A heated digester performs the same biological function as a lagoon in a much shorter time because of the higher temperature. This concentration can be achieved by underfloor scrapers or managed pull plug manure collection. A conservatively designed heated mixed digester is sized for a 20 day retention time. Per unit of volume, digester construction will cost more than lagoon construction. However, a heated digester requires a much smaller volume than a lagoon.
Costs vary between regions, soil types and digester types.
A lagoon system requires 20 times the volume as a heated digester to get the same treatment and that investment could be transferred into a digester construction. Substitution of a digester investment for lagoon treatment volume investment yields excess methane which is essentially a free energy source. Further investment in gas use equipment such as an engine generator should have a very rapid payback. Additional savings are realized by reduction of lagoon surface area, thereby reducing rainwater collection and land application costs. Also, the stored effluent is of uniform quality and will not be an odor source when land applied.