pH instability in relation to implementation of distribution system corrosion control

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Authors:Antoun, Edward N.; Hiltebrand, David J.; Gruber, Anthony D.
Author Affiliations:Primary:
AH Environmental Consultants, United States
Other:
Malcolm Pirnie, United States
Volume Title:Proceedings, 1997 annual conference; Volume C, Water research
Source:Proceedings, AWWA Annual Conference, Vol.1997(Vol. C), p.675-684; American Water Works Association, 1997 annual conference, Atlanta, GA, June 15-19, 1997. Publisher: American Water Works Association, Denver, CO, United States. ISSN: 0360-814X
Publication Date:1997
Note:In English. 7 refs.; illus.
Summary:With the highest lead levels in the country, and driven by regulatory compliance under the requirements of the Lead and Copper Rule (LCR), bench-scale testing followed by full-scale demonstration and implementation was conducted at a medium sized system in the southeast region of the US in an effort to reduce lead corrosion and bring the system into compliance. The system in question receives raw water from a large number of ground water wells often characterized by elevated chloride levels. Treatment consists of softening with lime using spiractors followed by disinfection. A bench-scale study was conducted using polarization flow cells which employ an accelerated aging technique that uses an electric current to speed-up corrosion processes by factors of up to 100 times the normal corrosion rate (Reiber, S. H., 1989. Edwards M. and J. F. Ferguson, 1992 Hidimi, L, et. al., 1994). The following were among the findings. - Higher salinity water increased corrosivity under all testing conditions; - Enhanced softening conducted at higher pH on low salinity water significantly reduced finished water corrosivity. Based on the results from the bench-scale testing, it was recommended to avoid to the greatest extent possible the use of high salinity wells. In addition, proceed with a fullscale implementation of: - Enhanced softening at a higher pH to reduce dissolved inorganic carbonate (DIC). To accomplish the reduction in DIC, it was necessary to increase the lime dosage, thereby increasing hardness removal. As part of the effort to implement this strategy, beginning in early May of 1995, the WTP increased the lime dosage sufficiently to produce an increase of 0.2 pH units per week in the finished water. This practice was continued until a pH of 9.0 to 9.2 was attained. However, several WTP upsets occurred between the months of May and August that resulted in wide variations in the finished water quality. These upsets were primarily due to a malfunction of the lime feed equipment. These variations resulted in instability in the passivation films and therefore prevented the observation of a steady decline pattern in lead level, an occurrence which was anticipated with a high level of confidence as it was successfully demonstrated (bench-scale as well as full-scale) for another system (model system) with identical raw water characteristics and treatment. This paper will not address the findings from the bench-scale testing in details. Instead, it will focus more on pH instability and its effects on implementation of the optimal corrosion control alternative.
Subjects:Chloride ion; Chlorine; Controls; Corrosion; Decontamination; Experimental studies; Geochemistry; Ground water; Halogens; Hydrochemistry; Laboratory studies; Lead; Metals; PH; Pollution; Testing; Water pollution; Water quality; Water wells
Record ID:1999013776
Copyright Information:GeoRef, Copyright 2018 American Geosciences Institute.
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