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Completely mixed reactors can be constructed of a variety of materials. In the U.S. most completely mixed reactors have a low profile with a diameter greater than the height. Some municipal digesters in the U.S., and most in Europe have an egg shape with a height much greater than the diameter. The egg shape enhances mixing while eliminating much of the stratification. Completely mixed reactors can have fixed covers, floating covers, or gas holding covers. Most municipal digesters have floating covers. Floating covers are more expensive than fixed covers. Mixing can be accomplished with a variety of gas mixers, mechanical mixers, and draft tubes with mechanical mixers or simply recirculation pumps. The most efficient mixing device in terms of power consumed per gallon mixed is the mechanical mixer. Most municipal digesters are intensely mixed to reduce the natural stratification that occurs in a low profile tank. A large amount of evidence has been accumulated over the past 10 years indicating that intense mixing may inhibit the bacterial consortia. But, intense mixing is required to keep sands and silts in suspension. The advantage of the completely mixed reactor is that it is a proven technology that achieves reasonable conversion of solids to gas. It can be applied to the treatment of slurry waste such as dairy manure (Ecotope 1979). The disadvantage of the completely mixed reactor is the high cost of installation, and the energy cost associated with mixing the digester. The completely mixed conventional anaerobic digester is a biomass growth based system. The process requires a constant conversion of a portion of the feed solids to anaerobic bacteria rather than gas. Since anaerobic bacteria are constantly wasted from the process, new bacteria must be produced to replace the lost bacteria. If the bacteria are retained, the portion of the waste that would have been converted to new bacterial cells will be converted to gas. For this reason, bacterial growth based systems are not as efficient as retained biomass systems. The advantage of the completely mixed thermophilic reactor is the rapid conversion of solids to gas and biomass (Ratkowsky, Olley et al. 1981). Some claim that the rate of conversion is three times greater with thermophilic reactors. Consequently, the HRT can be lower and the gas production greater. The disadvantage of the thermophilic reactor is the energy required to heat cold dairy manure to thermophilic temperatures. Additional costs are incurred in tank insulation and heat exchangers. Sufficient heat may not be available from the gas produced unless the solids are highly concentrated. Thermophilic digestion of dairy manure cannot be used with manure diluted with parlor or flush water since sufficient energy will not be available to meet the heat requirements. In addition, the higher temperature thermophilic reactors increase end product inhibition, especially ammonia and organic acids (Ahring, Ibrahim et al. 2001). © Environmental Energy Company 29 www.makingenergy.com 360-923-2000PDF Image | Dairy Waste Anaerobic Digestion Handbook Options for Recovering Beneficial Products From Dairy Manure
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