Anaerobic Digester Types and Designs

Anaerobic Digesters

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.

Floating cover applications

  • any type of gas collection from water basin
  • keep rain & snowmelt water separate from wastewater under the cover

Floating cover advantages:

  • provide a true “floating” cover, keeping the cover on the water surface avoiding damage from wind due to an inflated cover without lateral floats
  • accommodate fluctuation in water level
  • can be installed without interruption in basin use
  • can be installed on tanks or lagoons
  • eliminates rainwater pooling problems
  • eliminates inflating the cover and gas ballooning
  • hatches provide access under the cover for equipment

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