Fine-tuning the corrosion control targets of alkalinity and pH for Massachusetts Water Resources Authority

Massachusetts Water Resources Authority (MWRA) supplies metropolitan Boston with unfiltered surface water. The source water is corrosive. An interim corrosion control facility came on line in June 1996 to adjust the pH and alkalinity levels for better corrosion control. Copper levels have never been higher than the action limit of 1.3 mg/l and the 90th percentile copper is now less than 0.15 mg/l. The 90th percentile lead level is close to the action level of 0.015 mg/l, but is sometimes still in exceedance. The September 2002 sampling of lead and copper (after the paper was submitted) showed the 90th percentile of lead and copper to be 0.0113 and 0.117 mg/l. MWRA is exploring the option of changing pH and alkalinity targets to meet the lead rule. In addition to lead concerns, other water quality concerns were identified as being sensitive to alkalinity and pH targets. These include, for example, the amount of disinfection by-products formed, the amount of iron released, chloramine stability and nitrification, the buffering capacity of treated water (pH stability), virus inactivation and copper levels in the biosolids from wastewater treatment. The proper target level for pH and alkalinity is a balance between these protective public health and operational goals. Models were developed to assist in fine-tuning the current pH and alkalinity targets of 9.1 and 35 mg/l as calcium carbonate for optimum water quality.