§ 120-h. Further provisions as to apportionment of cost. Each of the contracting municipalities or districts shall pay its just and proportionate share for the public improvement authorized by this act and the general laws, including its just and proportionate share of the cost for the removal of sewage and of maintenance and carrying charges of the system. The manner of arriving at the share each local government shall bear and the method of payment thereof as hereinafter provided shall be determined by its local board or commissioners having charge of sewage, before such contract for construction or for sewerage removal becomes effective, as hereinafter provided.