No provision contained in this article shall be deemed to prevent any contract authorized
by city council between the city and any industrial user whereby an industrial waste
of unusual strength or characteristic may be accepted by the city for treatment, but
which will not violate or cause the city to violate federal or state discharge standards,
and which will not be harmful to the wastewater treatment plant, the treatment process
or the sewage lines. Waiver of national pretreatment standards may not be granted
unless such waiver is in compliance with the mechanisms established under the general
pretreatment regulations (40 CFR part 403).