Wash. Rev. Code § 36.94.190
Every county in furtherance of the powers granted by this chapter shall be authorized to contract with the federal government, the state of Washington, or any city or town, within or without the county, and with any other county, and with any municipal corporation as defined herein or with any other municipal corporation created under the laws of the state of Washington and not limited as defined in RCW 36.94.010, or political subdivision, and with any person, firm or corporation in and for the establishment, maintenance and operation of all or a portion of a system or systems of sewerage and/or water supply.
The state and such city, town, person, firm, corporation, municipal corporation and any other municipal corporation created under the laws of the state of Washington and not limited as defined in RCW 36.94.010, and political subdivision, is authorized to contract with a county or counties for such purposes.
RCW 36.94.190