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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.

This article develops and utilizes an in situ technique to estimate the Monod half-maximum rate concentration, K(s) and the maximum specific utilization rate constant, k, for biofilms. The technique employs a curve-matching method with kinetic results from several short-term experiments with completely mixed biofilm reactors. Use of the in situ method eliminates the two drawbacks of using conventional suspended-growth measurements to characterize biofilm: possible alteration of cell physiology and a major investment to run the suspended-growth tests. Results with five cultures of biofilm-forming oligotrophs demonstrated the in situ technique and supported the hypothesis that K(s) values were lower for the biofilm oligotrophs than for typical copiotrophs.

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