Anaerobic Digesters

Anaerobic Digesters

Anaerobic digesters produce gas made up of mostly methane (CH4) and carbon dioxide (CO2) via biological processes. Organic wastes such as livestock manure and food processing wastes are broken down by bacteria in an aerobic (oxygen-deprived) environment to produce the resulting biogas, which usually comprises between 55 and 75 percent pure methane. (California Energy Commission, ca.gov)

The process of anaerobic digestion involves three steps: 1. the decomposition of plant or animal matter, 2. the conversion of decomposed matter to organic acids, and 3. the conversion of acids to methane gas. (ibid)

The temperature of the digester may either be in the mesophillic range (95 to 105 degrees Fahrenheit) or in the thermophillic range (135 to 145 degrees F). The optimum temperature for digestion is usually 100 degrees F. Digesters in the mesophyllic range are more common than those in the thermophillic the hotter process is more difficult to monitor. (ibid)

Types of Digesters

Plug Flow
A plug flow digester is a long, narrow (usually 5 times longer than is wide) insulated and heated tank made of reinforced concrete, steel, or fiberglass with a cover for capturing the biogas. Plug flow digesters are able to operate at a mesophilic or thermophilic temperature, and work well with a scrape manure management system that contains little bedding and no sand. This type of digester has a retention time of about 15 to 20 days.

While manure in a plug flow digester does not mix longitudinally on its trip through the digester, one may envision it flowing as a plug, moving towards the outlet whenever new manure is added, at which point it discharges over an outlet weir that is designed to maintain a gas-tight environment. Realistically, the manure does not remain as a plug. Rather, portions of the manure flow through the digester faster than others, and some settles or floats in the digester. (ibid)

Example:

Mixed Plug Flow

Example: Keewaydin Farm in Stowe, VT is an example of an IFES with mixed plug flow digester. 80 milking cows and 50 heifers feed this digester each day, producing 1500 gallons of manure daily. (Keewaydin interview June 2015)

Complete Mix
Complete mixed digesters are kept at a constant temperature, either in the mesophilic or the thermophilic range. The digester vessel is usually a round insulated tank, and can be above or below ground. As with plug flow digesters, construction materials may include reinforced concrete, steel, or fiberglass. To maintain a proper internal temperature, the digester may contain heating coils with circulating hot water, or an external heat exchanger that moves the digestate. Again, like with plug flow digesters, a gas tight cover (floating or fixed) sits atop the vessel to trap the biogas. Manure that contains 3-10% solids is the best material for a complete mixed digester, and the retention time will generally be between 10 and 20 days. (Penn State EXTENSION)

Example: Cooperstown Holstein Corporation Farm in Cooperstown, NY, which has been operating for 30 years. The digester vessel is an above-ground, insulated, glass-lined tank where the resulting biogas powers an engine generator. The heat recovery from the engine generator maintains the digester temperature in the mesophilic range and heats sanitary wash water used in dairy production. Animal bedding is produced from solids that are separated from the digested effluent. The remaining liquid effluent is stored in a slurry tank for land application at a later date on crop land. (ibid)

Fixed-film
Fixed-film digesters are characterized by an inert medium that provides a large surface area to enable microbial growth. Anaerobic microbes attach themselves to the medium, creating biofilm (a thin layer of anaerobic bacteria). The microbes continue to grow, eating wastewater materials as the water flows by. Because bacteria remain attached to the plastic media when effluent is discharged, mixed film digesters have smaller reactor vessels, shorter retention times (3-5 days), and must be loaded with a feedstock that will flow through the media continuously without clogging. (ibid)

Example: Farber Farm, NY. This fixed-film digester has a 4-day retention time and treats separated liquid manure from 100 cows. When farmers here first installed the digester, odor control was a crucial incentive for them. See this Case Study to learn more about this fixed film digester. (ibid)

Covered Lagoon
This type of digester is a large anaerobic lagoon (not a manure storage pond or basin) with a long retention time (30-45 days or longer) and a high dilution factor. They are usually used with flush manure management systems that discharge manure at 0.5 to 2% solids. Gas-tight covers lay atop in-ground, earth, or lined lagoons, and can be flexible or floating. Covered lagoons are considered ambient temperature digesters (they are not heated). In climates such as those in the southern and western United States, where higher averages temperatures are the norm, covered lagoon digesters may produce stable, nutrient-rich effluent with a reduced odor. Very large lagoons in hot climates may produce sufficient quantity, quality, and consistency of gas to justify the use of an engine generator. In areas with cooler climates, however, waste digestion, odor control and gas production, will be less reliable, and one may need to flare off the low-quality gas throughout most of the year for odor control and greenhouse gas reduction. (ibid)

Example: Royal Farms in Tulare, CA. This covered lagoon digester is a “farrow-to-finish” swine operation that has been operating for 35 years. The farm contains a water-flush manure management system with a three-cell lagoon. The covered lagoon has reduced farm odor and pathogens, and produced biogas together with a stabilized nutrient-rich effluent. The resulting biogas is used for electricity to offset power costs. Excess electricity is sold to the grid. Heat is also recovered from the engine generator and used to supplement the heat in the nursery barns. Some of the effluent from the third lagoon is recycled back into the barn flush water, while the remainder is used for irrigation on corn, wheat, and silage. (ibid)