AWWA WQTC62370

In November 2000 the Washington Aqueduct, which is the water treatmentprovider to the District of Columbia, switched its disinfectant from chlorine tochloramines in order to reduce disinfection byproducts (DBP). However, accompanying this change was an increase in 90thpercentile 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 to150 ppb in homes with lead service lines. (Giani & Edwards 2004). Review of theprofiles indicated that the majority of the lead was dissolved and coming from the leadservice lines.During the spring, the Washington Aqueduct conducts an annual chlorine burnfor five consecutive weeks. During these five weeks, chlorine is added as the disinfectantwith free chlorine residuals maintained at approximately 3.5 mg/L in the distributionsystem. In 2004, the chlorine burn initiated in April. During this time, lead profilesconducted at customer homes showed a dramatic decrease in peak lead levels, < 15 ppbin a few weeks time frame, (Schock & Giani ???????? 2004). After the "burn" wascompleted, chloramine was reinstated. Throughout the next few months, peak leadprofile concentrations began to increase well above the action level again. During this same time frame, lead service lines were removed from the groundand shipped to the US Environmental Protection Agency's research laboratory inCincinnati, Ohio. X-Ray diffractions revealed that the majority of the scale consisted oflead oxide (Pb -IV), (Schock & Giani - 2004). Schock also revealed that the Pb-IVcould have formed due to high oxidation-reduction potential (ORP) caused by usingchlorine concentrations prior to 2000 in excess of 3.5 mg/L (free chlorine) inthe DCWASA system. When chloramines were introduced, the ORP decreased thuscausing the lead oxide to dissolve. Beginning in March 2003, DCWASA engaged in a series of pipeloop studies todetermine the most effective optimal corrosion control treatment. During these studies, itwas determined that the addition of orthophosphate was the most optimal treatment. Itwas also discussed that potential switch back to chlorine might help speed up the leadreduction process.Several pipeloops were being utilized at DCWASA's Fort Reno Water Qualityfacility to assist in optimizing corrosion treatment. Two of these pipeloops (Pipeloops #3and #6) were dedicated to study the effects of chloramines and phosphate, chlorine andphosphate and the switching back and forth of the disinfectants and their effects on leadrelease from the lead service lines. It was also a concern to study the effects a chlorineburn might have with regards to lead release once the phosphate has had a proper chanceto passivate the lead.This paper discusses the findings of pipeloops #3 and #6 with regards to leadleaching concentrations under the scenarios previously mentioned. Includes 3 references, figures.

Product Details

Edition:

Vol. - No.

Published:

11/01/2005

Number of Pages:

14

File Size:

1 file , 330 KB