The board of county commissioners in each year shall levy upon the property liable to taxation on account of the location and construction of any project as provided by this chapter a tax sufficient to pay the annual interest on any bonds which may have been issued for the purpose of locating and constructing such project. Separate sinking funds shall be provided for each separate project for the construction of which bonds shall have been issued. No funds in any such sinking fund shall be applied to any other purpose than the payment of the bonds for the payment of which such fund was created. No county shall be liable for the payment of any bonds issued under the provisions of this chapter but such bonds shall be paid only out of the sinking funds created as provided in this chapter.