AWWA WQTC64154

The community of Cave Creek relies on Central Arizona Project (CAP) surface water as a source tomeet its drinking water needs. CAP raw water is treated to drinking water standards through packagedMicrofloc Trident systems manufactured by USFilter. Currently there are two parallel treatment trainsthat each produce 600-700 gallons per minute (gpm) when in filtration mode. Under typical conditions,both trains treat CAP surface water. Under the old Arsenic Maximum Contaminant Level (MCL), sevenwells located throughout Cave Creek could be used as a supplemental water supply during times of peakdemand. Three of these wells were close to or exceeded the old Arsenic MCL of 50 micrograms perliter (µg/L) with the highest well producing Arsenic concentrations above 250 µg/L. The use of thesewells was accomplished under an approved blending plan where the well water was blended with thetreated CAP water to achieve a blended Arsenic concentration below 50 µg/L. With the implementationof the new Arsenic MCL of 10 µg/L, the Cave Creek WTP was no longer able to blend these wells withtreated CAP water.These wells could still be used as a limited backup to the CAP canal if one of the Microfloc Tridenttreatment trains could be optimized for Arsenic removal. Under this scenario, groundwater instead ofCAP water would be pumped into one of the treatment trains and Arsenic would be removed throughcoagulation filtration using a Ferric based coagulant. Initial full scale testing of this scenario wasapproved by Maricopa County and was conducted in early 2004. This testing showed that FerricChloride concentrations around 18.5 mg/L could remove Arsenic to below the new standard of 10 µg/L.Extended full scale testing of this Ferric Chloride dose was then accomplished in early 2005. Thisextended testing showed that although the finished water Arsenic concentration could be brought below10 µg/L there was difficulty in reliably producing Arsenic at this level. Based on this need to providemore reliable Arsenic removal, additional optimization was required.Further Arsenic treatment optimization was accomplished through two efforts that included: coagulation/filtrationbench scale testing; and, full-scale testing. Bench scale testing was conducted at the Cave Creek WTPsite in mid 2005. These results confirmed previous testing and formed the basis for recommendingmodifications to the full-scale process. Modifications were implemented in late 2005 and includedimproved coagulant and polymer mixing and improved process monitoring instrumentation. Final fullscale testing results have been received and results demonstrate that with treatment optimization androbust process monitoring, Arsenic concentrations can be reliably reduced from 180-200 µg/L to lessthan 10 µg/L. These results are useful to water systems looking to optimize existing treatment processes to meet multiple treatment objectives. Includes tables, figures, appendices.

Product Details

Edition:

Vol. - No.

Published:

11/01/2006

Number of Pages:

23

File Size:

1 file , 470 KB