Zero to Disinfection in Six Months Orange County Sanitation District Accomplishes Effluent Disinfection Goal By Dosing Primary Influent

The Orange County Sanitation District (District) made the environmentally and socially conscious decision to disinfect the treated wastewater. The goal of the District disinfection effort was to ensure compliance with the California Oceans Plan's bacteria requirements, as well as Ocean Discharge (NPDES) requirements within a six-month period. In order to accomplish the six-month disinfection goal by August 12, 2002, District staff had to plan, pilot-test, design, build, and commission the disinfection facilities for 245-MGD combined flow from two wastewater treatments plant in Orange County, California.The treated wastewater from the District's two wastewater treatment plants is discharged to the Pacific Ocean through a 120-inch outfall that extends five miles out at a depth of 220 feet. Currently, the District operates under the 301(h) provision of the Federal Clean Water Act, allowing for less than full secondary treatment based on receiving water environments. The District discharge an average of 50% secondary and 50% primary treated water. The District needed a strategy to reduce the combined primary and secondary effluent total coliform bacteria count from 2.5 × 107 MPN/100 ml to less than 180,000 MPN/100 ml.Sodium hypochlorite (bleach) was chosen as the disinfectant and sodium bisulfite is used to dechlorinate prior to ocean discharge. District staff evaluated bleach addition to the primary and secondary effluent but soon realized the existing primary effluent line only provided a 3-5 minute contact time. For this reason, bleach was added to the primary influent to gain contact time without large outlay of capital. Figures 1-3 refer to the plants effluent lines and chemical dosing locations. Dosing the primary effluent would require bleach dose rates of 25 to 35 mg/L with adequate mixing to achieve the disinfection goal. Furthermore, the chlorine residual at dechlorination point would be greater than 15 mg/L requiring 20-30 mg/L of sodium bisulfite for de-chlorination. This refinement in chemical addition location would require dosing the primary influent with 20 mg/l of bleach. The disinfection system was operational within 6-months and meeting project goals because the bleach is added to the influent of the primary and the construction of a contact basin or mixing systems was not required.