Microbial Selection During Granulation of Activated Sludge Under Wash-Out Dynamics

BiomassBiomass wash-outWash-out conditions stimulate granulationGranulation phenomenaPhenomena. However, deteriorationDeterioration in biomassBiomass settlingSettling and in nutrient removalNutrient removal has frequently been reported during the start-upStart-up of aerobic granular sludgeAerobic granular sludge bioreactorsBioreactor. The effectEffect of wash-outWash-out was investigated on bacterial selectionSelection, biomassBiomass settlingSettling, and metabolic activitiesActivity during the formation of early-stage granulesGranule from activated sludgeActivated sludge. SludgeSludge was collected from two different wastewater treatment plantsWastewater treatment plant designed for organic matter removalOrganic matter removal only (OMR-WWTPWWTP) and for full biological nutrient removalBiological nutrient removal (BNRBNR-WWTPWWTP). Five bubble-column sequencing batch reactorsSequencing batch reactor were operated with two different feast-famineFeast-famine regimes in either rapid pulse or slow anaerobic feedingAnaerobic feeding followed by aerobicAerobic starvationStarvation. Slow-settlingSettling fluffy granulesFluffy granule formed when an insufficient superficial air velocitySuperficial air velocity (SAV; 1.8 cm s−1) was applied, when the inoculationInoculation sludgeSludge was taken from the OMR-WWTPWWTP, or when reactorsReactor were operated at 30 °C. Fast-settlingSettling denseDense granulesGranule were obtained with 4.0 cm s−1 SAV, or when the inoculationInoculation sludgeSludge was taken from the BNRBNR-WWTPWWTP, or under anaerobic feedingAnaerobic feeding at 20 °C. However, among all C-N-P nutrientsNutrient supplied, only acetateAcetate was removed during start-upStart-up, and mainly during the aerationAeration phasePhase. Distinct phylotypesPhylotype dominated fluffy bioaggregatesBioaggregate (filamentous Burkholderiales) and denseDense granulesGranule (ZoogloeaZoogloea). ZoogloeaZoogloea dominated denseDense granulesGranule under both feedingFeeding regimes. Under anaerobic feedingAnaerobic feeding, the insufficient solidSolids retention time and leakage of acetateAcetate into aerationAeration under intensive biomassBiomass wash-outWash-out selected for ZoogloeaZoogloea spp. and resulted in the deteriorationDeterioration of BNRBNR. PolyphosphatePolyphosphate-accumulating organisms were outcompeted. Operational conditionsOperational conditions with a proper anaerobic selectorAnaerobic selector should be elucidated to maintain a balanceBalance between organisms with granulationGranulation propensity and with BNRBNR activityActivity to produce efficient granulesGranule in short start-upsStart-up.