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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

Diagram of a covered lagoon system.

Photo of a covered lagoon.

Photo: Environmental Credit Corp

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

Diagram of a plug flow digester.

Photo of a plug flow digester with a flexible cover.

Photos: AgSTAR

Photo of a plug flow digester with a rigid cover.

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

Photo of an externally mounted mixer.

Photo: Clear Horizons, LLC

Diagram of a complete mix digester.

Photo of two complete mix digester tanks.

Photo: AgSTAR

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.

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