Any such municipality may enter into contracts or agreements with any persons, firms or corporations for the operation and management of the facilities and properties of said sewerage system, or any part thereof, for such period of time and under such terms and conditions as shall be agreed upon between such municipality and such persons, firms or corporations. Such municipality shall have power to provide in the resolution authorizing the issuance of bonds hereunder, or in any trust indenture, securing such bonds, that such contracts or agreements shall be valid and binding upon the municipality as long as any of said bonds, or interest thereon, are outstanding and unpaid.