Like an anaerobic lagoon, the anaerobic digester is used to break down complex organic materials by a microbial population in the absence of DO. Anaerobic digesters can be designed and managed to optimize the bacterial decomposition of organic matter under more controlled conditions than those of anaerobic lagoons. Completely stirred tank reactors (CSTR) and plugflow (PF) reactors are commercially available. Covers floated on the surface of the manure are used to collect methane as well as to minimize odors. Approximately 60-80% of biogas is methane.
Temperature is crucial to the production of methane. As in other anaerobic processes, two optimal temperature peaks are 32-40°C (mesophilic) and 57-68°C (thermophilic). The mesophilic and thermophilic digesters are operated with retention times of 12-20
days and 6-12 days, respectively. Since higher temperatures can shorten the retention time, the digesters can be heated.
A complete anaerobic PF digestion system used on livestock manure is shown in Figure 8. It is suitable for animal manures that contain a solids concentration of 11-13%. The raw manure slurry enters one end of a rectangular tank and decomposes as it moves through the tank. New material added to the tank pushes older material to the opposite end. Coarse solids in manure form a viscous material as they are digested, limiting solids separation in the digester tank [1,4-6].
A PF digester requires normal maintenance. When digesters are operated at high temperatures (>120°F), they provide better treatment and biogas production. However, there
have also been some successful applications in 60-75°F, with lower treatment efficiencies offset by higher retention times. The typical hydraulic retention time (HRT) and the removal efficiencies for COD and TSS are 18-20 days, 35-70%, and 20-45%, respectively. A 99% removal for pathogen can be achieved.
Inside the digester, suspended heating pipes allow hot water to circulate and keep the slurry at 77-104°F, a temperature range suitable for methane-producing bacteria. The hot water can come from recovered waste heat from an engine generator fueled with digester gas or from burning digester gas directly in a boiler.
The CSTR digesters can handle manures with TSS of 3-10%, and in large volumes. The reactor is a large, vertical, poured concrete or steel circular container. Manure is collected in a mixing pit by either a gravity-flow or pump system and deliberately mixed within the digester reactor. The mixing process creates a homogeneous substrate that prevents the formation of a surface crust and keeps solids in suspension. Mixing and heating improve digester efficiency. A CSTR digester shown in Figure 9 can be operated at either mesophilic or thermophilic temperature ranges with an HRT of 12-20 days. A VSLR of CSTR is 1.3 g L-1 day -1 . The removal efficiencies for COD and TSS are 35-70, and 25-50%, respectively. A 99% reduction for pathogen can be achieved. However, CSTR digesters are more expensive to construct, operate, and maintain than PF digesters.
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