The sewer user rates for each user shall be based on user’s contribution to the total wastewater loading of the treatment facilities in comparison to a standard equivalent user as defined below. Each user (or user class) shall be assigned a number of equivalent users to be multiplied by a constant monthly cost factor to determine the basic user monthly rate. Special uses contributing sewage of excessive flow and/or strength, for which the tabulating equivalent users do not apply, shall be individually calculated.