The City of Yelm has historically relied on their two Downtown Wells for potable water supply. It is unlikely that these wells can provide enough capacity for future demands. Therefore, in anticipation of future growth, the City drilled the Southwest Well 1A in 2010, the first well in the Southwest Wellfield. The City plans on further developing and transferring water rights to this wellfield to provide reliable source that can keep up with growth.
Water quality testing of the Southwest Well 1A showed levels of manganese above the secondary maximum contaminant level (SMCL) and the presence of ammonia. Since the existing Downtown Wells have low to no manganese, the City has chosen to include manganese treatment as part of the Southwest Well 1A water system improvements. Other major improvements include a well pump station, a reservoir for additional storage, and a booster pump station.
Yelm retained RH2 to prepare preliminary design of these improvements. This started with a pilot study to determine oxidation and filtration treatment parameters to result in effective manganese removal and prolonged filter run times. Preliminary design also determined the design criteria of the 1,450 gpm well, the 600,000-gallon reservoir and the booster pump station. To minimize wastewater, the filter backwash water was to be captured in a storage tank where the solids would settle by gravity over time and the clarified water could be recycled for treatment. During the pilot study, RH2 determined that the filter backwash water consisted of suspended particles, likely due to the manganese, which would not settle naturally by gravity. Filter backwash samples were then shipped to a polymer supplier to determine polymer type and dose to assist with solids settling.
Full scale design of the 2,400 square foot facility was completed in spring 2015 and construction was completed in December 2016. The facility includes the Southwest Well 1A pump station, pyrolusite pressure filtration system, 48 pound per day on-site sodium hypochlorite generation and feed system, 50,000 gallon filter backwash storage tank and recovery system, laboratory, engine generator and booster pump station. This facility has space for future expansion to increase treatment capacity from 1,450 gpm to 2,100 gpm with the addition of an off-site well and another filter vessel. The at-grade steel reservoir is adjacent to the treatment facility on-site and provides the breakpoint chlorine contact time necessary to fully react with ammonia and minimize off tastes and odors.