Collection System Pretreatment Cost-Benefit Evaluation, Orange County Sanitation District (OCSD)

ProjectLine evaluated costs and benefits of upstream collection system chemical addition for odor and corrosion control. The goal of the evaluation was to quantify costs and benefits thereby providing for well-informed level of service decisions for OCSD’s collection system. OCSD’s collection system includes 11 major trunk sewers and 8,700 manholes valued at over $3 billion, and approximately $24 million per year is spent for asset rehabilitation and replacement. OCSD has been addressing collection system odor and corrosion concerns since 1985, and has since expanded and refined these efforts to include chemical pretreatment in four of its eleven interceptors that have the greatest capacity to generate hydrogen sulfide. In 2009, OCSD’s expenditures for chemicals, equipment, and related pretreatment program services were approximately $5 million per year, impacting 30% of OCSD’s collection system assets. The cost-benefit evaluation compared OCSD’s chemical expenditures to the cost of deferring anticipated maintenance / replacement costs for the impacted infrastructure assets. The conclusion is that at high levels of hydrogen sulfide (e.g. > 100 ppm vapor-phase), generally upstream chemical pretreatment in the collection system is expected to be economically justified. The project also included a benchmarking survey among three other large municipal agencies that use similar chemical pretreatment methods (LACSD, San Diego, and Phoenix), with the conclusion that OCSD’s approach to chemical pretreatment is similar to that of comparable agencies.