Mont. Code Ann. § 7-13-143

Notwithstanding any other section of this part or limitation imposed therein, when the board has applied pursuant to 7-13-142, the board may adopt a system of charges and rates to require that each recipient of treatment works services pays its proportionate share of the costs of operation, maintenance, and replacement and to require industrial users of treatment works to pay that portion of the cost of construction of the treatment works which is allocable to the treatment of that industrial user's wastes.