Gravimetric selection of activated sludge for settling properties improvement and granular sludge formation – full scale case study

With an increasing frequency of extreme weather events wastewater treatment plant operators face a need to increase plant hydraulic capacity while continuing to comply with effluent quality requirements. An economically attractive way to meet this challenge is to significantly improve secondary settling tank performance and therefore assure safe wet weather plant operation. The objective of this study was to evaluate gravimetric selection technology conducted in hydrocyclones as a way for improving activated sludge settling characteristics in continuous flow BNR with a long sludge retention time and assess its ability to form granular sludge. Long term operational data and sludge morphology monitoring data were analysed. A significant drop in sludge volume index (SVI) (to values under 50 ml/g) and increase in return activated sludge solids concentration (to values above 20 g/L) was observed. Hydrocyclone installation consistently, selectively retained denser, larger flocs in the system and after a start-up phase large granules begin to appear. Gravimetric selection did not prevent the seasonal filamentous biomass outgrowth and temperature drop at the beginning of winter resulted in rapid rise of SVI and SST sludge blanket height especially after heavy rain. Technology under study proved to be effective under certain process conditions but it needs further research to consistently maintain low SVI values throughout the whole year.