This paper deals with the evaluation of the substrate selectivity of the activated sludge as heterogeneous bacterial population. The substrate selectivity was evaluated by the substrate selectivity index (SSI) that was defined by the following equation. SSI=c/(a+b). Here, a, b, and c: the utilization rates of substrate A, B, and A·B mixture, respectively. For example, SSI=100% means that there are no metabolic competitions between the substrates and the utilization rate of each substrate is not influenced by the presence of other substrates. SSI=50% means that there exists the complete enzymatic inhibition as is known glucose effect.
The amino acids of 13 species were used for energy sources, because the matabolic path ways of these compounds are well known and the activated sludge was acclimated to peptone. SSI=55% was measured for a mixture of alanine and serine, 75% for glutamate and aspartate. The SSIs of more than 90% were measured for a mixture of two each essential amino acids . It was observed that the longer the metabolic pass ways of the amino acids to the TCA cycle intermediates, the larger the SSI. The extent of the substrate selectivity of the activated sludge was interpreted by the length of the metabolic pass ways of substrates to the TCA cycle intermediates.