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.
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