The metropolitan utilities district may, in its discretion, also afford, free of charge, water required for public use by each of the municipalities and schools within the limits of such municipalities. It shall be the duty of each of the municipalities and schools to reasonably conserve such water and to install and maintain all plumbing and services required in connection with such use in good condition and free from leaks, subject to the rules and bylaws governing water service in such district. If any flush tank maintained in connection with the sewage system of any such municipality uses more than fifty thousand gallons of water per month, as determined by meter measurement, the board of directors of the district may collect for the excess water used at the established rates maintained by the board.