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Anaerobic Digester Types and Designs

Factors to consider when designing an anaerobic digestion system include cost, size, local climate, and the availability and type of organic feedstock material.

Anaerobic digesters—also known as biodigesters—are made out of concrete, steel, brick, or plastic. They are shaped like silos, troughs, basins or ponds, and may be placed underground or on the surface. All anaerobic digestion system designs incorporate the same basic components:

• A pre-mixing area or tank
• A digester vessel(s)
• A system for using the biogas
• A system for distributing or spreading the effluent (the remaining digested material).

There are two basic types of digesters:

• Batch

  Batch-type digesters are the simplest to build. Their operation consists of loading the digester with organic materials and allowing it to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated.

• Continuous

  In a continuous digester, organic material is constantly or regularly fed into the digester. The material moves through the digester either mechanically or by the force of the new feed pushing out digested material. Unlike batch-type digesters, continuous digesters produce biogas without the interruption of loading material and unloading effluent. There are three types of continuous digesters: vertical tank systems, horizontal tank or plug-flow systems, and multiple tank systems.

  Proper design, operation, and maintenance of continuous digesters produce a steady and predictable supply of usable biogas. They may be better suited for large-scale operations.

Many livestock operations store the manure they produce in waste lagoons, or ponds. A growing number of these operations are placing floating covers on their lagoons to capture the biogas. They use it to run an engine/generator to produce electricity.

Anaerobic digester system costs vary widely. Systems can be put together using off-the-shelf materials. There are also a few companies that build system components. Some sophisticated systems have been designed by professionals whose major focus is research, not low cost.

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