The exercise of the powers granted by sections 7-136n to 7-136s, inclusive, shall be in all respects for the benefit of the inhabitants of the state, for the increase of their commerce and for the promotion of their safety, health, welfare, convenience and prosperity, and as the operation and maintenance of any project which the municipalities may undertake constitute the performance of an essential governmental function, no municipalities shall be required to pay any taxes or assessments upon any project acquired and constructed by it under the provisions of said sections; and the bonds, notes, certificates or other evidences of debt issued under the provisions of said sections, their transfer and the income therefrom, including any profit made on the sale thereof, shall at all times be free and exempt from taxation by the state and by any political subdivision thereof but the interest on such bonds, notes, certificates or other evidences of debt shall be included in the computation of any excise or franchise tax.