The volume of flow used for computing wastewater charges shall be based upon the metered water consumption as shown in the records of meter readings maintained by the city. If the person discharging wastes into the wastewater system procures any part, or all, of his/her water from sources other than the city water department, all or a part of which is discharged into the city wastewater system, the person shall install and maintain at his/her expense water meters of a type approved by the City Engineer for the purpose of determining the volume of water obtained from the other sources. Residential irrigation use shall be taken into account in the development of wastewater charges.