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A Framework for Developing pH Guidance for Drinking Water Treatment and Distribution
Citation:
Schock, M., F. Lemieux, AND N. Martinova. A Framework for Developing pH Guidance for Drinking Water Treatment and Distribution. Presented at 16th Annual Canadian National Conference on Drinking Water, Gatineau, QC, CANADA, October 26 - 29, 2014.
Impact/Purpose:
This presentation will give the Canadian audience the benefit of extensive research experience from EPA on the measurement of pH in drinking water, and the importance of pH to corrosion and multiple drinking water treatment processes. This is the result of a technical collaboration with Health Canada for the development of their upcoming updated pH Guideline.
Description:
Worldwide, many agencies have historically limited the range of pH values of distributed water between 6.5 and 8.5. Although this range is not a regulatory limit, many jurisdictions have used it as one. In some cases, the range has been a barrier to optimizing distribution system and premise plumbing corrosion control, especially for lead and copper release. In the absence of orthophosphate inhibitor, the solubility of Pb(II) and Cu(II) are minimized well above this pH range, which impacts human exposure. The formation of highly insoluble PbO2 that provides lead control in some water systems is facilitated by faster oxidation rates at pH above 8.5. Additionally, studies have established that the oxidation of iron and manganese markedly increases; the stabilization of monochloramine improves; and the propensity for nitrification is reduced at pH values above this limit. Since this range was established, there has been considerable advancement and knowledge of corrosion control science and interactions with other water treatment objectives that are pH-dependent. Variations in pH within a treatment plant can be significant and even desirable for various steps of the treatment train. However, once the treated water leaves the plant, the stability of the pH is of utmost importance in order to maintain water quality and corrosion control. Many different studies have shown associations of episodes of impaired water quality with mixing of waters of different pH and other chemical characteristics, or frequent fluctuations. Changes in pH in the distribution system can also be a good surrogate indicator of important changes in water quality. The current pH range should be re-examined in light of the new science to ensure that water quality is optimized. In the distribution system, buffering capacity (maintaining a stable pH) is at its minimum in the approximate range of pH 8.3-8.5, and considerable improvements are seen in practice at higher pH. In water treatment, every treatment process (e.g. coagulation, precipitation, disinfection and corrosion control) is dependent on pH. Measuring the pH of water at the various treatment steps and in the distributed water is critical for the optimization of each process. The accuracy of the measurement is important since each treatment step depends on a tight range for good control of the processes. However, the measurement accuracy is influenced by factors such as temperature, carbon dioxide concentration and the methodology used for sample collection, handling and analytical measurement of pH. The range of pH should be flexible to allow systems to determine the most appropriate pH for their water quality and treatment goals. Factors to be taken into consideration when selecting a pH for distributed water include treatment method used; secondary disinfectant; alkalinity; and dissolved inorganic carbon. These factors will be addressed in support of changing the range of pH range for one that reflects the most recent science. This presentation will include a discussion of practical guidance on factors that will identify vulnerability to pH change, and improvements to pH measurement techniques.
