Bonds for refunding one or more issues originally authorized for one or more purposes and bonds for any other purpose authorized in the Water and Sanitation District Act may be issued separately or issued in combination in one series or more by any district. Bonds payable solely from revenues of any utility or combination of utilities of the district shall not be refunded by general obligation bonds that are payable from general ad valorem property taxes unless authorized at an election as provided in the Water and Sanitation District Act.