organic substrate in many wastewaters. Thus a suitable application of the model in Table 6.1 would be to see how the nature of the substrate influences the performance of a single CSTR, such as that depicted in Figure 5.1. To do this, two situations were considered, one in which all influent organic matter was soluble, and the other in which it was all particulate. The total concentration was the same in both cases, 500 mg/L as COD, as was the flow rate, 1000 m7day (1.0 m3 = 1000 L), giving a total COD mass input rate of 500 kg/day. The solids retention time (SRT)/hydraulic residence time (HRT) ratio was held constant at 20 while the SRT was varied. In other words, the reactor volume was increased in proportion to the increase in SRT. This was done to make it easier to visualize the fate of particulate material, as well as the relative importance of growth and decay as SRT is changed. When the SRT/ HRT ratio is held constant, if particulate material does not undergo reaction it has a constant concentration in the system, regardless of the SRT. Furthermore, as growth occurs on particulate substrate, the concentration of suspended matter (particulate substrate plus biomass) will decrease because the yield is less than one. In addition, growth associated with increased soluble substrate removal in the bioreactor is reflected by an increase in biomass concentration, whereas an increase in the importance of decay is reflected by a decrease. The biomass separator was assumed to be perfect so that it removed all undegraded particulate substrate from the effluent and returned it to the bioreactor. Thus, undegraded particulate substrate was removed only through the wastage flow. The parameter values used to describe the reactions are those given in Table 6.3, with the exception of |iA, the maximum specific growth rate for autotrophic bacteria, which was set equal to zero to eliminate them from

Table 6.5 Process Kinetics and Stoichiometry Adopted for Considering the Impact of Particulate Substrate on the Performance of an Aerobic CSTR. Lysis:Regrowth Model for Biomass Loss



Process rate, r,


Death and lysis Hydrolysis

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