Spot-check Bioassay of a Full-Scale Gravity-Flow Open-Channel Vertical UV System At the Russian River Treatment Plant in Sonoma County, California

The focus of this paper is to detail the findings of a “spot-check” bioassay that was conducted on a full-scale, gravity-flow, open-channel, vertical UV system at the Russian River Treatment Plant in Sonoma County, California. Field testing was performed by Stantec Consulting Services, Inc. in April 2012.The new UV disinfection system is installed downstream of the tertiary filters and consists of two parallel reactor trains (UV channels). Each train contains eight banks (UV modules), each of which are equipped with forty (40) low-pressure high-output germicidal lamps.Under the assumption that UV channel 2 would perform equivalently, the “spot-check” bioassay was limited to UV channel 1. The UV channel was tested at the nominal minimum flow rate of 0.25 mgd, the average flow per train of 0.7 mgd, and the design peak flow rate per train of 1.85 mgd.In all eight tests the observed full-scale REDs are greater than the doses predicted using the model derived from the 2002 “off-site” bioassay. Placement of the UV modules did not affect performance of the UV disinfection system. De-rating is not needed for the UV system installed at the Russian River Treatment Plant.Correcting the dose values for both reactor validation and full-scale “spot-check” bioassay per the new standard MS2 dose-response equation recommended in the third edition of the NWRI/WRF “Ultraviolet Disinfection Guidelines for Drinking Water and Water Reuse” did not impact the outcome of this “spot-check” bioassay.