A separate fund shall be provided for each improvement to be financed by bonds payable from the consolidated improvement bond debt service fund, and the proceeds from the sale of any such bonds and from collection of special assessments and taxes levied for the improvement and any other monies appropriated thereto by the council shall be paid into such fund, and shall be used solely to defray expenses of the improvement and payment of principal and interest on any obligations issued to finance the improvement until completion and payment of all costs of the improvement. Thereupon, the improvement fund shall be discontinued, and any balance of the proceeds of the bonds remaining therein may be transferred by the council to the fund of any other improvement instituted pursuant to Minnesota statutes chapter 429. All monies not so transferred and all subsequent collections of special assessments and taxes levied for the improvement shall be credited and paid into the consolidated improvement bond debt service fund and are hereby pledged to the payment of principal and interest on all bonds payable therefrom. If the council shall have estimated that the proceeds of the bonds issued to finance an improvement and any other amounts appropriated to the separate fund established for such improvement will be sufficient to pay all costs of the improvement, special assessments collected for such improvement shall be deposited when received directly into the consolidated improvement bond debt service fund. (1981 Code 110 § 3)