Aerial view of proposed site of the Bremerton UV Treatment Facility. |
Due to the source water quality and the city's comprehensive watershed control program and effective land and forestry management, the Union River supply has been successfully operated under SWTR filtration avoidance requirements. For this project, it is assumed that the Union River source will retain its unfiltered status and that filtration will not be required for LT2ESWTR compliance or other drinking water regulations in the foreseeable future.
The LT2ESWTR requires that unfiltered surface water supplies be disinfected to inactivate Cryptosporidium, and that the treatment include a minimum of two separate disinfectants to meet the combined pathogen inactivation requirements of SWTR (2- or 3-log Giardia lamblia) and LT2SWTR (2- or 3-log Cryptosporidium). The application of chlorine disinfection alone, as currently practiced, would not be sufficient as it is not effective at inactivating Cryptosporidium. Although the City of Bremerton has never found Cryptosporidium in its surface water, LT2ESWTR compliance requires the inactivation of Cryptosporidium in unfiltered surface water supplies
The city's Union River surface supply originates at the Casad Reservoir. Water is released into a river and flows to the McKenna Falls Intake Facility, where it is screened, monitored for turbidity, flow-metered and chlorinated using one-ton gas containers for primary disinfection. The treated water enters a transmission main and flows through about 37,600 feet of piping to Reservoir 4, the current point of entry to the distribution system (W256 pressure zone). Travel time in the transmission main provides more than enough chlorine contact time to satisfy SWTR treatment requirements.
Project drivers
The city retained HDR to provide engineering design and construction support services for the Ultraviolet Disinfection Treatment Facility project. The objectives of the water system's improvement project were to:
- Provide multiple-barrier disinfection of the unfiltered Union River surface supply to ensure compliance with both SWTR and LT2ESWTR and enhance drinking water quality.
- Give the city flexibility to supply treated Union River water to the W517 zone in addition to the W256 zone currently used. The W517 zone is currently considered by the Department of Health to be a separate water system.
- Improve operations associated with flow control and diversion of unchlorinated Union River Water to the Twin Lakes Reservoir recharge basin.
Proposed Solution
Ultraviolet radiation (UV) was the recommended and chosen disinfection approach for LT2ESWTR compliance. Further evaluation of the advantages and disadvantages of various configurations of UV systems such as low pressure/high output versus medium pressure was conducted. The city chose to preselect and procure a medium pressure system after soliciting proposals and bids through a competitive process.