A sufficient amount of drainage tax shall be appropriated by the board of supervisors for the purpose of paying the principal and interest of such bonds and 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 per cent per annum from maturity until paid or until sufficient funds have been deposited at the place of payment and such interest shall be appropriated by the board of supervisors out of the penalties and interest collected on delinquent taxes 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 them shall be paid out of other funds in the hands of the district treasurer and collected for the purpose of meeting the expenses of administration.