Corrosion Control Aids
Residuals (cont'd)
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Results
and Discussion
Water used in flushing activities. Prior to polyphosphate-blend addition, flushing typically required 20 to 30 acre- ft/month, and occasionally, as much as 65 acre-ft/month. Following polyphosphate-blend addition that began June 1999, the required flushing volume dropped to 2 to 3 acre-ft; three years later, it has fallen as low as 0 to 1 acre-ft/month. A crew of two to three people flushed the area as frequently as twice per week. In general, flushing was done until water quality goals were met. Customer Complaints. One of the primary goals of the polyphosphate-blend addition was to reduce the number of customer complaints. During the summer of 1998, the number of complaints was as high as 35 per month. Because of its color-sequestering ability, the polyphosphate-blend addition had an immediate impact, reducing the number of complaints to fewer than five per month. The number of complaints has consistently, remained fewer than five per month, including a number of summer months in which zero complaints were received ((Figure 1). Total chlorine residuals. Total chlorine residual was monitored in the field, using pocket colorimeters with DPD-method procedures to determine if the District goal to maintain a total chlorine residual greater than 1.0 mg/L was being met. Prior to the addition of the polyphosphate-blend, as few as 30 percent of monthly samples had a measured chlorine residual greater than 1.0 mg/L. Approximately six months later, 90 to 100 percent of the residuals in the study area were greater than 1.0 mg/L. Also, less water was used in flushing to achieve the goal. HPC bacteriological analyses. HPC levels, which are the main indicator of bacterial activity in the study area, also decreased significantly following the addition of polyphosphate-blend. However, it took almost two years for the level to consistently remain below 100 colony forming Units/mL, which is the District goal. We suspect that initially the polyphosphate-blend agent dissolves the corrosion product-biofilm matrix on the pipe wall, resulting in higher HPC levels. Throughout the study period, all bacteriological samples were absent of coliform bacteria. For this system, a minimum of two years of polyphosphate-blend addition is required to establish low HPC levels. The length of time following polyphosphate-blend addition required to reduce HPC levels depends on the type and age of the water mains and characteristics of the biofilms and corrosion products. Newer systems, which may contain thinner biofilms or fewer corrosion by-products, may be able to reduce HPC concentrations in less time. Older systems that have unlined cast-iron and steel water mains with firmly established biofilms may require longer than two years (Figure 2). |
