The City of Scottsdale, Arizona manages a Centralized Groundwater Treatment Facility
(CGTF). The source water to the CGTF comes from four production wells. At this
facility, the City employs air stripping to remove trichloroethylene (TCE). The design
capacity of the CGTF is 13.5 million gallons per day (mgd) and the average operational
flow is approximately 11.5 mgd. The total dissolved solids (TDS) and hardness
concentrations 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 portion
of the CGTF effluent and blend the RO product water with the CGTF effluent to achieve
lower levels of TDS and hardness. In addition to lowering the TDS and hardness, the
City expects the RO treatment to reduce the levels of arsenic and nitrate in the blended
water to well below their target goals of 6 ug/L and 8 mg/L as N, respectively. The
existing 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% of
the 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. Corrosion
of galvanized iron pipe is the source for iron and zinc, while precipitation from water is
the source for calcium and silica. The City wants to avoid upsetting the existing scale or
excessively releasing corrosion products when the CGTF and RO blended water is
delivered through the old, rusty pipes.
Higher iron release was observed in pipes that were exposed to low pH (7.5) blended
waters. Blended water of TDS 550 mg/L had the least amount of iron release. Calcium
deposition to scale or dissolution from scale was minimal at pH 8.0 and TDS of 550
mg/L. Adding blended phosphates slightly reduced iron release and calcium
deposition. No visible color increase was observed in the first flush samples for all the
water quality conditions that were tested. A pH of 8.0 and a TDS of 550 mg/L are
recommended as the optimal water quality parameters for RO and CGTF blended
water, to minimize corrosion and scaling in the distribution system.
This paper outlines a successful approach that other drinking water purveyors can use
to identify water quality conditions that would minimize corrosion and scaling when
RO treated water is introduced into the distribution system. Includes 2 references, tables, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 2 MB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 19 |
| Published : | 06/15/2003 |
