Except as otherwise provided in subsections (c), (d), and (e) of this section, the Commission shall impose a sewer usage charge, based on the water consumption of each property, against all properties connected to the Commission’s sewer system to provide funds for: payment of principal and interest for bonds authorized under § 22–102(a) and (b) of this article; maintenance of the sewer system and disposal facilities, including the overhead expense and depreciation allowance; and payments to the District of Columbia for disposal of sanitary district sewage. The Commission shall impose a sewer usage charge against the property of a federal, State, or other unit of government that is: exempt from front foot benefit charges and ad valorem taxes imposed under this division; and connected to the Commission’s sewer system. In setting the charge, the Commission shall take into consideration the general tax, the front foot benefit charge imposed in the sanitary district, and the regular sewer usage charge as provided in this section. If the Commission furnishes sewer service to a property that is not connected to the Commission’s water system, the Commission shall impose a sewer usage charge that: fairly and ratably compensates the Commission for the use of the sewer system; and takes into consideration the sewer usage on the property and the sewer usage charge applicable to similar properties connected to the water system. The Commission shall bill for the amount of the sewer usage charge under this subsection monthly, twice a year, or once a year. Subject to paragraphs (2) and (5) of this subsection, if water furnished by the Commission to a lot or parcel of land is used exclusively for any purpose that results in the water not entering the Commission’s sewer system, the Commission may not impose a sewer usage charge to the owner, tenant, or occupant of a lot or parcel of land for the amount of the water furnished by the Commission that does not enter the Commission’s sewer system. The owner, tenant, or occupant of a lot or parcel of land exempt under paragraph (1) of this subsection shall pay the Commission: the cost of installing a separate metered connection; and an annual amount equal to the Commission’s annual water service charge for the size of the meter installed. The Commission shall determine the location for the installation of the required meter. The Commission may adopt regulations regarding the maintenance and control of the meter. The sewer usage charge for properties under paragraph (1) of this subsection shall be based on the total amount of water used as determined under § 25–502(a) and (b) of this subtitle, less the amount of separately metered water. A commercial, industrial, or multiresidential property may use a separate metered connection as provided in subsection (d) of this section, even though a portion of the separately metered water enters the sewer system of the Commission, provided that the owner, tenant, or occupant of the property requests to be billed according to a formula determined by the Commission. The formula determined by the Commission under paragraph (1) of this subsection shall: credit the owner, tenant, or occupant for separately metered water not entering the sewer system of the Commission; and be consistent with: manufacturers’ engineering standards for the class of equipment using the separately metered water supplied by the Commission; or industry standards for the class of operations using the separately metered water supplied by the Commission. The sewer usage charge for properties under paragraph (1) of this subsection shall be based on the sum of: the total amount of water used as determined under § 25–502(a) and (b) of this subtitle, less the amount of separately metered water; and the amount of separately metered water as adjusted by the formula described in paragraph (2) of this subsection. This section may not be construed to invalidate an existing contract between the Commission and a municipality located in the sanitary district without the consent of the municipality.