AWWA ACE58119

The City of Scottsdale, Arizona manages a Centralized Groundwater Treatment Facility(CGTF). The source water to the CGTF comes from four production wells. At thisfacility, the City employs air stripping to remove trichloroethylene (TCE). The designcapacity of the CGTF is 13.5 million gallons per day (mgd) and the average operationalflow is approximately 11.5 mgd. The total dissolved solids (TDS) and hardnessconcentrations of the CGTF effluent are respectively 830 mg/L and 390 mg/L.The City is planning to install a reverse osmosis (RO) treatment plant to treat a portionof the CGTF effluent and blend the RO product water with the CGTF effluent to achievelower levels of TDS and hardness. In addition to lowering the TDS and hardness, theCity expects the RO treatment to reduce the levels of arsenic and nitrate in the blendedwater to well below their target goals of 6 ug/L and 8 mg/L as N, respectively. Theexisting pipe in the distribution system that receives the CGTF water is 25-30 years old,galvanized iron pipe.Visual examination of the pipe has shown varying degrees of corrosion and scaling.There were locations where the internal tuberculation and scale filled almost 50-75% ofthe pipe opening. An elemental analysis of the scale revealed iron (~80% by weight),calcium (~10%), silica (2%) and zinc (~2%) to be the dominant compounds. Corrosionof galvanized iron pipe is the source for iron and zinc, while precipitation from water isthe source for calcium and silica. The City wants to avoid upsetting the existing scale orexcessively releasing corrosion products when the CGTF and RO blended water isdelivered through the old, rusty pipes.Higher iron release was observed in pipes that were exposed to low pH (7.5) blendedwaters. Blended water of TDS 550 mg/L had the least amount of iron release. Calciumdeposition to scale or dissolution from scale was minimal at pH 8.0 and TDS of 550mg/L. Adding blended phosphates slightly reduced iron release and calciumdeposition. No visible color increase was observed in the first flush samples for all thewater quality conditions that were tested. A pH of 8.0 and a TDS of 550 mg/L arerecommended as the optimal water quality parameters for RO and CGTF blendedwater, to minimize corrosion and scaling in the distribution system.This paper outlines a successful approach that other drinking water purveyors can useto identify water quality conditions that would minimize corrosion and scaling whenRO treated water is introduced into the distribution system. Includes 2 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

06/15/2003

Number of Pages:

19

File Size:

1 file , 2 MB