AWWA WQTC62370

In November 2000 the Washington Aqueduct, which is the water treatment provider to the District of Columbia, switched its disinfectant from chlorine to chloramines in order to reduce disinfection byproducts (DBP). However, accompanying this change was an increase in 90th percentile lead levels from 12 parts per billion (ppb) to 70 ppb. Lead profiles conducted by District of Columbia Water & Sewer Authority (DCWASA) showed average peak concentrations up to 150 ppb in homes with lead service lines. (Giani & Edwards 2004). Review of the profiles indicated that the majority of the lead was dissolved and coming from the lead service lines. During the spring, the Washington Aqueduct conducts an annual chlorine burn for five consecutive weeks. During these five weeks, chlorine is added as the disinfectant with free chlorine residuals maintained at approximately 3.5 mg/L in the distribution system. In 2004, the chlorine burn initiated in April. During this time, lead profiles conducted at customer homes showed a dramatic decrease in peak lead levels, < 15 ppb in a few weeks time frame, (Schock & Giani ???????? 2004). After the "burn" was completed, chloramine was reinstated. Throughout the next few months, peak lead profile concentrations began to increase well above the action level again. During this same time frame, lead service lines were removed from the ground and shipped to the US Environmental Protection Agency's research laboratory in Cincinnati, Ohio. X-Ray diffractions revealed that the majority of the scale consisted of lead oxide (Pb -IV), (Schock & Giani - 2004). Schock also revealed that the Pb-IV could have formed due to high oxidation-reduction potential (ORP) caused by using chlorine concentrations prior to 2000 in excess of 3.5 mg/L (free chlorine) in the DCWASA system. When chloramines were introduced, the ORP decreased thus causing the lead oxide to dissolve. Beginning in March 2003, DCWASA engaged in a series of pipeloop studies to determine the most effective optimal corrosion control treatment. During these studies, it was determined that the addition of orthophosphate was the most optimal treatment. It was also discussed that potential switch back to chlorine might help speed up the lead reduction process. Several pipeloops were being utilized at DCWASA's Fort Reno Water Quality facility to assist in optimizing corrosion treatment. Two of these pipeloops (Pipeloops #3 and #6) were dedicated to study the effects of chloramines and phosphate, chlorine and phosphate and the switching back and forth of the disinfectants and their effects on lead release from the lead service lines. It was also a concern to study the effects a chlorine burn might have with regards to lead release once the phosphate has had a proper chance to passivate the lead. This paper discusses the findings of pipeloops #3 and #6 with regards to lead leaching concentrations under the scenarios previously mentioned. Includes 3 references, figures.