When any city of over twenty thousand inhabitants has purchased or erected a waterworks system under sections 91.090 to 91.300 and issued bonds in payment therefor as herein provided, such bonds shall be subject to be deposited with the director of the department of commerce and insurance of Missouri, as provided by the statutes relating to the deposit of securities by trust and insurance companies, and such bonds may also be deposited with the treasurer of the state of Missouri, as provided in section 30.270, within the discretion of the governor, attorney general and treasurer of the state, as provided in said section 30.270; provided, that at the time that such bonds are offered for deposit, the waterworks system upon which they are secured has, for a period of five years last past, earned all necessary current running expenses, and in addition thereto an amount equal to eight percent per annum on the entire amount of such bonds issued for the payment of said waterworks system.