Screens, settling tanks, and skimming devices are most commonly used for the separation. Primary treatment removes 45 to 50 percent of the pollutants. After primary treatment, wastewater still contains solid materials either floating on the surface, dissolved in the water, or both. Under natural conditions, these substances would provide food for such organisms as fungi, algae, and bacteria that live in a stream or lake. Many public wastewater treatment plants now provide a second stage of treatment known as secondary treatment to remove more of the pollutants up to 80 or 90 percent altogether.
Secondary treatment is largely a biological process.
Air is supplied to stimulate the growth of bacteria and other organisms to consume most of the waste materials. The wastewater is then separated from the organisms and solids, disinfected to kill any remaining harmful bacteria, and released to a nearby lake, river or stream. You may have figured out by now that treatment of wastewater not only cleans the water that is released from the treatment plant to the stream, it leaves solids behind. These solids removed from wastewater do not just disappear. They can also be used as a landfill cover material, or placed in landfills for disposal.