The governing body of any county in which a sanitary district has been or may hereafter be created by general or special law shall have power, subject to the conditions and limitations of this article, to issue bonds of such sanitary district to an amount in the aggregate of not exceeding eighteen per centum of the assessed value of all real estate in the district subject to local taxation, for the purpose of raising the necessary funds to carry into effect any or all of the purposes specified in Article 1 (§ 21-112.22 et seq.) of Chapter 2 of Title 21, provided, however, that such limitation of eighteen per centum shall not apply if the petition required by § 21-123 states the maximum amount of bonds to be issued, and if such bonds are to be issued for a specific undertaking from which the sanitary district may derive revenue, but from and after a period to be determined by the governing body of the county, not exceeding five years from the date of the election authorized in § 21-123, whenever and for so long as such undertaking fails to produce sufficient revenue to pay the cost of operation and administration (including interest on bonds issued therefor), and the cost of insurance against loss by injury to persons or property, and an annual amount to be covered into a sinking fund sufficient to pay, at or before maturity, all bonds issued on account of such undertaking, all such bonds outstanding shall be included in determining such limitation.