Yosemite National Park – Innovative Solutions to Eliminate Chlorine Gas Wastewater Disinfection Hazard

The paper describes the process of evaluating disinfection alternatives to eliminate the chlorine gas disinfection system at the El Portal Wastewater Treatment Facility that serve Yosemite National Park. The disinfection alternatives considered included ozonation, sodium hypochlorite, and UV disinfection. Based on the evaluation of the available alternatives it was concluded that UV disinfection was the preferred option. The project components included modification of an existing chlorine contact tank to a new UV disinfection channel, providing necessary instrumentation and control features to provide fully automated operation, and connecting the new UV system to an existing diesel stand-by power generator. One of the key design approaches included evaluation of the major daily flow variations and seasonal flow changes, to ensure that the required effluent criteria are met at all plant operating conditions and to optimize the design of the UV system.The paper also highlights some of the major design issues applied to achieve the required effluent criteria and to minimize impacts to existing plant operations during construction of new facilities. The UV disinfection system was designed to adjust UV lamp output automatically to provide the appropriate UV dose while minimizing power consumption. Since this project was a retrofit of the exiting facility, which must be operated during construction of the new UV system, an evaluation of the construction mitigation measures and plant operation during construction, to achieve adequate disinfection at all times was also undertaken.