Case Study in Optimizing the Use of Existing Infrastructure and Plant Carbon Sources to Reduce the Effluent Total Nitrogen: Upgrade of the Tapia Water Reclamation Facility

The Las Virgenes Municipal Water District/Triunfo Sanitation District Joint Powers Authority (JPA) (Calabasas, CA) operates the Tapia Water Reclamation Facility (WRF), which currently treats an average dry weather flow of approximately 34,000 m3/day (9 MGD). In May 2010, the facility was required to comply with a new maximum monthly average NOx-N concentration limit of 8 mg-N/L. In anticipation of this lower limit, various minor low-cost improvements were implemented and plant staff modified the secondary process operations in an attempt to demonstrate that the facility could consistently comply with the future lower NOx-N limit. Although these changes resulted in the reduction of the effluent NOx-N to a range of 8-12 mg-N/L from 12-20 mg-N/L, further facility improvements were required.After an extensive analysis of the WRF secondary process and spare tanks available at the WRF and at the Rancho Las Virgenes Composting facility, the most cost-effective options to improve NOx-N removal were: (1) the implementation of RAS denitrification (endogenous) in spare tanks formerly used for aerobic digestion of primary and waste activated sludges at the WRF; (2) retrofitting tanks at the composting facility to equalize and biologically treat the high strength recycle stream generated by the dewatering of the anaerobically-digested sludge (“centrate”); and (3) conversion of the main secondary process to a 4-stage Bardenpho configuration. The combined effect of these three modifications resulted in the reduction in the Tapia WRF effluent NOx-N concentration to concentrations consistently below the new maximum monthly permit limit of 8 mg-N/L.Two existing raw sludge storage tanks at the composting facility were retrofitted for equalization and biological treatment of the centrate nitrogen load, which was contributing approximately 5 to 7 mg/L of ammonium-N to the plant influent or about 15% of the TKN load. The biological reactor is operated as a Sequencing Batch Reactor (SBR) with intermittent aeration. To avoid the purchase, storage and use of an external carbon source to enhance denitrification and generate alkalinity for nitrification in the SBR, a portion of the raw combined sludge was found to be a suitable “in-house” source of carbon for the process. Approximately 6.5% of the raw combined sludge is diverted to the centrate treatment process for this purpose. Ammonium-N and Total Inorganic Nitrogen (ammonium plus NOx-N) removal efficiencies up to 90% and 85%, respectively, were demonstrated.