If a lot discharging sewage uses over 7,000 gallons of water per month, and it is shown to the city’s satisfaction that a portion of the measured water does not enter the sewerage system, the city may determine the percentage of metered water entering the sanitary sewage system. That percentage shall constitute the basis of sewage service charges. However, the city may require or permit the installation of additional meters at the owner’s expense so as to determine the quantity of water actually entering the sewerage system.