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

摘要

The Las Virgenes Municipal Water District/ Triunfo Sanitation District Joint Powers Authorityrn(JPA) (Calabasas, CA) operates the Tapia Water Reclamation Facility (WRF), which currentlyrntreats an average dry weather flow of approximately 34,000 m3/day (9 MGD). In May 2010, thernfacility was required to comply with a new maximum monthly average NOx-N concentrationrnlimit of 8 mg-N/L. In anticipation of this lower limit, various minor low-cost improvementsrnwere implemented and plant staff modified the secondary process operations in an attempt torndemonstrate that the facility could consistently comply with the future lower NOx-N limit.rnAlthough these changes resulted in the reduction of the effluent NOx-N to a range of 8-12 mg-rnN/L from 12-20 mg-N/L, further facility improvements were required.rnAfter an extensive analysis of the WRF secondary process and spare tanks available at the WRFrnand at the Rancho Las Virgenes Composting facility, the most cost-effective options to improvernNOx-N removal were: (1) the implementation of RAS denitrification (endogenous) in sparerntanks formerly used for aerobic digestion of primary and waste activated sludges at the WRF;rn(2) retrofitting tanks at the composting facility to equalize and biologically treat the high strengthrnrecycle stream generated by the dewatering of the anaerobically-digested sludge (“centrate”);rnand (3) conversion of the main secondary process to a 4-stage Bardenpho configuration. Therncombined effect of these three modifications resulted in the reduction in the Tapia WRF effluentrnNOx-N concentration to concentrations consistently below the new maximum monthly permitrnlimit of 8 mg-N/L.rnTwo existing raw sludge storage tanks at the composting facility were retrofitted for equalizationrnand biological treatment of the centrate nitrogen load, which was contributing approximately 5 torn7 mg/L of ammonium-N to the plant influent or about 15% of the TKN load. The biologicalrnreactor is operated as a Sequencing Batch Reactor (SBR) with intermittent aeration. To avoid thernpurchase, storage and use of an external carbon source to enhance denitrification and generaternalkalinity for nitrification in the SBR, a portion of the raw combined sludge was found to be arnsuitable “in-house” source of carbon for the process. Approximately 6.5% of the raw combinedrnsludge is diverted to the centrate treatment process for this purpose. Ammonium-N and TotalrnInorganic Nitrogen (ammonium plus NOx-N) removal efficiencies up to 90% and 85%,rnrespectively, were demonstrated.