If fifty or more of the subscribers, but not more than one from any one household, who have contracts to purchase water from a water system or for sewerage service from a sewerage system file with the Commission a petition alleging that the service furnished by the system is inadequate and ought to be improved, the Commission shall after notice to the operators of such system investigate the complaint and formulate an opinion whether in the light of the successful performance of sewerage or water systems of similar design and purpose, the system is capable of serving the reasonable domestic needs of the persons or properties served.