One faithful reader has asked “How do factories control air pollution”? There are two basic options: pollution prevention and pollution control. Pollution prevention means changing the factory’s operations so that it creates little or none of a particular pollutant. For example, a cadmium plating factory can prevent cadmium pollution by switching to a different plating process that doesn’t use cadmium. That process will use other metals, but hopefully ones that are less toxic. Factories that switch from coal to natural gas fuel can dramatically reduce emissions of mercury, particulates, greenhouse gasses, and other contaminants such as sulphur.
The second major option is pollution control. This is equipment installed, usually “end of pipe”, to capture pollutants before they are released into air or water. A common example is a baghouse, where dust in the air is forced through filters before being released to the outdoors. The filters catch most of the dust, which is then removed for reuse, recycling or disposal.
For more information, see the free online air pollution control technology manual and http://en.wikipedia.org/wiki/Air_pollution#Control_devices.