Posts Tagged ‘covered lagoons’

Part 2 of Biogas and Anaerobic Digestion

October 26, 2010

Biogas and Anaerobic Digestion | Covered Lagoon

A covered lagoon digester is a large anaerobic lagoon (not a manure storage pond or basin) with a long retention time and a high dilution factor.  Typically covered lagoons are used with flush manure management systems that discharge manure at 0.5 to 2 percent solids.  The in-ground, earth or lined lagoon is covered with a flexible or floating gas tight cover.  They are not heated and considered ambient temperature digesters.  Retention time is usually 30-45 days or longer depending on lagoon size.

In climates that have elevated year round temperatures, such as southern and western U.S., these digesters can produce stable, reduced odor, nutrient rich effluent for application on fields and crops; pathogen and weed seed reduction and; produce biogas for farm energy use.  Heat recovery from the biogas can be used to heat nurseries on swine farms and warm milking parlors on dairy farms.  Very large lagoons in hot climates may produce sufficient quantity, quality and consistency of gas to justify use in an engine generator.  In areas with cooler climates, waste digestion, odor control and gas production will be less consistent and the low quality gas may need to be flared off much of the year for odor control and greenhouse gas reduction.

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.