Anaerobic & anaerobic contact process as wastewater pretreatment

Anaerobic waste pretreatment extends back to the earliest Imhoff and septic tank waste treatment systems. While these early systems were passive with no mixing or biosolids recycling, they biochemically operate in the same manner as the latest in fluidized bed anaerobic reactors. Unlike digesters where methane production and solids stabilization is the primary goal, anaerobic pretreatment is concerned with hydrolysis of complex molecules converting the complex and insoluble organics into short chain volatile fatty acids (VFA). The conversion into VFA does reduce BOD/COD concentration via creation and maintenance of microbial cells. If the anaerobic pretreatment system produces methane - from the growth of methanogens on VFA - the BOD/COD reduction is much greater than a system with no methanogen activity. 

We see anaerobic pretreatment with high strength wastewater that would be difficult and cost prohibitive to treat in a standard aerobic system - maintaining D.O. would require a massive amount of energy. One common application of anaerobic pretreatment is in meat processing wastewater. Often these systems have an anaerobic basin with a grease cap or physical cover following DAF or other pretreatment system. The anaerobic basin microbes grow on soluble organics and via anaerobic biochemistry begin to reduce the carbon chain length making insoluble fatty acids more soluble. This is a much slower process under anaerobic conditions than in an aerobic system resulting in the "grease cap".  With a relatively long residence time, the anaerobic pretreatment removes a substantial portion of influent BOD and converts proteins into ammonia which is amenable to rapid treatment in the following activated sludge unit.

While meat processing is a common application of anaerobic pretreatment, it is also useful in many waste streams with long chain organic compounds that require solubilization to become BOD5. Examples include cellulose, fats, and large particulate material.

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