AWWA WQTC58831

The City of Billings, Montana Water Treatment Plant (WTP) is a surface water treatmentfacility with the Yellowstone River as the source water. Due to the geology ofYellowstone Park located upstream of the City's river intake, Billings contends witharsenic in their surface water supply. Since the 1980s, the plant staff has used ferricchloride as the primary coagulant for sedimentation, which also served to reduce theconcentration of arsenic.The City of Billings began experimenting on a bench scale basis with polyaluminumchloride (PACL) in January of 2000. In September of 2001, the City began a full-scaleswitchover from ferric chloride to PACL, while maintaining use of the same coagulantpolymer. Various PACL doses, ranging from 7 mg/L to 15 mg/L, were tested in responseto the changes in river turbidity.The PACL and coagulant polymer combination showed similar turbidity removalcapability to that of ferric chloride and coagulant polymer. In addition, pH in the finishedincreased 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 inconcentration to 9 ppb in the treated water.Although the primary goal was to eliminate ferric chloride use altogether, preliminaryresults suggested that maintaining a small dosage of ferric chloride may be required toenhance the arsenic removal provided by PACL. In one test, 2 mg/L of ferric chloridewas 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 of14 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 takinginto account the elimination of the need for caustic soda, the overall estimated chemicalcost increased only by 7 to 25%. For the PACL/ferric chloride combination, theestimated cost increases by 10 to 28%. Includes tables, figures.

Product Details

Edition:

Vol. - No.

Published:

11/02/2003

Number of Pages:

11

File Size:

1 file , 450 KB