a cost of 50 cents per gallon.) Cost/gal. x Step C.3 ($0.50/gal. x 41,385 gal.) = $20,692 _________ 2. Cost for digester insulation on cover and sidewalls. a.Insulation covering. [One way to insulate is to construct a larger diameter tank around the digester and insulate the space between. The outer tank does not need to be as watertight or sturdy as the inner tank. Assume cost of the exterior tank is 1/2 the digester tank.) Step D.1 x 1/2 ($20,692 x 1/2) = $10,346 _________ b. Digester sidewall surface area. Step C.4 x Step C.5 X 3.14 (14 ft x 24 ft x 3.14) = 1055 sq.ft. _________ c. Digester cover area. (Step C.5)2 x 0.79 (24 ft x 24 ft x O.79) = 455 sq.ft. _________ d. Total digester surface area. Step D.2.b + Step D.2.c (1055 sq.ft + 455 sq.ft) = 1510 sq.ft _________ e. Insulation cost. (Assume $1 per square foot of insulation installed. Because this cost is extremely variable, check with local builders when evaluating a digester for your situation.) Insulating cost/sq.ft. x Step D.2.d ($1/sq.ft x 1510 sq.ft) = $1510 _________ 3. Cost of the water heater. a. Heater sized to supply 30 BTU per hour per cubic foot of digester liquid volume. Heater capacity/cu.ft. x Step 0.1 (30 BTU/hr/cu.ft x 2678 cu.ft.) = 80,340 BTU/hr. _________ b. Heater cost. (1980 price for this size heater with water pipe is about $1000) = $1000 __________ 4. Digester mixer cost. a. Mixer sized to stir digester contents having about 10 percent solids (see Table 1). Step C.1/1000 x pct. solids x 0.185 (2678 cu.ft./1000 x 10% x 0.185) = 5 hp _________ b. Mixer cost. (Assume a 3-in. diaphragm pump and piping system to recirculate digester contents, costing about $2500.) = $2500 _________ 5. Total digester cost. Steps D.1 + D.2.a + D.2.e + D.3.b + D.4.b ($20,692 + $10,346 + $1510 + $1000 + $2500) = $36,048 _________E. Determine the cost of holding digester effluent until spread. 1. Manure produced per day in pounds (from Table 1). Manure/1000 lb. animal wt./day x avg. wt./hd. x no. hd. (82 lb./1000 lb./day x 1300 lb. x 100 hd.) = 10,660 lb./day _________ 2. Manure produced per day in cubic feet. Step E.1 / lb./cu.ft. (10,660 lb./day 60 lb./cu.ft) = 178 cu.ft./day _________ 3. Storage volume needed for 180-day capacity in cubic feet. Days capacity x Step E.2 (180 days x 178 cu.ft./day) = 32,040 cu.ft. _________ 4. Storage volume needed for 180-day capacity in gallons. Gals./cu.ft. x Step E.3 (7.5 gal./cu.ft. x 32,040 cu.ft.) = 240,300 gal./ _________ 5. Cost for a prefabricated storage structure. [Assume 15 cents per gallon.) Construction cost/gal. x Step E.4 ($0.15/gal. x 240,300 gal.) = $36,045 _________ 6. Cost for an earthen storage structure. (Assume 5 cents per cubic foot.) Construction cost/gal. x Step E.3 ($0.05/cu.ft. x 32,040 cu.ft.) = $1602 __________Note. It is questionable whether cost of the manure storage unitshould be charged to the digester, since many dairy farms already haveone or will require one whether or not a digester is used.F. Determine cost of a gas storage unit. A gas storage unit must also be constructed or else a use developed which consumes gas at the rate it is produced. For this example, assume no gas storage is needed. = 0 __________G. Determine total cost of this methane generation system. 1. Total cost with a prefabricated storage. Step D.5 + Step E.5 ($36,048 + $36,045) = $72,093 ___________ 2. Total cost with an earthen storage. Step D.5 + Step E.6 ($36,048 + $1602) = $37,650 ___________H. Determine the economic feasibility. [The dollar value of methane produced in Step B.4 can be used to determine the payback period. Table 7 also assists in determining how much capital can be justified for digester construction.) 1. Capital investment per head that can be paid back in 7 years [from Table 7 at 15 percent interest rate and 60 cents per gallon energy value]. = $285/hd ___________ 2. Total breakeven investment. Step H.1 x no. hd. ($285/hd. x 100 hd.) = $28,500 ___________ --------------------------------------------------------------------------------------------------

SUMMARIZING THE EXAMPLE RESULTS

Under the best digester operating conditions at 15 percentinterest, the break-even point for construction of the earthen holdingpit ($37,650) will not be reached in less than 7 years based on Table7. Current tax credits and government demonstration grants couldshorten the pay-back period somewhat.

Table 7. Maximum Initial Investment per Head in an Anaerobic Digester with a 7-Year Pay-Back at Various Propane Equivalent Costs and Interest Rates for Dairy, Beef and Swine.*

 Per-head investment at a propane equivalent price and interest rate of- --------------------------------------------------- $ .60/gal. $ .90/gal. $1.20/gal. --------- -------- -------- Species 10% 15% 10% 15% 10% 15%------------------------------------------------------------------- Dairy $330 $285 $495 $427 $660 $570 Beef 184 161 276 242 368 222 Swine 26 21 38 32 52 42------------------------------------------------------------------- * The investment price is based on a 7 year pay-back with 4 percentof the initial investment allocated to repairs, insurance andtaxes. There is no profit or return to labor included in thesefigures, nor are any tax incentives considered. Current tax laws arein a state of flux. At present, if the methane is used on the farm,only investment tax credit applies, while digesters which produce gasfor off-farm sale can receive investment tax credit as well as $3 per5.8 million BTU sold.

It may also be argued that less gas must be diverted for heat duringwarm months. But this is offset somewhat by the fact that ourestimates for gas production are based on ideal conditions, and thatall labor costs and profits were ignored. It therefore appears to beuneconomical to construct a digester in our example. (Methanegeneration will be more attractive for large livestock productionunits, which have the potential tor considerable `economy of scale'saving.)

Table 7 was prepared to aid you in considering the effect ofchanging interest rates and fuel prices. This table presents themaximum initial investment that could be made if you expected to payfor the digester in 7 years.

The promoters of commercial anaerobic digesters sometimes add onother economic returns in order to balance larger investments. Oneexample is to give the digested sludge a value as either a feed or afertilizer. The feed value of sludge is reported to be as high as$100 per ton of dried digester solids (1980 price). Some proponentsclaim that the feed value is as much as ten times greater than thevalue of the gas produced. At present, however, few Indiana farmersare willing or able to feed the digester effluent to their livestock.

Another technique is to credit the digester with the fertilizervalue of the livestock waste. This fertilizer value is about 1 centper gallon of digester effluent (1980 price), which works out to 1.5times the fuel value of the methane. It should be noted, however, thatthe fertilizer value will be present even if the waste is not passedthrough a digester, making this credit rather questionable.

In addition, all digesters require some management and labor tocontrol the process. Successful operation for a typical on-farmdigester will require a minimum of 1-2 hours per day for monitoring,loading, unloading and performing general maintenance-some systemseven more!

ADDITIONAL INFORMATION ON METHANE GENERATION

Several private organizations and individuals have written about theprinciple and practice of methane generation. The following listshould be useful to those desiring more information.

1. Ram Bux Singh, "Bio-Gas Plant-Generating Methane from Organic Wastes." 70 pages. Available for $6.00 from the Gobar Gas Research Station, Ajitmal (Etawah) Upper Pradesh, India.

2. Les Auerbach, William Olkowski and Ben Katz, "Manual on Home Methane Generation." Available for $5.00 from Les Auerbach, 242 Copse Road, Madison, Connecticut 06442.

3. "Livestock Waste Management with Pollution Control." MWPS-19. North Central Regional Research Publication 222. June, 1975. Write Midwest Plan Service, Agricultural Engineering Department, Iowa State University, Ames, IA 50011, regarding availability and cost.

4. R. Douglas Kreis. "Recovery of By-Products from Animal Waste." A Literature Review, EPA-600/2-79-142, August 1979, National Technical Information Service, Springfield, VA 22161.

Acknowledgment

The authors wish to express their appreciation to David H. Bache and George F. Patrick of the Agricultural Economics Department Purdue University tor their review and suggestions in the economic aspects of this publication.

New 9/80

Cooperative Extension work in Agriculture and Home Economics, state of Indiana, Purdue University, and U.S. Department of Agriculture cooperating; HA. Wadsworth, Director, West Lafayette. IN. Issued in furtherance of the acts of May 8 and June 30, 1914. Purdue University Cooperative Extension Service is anequal opportunity/equal access institution.