AWWA WQTC62508

A drought cycle began in Salt Lake City in 1999 and persisted through 2004. In 1999, the reserves in the Upper Reservoir were drawn down to provide drinking water. It was anticipated that runoff would happen again sometime in the foreseeable future and fill it back up. The next four years did not see much in the way of runoff. The decision was made in 2000 to minimally utilize Parleys Treatment Plant and purchase water from neighboring utilities that had more storage. The flow through volumes in the Lower Reservoir went from 20,000 acre-ft to less than 5,000 acre-ft annually. The Upper Reservoir did not fill until the spring of 2005 after a heavy winter. No water was released and very little was treated from the Lower Reservoir during 2000 through 2004. Very small releases were made from the Upper to the Lower Reservoir to provide what was absolutely necessary for treatment during 2000 to 2004. The Lower Reservoir became an essentially closed system, and water quality problems started to occur in 2002. In late October of 2002 Parleys Treatment Plant was called upon to produce water after being off for about nine months. The algae counts in the raw water were about 202,000,000/100L. The predominant organism was Oocystus Gigal. While this organism is not a taste and odor problem, it forms a slime coating which made it very difficult to coagulate and remove in the treatment process. Good floc appeared to form just fine. Then the floc broke up across the sedimentation basin, and unfilterable water resulted at the end. Eventually, 60 mg/L of ferric chloride produced a settleable and filterable floc. The normal dose for the treatment plant was 5-10 mg/L. This incident prompted a closer look at what had been happening to water quality on the reservoir during the drought. In general, DO decreased and TDS increased as the reservoir annual flow through amounts decreased. This paper discusses improvements due to measures that were taken during the summer of 2004. Includes figures.