Role of pH for Phosphate-Based Lead Control in Windsor, Ontario, Canada

摘要

This paper presents results from an eight month pilot-scale study in Windsor, Ontario. The pipe looprnsystem consisted of eleven 20 mm (?-inch) lead service lines (LSLs) that were harvested from therndistribution system. A Corrosion Control Plan was prepared in 2010 that recommended pipe loop testingrnof phosphate-based inhibitors at different pH values, because Windsor’s distribution mains consistrnpredominantly of iron-based materials, and polyphosphates may be required to control coloured water.rnThe water supply is obtained from the Detroit River and its quality is comparable to Great Lakes waterrn(i.e., it has moderate alkalinity and low organics/turbidity), and is treated with conventional processes,rnwith polyaluminum chloride as the coagulant and free chlorine for residual maintenance. The pH ofrncoagulation is controlled with carbon dioxide; the typical pH of finished water is 7.1, which is slightlyrnbelow the desired range for phosphate-based inhibition of 7.2 to 7.8. Of interest was whether or notrnadequate corrosion control could be provided without adding a second chemical to achieve a higher pH.rnFive influent conditions were tested with the LSLs: water treatment plant treated water (with anrnambient pH of 7.1), orthophosphate with and without pH adjustment to 7.6, and orthopolyphosphaternwith and without pH adjustment to 7.6. Orthophosphate and (the orthophosphate component of)rnorthopolyphosphate were dosed between 0.5 and 2.0 mg/L as orthoPO_4 during the testing. Thernevaluation of compliance is based on the Ontario Ministry of Environment’s O.Reg. 170/03 Schedulern15.1 sampling schedule of ≤10 μg/L after 30 minutes of stagnation, in 90 percent of samples collectedrnduring one of two sampling periods (four months in cold water conditions, and four months in warmrnwater conditions).rnBoth orthophosphate and orthopolyphosphate effectively controlled lead release with a dose of about 1rnmg/L as orthoPO_4. More time was needed for the orthophosphate to reduce the magnitude andrnvariability of lead release. Continued exposure of the LSLs to phosphates is expected to result inrncomparable performance from both products. Maintaining the pH at 7.6 served to reduce thernoccurrence and variability of intermittent lead spikes, which are suspected to be associated with thernrelease of iron particles. However, with observed lead levels close to the method detection limit,rnincluding during warm water conditions, operating with phosphate-based inhibitors at the ambient pHrnof treated water produced by the plant is effective at meeting the regulation. Although some utilities are examining the benefits of operating with a higher phosphate dosern(i.e., ＞ 2 mg/L as orthoPO_4 as is the practice in some systems in the United Kingdom), the results fromrnthis study on this water suggest that a lower dose of 1 mg/L as orthoPO_4 is effective. Monitoring fullscalernperformance after implementation can be used to optimize the phosphate dose.