Revenues derived from the operation or rental of any such facility (exclusive of land rentals) shall be paid into the treasury of the issuing municipality and kept in a separate fund and shall not be used except for the purpose of paying the cost of operation, maintenance and improvement of such special facility, providing an adequate depreciation fund and paying the principal of and the interest upon said bonds, all as may be agreed upon with said bondholders.
K.S.A. 3-156