Integrating Ozonation and UV Disinfection for an Unfiltered System to Comply with LT2ESWTR

摘要

The Bangor Water District (BWD) has evaluated the treatment requirements for compliance with the US EPA's Long Term 2 Surface Water Treatment Rule (LT2SWTR). The LT2ESWTR will supplement existing regulations by mandating additional Cryptosporidium inactivation requirements for higher-risk systems. BWD is an unfiltered water system that currently uses ozonation and chloramination. After an evaluation and selection process, BWD is in the process of adding UV disinfection to meet the LT2ESWTR treatment requirements. The UV transmittance (UVT) data collected to date demonstrates the significant impact of ozonation. The raw water 99th percentile for UVT was 70 percent, where as the 99th percentile for the ozonated water was 83 percent. It was determined to put UV after ozone, however, there was a considerable difference between designing the UV system with or without ozone, because of the difference in transmittance. In many cases it meant going from two reactors (one duty and one standby) to 3 or even 4 reactors. It was decided to let the market decide the true cost of being "conservative" and designing the UV system for the raw water transmittance. So the final design of the UV system included pre-purchase of the UV system. The UV system bid included a based bid with a "high" UVT assuming ozone is in service and an alternate bid with a "low" UVT assuming ozone was out of service. The UV manufacturers were also required to submit detailed information on the operating envelope of the base bid so it could be determined how much "extra" capacity the base bid had by using more power or adding more lamps to the reactor. The allowed BWD to balance the initial cost savings of the base bid, while having a contingency plan should the need to treat lower UVT water in the future