In primary Treatment, sand, grit, and the larger solids in the wastewater are separated from the liquid. Screens, settling tanks, and skimming devices are most commonly used for the separation. Primary treatment removes 45 to 50 percent of the pollutants. Under natural conditions, these substances would provide food for such organisims 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 85 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. 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.