Al Hartmann | The Salt Lake Tribune Sanitation truck fills up at Salt Lake County Fleet Management's new CNG (compressed natural gas) filling station Tuesday, Nov. 3. The new CNG filling stations will service its large fleet of trucks that are switching from diesel to the cleaner, cheaper charged natural gas. The change to this fuel is expected to reduce the fleet's contribution to air pollution in the Salt Lake Valley — just as residents brace for on the onset of inversion season.