The kinetics of endogenous decay, death and lysis for viable organic solids.

The long-term kinetic rates of solubilization/utilization of proteins, carbohydrates, and lipids in secondary solids from three municipal wastewater treatment plants were measured. Temperature, input sludge retention time (SRT₀), and terminal electron acceptor were used as control variables for two four-month digestion studies employing seven mixed batch reactors. Three types of adenosine triphosphate (ATP) measurements were used to differentiate between viable endogenous biomass, viable anaerobic/facultative biomass and intact non-viable solids. Solids solubilization was significantly affected by all three control variables. Temperature and terminal electron acceptor showed greater effects on solubilization than SRT₀. The effect of temperature was time-dependent and differed for the two types of secondary solids evaluated. Proteins were removed to the greatest extent, lipids to an extent comparable to that of volatile suspended solids (VSS), and carbohydrates were the most resistant component. Direct sample ATP or VSS measurements were poor indicators of the time-dependent potential of mixed microbial population for further waste stabilization. However, the analysis of the ratios of recovery ATP to sample ATP (ATP recovery ratio) provided more insights into the microbial behavior during sludge digestion. Under aerobic conditions, ATP recovery ratios generally increased with time. This result implied that during the initial phase of digestion the volatile solids consisted of a higher fraction of intact non-viable material, but after prolonged digestion the solids included a greater fraction of viable endogenous biomass. Under anaerobic conditions, ATP recovery ratios increased and then decreased with time, implying that the fermentative microbial fraction was highest at some intermediate point in the time frame employed. Four phase degradation patterns were observed in all cases, indicating an inability for any single equation to effectively model the long-term solubilization process