A separate account shall be kept by the municipality of the funds derived from such waterworks system and of their disposition, which account shall be audited annually by a competent auditor, and a report of such audit shall be open to public inspection. The treasurer of the municipality shall be the custodian of such funds and shall give bond to the satisfaction of the legislative body for the faithful discharge of his duties. Such funds shall be kept separate from other funds of such municipality and shall be used for such waterworks system and for no other purpose.