AWWA ACE54326

The Washington Aqueduct, owned and operated by the U.S. Army Corps of Engineers, operates the Dalecarlia and McMillan Water Treatment Plants. The McMillan plant is a high rate filtration plant with a hydraulic capacity of up to 180 MGD. It is unique because chemical addition, including alum and fluoride, are added six miles upstream of the last sedimentation basin. After the chemical addition, the water flows by gravity two miles to the Georgetown Reservoir where sedimentation occurs. The water then flows through the four mile Washington City tunnel to the McMillan Reservoir. Water is pumped from the McMillan reservoir through 12 filters. The unusual chemical addition location gives the plant characteristics of a direct filtration plant. The filter influent, from the McMillan reservoir, has a very low turbidity and very few particles. During the summer, warm temperatures coupled with the large surface area and low turbidity of the reservoir promotes algae growth. The presence of algae increases both the organic load and the number of particles onto the filters. This organic material and the additional particles are being added downstream of the point of coagulant addition. Cationic polymer was used as a filter-aid to help reduce the composite filter effluent turbidity (non-ionic polymer caused the filters to bind). The cationic polymer, however, was ineffective in keeping the turbidity below 0.1 NTU. With the promulgation of the Interim Enhanced Surface Water Treatment Rule (IESWTR), it has become increasingly important to keep the composite filter effluent below 0.1 NTU and the individual filter effluent below 0.3 NTU. This study evaluates various other chemical filter-aids to achieve the treatment goal. Includes table, figures.