Nothing in Sections 21-27-11 through 21-27-69 shall be construed to prohibit the municipality from appropriating and using any part of its available income or revenues derived from any source other than from the operation of such system or combined system in paying any immediate expenses of operation and/or maintenance of any such system or combined system. Nothing in Sections 21-27-11 through 21-27-69 shall be construed, however, to require the municipality to do so.