Wash. Rev. Code § 35.67.300
Any city, town, or organized and established water-sewer district owning or operating its own sewer system, whenever topographic conditions shall make it feasible and whenever such existing sewer system shall be adequate therefor in view of the sewerage and drainage requirements of the property in such city, town, or water-sewer district, served or to be served by such system, may contract with any other city, town, or organized and established water-sewer district for the discharge into its sewer system of sewage from all or any part or parts of such other city, town, or water-sewer district upon such terms and conditions and for such periods of time as may be deemed reasonable.
Any city, town, or organized and established water-sewer district may contract with any other city, town, or organized and established water-sewer district for the construction and/or operation of any sewer or sewage disposal facilities for the joint use and benefit of the contracting parties upon such terms and conditions and for such period of time as the governing bodies of the contracting parties may determine. Any such contract may provide that the responsibility for the management of the construction and/or maintenance and operation of any sewer disposal facilities or part thereof covered by such contract shall be vested solely in one of the contracting parties, with the other party or parties thereto paying to the managing party such portion of the expenses thereof as shall be agreed upon.
RCW 35.67.300
Part headings not law-1999 c 153: See note following RCW 57.04.050.