Nothing contained in sections 6.501 to 6.521 of this code shall be construed to prohibit an agreement between the city and any person whereby a discharge which would otherwise be prohibited by sections 6.501 to 6.596 of this code may be admitted to the city wastewater system. Such an agreement may be made when, in the opinion of the city manager, special circumstances justify such agreement, provided that no interference or pass through results from the discharge and no additional costs are incurred by the city without recompense by the person. Categorical pretreatment standards shall not be waived by special agreement of the parties.