Whenever the sewers of one or more sewer districts under the direction and control of any board of county commissioners drain into the treatment plant of another municipality either within or outside the state, and the municipality imposes a charge upon the sewer district for a proportionate share of the expense of operating the treatment plant, the governing body of the sewer district, by resolution, may establish an additional user charge to cover the total cost of the charge imposed upon the district.
K.S.A. 19-27a25