Sequencing batch reactor: Influence of periodic operation on performance of activated sludges in biological wastewater treatment

This paper presents the main results obtained during several years of studi es with periodically fed biological reactors (sequencing batch reactors, SB Rs) from the perspective of wastewater treatment. Substrate (acetate) remov al mechanisms under transient conditions were studied both in the SBR and i n 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 a noxic conditions). The most general evidence was the relevant role of stora ge (usually representing about 70% of the overall observed yield) and the n egligible 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 co uld become more important. The applied operating conditions affected the bi omass 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 ba tch tests decreased, whereas with an increase in the feed length (other con ditions being the same), the relevance of the storage response decreased. T he role of dynamic conditions in selecting a floc-forming or filamentous bi omass was also investigated. Even though a floc-forming biomass usually dev eloped, filament growth was also sometimes observed. Thus, the usual assump tion 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.