A sufficient amount of the assessment levied under the provisions of sections 243.550 to 243.553 shall be appropriated by the county commissioners for the purpose of paying the principal and interest of the bonds and the same shall, when collected, be preserved in a separate fund for that purpose and no other. All bonds and coupons not paid at maturity shall bear interest at the rate of six percent per annum from maturity until paid, or until sufficient funds have been deposited at the place of payment, and the interest shall be appropriated by the county commission out of the penalties and interest collected on delinquent assessment or any other available funds of the district. Any expense incurred in paying the bonds and interest thereon, and a reasonable compensation to the bank or trust company for paying same, shall be paid out of other funds of the district collected for the purpose of meeting the expenses of administration.