Excerpted from Biogas From Municipal Solid Waste, IEA Bioenergy, Energy Recovery from MSW Task, Anaerobic Digestion Activity, 1996.

A wide variety of engineered systems have been specifically developed for anaerobic digestion of different waste streams. The oldest and simplest form of anaerobic digestion is the uncovered, unmixed lagoon. To make practical use of the lagoon technology and shorten the digestion time requirements, methods were devised to improve the contact between the organic matter and the bacterial biomass by developing either plug flow or well mixed conditions. This "low loaded" type of system now widely used for agricultural and some industrial wastes also can include coverings for gas collection. Following are a number of other leading concepts for industrial wastewaters and municipal solid waste:

A Completely Mixed Stirred Tank (also called the CSTR or "contact" process) is similar to the conventional activated sludge aerobic process widely used in the treatment of municipal wastewater. It became the industry standard in the 1970s and represents an effort to improve the design of anaerobic lagoons. The CSTR reactor contains a mixer to maintain good contact between biomass and the organic material to be digested, and a postclarification step with biomass return to insure a steady quantity of mixed liquor suspended solids within the reactor. 

The anaerobic filter was commercialized in the late 1970s and, as the name implies, relies on a media substrate to retain the biomass within the reactor vessel. Different types of substrate materials have been utilized for this purpose, and different "flush out" methods have been developed to reduce the possibility of plugging.

Upflow Anaerobic Sludge Blanket (UASB) technology basically combines the well-mixed attributes of the contact system with an internal gas separation and clarification mechanism. The mixing within the reactor results from the gassing which occurs as the organic components are distributed within the biomass bed at the bottom. The reactor contains no mechanical components, but does have a topworks baffle arrangement which acts to separate the gas, liquid and solid phases.

Upflow Fluidized Bed processes reduce the loading rates and reactor size significantly. Earlier systems (late 1970s) attached biomass to heavier particulate matter (e.g. sand) so that the bacteria would not be swept out of the reactor by very strong hydraulic flowthrough rates. A later generation of "ultra high rate" reactors eliminate the need for carrier material, but still maintain a fluidized or expanded bed to facilitate biomass contact.

Dry Continuous Digestion involves a continuously fed digestion vessel with a digestate dry matter content of 20 to 40 percent. Both completely mixed and plug flow systems are available. Plug flow systems rely on external recycle of a proportion of the outgoing digestate to inoculate the incoming raw feedstock. In both cases, the requirement for only minimal water additions makes the overall heat balance very favorable for operation at thermophilic digestion temperatures (50-55 degrees C).

Dry Batch Digestion is closest to the accelerated landfill concept. While the containment vessel is being loaded with raw MSW, it is inoculated with digestate from another reactor. It is then sealed and left to digest naturally. During this closure period, leachate from the base of the vessel is recirculated to maintain a uniform moisture content and redistribute soluble substrates (volatile fatty acids) and methane bacteria throughout the mass of MSW within the vessel. When digestion is complete, the vessel is reopened, unloaded and refilled with a fresh charge of raw MSW feedstock. The main advantages are the simplicity of the containment vessel and the need for only minimal feed preparation and mechanical handling. Disadvantages are the batchwise nature of the operation and (possibly) increased posttreatment to provide a suitable refined digestate product. 

Leach-Bed Processes concept is generally similar to dry batch digestion, except that leachate from the base of the vessel is exchanged between established and new batches to facilitate start-up, inoculation and removal of volatile acids in the active reactor. After a while, when methanogenesis becomes established in the solid waste, the industrial digester is uncoupled and reconnected to fresh solid waste in a second vessel. This concept has also been described as "sequential batch anaerobic composting" (SEBAC).

Wet Continuous Digestion. MSW feedstock involves slurrying with a large proportion of water to provide a dilute (10 percent dry solids) feedstock that can be fed to a conventional completely mixed digester similar to those commonly used for sewage sludges or farm slurries. Effective removal of glass and stones is required in the feed preparation stages to prevent their rapid accumulation in the bottom of the main digestion tank. When used for MSW digestion alone, filter pressing of the wet digestate to recover liquor to recycle for feed preparation is required to avoid generating and excessive volume of diluted digestate for disposal. alternatively, this concept lends itself towards codigestion of MSW with more dilute feedstocks such as sewage sludges or animal manures. The concept of codigestion is especially well established in Denmark where there is a major program of cooperative biogas plants for animal manures.

Multistage Wet Digestion includes a range of proprietary multistage wet digestion processes where the MSW is slurried with water or recycled liquor and fermented by hydrolytic and fermentative bacteria to release volatile fatty acids which are then converted to biogas in a specialist high rate industrial anaerobic digester, usually an anaerobic filter or an UASB reactor.

[Resource Development Associates, 240 Ninth Street, NE, Washington, DC 20002 6110. (202) 546-6283; and Proceedings of Second Biomass Conference of the Americas, National Renewable Energy Laboratory, Golden, Colorado]