Sludge settling and online NAD(P)H fluorescence profiles in wastewater treatment bioreactors operated at low dissolved oxygen concentrations

Biological nitrogen removal via simultaneous nitrification and denitrification (SND) may be achieved in the single-tank bioreactors operated at low dissolved oxygen concentrations (DO). The continuous-stirred tank reactor (CSTR) configuration and the low DO environments employed are; however, more conducive to growth of filamentous bacteria and, thus, poor sludge settling in clarifiers. In this work, a synthetic wastewater was treated in bench-scale (∼6 L) bioreactors under either cyclic or constant-rate aeration, at various sludge retention times (SRT) and DO. The objective was to evaluate the effects of these factors on the sludge settling indicated by sludge volume index. The cyclic aeration was carried out by alternating the aeration between a higher rate for 1 h and a lower (or zero) rate for 30 min. In different experiments, the DO during the period of higher aeration (HDO) was 0.4, 0.6, 0.8, or 2.0 mg/L and the DO during lower aeration (LDO) was 0.0 or 0.2 mg/L. The sludge established under the cyclic aeration was found to settle better than that under constant-rate aeration. Shortening SRT also improved the sludge settling significantly. NAD(P)H fluorescence profiles in these bioreactors were monitored using an online fluorometer. A procedure was developed to quantitatively describe the metabolic state of sludge's heterotrophic population on a 0–1 scale using the fluorescence profile, with “0” corresponding to the fully anoxic-denitrifying state and “1” to the fully aerobic state.