ENHANCED BNR WITH MBR – A UNIQUE COMBINATION

Over the past decade the application of membrane bioreactor (MBR) technology has grown exponentially in terms of number of plants and capacity of wastewater treated. Most of these facilities are designed for nitrification/denitrification and some for phosphorous removal using chemical precipitation. More recently there has been a significant interest in enhanced biological nutrient removal (EBNR) using MBR technology to capitalize on the benefits of these two advanced technologies. Enhanced BNR and MBR is a unique combination since the enhanced BNR process provides excess phosphorous accumulation in the biomass and the MBR process provides excellent solids-liquid separation which ensure virtually no solids in the treated effluent.The Cauley Creek WRF (Georgia, U.S.A.) utilizing ZeeWeed® MBR technology has been operating since May 2002 using ferric chloride addition to achieve effluent TP less than 0.13 mg/L. As part of the planning process to double the capacity to 5 mgd, the Owner wanted to reduce operating costs by converting the plant to an enhanced BNR process with chemical trimming for TP removal. Detailed wastewater characterization and Bio Win modeling showed that a Johannesburg type configuration was the preferred approach. The existing bioreactor tankage was retrofitted to an enhanced BNR process to treat the same capacity of 2.5 mgd by installing new concrete baffles and flow splitter boxes. A new mirror image plant was built for the additional 2.5 mgd. The results from actual operation during the first fall/winter showed that the dosage of ferric chloride and caustic were reduced by approximately 50% as compared to chemical P removal to achieve the same effluent TP limit, indicating significant Bio-P removal. The plant also achieved excellent nitrification/denitrification with effluent ammonia-N below 0.5 mg/L and nitrate-N below 5 mg/L.