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What Are the Different Types of Anaerobic Digesters? Several different types of anaerobic digesters are used throughout the world for municipal waste treatment, industrial waste treatment, food waste and agricultural residual treatment. This page will focus on those best suited for agricultural residual treatment.
Covered Lagoon/In-Ground Digester
Typically found on farms, these digesters are usually converted manure holding lagoons for flushed hog or dairy manure (<5% solids). The capital cost for installation of these units is small compared to other designs; most of the expense is in the lagoon liner and cover. These digesters are generally not mixed and are operated at ambient temperature; the warmer the temperature the better the performance. Compared to more advanced units, conversion of solids into methane gas in covered lagoons is low. These units work well to reduce odor, but biogas recovery may not be feasible for northern climates. North Carolina State has done research on a covered in-ground unit on Barham Farm in Raleigh NC. [Link to Research]
Plug-Flow Digesters
These digesters are built to handle high solids materials (10-15%), such as manure and bedding from scraped dairy operations. Plug-flow digesters are suitable for both on-farm and large-scale regional applications. They are generally operated at ambient, mesophilic or thermophilic temperatures. The digesters are usually constructed of concrete and topped with polyethylene covers to collect the biogas. The concrete tank is rectangular in shape and at or below ground level. Material fed into one end of the digester pushes an equivalent amount of effluent out of the other end. The capital cost for these units is more than the covered lagoons and less than the complete mix tank reactors, (CSTR's). The conversion of solids to methane is usually better than the covered lagoons and increases at higher temperatures.
CSTR Digesters
CSTR is an acronym for continuously stirred tank reactor. There are several types of CSTR digesters. The typical digester is a large steel or concrete tank that is mixed by impellers or forced gas. These digesters are operated at mesophilic or thermophilic temperatures and are the most efficient design to convert solids into methane. The capital investment and operating cost is much higher than the other two designs, therefore these digesters are only suitable for large scale or regional use. CSTR digesters are fed material ranging for 2-10% solids. The pilot digester on the campus of West Virginia State University is a CSTR design and operated at thermophilic temperatures.
The following page indicates other types of digesters suitable for treating food waste and waste materials in solution.
Anaerobic Filters.
These are digesters that contain a solid matrix such as random packing materials and are able to hold a high concentration of attached anaerobic bacteria on the surface of the plastic packing. In this way dissolve organics can be passed through the packing with high activity microbes present. This enables low hydraulic retention times, (less than 5 days), to be achieved and a high rate of BOD/COD removal from solution. This type is suitable for operation with sugar, brewery, distillery, food processing and some industrial wastewaters. The cost is overall about the same as with CSTR's as the packing material adds to the cost, but the footprint is low and the digester can be installed in a smaller area. They are normally operated within the mesophilic range, but ambient and thermophilic systems are also available.
Fluidized Bed Digesters.
The fluidized bed digesters build the anaerobic microbes around sand or carbon particles and have rapid flow through the particle bed with a high rate fluidization pump as well as a high rate feed pump. In this way contained soluble waste materials are rapidly absorbed by the bed and reacted to produce biogas from the waste. The hydraulic retention times are usually less than 1 day and are operated at mesophilic and thermophilic temperatures. They have a small footprint, but require careful control.
Upflow Anaerobic Sludge Blanket Digesters.
These systems are generally used for soluble wastes and have been used very widely for sugar processing and other similar effluents. The microbes produce anaerobic sludge flocs naturally over a specified time period and the bed expands as the microbes produce biogas from the incoming waste stream. When the flocs reach the top of the digester and come into contact with the gas collection tank roof the bubbles are dislodged and the flocs sink. They then pick up more dissolved organics and begin producing more biogas. These units are usually mesophilic and have good reliability for these types of wastes, where the retention time can be between 5-10 days.
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