Title

The Use Of Inhibitors To Control Copper Release In A Changing Water Quality Environment

Abstract

Water scarcity is forcing many utilities to develop alternative water supplies to supplement conventional sources. Multiple sources are blended in seasonally varying ratios causing the distribution system to see changes in water quality. This can have a significant impact on the metal release in the distribution system and therefore, at the tap. The purpose of this study was to observe the effects of corrosion inhibitor type and dose on copper release. The effects of blending finished waters produced from ground, surface and saline sources along with addition of phosphate and silicate inhibitors were evaluated in a large scale investigation supported by the Water Research Foundation (then AwwaRF) and Tampa Bay Water (TBW). The pilot distribution systems were made from actual pipes extracted from drinking water distribution systems. Blended finished waters were produced and distributed to fourteen pilot distribution systems (PDSs) operated at a 2-day hydraulic residence time for a period of twelve months. The aged pipe materials used in this pilot study included PVC, lined cast iron, unlined cast iron, and galvanized iron. Copper loops were housed in a shed at the effluent of the pilot distribution systems. Copper loops were flushed daily and samples were taken after a 6-hour stagnation time. Corrosion inhibitors included blended poly/orthophosphate, sodium orthophosphate, zinc orthophosphate, and sodium silicate. Two pilot distribution systems were operated without inhibitor at different pHs. The high alkalinity blends triggered copper release exceeding the 1.3 mg/L action level in PDSs without inhibitor addition. pH ranged from 7.4 to 8.2 and alkalinity ranged from 80 to 200 mg/L as CaCO3. Both phosphate and silicate inhibitors mitigated the adverse effects of higher alkalinity on copper release. At a dose of 1 mg/L as P the phosphate inhibitors reduced total copper to less than 0.5 mg/L for all blends tested. The inhibitor doses tested were 0.5, 1.0, and 2.0 mg/L as P and 3, 6, and 12 mg/L as SiO2 for phosphate and silicate inhibitors, respectively. Generally the reduction in copper release for phosphate inhibitors was independent of inhibitor type at 1.0 and 2.0 mg/L doses. The inhibitor type was observed to be statistically different for only the lowest dose of 0.5 mg/L. The silicate inhibitor also reduced total copper release but was less effective at the doses tested. Coupon surface characterization studies revealed changes in the copper surface with inhibitor addition. Scanning electron micrographs showed visible changes in copper surface morphology with the addition of different inhibitors. Energy dispersive x-ray spectroscopy confirmed the deposition of phosphate and silicate films on the copper surface. Surfaces exposed to inhibitors had blue-green copper (II) scales whereas surfaces not exposed to inhibitor had dull red copper (I) scales. A statistical regression model (R2 = 0.76) was developed for predicting total copper release as a function of water quality and inhibitor dose. This model identifies the water quality parameters critical to copper release in the blends tested and provides a practical tool to conduct desktop evaluations. This research confirms that utilities can control copper corrosion in distribution systems with varying water sources with the use of phosphate or silicate corrosion inhibitors. © 2009 American Water Works Association - AWWA DSS Conference Proceedings All Rights Reserved.

Publication Date

12-1-2009

Publication Title

Distribution Systems Symposium and Exhibition 2009, DSS 2009

Number of Pages

1243-1294

Document Type

Article; Proceedings Paper

Personal Identifier

scopus

Socpus ID

84871506562 (Scopus)

Source API URL

https://api.elsevier.com/content/abstract/scopus_id/84871506562

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