Many facilities use aerobic digesters to reduce volumes and improve sludge handling characteristics of biological solids produced in the wastewater treatment system. The aerobic digester works on the same biochemical principals as the general wastewater treatment system in that microbes convert influent organics into carbon dioxide, water and new cells. Of course there are many steps and intermediates produced, but the digesting process results in a reduction in mass - as the low F/M conditions produce fewer new cells and much of the energy is used for cell maintenance rather than new cell production.
Aerobic digesters with a 40 day retention time average 38 - 50% reduction in VSS compared to anaerobic digesters reducing VSS by 56 - 65% with a 30 day retention time. Just as important as solids reduction is improved dewatering/drying characteristics of the biosolids. Often organics such as grease and surfactants can create high levels of water and polysaccharides in the biosolids (floc). An efficient digester will act to continue the biological degradation of these compounds that was left incomplete in the main biological treatment unit.
With the high costs of removing water, handling, and disposing of biosolids, wastewater system engineers often look for ways to enhance solids destruction in the digester. Options include adding more aeration, mixers, and often "bioadditives". In bioadditives, I am lumping in both bioaugmentation cultures (i.e. bugs) and various enzyme or micronutrient products. The big question is ... can reduced sludge volume/mass justify the expense of adding these products?
While my company, Aster Bio, manufactures multiple biochemicals, I will say it all depends upon the makeup of the biosolids as to how well any product will help with the digester performance. In systems with high levels of entrained water or polysaccharides (biopolymers), the addition may very well give great results and easily justify the costs of addition. Other times, the improved dewatering properties may help reudce polymer usage or moisture in the solids which can justify the product cost. In all cases, we can screen the products using an aerated, temperature controlled lab test. This lab test should be done before committing to any full scale implementation.
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
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