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

ABSTRACT

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 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 (Table 1). 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.

KEYWORDS: Sulfide control, iron salts, Chemically Enhanced Primary Treatment (CEPT), hydrogen peroxide, PRI-SC, Peroxide Regenerated Iron.

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