Improving the settling properties of activated sludge by gradually decreasing the settling time

The study aims at evaluating the impact of gradually decreasing the settling time on settling properties of activated sludge removing organic matter. For that, a lab-scale sequencing batch reactor (SBR), fed with soluble starch and acetate, was seeded with floccular biomass. As the selected strategy to washout slowly settling flocks and retain biomass with improved settling capacity, settling time (T_s) was decreased gradually from 45 to 30, 15, 5 and 1 min. Long-term monitoring showed that each time the T_s was decreased to the next lower level, there was biomass washout. Yet high amount of biomass (10 g MLVSS/L) with improved settling properties (sludge volume index ≤ 16 mL/g) and high chemical oxygen demand (COD) removal performance (97%) was maintained with a T_s as low as 5 min. Further decreasing T_s to 1 min did not impose any negative effect. Biomass maintenance, settling properties, and COD removal performance of the system were comparable with those of aerobic granular biomass. Yet, minimum settling velocities corresponding to 5 and 1 min settling times applied at the last two phases of operation (0.4 and 2.0 cm/min) were lower than those reported for granular biomass systems, hence the biomass remained floccular but the system had an exceptionally good settling capacity and COD removal efficiency.