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.
|
 |
|
|
|
|
| Horizontal |
|
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.
|
|
|
|
|
