Each nonresidential customer's wastewater treatment cost contributions as determined in Sections 13.20.030 and 13.20.040 shall be added together to determine such customer's annual wastewater service charge. Residential customers may be considered to be one class of customer and an equitable service charge may be determined for each customer based on an estimate of the total water consumption of this class of customer. (Ord. 603 (part), 2004)