Whenever the municipalities so jointly contracting as provided in this article, shall have entered into a contract with the authorities of any other municipality, granting the privilege of connecting the sewers of such other municipality with such joint improvements or works and under the terms of said contract, such other municipality or municipalities is entitled to discharge through said connection into such joint improvements or works, sewage matter of a fixed quantity per diem, and such other municipality shall deem it expedient to purchase an additional sewage capacity in such joint improvements or works and to discharge therein an additional quantity of sewage, the municipalities so jointly contracting as aforesaid, may be resolution of the joint meeting of the governing bodies or boards thereof, having power to construct sewers or drains therein, authorize the execution of a further or supplemental contract with such other municipality granting to such other municipality the privilege to increase its sewage capacity in said joint improvements or works, and to discharge therein an additional quantity of sewage matter upon such terms and for such consideration and for such length of time as may be mutually agreed upon between the joint meeting and such other municipality, and the governing board or body of such other municipality, having power to construct sewers and drains therein, may by ordinance or resolution authorize the execution of such further or supplemental contract with the municipalities represented in such joint meeting, on behalf of such other municipality.