Cost Savings and Performance Improvement of Large System Iron Salt Use for Integrated Sulfide Control and Chemically Enhanced Primary Treatment by Using Peroxide Regenerated Iron Technology
San Diego's Point Loma WWTP
San Diego's Point Loma WWTP is a 160 MGD (240 MGD permitted), 100% advanced primary treatment plant that has historically used iron salts for collection system sulfide control and chemically enhanced primary treatment. Beginning in 2006, a PRI-SC® (Peroxide Regenerated Iron - Sulfide Control) program was implemented by adding H2O2 at the intermediate pump station PS2 (in place of the FeCl3), and again to the plant influent (ahead of FeCl3 addition for CEPT). The application of PRI-SC® in the Point Loma system was designed to provide at least $685/day in cost savings, to be achieved through reduced ferric chloride use at PS2 and Point Loma, while improving sulfide control and CEPT performance. Since integrating the PRI-SC® program full-time in 2008, SDMWD is realizing savings of approximately $4,700 per day (~$1.72 million/yr) compared to the 2007 baseline iron salts program. At the same time, both sulfide control and CEPT performance has improved. The cost savings were helped by the hedging aspect of the PRI-SC® program - iron salt price volatility in 2008 and 2009 was upwards of 45%. The PRI-SC® program has reduced the total iron salt use from the 2007 baseline rate of 32.5 dry tons per day to approximately 19.3 dry tons per day in 2009, with the core savings coming from an overall reduction in ferric chloride use at PS2 and the treatment plant.
Significantly, ferric chloride use at PS2 was eliminated and, for CEPT, it was reduced from 24 mg/L to 10 mg/L (16.6 to 6.8 dry tons per day) with no loss in performance. In addition, total sulfide removal has improved over baseline levels, and average CEPT performance exceeds the permit levels at 89% for TSS and 65% for BOD, and effluent water quality has improved (with 60% less spent iron (as FeS) present in the ocean discharge). For the most part, digester biogas H2S levels were maintained below the permit requirement of < 40 ppm, but required approximately twice the baseline FeCl2 feed rate. Even so, the overall program has maintained the stated savings benefit.
Reprinted with permission from Proceedings of WEFTEC® ‘10, the 83rd Annual Water Environment Federation Technical Exhibition and Conference, New Orleans, LA, October 2 - 6, 2010. Copyright © 2010 Water Environment Federation, Alexandria, Virginia.