Washington Aquaduct Water Treatment Process
The design included an investigation into the most feasible method to feed chloramine, as a secondary disinfectant, at both plants. The Dalecarlia and McMillan Water Treatment Plants produce an average of 220 MGD for the District of Columbia, Arlington, VA, Falls Church, VA, and portions of Fairfax County, VA. The design of the chloramination process consists of revisions and upgrades to the existing chlorination systems (primary disinfection), and the addition of new facilities for ammonia chemical storage, feed and monitoring (secondary disinfection). Improvements to the existing chlorination process includes replacement chlorinators and evaporators, additional chemical feed lines (chlorine for primary disinfection and lime for pH adjustments), clearwell baffle wall configuration design (maximize contact time in existing WTP finished water storage), and improvements to process monitoring.
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The design process included a chemical kinetics treatability study and a feasibility study for both WTP facilities. These concurrent studies determined the most efficient and cost effective method of providing ammonia storage, feed, and monitoring and utilizes bulk aqua ammonia storage and feed facilities. The kinetics testing determined that the ammonia feed system will minimize DBP formations following the primary disinfection phase of the upgraded water treatment process. The new ammonia facilities at each plant will require 3,000 sf for chemical storage, safety scrubbers, chemical feed equipment, and monitoring/header piping devices.
The project design scope was expanded to address other operational issues at the Dalecarlia and McMillan Water Treatment Plants. The additional scope items for the Dalecarlia WTP included the conversion of the bulk lime handling facility from bucket elevator and screw conveyor to a pneumatic conveying system; the replacement of the polymer feed and storage facilities; the conversion of the existing dry alum storage and feed facilities with a liquid coagulant storage and feed system; replacement of an existing chlorine gas scrubber with a deep bed dry chemical type scrubber; and water proofing the top slab of a buried 30 million gallon clearwell. The additional scope items for the McMillan WTP included replacement of three lime feeders and slakers and waterproofing the top slab of a buried 30 million gallon clearwell. The additional scope items for the McMillan WTP included replacement of three lime feeders and slakers and waterproofing the top slab of a buried clearwell.