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Coastal communities with aging sewer systems face special challenges for effluent disposal. Peak wet weather flows during storms are often many times greater that those during dry weather conditions. Disinfection for such communities has been accomplished historically by applying gaseous chlorine, an approach highly suitable for varying flows. Safety concerns and more stringent regulations regarding gaseous chlorine use have forced reconsideration of disinfection practices, focused on ultraviolet (UV) radiation as a most attractive alternative. Varying flows are a particular challenge for UV systems. This paper reports on implementation of UV disinfection at two coastal communities carried out in response to these challenges.

City of Santa Cruz, California

The City of Santa Cruz recently updated its water pollution control facility (WPCF) from advanced (chemical) primary treatment to secondary treatment using the trickling filter/solids contact process. A highly efficient outfall and diffuser disperse effluent into deep water over a mile offshore. The effluent field rarely surfaces, achieving high initial dilution when it does. Since the WPCF is located near the center of town, the city requested disinfection alternatives that would eliminate its 45-year history of gaseous chlorine disinfection. Sodium hypochlorite solution and medium pressure UV systems were compared on a life cycle cost basis and were found to be cost competitive. The UV system greatly reduced chemical deliveries and eliminated the potential for disinfectant byproduct formation. UV was implemented as part of the secondary treatment upgrade.

Peak wet weather flow more than 6 times average flows required a special strategy. Prior regulatory negotiations allow the city to bypass part of the PWWF around the new secondary treatment facility (up to 50 mgd through secondary treatment and up to 30 mgd bypass). Site constraints prevented treating all flow through the proposed UV system. The final scheme includes sodium hypochlorite addition to the raw sewage for odor control and some disinfection, secondary treatment and UV disinfection for up to 50 mgd, and supplemental hypochlorite addition to bypassed primary effluent, with the effluent outfall used to achieve chlorine contact. The design parameters for the UV systems are a minimum dose rate of 30 mW-s/sc, an effluent transmittance of 55 percent, and an effluent TSS of 30 mg/L, to achieve an effluent total coliform concentration of no more than 100,000 MPN per l00 mL. Typical performance of the secondary treatment system and UV disinfection have exceeded expectation allowed easy permit compliance. Effluent TSS and transmittance are typically 7 mg/L and about 55 percent, respectively. Total coliform counts rarely exceed 10,000 MPN/100mL with a UV dose of 29 mW-s/sc.

City and County of Honolulu, Hawaii

The City and County of Honolulu operates a primary wastewater treatment plant at Sand Island located on the south coast of the Island of Oahu. It discharges to Mamala Bay through a deep ocean outfall and is Hawaii's largest wastewater treatment facility. The Sand Island Wastewater Treatment Plant (SIWWTP) has average daily flow rates of 70 million gallons per day (MGD), and peak wet weather flow rates of over 200 MGD. The plant has a 301(h) waiver of secondary treatment. Treated effluent from SIWWTP discharges to Mamala Bay via a 10,573-ft. long, 84- inch diameter outfall terminating in a 3,398-foot long diffuser section. The diffuser varies in diameter from 84 inches down to 48 inches and discharge about 1-3/4 miles offshore at a depth of approximately 240-ft. Its capacity is between 260 and 300 MGD.

Ocean dispersion modelling have suggested the possibility that bacterial contaminants in the effluent could flow onto Honolulu's beaches and shoreline, although this has not been validated and no evidence has shown that it has in the past. The most recent NPDES permit renewal added a bacterial standard for the first time. The plant's NPDES permit requires that disinfection be provided to meet a daily maximum Enterococci limit of 18,000 CFU/100 ml. Brown and Caldwell and the University of Hawaii Water Resources Research Center (UH WRRC) have performed a yearlong study to characterize the effluent and determine its suitability for chlorination and UV disinfection. Primary effluent presents a challenge for any type of disinfection due to the high concentration of solids and organic matter (BOD), and particularly the high variability in these concentrations. In addition, a large portion of the aging collection system is located at low elevations, near or below sea level, so there is significant seawater infiltration during the low flow periods. As expected the chlorine demand is variable and high (near 15 mg/l), which would require a dose of 20 to 25 mg/l dose. Similarly, the UV transmittance (UVT) is low (as low as 15 to 20 percent UVT but typically in the mid to high 20's). Collimated beam testing for UV disinfection indicates that the necessary effluent bacterial density to meet permit requirements is still in the “free-swimmer” portion of the log-survival curve not in the particulate associated portion.

UV disinfection was determined to be the best disinfection option for several reasons including the potential health and safety concerns associated with transporting large amounts of chlorine and dechlorination chemicals to the plant from the mainland. Process control for UV disinfection is also more reliable than it would be for large-scale chlorination/dechlorination. The possible adverse impact of chlorination byproducts on the ocean environment was also a significant consideration in the selection.

Document Type: Research Article


Publication date: January 1, 2000

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