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