Any municipality may use any sinking fund, reserve fund, or surplus fund to purchase any bond hereunder issued, and shall cancel and retire the same when, in the judgment of the governing authorities of such municipality, the interest of such municipality will be subserved thereby. Any surplus income from said enterprise arising through its operation or from its disposition, accruing to the municipality over and above the amount necessary to pay for repairs, replacements, bonds herein authorized which may be issued and interest thereon, may be applied by the governing board of the municipality upon any of the other outstanding debts or obligations of the municipality.