The type of digester chosen depends upon the type of waste available and the method of collecting that waste. There are essentially three types of methane digester designs:

• covered lagoon
• complete mix digesters
• plug flow digesters

A common method of waste collection in dairy and swine operations is gutter flushing. The addition of large volumes of water to flush the waste from concrete floors into a collection structure dilutes the waste (solids less than 3%). This flushing process is usually associated with an anaerobic lagoon from which flush water is recycled. The surface of the anaerbogic lagoon can be fitted with an impermeable cover to trap biogas for energy use. These digesters work best where the climate is warm year round. Cold winter temperatures in northern climates greatly restrict biological activity. This reduces biogas production for energy use. Odor control becomes the optimum benefit for employing anaerobic digestion in these regions.

Farm operations which collect wastes as slurries (3-8% solids) in pits or tanks can utilize complete mix digesters. Complete mix digesters are typically circular concrete or steel tanks of a constant volume which are mixed and heated to a constant temperature (35 degrees C). Because they are mixed and heated, the volume required for digestion is much smaller than covered lagoons, and they can be used in any climate.

Plug flow digesters are normally used where wastes are collected as solids (solids greater than 11%). Plug flow digesters are long tanks (often built into the ground) with an impermeable plastic cover. The contents are usually heated, but not mixed as the contents move through the digester as a combined mass or a "plug". Plug flow digesters have been used mostly with scraped dairy wastes, but a few have been applied to swine wastes.

Anaerobic digestion can be applied to any size of operation for waste treatment and odor control. If energy recovery (biogas) is the primary concern, then size of operation becomes important. For consistent energy recovery livestock facilities with a minimum of 300 dairy or beef animals, or 2000 swine should be considered. These animals should be in confinement where 100% of the waste can be collected on a regular basis.

To make another selection from the Components of a Biogas System, click here.

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