In primary treatment, sand, grit, and the larger solids in the wastewater are separated from the liquid. Screens, settling tanks, and skimmering 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.
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. For example, the material that is removed from wastewater doesn t just disappear. It is called sludge. Sludge requires proper treatment and disposal, and can often be reused.