This section may not be construed to invalidate a contract between the Commission and a municipality located in the sanitary district without the consent of the municipality. Except as provided in subsection (c) of this section, the Commission shall charge and collect its service rate for water, including a ready to serve charge, for all water used by: the State; Montgomery County; Prince George’s County; and a municipality in Montgomery County or Prince George’s County. Notwithstanding any other law, a fire department or rescue squad that is recognized by Montgomery County or Prince George’s County or that is maintained and operated by a municipality in Montgomery County or Prince George’s County is exempt from all Commission water and sewer usage charges and meter service charges.