ALTERNATIVES FOR DISINFECTING EFFLUENT GENERATED AT ORANGE COUNTY SANITATION DISTRICT WASTEWATER TREATMENT PLANTS
Abstract:In 2000, the Orange County Sanitation District (OCSD) initiated a conceptual study for long-term disinfection of the effluent from its two wastewater treatment plants. The study was prompted by concerns about elevated bacteria levels on the beach following implementation of beach monitoring required by Assembly Bill 411 (AB 411) in 1999. Although no beach closure had ever been correlated to the wastewater effluent discharged from the deep ocean outfall, the OCSD board of directors directed that all wastewater be disinfected prior to discharge to eliminate any possible adverse effect to recreational waters from ocean discharge of treated wastewater. In 2002, OCSD implemented chlorine (sodium hypochlorite) disinfection and sodium bisulfite dechlorination using existing chemical storage and feed facilities. The chlorination-dechlorination disinfection approach was considered an interim measure, during which OCSD continued to evaluate alternatives that could be implemented after expanded secondary treatment facilities had been completed.
The Initial Study, completed in January 2003, identified and evaluated 14 different disinfectants and disinfection methods and recommended that ultraviolet (UV) light and chlorine disinfection be further evaluated for implementation. Pilot testing of three UV disinfection technologies was conducted between September 2003 and March 2004. The three UV technologies were:
Low-pressure/high-intensity (LPHI) UV light
Medium-pressure/high-intensity (MPHI) UV light
Microwave-induced UV light
Pilot testing, including field measurements, water quality analyses, UV dose analyses, lamp sleeve fouling, hydraulic losses, power requirements, and heat generation, yielded the following observations:
A UV dose of 25 milliJoules per square centimeter (mJ/cm2), as achieved by the Trojan pilot unit, was sufficient to accomplish the targeted goal of 3-log inactivation of total coliform.
The secondary effluent exhibited a high potential for quartz sleeve fouling, which confirms that a proper mechanical/chemical cleaning system is an essential component for this application.
Of the three units tested, only the Trojan pilot unit performed satisfactorily in all of the tests conducted.
No disinfection by-products were identified in the UV-disinfected effluent during the pilot testing.
From the bioassay marine acute and chronic testing, no apparent difference in toxicity was observed between UV-disinfected effluent and bleach-disinfected effluent.
Pilot test results, recent data from the existing disinfection system, and conclusions and recommendations from the Initial Study were used to identify long-term disinfection alternatives for OCSD. Given the success of the existing bleach disinfection system at OCSD, bleach disinfection has been identified as the best disinfection alternative at Reclamation Plant No. 1. For Treatment Plant No. 2, in-line bleach disinfection for the existing activated sludge effluent and bleach disinfection with chlorine contact tank for trickling filter effluent is the most cost-effective disinfection alternative.
Document Type: Research Article
Publication date: 2005-01-01
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