Industrial users shall be charged by measuring the volume of sewage discharged, and their charges may be regulated by an agreement between the user and the city. Any industrial user who discharges sewage with a strength higher than two hundred fifty mg/1 for either biochemical oxygen demand (BOD) or total suspended solids (TSS) will pay an additional charge for the extra strength of the sewage. The amount of the charge is five and three-tenths cents per twentyfive mg/1 per one thousand gallons of metered water usage. (Ord. 489, 2002).