After payment of the expenses of conducting and managing the water works, any surplus of a municipal corporation may be applied to the repairs, enlargement, or extension of the works or of the reservoirs, the payment of the interest of any loan made for their construction, or for the creation of a sinking fund for the liquidation of the debt. In those municipal corporations in which water works and sewerage systems are conducted as a single unit, under one operating management, a sum not to exceed ten per cent of the gross revenue of the water works for the preceding year may be taken from any surplus remaining after all of the preceding purposes have been cared for and may be used for the payment of the cost of maintenance, operation, and repair of the sewerage system and sewage pumping, treatment, and disposal works and for the enlargement or replacement thereof. Each year a sum equal to five per cent of the gross revenue of the preceding year shall be first retained from said surplus as a reserve for water-works purposes.

The amount authorized to be levied and assessed for water-works purposes shall be applied by the legislative authority to the creation of the sinking fund for payment of any indebtedness incurred for the construction and extension of water works and for no other purpose; provided, where such municipal corporation does not operate or maintain a water works or a sewage pumping, treatment, and disposal works, any or all such surplus may be transferred to the general fund of the municipal corporation in the manner provided for in sections 5705.15 and 5705.16 of the Revised Code.