The North end WWTP in Tacoma, WA, was initially designed as a standard primary plant but was converted to a chemical primary treatment plant using alum and polymer plus fly ash for precipitation of TSS and associated BOD5 in the early 80's. After extensive testing to
improve the performance it was converted to just alum and polymer treatment in 1985. In 1992 the City of Tacoma entered into an agreement with State of Washington, Department of Ecology and U.S. EPA to upgrade the plant to meet the effluent requirements of 30 mg/L BOD5 and TSS
and a minimum of 85% removal of these contaminants. The plant could not achieve 85% removal of BOD5 consistently. Studies of alternatives resulted in a trickling filter installed to remove soluble BOD5 and a novel flushing-recycle mode to recover a portion
of the TSS produced without the need for a secondary clarifier. During a 12 month optimization period and a subsequent 24 month consent order test period, the plant demonstrated its capability to produce an effluent averaging 13 mg/L BOD5 and 5 mg/L TSS with ≥92%
removal of these components. The prototype solids recovery system removed 80% of the TSS without need for a follow-on clarifier or any additional clarification area. This novel improvement proved its worth during the performance evaluation tests at operating conditions which exceeded
the basis of design.
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