AWWA WQTC63966

Orthophosphate is being used by many water utilities as a metal corrosion inhibitor. The substance binds withmetal ions, creating a film on the outer pipe scale to prevent corrosion. After the Washington Aqueduct (WA),the District of Columbia Water and Sewer Authority's (WASA) finished water supplier changed disinfectionfrom free chlorine to chloramines, lead leaching accelerated in the water distribution system. WA began to addphosphoric acid in response to the reported higher lead levels. The initial target residual was 3.2 mg/Lphosphate, which WASA intended to maintain until the corrosion control treatment optimized. The 90thpercentile lead levels declined from 59 to 15 ppb within eight months of phosphate addition.WASA began to receive customer complaints of white cloudy water after a year of continuous phosphatetreatment. Water quality technicians collected and tested water samples from taps and hydrants in response tothese complaints. Several hydrant samples had a white cloudy appearance and color_apparent exceeding 200 Pt-Counits. Significant higher levels of phosphate, iron, and aluminum were also measured in these hydrant samples.Filtering the hydrant samples showed nearly all the iron and aluminum and a majority of the phosphate were inparticulate form. Interestingly, technicians rarely captured this cloudy appearance inside tap samples. WASAconducted a study, involving field and bench-scale analyses, to determine the cause of the white cloudy water. This paper outlines the investigation of the white cloudy water phenomena within WASA's distribution system.WASA conducted field analyses to determine the extent of the problem and gather current data. Further benchscaleexperiments were performed to characterize the physical properties of the aluminum, calcium, iron, andphosphate interactions under WASA's typical conditions. Bench-scale particulate volume measuring andsettling tests were also performed. X-ray diffraction with computer modeling of laboratory formulated and fieldprecipitates are presented to further describe, compare, and identify the major elemental species that contributeto this white cloudy water. Includes 7 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

11/01/2006

Number of Pages:

14

File Size:

1 file , 420 KB