Effect of anaerobic COD utilization on characteristics and treatment performance of aerobic granular sludge in anaerobic/anoxic/oxic SBRs

Influence of anaerobic COD consumption on the properties and nutrient removal performance of aerobic granules was studied. Three identical sequencing batch reactors (SBR) were operated under two operation strategies over four months. Results indicated that, the applied anaerobic contact provided better mass transfer during the anaerobic phase and improved the nutrient removal efficiency of the granules. Anaerobic contact increased anaerobic COD utilization from 17-24% to 45-53% and improved phosphorus removal from 63 to 93%. The anaerobic contact also decreased effluent solids from 87 to 46 mg/L VSS with preventing the growth of biomass in suspension associated with rapidly-growing organisms. With the application of anaerobic contact, slowly growing organisms dominated the system and solid retention time (SRT) increased from 15 to 32 days. Alerting anaerobic continuous feeding with anaerobic contact worsened settling properties of the granular biomass. However, good-settling granules were obtained after acclimation of the biomass to the new condition. The application of anaerobic contact also affected granules' size and extracellular polymeric substance (EPS) content. More compact granules with less net EPS content and lower protein-to-carbohydrate ratio were obtained under increased anaerobic contact condition compared with operating with anaerobic continuous feeding. Results suggested that appropriate anaerobic contact could enhance the nutrient removal efficiency and effluent quality of mature aerobic granules treating low-strength wastewater. Therefore, design considerations should be taken into account to provide sufficient anaerobic contact in granular sequencing batch reactors (SBRs) when applied for the treatment of low-strength wastewater.