In any buildings, structures or premises in which the house drain is too low to permit gravity flow to the sewer, the sewage shall be lifted by artificial means and discharged to the sewer. Wherever a situation exists involving an unusual danger of backup, the city may prescribe the minimum elevation at which the house drain may be discharged to the public sewer. Sewers below such minimum elevation shall be lifted by artificial means. However, if approved by the superintendent, the property owner may install a backwater sewage valve, which shall be installed and maintained at his or her expense. (Prior code § 4.130)