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Covered Lagoon Digester  A covered lagoon digester consists of a manure treatment lagoon with an impermeable cover and is generally not heated.  The cover traps gas produced during decomposition of the manure. Covered lagoon digesters are used for liquid manure (less than 2 percent solids) and require large-volume lagoons.  Because the methane production rate is dependent on ambient temperatures with a covered lagoon system, it is not considered cost-effective to use the biogas for energy production in Minnesota’s climate.  It has been used in cold climates for odor control, however, including in Wisconsin.  This type of digester is the least expensive of the three.   

Complete Mix Digester A complete mix digester is suitable for manure that is 3 to 10 percent solids, such as swine manure or dairy manure collected by a flush system.  Complete mix digesters process manure in a heated tank above or below ground.  A mechanical or gas mixer keeps the solids in suspension. However, complete mix digesters are expensive to construct and cost more than a plug-flow digester to operate and maintain.

Plug-Flow Digester Plug-flow digesters are suitable for ruminant animal manure having a solids concentration of 11 to 14 percent, such as cow manure collected by scraping.  A flush system for manure collection is not appropriate for this system, since this would reduce the total solids content of the manure below specified levels.  In manure with lower solids concentrations, such as swine manure, solids cannot stay in solution and tend to settle to the bottom of the tank, limiting their digestion.  A plug-flow digester has few moving parts and requires minimal maintenance. 

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Other Digester Types 

Besides the three digester types discussed above, there are many other anaerobic digester designs that have been used for processing municipal sewage as well as industrial waste.6 Most of them treat waste streams with a low solids content, and thus have found various ways to speed up the digestion process or increase solids content in order to reduce the volume required for digesting, thereby reducing costs.  Without providing details of how they work, other digester designs include: 

  •  1.        Batch-fed reactor, such as the anaerobic sequential batch reactor (ASBR);
  • 2.        Temperature-phased anaerobic digester (TPAD);
  • 3.        Suspended particle reactor; 
  • 4.        Anaerobic filter reactor; 
  • 5.        Upflow solids reactor;
  • 6.        Continuously stirred tank reactor with solids recycle; 
  • 7.        Upflow anaerobic sludge blanket reactor; 
  • 8.        Anaerobic pump digester;
  • 9.        Fluidized- and expanded-bed reactors,
  • 10.     Fixed-film anaerobic digester.

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