Every person owning or occupying premises receiving its water supply from the city shall maintain such water supply free from any connection, either of a direct or of an indirect nature, with a water supply from a foreign source or by any manner of connection with any fixture or appliance whereby water from a foreign supply or the waste from any fixture, appliance, waste, or soil pipe may flow, be siphoned, or pumped into the piping of the water system. All existing private fire protection systems having cross-connection with private supplies must maintain the double check and gate valves free from leakage or defect of any nature.