AWWA WQTC63966

Orthophosphate is being used by many water utilities as a metal corrosion inhibitor. The substance binds with metal 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 disinfection from free chlorine to chloramines, lead leaching accelerated in the water distribution system. WA began to add phosphoric acid in response to the reported higher lead levels. The initial target residual was 3.2 mg/L phosphate, which WASA intended to maintain until the corrosion control treatment optimized. The 90th percentile 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 phosphate treatment. Water quality technicians collected and tested water samples from taps and hydrants in response to these complaints. Several hydrant samples had a white cloudy appearance and color_apparent exceeding 200 Pt-Co units. 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 in particulate form. Interestingly, technicians rarely captured this cloudy appearance inside tap samples. WASA conducted 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 benchscale experiments were performed to characterize the physical properties of the aluminum, calcium, iron, and phosphate interactions under WASA's typical conditions. Bench-scale particulate volume measuring and settling tests were also performed. X-ray diffraction with computer modeling of laboratory formulated and field precipitates are presented to further describe, compare, and identify the major elemental species that contribute to this white cloudy water. Includes 7 references, tables, figures.