Posts Tagged ‘anaerobic digestion’

Methane (Biogas) from Anaerobic Digesters

November 9, 2010

Methane (Biogas) from Anaerobic Digesters

Methane, or biogas, can be produced from the digestion of organic material by anaerobic bacteria. This gas can be used for a variety of energy needs.

Methane is a gas that contains molecules of methane with one atom of carbon and four atoms of hydrogen (CH4). It is the major component of the “natural” gas used in many homes for cooking and heating. It is odorless, colorless, and yields about 1,000 British Thermal Units (Btu) [252 kilocalories (kcal)] of heat energy per cubic foot (0.028 cubic meters) when burned. Natural gas is a fossil fuel that was created eons ago by the anaerobic decomposition of organic materials. It is often found in association with oil and coal.

The same types of anaerobic bacteria that produced natural gas also produce methane today. Anaerobic bacteria are some of the oldest forms of life on earth. They evolved before the photosynthesis of green plants released large quantities of oxygen into the atmosphere. Anaerobic bacteria break down or “digest” organic material in the absence of oxygen and produce “biogas” as a waste product. (Aerobic decomposition, or composting, requires large amounts of oxygen and produces heat.) Anaerobic decomposition occurs naturally in swamps, water-logged soils and rice fields, deep bodies of water, and in the digestive systems of termites and large animals. Anaerobic processes can be managed in a “digester” (an airtight tank) or a covered lagoon (a pond used to store manure) for waste treatment. The primary benefits of anaerobic digestion are nutrient recycling, waste treatment, and odor control. Except in very large systems, biogas production is a highly useful but secondary benefit.

Biogas produced in anaerobic digesters consists of methane (50%-80%), carbon dioxide (20%-50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide. The relative percentage of these gases in biogas depends on the feed material and management of the process. When burned, a cubic foot (0.028 cubic meters) of biogas yields about 10 Btu (2.52 kcal) of heat energy per percentage of methane composition. For example, biogas composed of 65% methane yields 650 Btu per cubic foot (5,857 kcal/cubic meter).

Digester Designs

Anaerobic digesters are made out of concrete, steel, brick, or plastic. They are shaped like silos, troughs, basins or ponds, and may be placed underground or on the surface. All designs incorporate the same basic components: a pre-mixing area or tank, a digester vessel(s), a system for using the biogas, and a system for distributing or spreading the effluent (the remaining digested material).

There are two basic types of digesters: batch and continuous. Batch-type digesters are the simplest to build. Their operation consists of loading the digester with organic materials and allowing it to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated.

In a continuous digester, organic material is constantly or regularly fed into the digester. The material moves through the digester either mechanically or by the force of the new feed pushing out digested material. Unlike batch-type digesters, continuous digesters produce biogas without the interruption of loading material and unloading effluent. They may be better suited for large-scale operations. There are three types of continuous digesters: vertical tank systems, horizontal tank or plug-flow systems, and multiple tank systems. Proper design, operation, and maintenance of continuous digesters produce a steady and predictable supply of usable biogas.

Many livestock operations store the manure they produce in waste lagoons, or ponds. A growing number of these operations are placing floating covers on their lagoons to capture the biogas. They use it to run an engine/generator to produce electricity.

The Digestion Process

Anaerobic decomposition is a complex process. It occurs in three basic stages as the result of the activity of a variety of microorganisms. Initially, a group of microorganisms converts organic material to a form that a second group of organisms utilizes to form organic acids. Methane-producing (methanogenic) anaerobic bacteria utilize these acids and complete the decomposition process.

A variety of factors affect the rate of digestion and biogas production. The most important is temperature. Anaerobic bacteria communities can endure temperatures ranging from below freezing to above 135° Fahrenheit (F) (57.2° Centigrade [C]), but they thrive best at temperatures of about 98°F (36.7°C) (mesophilic) and 130°F (54.4°C) (thermophilic). Bacteria activity, and thus biogas production, falls off significantly between about 103° and 125°F (39.4° and 51.7°C) and gradually from 95° to 32°F (35° to 0°C).

In the thermophilic range, decomposition and biogas production occur more rapidly than in the mesophilic range. However, the process is highly sensitive to disturbances such as changes in feed materials or temperature. While all anaerobic digesters reduce the viability of weed seeds and disease-producing (pathogenic) organisms, the higher temperatures of thermophilic digestion result in more complete destruction. Although digesters operated in the mesophilic range must be larger (to accommodate a longer period of decomposition within the tank [residence time]), the process is less sensitive to upset or change in operating regimen.

To optimize the digestion process, the digester must be kept at a consistent temperature, as rapid changes will upset bacterial activity. In most areas of the United States, digestion vessels require some level of insulation and/or heating. Some installations circulate the coolant from their biogas-powered engines in or around the digester to keep it warm, while others burn part of the biogas to heat the digester. In a properly designed system, heating generally results in an increase in biogas production during colder periods. The trade-offs in maintaining optimum digester temperatures to maximize gas production while minimizing expenses are somewhat complex. Studies on digesters in the north-central areas of the country indicate that maximum net biogas production can occur in digesters maintained at temperatures as low as 72°F (22.2°C).

Other factors affect the rate and amount of biogas output. These include pH, water/solids ratio, carbon/nitrogen ratio, mixing of the digesting material, the particle size of the material being digested, and retention time. Pre-sizing and mixing of the feed material for a uniform consistency allows the bacteria to work more quickly. The pH is self-regulating in most cases. Bicarbonate of soda can be added to maintain a consistent pH, for example when too much “green” or material high in nitrogen content is added. It may be necessary to add water to the feed material if it is too dry, or if the nitrogen content is very high. A carbon/nitrogen ratio of 20/1 to 30/1 is best. Occasional mixing or agitation of the digesting material can aid the digestion process. Antibiotics in livestock feed have been known to kill the anaerobic bacteria in digesters. Complete digestion, and retention times, depend on all of the above factors.

Producing and Using Biogas

As long as proper conditions are present, anaerobic bacteria will continuously produce biogas. Minor fluctuations may occur that reflect the loading routine. Biogas can be used for heating, cooking, and to operate an internal combustion engine for mechanical and electric power. For engine applications, it may be advisable to scrub out hydrogen sulfide (a highly corrosive and toxic gas). Very large-scale systems/producers may be able to sell the gas to natural gas companies, but this may require scrubbing out the carbon dioxide.

Using the Effluent

The material drawn from the digester is called sludge, or effluent. It is rich in nutrients (ammonia, phosphorus, potassium, and more than a dozen trace elements) and is an excellent soil conditioner. It can also be used as a livestock feed additive when dried. Any toxic compounds (pesticides, etc.) that are in the digester feedstock material may become concentrated in the effluent. Therefore, it is important to test the effluent before using it on a large scale.

Economics

Anaerobic digester system costs vary widely. Systems can be put together using off-the-shelf materials. There are also a few companies that build system components. Sophisticated systems have been designed by professionals whose major focus is research, not low cost. Factors to consider when building a digester are cost, size, the local climate, and the availability and type of organic feedstock material.

In the United States, the availability of inexpensive fossil fuels has limited the use of digesters solely for biogas production. However, the waste treatment and odor reduction benefits of controlled anaerobic digestion are receiving increasing interest, especially for large-scale livestock operations such as dairies, feedlots, and slaughterhouses. Where costs are high for sewage, agricultural, or animal waste disposal, and the effluent has economic value, anaerobic digestion and biogas production can reduce overall operating costs. Biogas production for generating cost effective electricity requires manure from more than 150 large animals.

Below-ground, concrete anaerobic digesters have proven to be especially useful to agricultural communities in parts of the world such as China, where fossil fuels and electricity are expensive or unavailable. The primary purpose of these anaerobic digesters is waste (sewage) treatment and fertilizer production. Biogas production is secondary.

The most common means of collecting and storing the gas produced by a digester is with a floating cover—a weighted pontoon that floats on the liquid surface of a collection/storage basin.

Source: U.S. Department of Energy

How Anaerobic Digestion (Methane Recovery) Works

November 4, 2010

Methane and Anaerobic Bacteria

Biodigesters recover methane from animal manure through a process called anaerobic digestion. Here’s how it works.

Methane is a gas that contains molecules of methane with one atom of carbon and four atoms of hydrogen (CH4 ). It is the major component of the “natural” gas used in many homes for cooking and heating. It is odorless, colorless, and yields about 1,000 British Thermal Units (Btu) [252 kilocalories (kcal)] of heat energy per cubic foot (0.028 cubic meters) when burned. Natural gas is a fossil fuel that was created eons ago by the anaerobic decomposition of organic materials. It is often found in association with oil and coal.

The same types of anaerobic bacteria that produce natural gas also produce methane today. Anaerobic bacteria are some of the oldest forms of life on earth. They evolved before the photosynthesis of green plants released large quantities of oxygen into the atmosphere. Anaerobic bacteria break down or “digest” organic material in the absence of oxygen and produce “biogas” as a waste product. (Aerobic decomposition, or composting, requires large amounts of oxygen and produces heat.)

Anaerobic decomposition occurs naturally in swamps, water-logged soils and rice fields, deep bodies of water, and in the digestive systems of termites and large animals. Anaerobic processes can be managed in a “digester” (an airtight tank) or a covered lagoon (a pond used to store manure) for waste treatment. The primary benefits of anaerobic digestion are nutrient recycling, waste treatment, and odor control. Except in very large systems, biogas production is a highly useful but secondary benefit.

Biogas produced in anaerobic digesters consists of methane (50%–80%), carbon dioxide (20%–50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide. The relative percentage of these gases in biogas depends on the feed material and management of the process. When burned, a cubic foot (0.028 cubic meters) of biogas yields about 10 Btu (2.52 kcal) of heat energy per percentage of methane composition. For example, biogas composed of 65% methane yields 650 Btu per cubic foot (5,857 kcal/cubic meter).

Anaerobic Digestion

Anaerobic decomposition is a complex process. It occurs in three basic stages as the result of the activity of a variety of microorganisms. Initially, a group of microorganisms converts organic material to a form that a second group of organisms utilizes to form organic acids. Methane-producing (methanogenic) anaerobic bacteria utilize these acids and complete the decomposition process.

A variety of factors affect the rate of digestion and biogas production. The most important is temperature. Anaerobic bacteria communities can endure temperatures ranging from below freezing to above 135° Fahrenheit (F) (57.2° Centigrade [C]), but they thrive best at temperatures of about 98°F (36.7°C) (mesophilic) and 130°F (54.4°C) (thermophilic). Bacteria activity, and thus biogas production, falls off significantly between about 103° and 125°F (39.4° and 51.7°C) and gradually from 95° to 32°F (35° to 0°C).

In the thermophilic range, decomposition and biogas production occur more rapidly than in the mesophilic range. However, the process is highly sensitive to disturbances, such as changes in feed materials or temperature. While all anaerobic digesters reduce the viability of weed seeds and disease-producing (pathogenic) organisms, the higher temperatures of thermophilic digestion result in more complete destruction. Although digesters operated in the mesophilic range must be larger (to accommodate a longer period of decomposition within the tank [residence time]), the process is less sensitive to upset or change in operating regimen.

To optimize the digestion process, the biodigester must be kept at a consistent temperature, as rapid changes will upset bacterial activity. In most areas of the United States, digestion vessels require some level of insulation and/or heating. Some installations circulate the coolant from their biogas-powered engines in or around the digester to keep it warm, while others burn part of the biogas to heat the digester. In a properly designed system, heating generally results in an increase in biogas production during colder periods. The trade-offs in maintaining optimum digester temperatures to maximize gas production while minimizing expenses are somewhat complex. Studies on digesters in the north-central areas of the country indicate that maximum net biogas production can occur in digesters maintained at temperatures as low as 72°F (22.2°C).

Other factors affect the rate and amount of biogas output. These include pH, water/solids ratio, carbon/nitrogen ratio, mixing of the digesting material, the particle size of the material being digested, and retention time. Pre-sizing and mixing of the feed material for a uniform consistency allows the bacteria to work more quickly. The pH is self-regulating in most cases. Bicarbonate of soda can be added to maintain a consistent pH; for example, when too much “green” or material high in nitrogen content is added. It may be necessary to add water to the feed material if it is too dry or if the nitrogen content is very high. A carbon/nitrogen ratio of 20/1 to 30/1 is best. Occasional mixing or agitation of the digesting material can aid the digestion process. Antibiotics in livestock feed have been known to kill the anaerobic bacteria in digesters. Complete digestion, and retention times, depend on all of the above factors.

Sludge or Effluent

The material drawn from the anaerobic digester is called sludge, or effluent. It is rich in nutrients (ammonia, phosphorus, potassium, and more than a dozen trace elements) and is an excellent soil conditioner. It can also be used as a livestock feed additive when dried. Any toxic compounds (pesticides, etc.) that are in the digester feedstock material may become concentrated in the effluent. Therefore, it is important to test the effluent before using it on a large scale.

The most common means of collecting and storing the gas produced by a digester is with a floating cover—a weighted pontoon that floats on the liquid surface of a collection/storage basin.

Source: energysavers.gov

Basic Types Of Anaerobic Digesters

November 2, 2010

Anaerobic Digesters

Basic Types:

While many different types of biogas recovery systems are available, the three designs most commonly used at U.S. farms are described below.

Covered anaerobic lagoon

An anaerobic lagoon is sealed with a flexible cover, and the methane is recovered and piped to the combustion device. Some systems use a single cell for combined digestion and storage.

Plug flow digester

A plug flow digester has a long, narrow concrete tank with a rigid or flexible cover. The tank is built partially or fully below grade to limit the demand for supplemental heat. Plug flow digesters are used only at dairy operations that collect manure by scraping.

Complete mix digester

A complete mix digester is an enclosed, heated tank with a mechanical, hydraulic, or gas mixing system. Complete mix digesters work best when there is some dilution of the excreted manure with water (e.g., milking center wastewater). The photo on the left shows an externally mounted mixer.

Additional digester types:

Several other digester types have also been constructed in recent years, such as induced blanket reactors, fixed film digesters and batch digesters.

  • Induced Blanket Reactors are digesters in which a blanket of sludge develops that retains anaerobic bacteria, providing a bacteria-rich environment through which influent must pass.
  • Fixed film digesters contain plastic media (e.g., pellets) on which bacteria attach and grow, instead of relying solely on suspended bacteria to break down the digester influent.
  • A batch digester is the simplest form of digestion, where manure is added to the reactor at the beginning of the process in a batch and the reactor remains closed for the duration of the process.

The most common means of collecting and storing the gas produced by a digester is with a floating cover—a weighted pontoon that floats on the liquid surface of a collection/storage basin.

Source: epa.gov


Part 1 of Biogas and Anaerobic Digestion

October 21, 2010

Biogas is formed solely through the activity of bacteria, unlike composting in which fungi and lower creatures are also involved in the degradation process.  Microbial growth and biogas production are very slow at ambient temperatures.  They tend to occur naturally wherever high concentrations of wet organic matter accumulate in the absence of dissolved oxygen, most commonly in the bottom sediments of lakes and ponds, in swamps, peat bogs, intestines of animals, and in the anaerobic interiors of landfill sites.

The overall process of anaerobic digestion (AD) occurs through the symbiotic action of a complex bacteria consortium as show in diagram.  Hydrolytic microorganisms, including common food spoilage bacteria, break down complex organic wastes.  These subunits are then fermented into short-chain fatty acids, carbon dioxide, and hydrogen gases.

Syntrophic microorganisms then convert the complex mixture of short-chain fatty acids to acetic acid with the release of more carbon dioxide, and hydrogen gases.  Finally, methanogenesis produces biogas from the acetic acid, hydrogen and carbon dioxide.  Biogas is a mixture of methane, carbon dioxide, and numerous trace elements.  According to some, the two key biological issues are determining the most favorable conditions for each process stage and how non-optimal circumstances affect each stage as a whole, and the governing role of hydrogen generation and consumption.

Benefits and Challenges of Biogas Technology

October 14, 2010

Biogas Technology

Anaerobic digestion can convert organic wastes into profitable byproducts as well as reduce their environmental pollution potential. Anaerobic digestion offers the following benefits to an animal feeding operation and the surrounding communities:

  • Electric and thermal energy.
  • Stable liquid fertilizer and high-quality solids for soil amendment.
  • Odor reduction.
  • Reduced groundwater and surface water contamination potential.
  • Potential revenue from sales of digested manure (liquid and solids) and excess electricity and/or processing off-site organic waste.
  • Reduction of greenhouse gas emissions; methane is captured and used as a fuel.
  • Revenue from possible reuse of digested solids as livestock bedding.
  • Potential revenue from green energy and carbon credits.

The cost of installing an anaerobic digester depends on the type and size of system, type of livestock operation, and site-specific conditions (EPA AgStar, 2006).  In general, consider the following points when estimating installation/operating costs:

  • Estimate the cost of constructing the system.
  • Estimate the labor and cost of operating the system.
  • Estimate the quantity of gas produced.
  • Estimate the value of the gas produced.
  • Compare operation costs to benefits from operation (include value as a waste-treatment system and the fertilizer value of the sludge and supernatant).

The main financial obligations associated with building an anaerobic digester include capital (equipment and construction and associated site work), project development (technical, legal, and planning consultants; financing; utilities connection; and licensing), operation and maintenance, and training costs.

In making a decision to install a digester, one must realize that the system will require continuous monitoring and routine maintenance and repair that should not be underestimated.  Components should be maintained as recommended by the manufacturers because manure and biogas can be corrosive on metal parts.  In fact, the majority of digester failures over the past few decades were the result of management, not technological, problems.

Types Of Anaerobic Digesters Part 2

June 3, 2010

Anaerobic Digesters

The process of anaerobic digestion occurs in a sequence of stages involving distinct types of bacteria.  Hydrolytic and fermentative bacteria first break down the carbohydrates, proteins and fats present in biomass feedstock into fatty acids, alcohol, carbon dioxide, hydrogen, ammonia and sulfides.  This stage is called “hydrolysis” (or “liquefaction”).

Next, acetogenic (acid-forming) bacteria further digest the products of hydrolysis into acetic acid, hydrogen and carbon dioxide.  Methanogenic (methane-forming) bacteria then convert these products into biogas.

The combustion of digester gas can supply useful energy in the form of hot air, hot water or steam.  After filtering and drying, digester gas is suitable as fuel for an internal combustion engine, which, combined with a generator, can produce electricity.  Future applications of digester gas may include electric power production from gas turbines or fuel cells.  Digester gas can substitute for natural gas or propane in space heaters, refrigeration equipment, cooking stoves or other equipment.  Compressed digester gas can be used as an alternative transportation fuel.

Manure Digesters

Anaerobic digestion and power generation at the farm level began in the United States in the early 1970s. Several universities conducted basic digester research.  In 1978, Cornell University built an early plug-flow digester designed with a capacity to digest the manure from 60 cows.

In the 1980s, new federal tax credits spurred the construction of about 120 plug-flow digesters in the United States. However, many of these systems failed because of poor design or faulty construction.  Adverse publicity about system failures and operational problems meant that fewer anaerobic digesters were being built by the end of the decade.  High digester cost and declining farm land values reduced the digester industry to a small number of suppliers.

Wastewater

Municipal sewage contains organic biomass solids, and many wastewater treatment plants use anaerobic digestion to reduce the volume of these solids.  Anaerobic digestion stabilizes sewage sludge and destroys pathogens.  Sludge digestion produces biogas containing 60-percent to 70-percent methane, with an energy content of about 600 Btu per cubic foot.

Most wastewater treatment plants that use anaerobic digesters burn the gas for heat to maintain digester temperatures and to heat building space.  Unused gas is burned off as waste but could be used for fuel in an engine-generator or fuel cell to produce electric power.

Landfill Gas

The same anaerobic digestion process that produces biogas from animal manure and wastewater occurs naturally underground in landfills.  Most landfill gas results from the decomposition of cellulose contained in municipal and industrial solid waste.  Unlike animal manure digesters, which control the anaerobic digestion process, the digestion occurring in landfills is an uncontrolled process of biomass decay.

The efficiency of the process depends on the waste composition and moisture content of the landfill, cover material, temperature and other factors.  The biogas released from landfills, commonly called “landfill gas,” is typically 50-percent methane, 45-percent carbon dioxide and 5-percent other gases.  The energy content of landfill gas is 400 to 550 Btu per cubic foot.

Capturing landfill gas before it escapes to the atmosphere allows for conversion to useful energy.  A landfill must be at least 40 feet deep and have at least one million tons of waste in place for landfill gas collection and power production to be technically feasible.

A landfill gas-to-energy system consists of a series of wells drilled into the landfill.  A piping system connects the wells and collects the gas.  Dryers remove moisture from the gas, and filters remove impurities.  The gas typically fuels an engine-generator set or gas turbine to produce electricity.

The gas also can fuel a boiler to produce heat or steam. Further gas cleanup improves biogas to pipeline quality, the equivalent of natural gas.  Reforming the gas to hydrogen would make possible the production of electricity using fuel cell technology.

Types of Anaerobic Digesters Part 1

June 1, 2010

Types Of Digesters

There are three basic digester designs. All of them can trap methane and reduce fecal coliform bacteria, but they differ in cost, climate suitability and the concentration of manure solids they can digest.

A covered lagoon digester, as the name suggests, consists of a manure storage lagoon with a cover.  The cover traps gas produced during decomposition of the manure.  This type of digester is the least expensive of the three.

Covering a manure storage lagoon is a simple form of digester technology suitable for liquid manure with less than 3-percent solids.  For this type of digester, an impermeable floating cover of industrial fabric covers all or part of the lagoon.  A concrete footing along the edge of the lagoon holds the cover in place with an airtight seal.  Methane produced in the lagoon collects under the cover.  A suction pipe extracts the gas for use.  Covered lagoon digesters require large lagoon volumes and a warm climate.  Covered lagoons have low capital cost, but these systems are not suitable for locations in cooler climates or locations where a high water table exists.

A complete mix digester converts organic waste to biogas in a heated tank above or below ground.  A mechanical or gas mixer keeps the solids in suspension.  Complete mix digesters are expensive to construct and cost more than plug-flow digesters to operate and maintain.

Complete mix digesters are suitable for larger manure volumes having solids concentration of 3 percent to 10 percent.  The reactor is a circular steel or poured concrete container.  During the digestion process, the manure slurry is continuously mixed to keep the solids in suspension.  Biogas accumulates at the top of the digester.  The biogas can be used as fuel for an engine-generator to produce electricity or as boiler fuel to produce steam.  Using waste heat from the engine or boiler to warm the slurry in the digester reduces retention time to less than 20 days.

Plug-flow digesters are suitable for ruminant animal manure that has a solids concentration of 11 percent to 13 percent.  A typical design for a plug-flow system includes a manure collection system, a mixing pit and the digester itself.  In the mixing pit, the addition of water adjusts the proportion of solids in the manure slurry to the optimal consistency.  The digester is a long, rectangular container, usually built below-grade, with an airtight, expandable cover.

New material added to the tank at one end pushes older material to the opposite end.  Coarse solids in ruminant manure form a viscous material as they are digested, limiting solids separation in the digester tank. As a result, the material flows through the tank in a “plug.”  Average retention time (the time a manure “plug” remains in the digester) is 20 to 30 days.

Anaerobic digestion of the manure slurry releases biogas as the material flows through the digester.  A flexible, impermeable cover on the digester traps the gas.  Pipes beneath the cover carry the biogas from the digester to an engine-generator set.

A plug-flow digester requires minimal maintenance.  Waste heat from the engine-generator can be used to heat the digester.  Inside the digester, suspended heating pipes allow hot water to circulate.  The hot water heats the digester to keep the slurry at 25°C to 40°C (77°F to 104°F), a temperature range suitable for methane-producing bacteria.  The hot water can come from recovered waste heat from an engine generator fueled with digester gas or from burning digester gas directly in a boiler.

There are three basic digester designs.  All of them can trap methane and reduce fecal coliform bacteria, but they differ in cost, climate suitability and the concentration of manure solids they can digest.

A covered lagoon digester, as the name suggests, consists of a manure storage lagoon with a cover.  The cover traps gas produced during decomposition of the manure.  This type of digester is the least expensive of the three.

Covering a manure storage lagoon is a simple form of digester technology suitable for liquid manure with less than 3-percent solids.  For this type of digester, an impermeable floating cover of industrial fabric covers all or part of the lagoon.  A concrete footing along the edge of the lagoon holds the cover in place with an airtight seal.  Methane produced in the lagoon collects under the cover.  A suction pipe extracts the gas for use. Covered lagoon digesters require large lagoon volumes and a warm climate.  Covered lagoons have low capital cost, but these systems are not suitable for locations in cooler climates or locations where a high water table exists.

A complete mix digester converts organic waste to biogas in a heated tank above or below ground.  A mechanical or gas mixer keeps the solids in suspension. Complete mix digesters are expensive to construct and cost more than plug-flow digesters to operate and maintain.

Complete mix digesters are suitable for larger manure volumes having solids concentration of 3 percent to 10 percent. The reactor is a circular steel or poured concrete container. During the digestion process, the manure slurry is continuously mixed to keep the solids in suspension. Biogas accumulates at the top of the digester. The biogas can be used as fuel for an engine-generator to produce electricity or as boiler fuel to produce steam. Using waste heat from the engine or boiler to warm the slurry in the digester reduces retention time to less than 20 days.

Plug-flow digesters are suitable for ruminant animal manure that has a solids concentration of 11 percent to 13 percent. A typical design for a plug-flow system includes a manure collection system, a mixing pit and the digester itself. In the mixing pit, the addition of water adjusts the proportion of solids in the manure slurry to the optimal consistency. The digester is a long, rectangular container, usually built below-grade, with an airtight, expandable cover.

New material added to the tank at one end pushes older material to the opposite end. Coarse solids in ruminant manure form a viscous material as they are digested, limiting solids separation in the digester tank. As a result, the material flows through the tank in a “plug.” Average retention time (the time a manure “plug” remains in the digester) is 20 to 30 days.

Anaerobic digestion of the manure slurry releases biogas as the material flows through the digester. A flexible, impermeable cover on the digester traps the gas. Pipes beneath the cover carry the biogas from the digester to an engine-generator set.

A plug-flow digester requires minimal maintenance. Waste heat from the engine-generator can be used to heat the digester. Inside the digester, suspended heating pipes allow hot water to circulate. The hot water heats the digester to keep the slurry at 25°C to 40°C (77°F to 104°F), a temperature range suitable for methane-producing bacteria. The hot water can come from recovered waste heat from an engine generator fueled with digester gas or from burning digester gas directly in a boiler.

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The Process of Anaerobic Digestion

The process of anaerobic digestion occurs in a sequence of stages involving distinct types of bacteria. Hydrolytic and fermentative bacteria first break down the carbohydrates, proteins and fats present in biomass feedstock into fatty acids, alcohol, carbon dioxide, hydrogen, ammonia and sulfides. This stage is called “hydrolysis” (or “liquefaction”).

Next, acetogenic (acid-forming) bacteria further digest the products of hydrolysis into acetic acid, hydrogen and carbon dioxide. Methanogenic (methane-forming) bacteria then convert these products into biogas.

The combustion of digester gas can supply useful energy in the form of hot air, hot water or steam. After filtering and drying, digester gas is suitable as fuel for an internal combustion engine, which, combined with a generator, can produce electricity. Future applications of digester gas may include electric power production from gas turbines or fuel cells. Digester gas can substitute for natural gas or propane in space heaters, refrigeration equipment, cooking stoves or other equipment. Compressed digester gas can be used as an alternative transportation fuel.

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Manure Digesters

Anaerobic digestion and power generation at the farm level began in the United States in the early 1970s. Several universities conducted basic digester research. In 1978, Cornell University built an early plug-flow digester designed with a capacity to digest the manure from 60 cows.

In the 1980s, new federal tax credits spurred the construction of about 120 plug-flow digesters in the United States. However, many of these systems failed because of poor design or faulty construction. Adverse publicity about system failures and operational problems meant that fewer anaerobic digesters were being built by the end of the decade. High digester cost and declining farm land values reduced the digester industry to a small number of suppliers.

The Tillamook Digester Facility (MEAD Project) began operation in 2003. The facility is located on a site once occupled by a Navy blimp hanger on property owned by the Port of Tillamook Bay. The facility consists of two 400,000-gallon digester cells. The facility uses the biogas to run two Caterpillar engines, each coupled to a 200 kilowatt generator. The facility sells its electric output to the Tillamook PUD. Manure is brought to the facility by truck from participating dairy farms in the Tillamook area.

Anaerobic Digesters For Lagoons Part 2

May 27, 2010

Anaerobic Digester Lagoon With Methane Gas Recovery

Anaerobic lagoons are perhaps the most trouble free, low maintenance systems available for treatment of animal waste. This is particularly true in the southern U.S.where winter temperatures are mild, permitting anaerobic digestion the year around.  The effluent from the digester is a valuable source of nitrogen for plants that can be field applied for improved crop production.  Placing a cover over the lagoon for collecting biogas virtually eliminates odor from the lagoon.  The collected biogas, a byproduct of the digestion process, is typically 60 to 70 percent methane that can be utilized as a valuable energy resource.  Limited experience indicates that odor from field application of effluent from two cell covered lagoons is much reduced from what might be expected when applying untreated or uncovered lagoon effluent.  A properly designed, constructed and operated anaerobic digester is a low maintenance system that is very forgiving and not likely to create emergency situations that can be expected with many alternative waste management systems.  Adding methane recovery to the anaerobic digester increases maintenance, but even in the event of failure of the gas collection system, it will not interrupt the waste stream and digestion process.  It is well suited to the livestock industry.

Anaerobic Digesters For Lagoons Part 1

May 25, 2010

What Is An Anaerobic Lagoon?

An anaerobic lagoon is an earthen impoundment receiving manure from an animal feeding operation in which manure is stored and stabilized by bacterial activity operating without oxygen (compare with an aerobic structure). The statute specifically provides that an anaerobic lagoon does not include a confinement feeding operation structure such as an earthen manure storage basin; a basin connected to unroofed operations (feedlots) which collects and stores runoff produced by rain or a system which collects and treats off gases.

Covered lagoon digesters are the simplest AD system.  These systems typically consist of an anaerobic combined storage and treatment lagoon, an anaerobic lagoon cover, an evaporative pond for the digester effluent, and a gas treatment and/or energy conversion system.  Figure 1 shows a typical schematic for a floating covered anaerobic lagoon.

Covered lagoon digesters typically have a hydraulic retention time (HRT) of 40 to 60 days. The HRT is the amount of time a given volume of waste remains in the treatment lagoon.  A collection pipe leading from the digester carries the biogas to either a gas treatment system such as a combustion flare, or to an engine/generator or boiler that uses the biogas to produce electricity and heat.  Following treatment, the digester effluent is often transferred to an evaporative pond or to a storage lagoon prior to land application.

Climate affects the feasibility of using covered lagoon digesters to generate electricity.  Engine/generator systems typically do not produce sufficient waste heat to maintain temperatures high enough in covered lagoon digesters in the winter to sustain consistently high biogas production rates.  Using propane or natural gas to provide additional heat for the lagoon contents is typically not an economically viable option.  Without that additional heat, most covered lagoon digesters produce less biogas in colder temperatures, and little or no gas below 39 FACE= “Symbol”>° F.  As a result, covered lagoon digesters are most appropriate for use in warm climates if the biogas is to be used for energy or heating purposes.

Complete mix digester systems consist of a mix tank, a complete mix digester and a secondary storage or evaporative pond.  The mix tank is either an aboveground tank or concrete in-ground tank that is fed regularly from underfloor waste storage below the animal feedlot.  Waste is stirred in the mix tank to prevent solids from settling in the waste prior to being fed to the digester.  The complete mix digester is essentially a constant-volume aboveground tank or in-ground covered lagoon that is fed daily from the mix tank.  Complete mix digesters with in-ground lagoons often employ covers similar to those used in covered lagoon digesters. In the digester, a mix pump circulates waste material slowly around the heater to maintain a uniform temperature.  Hot water from an engine/generator cogeneration water jacket or boiler is used to heat the digester.  A cylindrical aboveground tank, such as that shown in Figure 2, optimizes biogas production, but is more capital intensive than in-ground tanks.
Source: EPA. Manual for Developing Biogas Systems at Commercial Farms in the United States

Biogas Production

May 18, 2010

Biogas Production

Biogas production using anaerobic (oxygen free) digestion is a biological treatment process to reduce odor, produce energy and improve the storage and handling characteristics of manure.  A biogas production system must be specially designed and requires regular attention by someone familiar with the needs and operation of the digester.  Associated manure handling equipment and gas utilization components are also required.  The digester does not remove significant nutrients and requires environmentally responsible manure storage and handling system.

A well designed and operated digester will require modest daily attention and maintenance.  The care and feeding of a digester is not unlike feeding a cow or a pig; it responds best to consistent feeding and the appropriate environmental (temperature and anaerobic- oxygen free) conditions.  The earlier a problem in operation is identified the easier it is to fix and still maintain productivity.