Modeling of energy spilling in substrate-sufficient cultures

A concept of an energy spilling coefficient between anabolism and catabolism was developed. Based on this concept, two models were proposed to describe quantitatively the effect of a substrate on the coefficient for substrate-sufficient continuous and batch cultures. The models have been well verified using literature data obtained from the continuous cultures of Bacillus strains and the writers' own data obtained from both anoxic and aerobic batch cultures of activated sludge microorganisms. It has also been demonstrated that in both types of cultures, with an increasing residual substrate concentration or increasing initial food-to-microorganisms ratio, a decline of observed growth yield (Y-obs) was mainly due to energy spilling. The effect of the substrate on the energy spilling coefficient in the substrate-sufficient cultures can be expressed by a Monod type curve. In the continuous cultures, the maximum energy spilling coefficient was found to be 0.7 when the residual methanol concentrations were greater than 10 mmol/L. In the batch cultures, the efficiency of anoxic sludge reached 0.8 as the initial food-to-microorganisms ratio exceeded 10 mg chemical oxygen demand/mg mixed-liquor suspended solids, while that of aerobic sludge was only 60% with the same ratio. These findings reveal that anoxic activated sludge has a higher potential of energy spilling than does aerobic sludge.