All consumers who purchase from the City only a part of the water consumed or used by them and all persons who purchase or acquire water from any other source, regardless, shall pay based on such volume as set forth above as would have been assessed against such consumers had they been furnished all their water from the City's water system for disposal of industrial sewage. In order to determine the amount of water used by such consumers, regardless of its source, when less than the entire amount of water so used is furnished through the City's water system, such customers shall install water meters of a type currently used by the City's Water and Sewer Department, so that the same may be read in like manner as the City water meters are now read, and the proper charge for industrial sewage can be fixed and determined on a monthly basis as if such water had actually been purchased each month from the City at the City's prevailing rates, and in addition thereof, the surcharges as are provided for in the subdivision.