A wastewater treatment plant is a facility where dirty water is directed to be cleansed of contaminants. Some plants treat sewage while others handle industrial or agricultural wastewater. Depending on the nature and degree of the contamination, a wastewater treatment plant may use physical, chemical, or biological methods to clean the water.
Often, the first step in wastewater treatment is pumping the water through one or more filters. Some filtration methods use tightly woven cloth or other screens. This removes any debris from the water, including solid waste, rocks, and decaying organic matter, such as leaves, branches, and dead animals. It also prevents clogs and other problems later in the wastewater treatment process. Sand may also be used as a filter, and is effective at removing smaller particles such as bacteria from the water.
Other physical processes may be used to remove smaller solids and gases from the water, too. By aerating it, a wastewater treatment plant allows gases to escape from the water and restores oxygen that may have been consumed as organic matter decayed in it. It also causes sand, coffee grounds, and other fine particles to settle so they can be removed. This effect can also be achieved by sedimentation, the process where the force of gravity causes solid particles, which are heavier than water molecules, to settle to the bottom of the wastewater treatment tank. Meanwhile, lighter particles such as oil, soap, and plastic float to the top, where they can be skimmed off.
If the water cannot be sufficiently purified through physical means, a wastewater treatment plant may apply chemicals to clean it. Chlorine is most commonly used for chemical wastewater treatment, though ozone is sometimes used and has a similar bacteria-killing effect. If the chlorine isn't used up in the process of killing the bacteria, it may have to be neutralized through the addition of other chemicals. Neutralizing chemicals are also used to balance the pH of wastewater.
A wastewater treatment plant may also employ biological wastewater treatment to decontaminate water. Typically, specially selected bacteria are added to the water, where they convert the pollutants into sludge, gas, or other by-products that can more easily be removed. In anaerobic wastewater treatment, no oxygen is required for this conversion to take place. The end product of anaerobic wastewater treatment is methane gas, which is valuable. In contrast, aerobic wastewater treatment requires oxygen to be added to the water, and may create a lot of sludge, which is expensive and difficult to dispose.
The solids removed from wastewater may also be treated with bacteria. Often, they are held in a digester, a large, heated tank, where bacteria can break the solids down into a less noxious substance. From there, the waste may be sent to a landfill, or it may be used as fertilizer. The cleaned water is then typically releases it into the local water supply.