No lot, parcel of land, building, or premises now or hereafter used by the city for Municipal purposes shall be subject to the sewer charges established in this chapter; nor shall any water supplied by the Municipal waterworks system for extinguishing fires, cleaning fire apparatus, or furnishing or supplying water to fire hydrants be used to determine any sewer charge as set forth in § 51.26.
(`59 Code, § 927.11)