Verification and Scale-Up of Sewer Overflow Chemical Oxidation for Urban Outfalls

Abstract

Discharge of untreated combined or sanitary sewer overflows (CSOs or SSOs) during high precipitation events is an environmental concern. To address this, a decentralized wet weather–flow peaker facility, designed to treat overflows, was investigated. The technology was engineered for quick start-up, extended periods of inactivity, a minimal urban footprint, and the ability to meet treatment goals. Feasibility was assessed based on 4 years of bench- and pilot-scale data. A combination of enhanced primary treatment and ozonation met discharge requirements for total suspended solids and Escherichia coli inactivation in less than 35 min of treatment at three different scales: 1-L bench-scale batch, 10-L bench-scale continuous flow, and 650-L pilot-scale continuous flow. The 1-L bench-scale batch and 650-L pilot-scale systems met discharge permit requirement for biochemical oxygen demand, whereas the 10-L bench-scale continuous-flow system did not due to poor solids removal associated with the small clarifier employed. Chemical oxidation pseudo-first-order kinetic results were identical at bench and pilot scales. Pseudo-first-order rate constants for chemical oxygen demand removal and the Arrhenius temperature coefficient value were determined. A full-scale preliminary design for sewer overflow end-of-pipe treatment had chemical and energy costs of $0.15/m3 treated, which was within the same order of magnitude as conventional wastewater treatment costs. Untreated water from municipal sewers sometimes is released to lakes and rivers. This may cause poor lake and river water quality that interferes with fishing, boating, and swimming. The releases typically occur during large rainstorms when sewers fill with water and existing treatment facilities cannot handle the high flow. It would be beneficial to have technologies that could be added to sewer systems to quickly start up and treat the excess sewer water when needed, but that can remain unused during dry weather. This paper describes a sewer water treatment system that can meet these requirements. The technology is called a peaker facility, because it treats excess flow during periods of peak flow, but is idle during normal, base-flow conditions. The system comprises a solids removal step followed by chemical oxidation using ozone to destroy pollutants before water is discharged to lakes and rivers. Benefits and challenges of scaling-up the technology from the laboratory to pilot-scale are described. In the future, peaker facilities may be implemented to protect lake and river water quality. However, more work must be done to quantify treatment costs and develop practical operating procedures.