ABSTRACT
If the characteristics of the trickling filter effluent, i.e., Sse and X8 ,H,TFE, could be defined, the relationships presented in Section 5.2.3 could be used to size the suspended growth bioreactor of a coupled TF/AS system. Unfortunately this cannot be done easily. First, we saw in Section 19.3.1 that trickling filter design procedures focus on the removal of substrate rather than on the growth of biomass. Thus, while Sse may be well defined, X8 ,H,TFE is not. If an attempt were made to estimate biomass growth by using the true growth yield alone and neglecting cell decay, X8 ,H,TFE would be overestimated, thereby causing the substrate removal capability of the suspended growth bioreactor to be overestimated. Second, the suspended solids in the effluent from the trickling filter consist of heterotrophic biomass, cell debris, inert influent suspended solids, and unmetabolized substrate, with their relative quantities depending on the characteristics of the influent wastewater, as well as on the TOL and THL of the trickling filter. Precise prediction of the concentrations of these constituents, which are required for use of the equations in Section 5.2.3, is not currently possible. Third, both aerobic and anaerobic metabolism can occur within the biofilm of a trickling filter. Although the outer portion of the biofilm is aerobic, the inner portion may be anaerobic. Moreover, the relative importance of aerobic and anaerobic metabolism will vary depending on the nature and concentration of the biodegradable organic matter in the influent wastewater, the availability of oxygen, and hydraulic conditions affecting biofilm thickness. Since yields are quite different under aerobic and anaerobic conditions, biomass production can vary significantly. This further complicates prediction of the concentrations of various types of suspended solids in the trickling filter effluent. Finally, biomass can accumulate within a trickling filter and be sloughed periodically, as discussed in Section 19.4. This results in timevariant concentrations of biomass and other particulate constituents in the trickling filter effluent. In many instances, sloughing cycles occur over the course of several days, or even several weeks, a time interval that can significantly exceed the suspended growth bioreactor SRT. In fact, significant variations in suspended growth bioreactor MLSS concentrations have been observed as a result of trickling filter sloughing cycles. 10'22 These variations can affect the performance of the suspended growth bioreactor in a significant manner. Consequently, the suspended growth bioreactor is typically sized by using the net process yield approach as illustrated in Examples 19.3.6.1 and 19.3.6.2.
