In this section, “charitable institution” means an institution: that is located in Montgomery County or Prince George’s County; and whose purpose is primarily charitable. Except as provided in subsection (c) of this section, if a charitable institution does not discriminate against the entrance or care of residents of Montgomery County or Prince George’s County, the Commission: shall supply without cost up to 100 gallons of water per day for each resident of the charitable institution; and may not charge the charitable institution a fee for sewer usage for the water supplied under item (1) of this subsection. The charitable institution shall install and maintain a water meter at its own cost. The Commission may charge the charitable institution for: the cost of reading the water meter; and sewer usage and water used in excess of 100 gallons per day for each resident of the charitable institution.