A critical evaluation of the characteristics of soluble microbial products (SMP) indicates that SMP is comprised of many different types and sizes of molecules and is biodegradable. A portion of SMP is formed at a rate proportional to the rate of substrate utilization (UAP), while the rest is formed at a rate proportional to the concentration of active biomass (BAP). These characteristics are incorporated into a mathematical model that includes the following components: substrate utilization and biomass growth according to Monod kinetics, SMP formation kinetics in proportion to substrate utilization rate and to biomass accumulation, and SMP degradation according to a semi-empirical multi-component degradation model. The SMP formation/degradation model successfully describes the fractional conversion of substrate into SMP, including the observed trends of increasing conversion at high and low sludge ages. In addition to activated sludge, the model is applied to anaerobic treatment and to biofilm processes.

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