Any municipal corporation in this State which is about to enlarge, extend or establish a system of sewerage therein may, by its mayor and aldermen, intendant and wardens or city or town council, elect five or seven of its citizens, who shall be freeholders therein, as a sewerage commission, which shall be known and designated as the sewerage commission of such municipal corporation; provided, that not more than three persons so elected as members of said commission shall be members of the body electing such commission. The members of the sewerage commission shall continue as such for a term of two years and until their successors are elected or until the enlarging, extending or establishment of the system of sewerage is fully completed, as contemplated under the laws and ordinances providing therefor.