BNR Process using RAS Anaerobic Zone

The increasingly stringent limits imposed on wastewater treatment plant discharges demand reductions in Total Nitrogen and Total Phosphorus and improvements in process stability.Although enhanced biological phosphorus removal (EBPR) process is considered a potentially efficient process with economic and environmental advantages compared to traditional chemical phosphorus removal alone, the benefits are often offset, in practice, by the needs to have standby chemical systems to achieve reliable and consistent performance.Total Nitrogen removal using Anoxic/Aerobic sequencing without chemical addition has been successfully achieving effluent Total Nitrogen values of < 6 mg/L. However, when combined in an EBPR process, a competition sets up for utilization of the available carbon between the biological nitrogen and phosphorus removing processes. The use of external sources of the carbon has proven effective in stimulating de-nitrification, VFA production and Biological Phosphorous removal, however adds significantly to the treatment costs.Side-stream RAS Anaerobic zones offers an innovative alternative to address the carbon competition, reduce the need for external carbon addition and minimize chemical usage for back-up P removal. Normally, a RAS only Anaerobic Zones requires up to 24 hour retention time, however when augmenting VFA and fermenting bacteria to this zone through the use of Primary Sludge Fermenters, the required retention time can be reduced to less than 2 hours and stabilize the Biological Phosphorus removal process. The use of a side stream Anaerobic Zone thus allows for all Primary Effluent to be directed to the denitrification stage of the process and enhance the overall Total Nitrogen removal.