AWWA WQTC58831

The City of Billings, Montana Water Treatment Plant (WTP) is a surface water treatment facility with the Yellowstone River as the source water. Due to the geology of Yellowstone Park located upstream of the City's river intake, Billings contends with arsenic in their surface water supply. Since the 1980s, the plant staff has used ferric chloride as the primary coagulant for sedimentation, which also served to reduce the concentration of arsenic. The City of Billings began experimenting on a bench scale basis with polyaluminum chloride (PACL) in January of 2000. In September of 2001, the City began a full-scale switchover from ferric chloride to PACL, while maintaining use of the same coagulant polymer. Various PACL doses, ranging from 7 mg/L to 15 mg/L, were tested in response to the changes in river turbidity. The PACL and coagulant polymer combination showed similar turbidity removal capability to that of ferric chloride and coagulant polymer. In addition, pH in the finished increased by 0.5 units, eliminating the need to adjust pH with caustic soda. However, arsenic removal was significantly less with PACL as it on occasion increased in concentration to 9 ppb in the treated water. Although the primary goal was to eliminate ferric chloride use altogether, preliminary results suggested that maintaining a small dosage of ferric chloride may be required to enhance the arsenic removal provided by PACL. In one test, 2 mg/L of ferric chloride was added along with 7 mg/L of PACL, with the coagulant polymer dosage eliminated. This approach reduced the arsenic levels back down to around 2 ppb from river levels of 14 ppb. Turbidity onto the filters remained unchanged and pH dropped slightly. The chemical cost of PACL is about 2.5 times more than ferric chloride, but when taking into account the elimination of the need for caustic soda, the overall estimated chemical cost increased only by 7 to 25%. For the PACL/ferric chloride combination, the estimated cost increases by 10 to 28%. Includes tables, figures.