Wash. Rev. Code § 35.92.410
A city or town contiguous with Canada may contract with a Canadian corporation for the discharge of sewage from all or any portion of the city's or town's sewer service area into the sewer system of the Canadian corporation. A city or town contiguous with Canada may contract with a Canadian corporation for the construction, operation, or maintenance of sewers and sewage treatment and disposal facilities for their joint use and benefit upon such terms and conditions and for such period of time as the contracting parties may determine, which may include vesting one of the contracting parties with the sole authority to construct, operate, or maintain the facilities with the other contracting party or parties paying an agreed-upon portion of the expenses to the party with sole authority to construct, operate, or maintain the facilities.
RCW 35.92.410