Okaloosa County's Transition to UV Disinfection

Population growth and increased attention on water quality are forcing communities to increasingly look to reclaimed water for landscape irrigation to offset potable demand. Due to the requirements to provide high level disinfection with a chlorine residual for public access reuse, use of UV disinfection is limited, because it would have to be supplemented with chlorine disinfection. Operating two disinfection systems is prohibitive from both a cost and operations stand-point at most facilities. However, in certain circumstances the use of UV disinfection can be a feasible disinfection alternative.The new Okaloosa County Arbennie Pritchett Water Reclamation Facility (WRF) presents a situation where UV disinfection is the ideal alternative. Located in the Florida panhandle, Okaloosa County is currently served by the aging Garniers wastewater treatment plant. Due to a number of odor issues and the residential location of the Garniers plant, the County commissioned the Arbennie Pritchett WRF to be constructed on the site of its existing effluent spray fields, property leased from Eglin Air Force Base. Several new rapid infiltration basins (RIBs) have been constructed for effluent disposal in the immediate vicinity of the plant. Because no public access is currently planned for the Arbennie Pritchett WRF, effluent limits for the RIBs system are considerably less stringent than for areas with public access. No chlorine residual is required per the facility's permit from the Florida Department of Environmental Protection (FDEP).While the Garniers plant currently uses chlorine gas for disinfection, it was not a practical option for the Arbennie Pritchett WRF. Due to increasing regulations and safety issues related to the use of chlorine gas and concerns expressed by the Air Force regarding chlorine gas on base, UV disinfection was determined to be the best alternative for the Arbennie Pritchett WRF. The facility includes a four-stage Bardenpho Carrousel® process with secondary clarifiers and a UV disinfection system, with provisions for future installation of cloth-disk filters prior to the UV disinfection system included in the design. Disinfected effluent flows into a wet well where it is pumped to the RIBs system.Considering the effluent disposal location and permit requirements, UV disinfection was selected as the preferred alternative at the Arbennie Pritchett WRF. The UV disinfection system is sized for an average daily flow of 10 MGD and is readily expandable to 15 MGD to meet projected future demand. The UV modules will be installed in two concrete channels, with a third empty channel available for the expansion. Design parameters for the system include 65 percent UV transmittance without the presence of filters, and an effluent limit of 200 fecal coliforms per 100 mL, per Florida Statute 62–600. Additionally, the absence of chlorine addition and associated disinfection by-products provides a major advantage over chlorine disinfection.This paper will discuss the disinfection alternatives evaluated as well as the selection of UV disinfection for the Okaloosa County Arbennie Pritchett WRF. This paper will also highlight the differences between chlorine and UV disinfection systems and the reasons behind selecting one over the other.