Same; rates for water; how determined; sinking fund

The waterworks board shall fix such rates for water furnished to consumers as will secure an income sufficient to pay all salaries and wages of all officers and employees in such department; to cover the cost of all materials and supplies used in the operation of the plant; to cover the cost of all repairs; to cover all miscellaneous expenses; to pay all interest charges upon all indebtedness of the city created for the purpose of purchasing, improving or extending the waterworks, and to provide a sinking fund sufficient for the purpose of paying off such indebtedness at maturity, and to cover the cost of all repairs and renewals of the plant and all material used, together with a reasonable allowance for emergency and unforeseen expenses: Provided, That the waterworks board may fix special rates for water furnished to the governing body of the city for public purposes, or to organized institutions of charity.