Interest shall be charged on all unpaid principal amounts of in-lieu of assessment charges at the rate of 8 percent per year, simple interest until paid. Such interest on unpaid charges shall be deposited to the Pierce County Sewer Maintenance Fund and used first to defray the costs of administering, maintaining records of, and collecting moneys into and on behalf of the fund, and thereafter for such other purposes and uses as may be deemed necessary or desirable by the Utilities Department.