AWWA WQTC62498

The Santa Clara Valley Water District owns and operates three treatment plants and two distribution pipelines. The District installed intra-basin baffles in two of its treated water clearwells, one in 2002 and another in 2004. While baffling improves circulation and theoretically should improve water quality, side effects of baffling cannot be ignored. With more surface area for microbial attachment, and under prime water quality conditions, the baffles can become a magnet for biofilms. In addition, the district boosted its chloramine residual from 1.4 mg/L to 2.3 mg/L in 2003 as a preventative measure against nitrification. As a result of these changes, the District began seeing a rise in heterotrophic plate count (HPC) in one of its distribution pipelines despite maintaining a healthy chloramine residual of 2.0 mg/L throughout the pipeline. Various causes of the high HPC occurrence were investigated, one of which was the assimilable organic carbon (AOC) level in the treated water. Each treatment plant chose a different location to boost its in-plant chlorine dose in order to achieve the higher chloramine residual in the distribution system. It was discovered that depending on the chlorine application scheme employed at the treatment plant, AOC levels varied accordingly. One of the plants modified its chlorine application scheme and subsequently lowered AOC in its treated water. This paper presents the District's experience, data, and various observations made correlating several water quality parameters to the microbiological quality of its treated waters. Includes 5 references, tables, figures.