Sequencing batch reactor: influence of periodic operation on performance of activated sludge in biological wastewater treatment

This paper presents the main results obtained during several years of studies with periodically fed biological reactors (sequencing batch reactors, SBRs) from the perspective of wastewater treatment. Substrate (acetate) removal mechanisms under transient conditions were studied both in the SBR and in batch tests, as were their dependence on the applied operating conditions in the SBR (organic load rate and/or sludge age, feed length, aerobic or anoxic conditions). The most general evidence was the relevant role of storage (usually representing about 70% of the overall observed yield) and the negligible role of growth (usually less than 10%) during acetate removal for all tested conditions. However, when acetate was present for a long enough time, physiological adaptation could occur, and the growth contribution could become more important. The applied operating conditions affected the biomass behavior. In particular, with an increase in the applied organic load rate, the observed yield in the SBR and the acetate removal rate in the batch tests decreased, whereas with an increase in the feed length (other conditions being the same), the relevance of the storage response decreased. The role of dynamic conditions in selecting a floc-forming or filamentous biomass was also investigated. Even though a floc-forming biomass usually developed, filament growth was also sometimes observed. Thus, the usual assumption that filaments are less able than floc-formers to store the substrate should not be considered as an absolute rule. Finally, an empirical kinetic model, including growth and storage both in parallel and in sequence, was defined and applied to describe and interpret the experimental results.