It is interesting when academic research confirms something that you have observed in the field for many years. In this case, aerobic digesters stabilizing MLSS from municipal and industrial systems are modeled on the removal of VSS which contains adsorbed organics, cellular byproducts, and cells. The models maintain that the digester will remove the degradable portion of the VSS leaving an non-degradable VSS residual. The residual was based on looking at COD vs BOD20 which is also called ultimate BOD.
However, certain components of the VSS are more recalcitrant to biodegradation than others. Some parts of the VSS, or any water sample, may take longer than 20 days to be metabolized by microbes. We also see this when simple soluble organics such as sugars inhibit the degradation of fatty acids or grease. Like kids at Halloween, the bacteria go after higher energy yielding "candy" before using complex, lower energy yield compounds.
So when under starvation or very low F/M conditions, the degradation of "resistant" organics begins. This is not well modeled in conventional activated sludge equations. The paper below gives estimates that the degradation rates of “unbiodegradable” VSS between 0.006 to 0.029 d−1.
Friedrich, M. et al. "Experimental Assessmentof the Degradation of "Unbiodegradable" Organic Solids in Activated Sludge" Water Environmental Research Vol. 88, Number 3, 1 March 2016. Abstract
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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