(1) At the end of each fiscal year of the operation of an electric power plant or waterworks system, the commissioners of a municipal improvement board shall set aside, in a special account out of the profits from the operation of the plant or system, such sum as may be necessary to pay all bonds and interest of the improvement district which shall accrue during the next ensuing fiscal year so that the district shall, at all times, have in the special account a sum sufficient to pay all principal bonds and interest which may accrue within the next fiscal year.
(2) The funds so deposited in the special account shall be used for no other purpose except the payment of bonds and interest of the district.