Section 29. Each district shall be a body politic and corporate and shall have a commission to be known as the district commission.

Except as provided in the following paragraph, the district commission shall consist of two members from each town included in whole or in part in the district selected by the board of selectmen of such town and two members from each city included in the district selected by the city council. If only part of a town is included in the district, at least one of the two members so selected shall, unless impracticable, be a resident in the part so included; provided, however, that whenever a town or city has a population exceeding twenty thousand persons resident within such district, the board of selectmen or the mayor, but in the case of the city having a Plan D or Plan E charter the city manager, shall, if the resident population does not exceed fifty thousand persons, designate one additional person, or, if it exceeds fifty thousand persons, two additional persons, to serve as members of the district commission.

In the event of the formation of a mandatory district, the district commission shall consist of three members appointed by the director, with the approval of the water resources commission.

Each member of the commission shall be sworn and shall serve for three years or until his successor is selected and qualified.

The district commission shall employ a registered professional engineer who shall be the executive director of the district, and a person with accounting and financial experience who shall be the treasurer of the district, and shall fix their respective salaries. Said director shall be the executive and administrative head of the district, and the treasurer shall give a bond for the faithful performance of his duties in an amount required by the director.

The executive director may, with the approval of the district commission, employ such other persons as he shall deem necessary, and such persons shall be subject to the provisions of chapter thirty-one.