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After the completion of the Phase II Combined Sewer Overflow (CSO) control improvements, the City of Richmond, Virginia conducted a re-evaluation study on its original 1988 CSO Long Term Control Plan (LTCP). The purpose of the study was to reassess the last phase (Phase III) of the original LTCP in light of EPA's 1994 National CSO Control Policy and state-of-the-art technologies. The study identified that after the completion of the Phase II CSO controls approximately 79% of the entire CSO volume will be discharged through the City's largest CSO outfall, Shockoe Creek, at a peak flow rate of 5,000 MGD. The study acknowledged that reliable disinfection of such a large flow rate and volume would be challenging and that inactivation efficiencies greater than 80% may be difficult to achieve and would require further investigations. The receiving water quality model showed that an 80% reduction of bacteriological loading would result in a significant improvement in water quality of the James River.

Under the recommendation of the re-evaluation report, the City conducted a comprehensive disinfection pilot study that evaluated the feasibility of two technologies – ultraviolet (UV) irradiation and chlorination/dechlorination – to achieve a minimum of 80% or possibly higher disinfection efficiency at the Shockoe outfall. The UV multivariable linear regression model and the chlorine bacteria reduction model, developed as part of the CSO disinfection pilot study, were used to determine the bacteriological water quality that could be achieved for a given Shockoe Disinfection Facility. The bacteria reduction models are applied to the dynamic flows from the combined sewer system hydraulic model, which will more accurately account for the changes in disinfection performance as a function of flow.

The pilot studies document that reliable disinfection levels of 80% (bacteriological reduction at Shockoe) and higher can be achieved using UV or sodium hypochlorite (NaOCl) disinfection. Preliminary cost estimates suggest that the use of NaOCl is more economical. However, the complete life cycle cost analyses must also include tangible costs, intangible factors, O&M considerations and input from the City's staff. Alternatives for cost reductions with UV and with NaOCl include the potential disinfection of lower flow rates at higher levels of bacteriological reductions.

Document Type: Research Article


Publication date: 2005-01-01

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