At such time as a public sewer becomes available to a property served by a private sewage disposal system as provided in § 38-3-6, the building sewer shall be connected to the sewer within 90 days, and the private sewage disposal system shall be cleaned of sludge and waste material, which shall be disposed of in a lawful, sanitary and proper manner, and the private disposal system shall be filled with clean bank- run gravel or dirt.