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.
| Edition : | Vol. - No. |
| File Size : | 1
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, 450 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 11 |
| Published : | 11/02/2003 |
