The director may, with the approval of the council, enter into a contract with persons discharging industrial wastes who desire to reserve a portion of the design capacity of the sanitary sewer system or water pollution control plant. The contract shall contain a provision stating that an annual amount, representing the person's proportionate share of the City's net annual capital investment cost in the facilities reserved, shall be paid by the person signing the contract. The contract shall also contain a provision that if the person's average daily flow varies more than 20 percent on a yearly basis, either party may open the contract for renegotiation of the minimum payment.