Sewerage for the City of Yakima was first provided in 1886. By 1891 most of the original central business district was sewered and the area's raw waste conveyed to the Yakima River. Nearly all the City's approximately 25,000 residents were connected to this system by 1936, when the original primary treatment plant was constructed, providing treatment for 2.0 mgd. In 1955 food processing flow (fruit and vegetable waste) was separated and sent to the 100- acre spray field which still borders the plant on the south and east. Although the spray field eliminated a major source of organic loading, deteriorating river water quality led the Washington State Pollution Control Commission to direct the city to provide secondary treatment. In 1965, the city implemented secondary treatment by building two 170-foot diameter trickling filters. The new secondary treatment facility was capable of treating a flow of 15.4 mgd. As a result of 1972 amendments to the Federal Water Pollution Control Act, the city was again required to evaluate the performance of its wastewater treatment facility. This eventually led to further facility upgrades including the addition of an activated sludge system. Intergovernmental agreements were formulated whereby the treatment systems in Union Gap and Terrace Heights were abandoned and their wastes treated at the city owned and managed Yakima Regional Wastewater Treatment Plant. Union Gap and Terrace Heights operate their own collection systems. The collection system owned and maintained by the City of Yakima consists of over 275 miles of sewer lines in sizes up to 48 inches. Preliminary treatment at the existing Regional Wastewater Plant consists of mechanically cleaned bar screens and screenings compaction followed by grit removal. After flow measurement through Parshall flumes, wastewater is directed to primary clarifiers. The four 90-foot diameter primary clarifiers are part of the original 1936 construction project. After primary clarification, wastewater gravity flows to the trickling filter pump station where it is distributed over the two trickling filters. The biological treatment in the trickling filters is followed by additional biological treatment in the activated sludge system. Air is supplied to the four aeration basins by 400 horse power blowers through fine bubble ceramic disk diffusers. The four aeration basins are 28 feet deep and hold 1 million gallons each. Two 140-foot diameter secondary clarifiers follow the activated sludge aeration basins. Effluent is disinfected with chlorine and de-chlorinated with sulfur dioxide before final discharge to the Yakima River. An odor control system was added to the facility in 1992. Geodesic domes were constructed over the trickling filters as part of this project. Foul air is ducted from certain areas of the plant and treated in two packed tower odor scrubbers. Sludge settled out in the primary clarifiers and wasted from the activated sludge system is pumped to the anaerobic digesters for stabilization. There are three heated and mechanically mixed primary digesters with a combined capacity of 1.6 million gallons and three secondary digesters with flexible membrane gas holder covers. After stabilization in the anaerobic digesters, sludge is dewatered by centrifuge and recycled on permitted farmland as a soil conditioner and fertilizer. In 2000, the plants capacity was again expanded to accomodate the closure of the spray field and bring the remaining food processing wastewater into the plant for treatment. The City's delegated Pretreatment Program consists of an extensive monitoring, public education, and if necessary, enforcement program to reduce and/or eliminate prohibited pollutants which could pass through, interfere, or contaminate the plant's effluent and/or sludge or cause harm or damage to city workers or collection or facility equipment. The Yakima Wastewater Plant was issued a new NPDES permit effective June 1, 2003. The wastewater plant currently receives an average monthly flow near 13 mgd. Peak flows are during irrigation season, a result of infiltration caused by an aging irrigation system. Infiltration adds approximately 4 mgd to the warm weather flows. The city's sewer rehabilitation program of pipe replacement and repair over the past few years has reduced flow by over 2 mgd. The current plant capacity is rated near 22 mgd.

Annual Biosolids Reports

Business Applicants get your Industrial Waste Surveys here: Pretreatment Department

Also available: pdf documents of Municipal Codes 7.60 and 7.65 which were passed before Council in December 2003.

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