Any municipality which may own and operate any such system may improve, enlarge, extend or repair it; and, whenever there shall be in force an agreement between any municipality owning a waterworks system and any public authority or agency authorized to sell water at wholesale which agreement requires payments of money or a loan by the municipality to the authority, or both, in return for a commitment on the part of the public authority or agency to furnish water to the municipality in quantity for a period of not less than twenty years, the benefits to be derived by the municipality under such agreement shall be deemed an improvement to and extension of the waterworks system of the municipality so obligating itself, to finance which bonds may be issued pursuant to this chapter to obtain the moneys required by the municipality to discharge its undertakings.