Wastewater is any water that requires cleaning after it is used. This includes water that has been used for laundry, bathing, dishwashing, toilets, garbage disposals, and industrial purposes. Wastewater also includes rainwater that has accumulated pollutants as it runs into oceans, lakes, and rivers. Pollutants are unwanted chemicals or materials that contaminate air, soil, and water.
The goal of wastewater management is to clean and protect water. This means that water must be clean enough so that it can be used by people for drinking and washing, and by industry for commercial purposes. It also must be clean enough to release into oceans, lakes, and rivers after it has been used.
Wastewater is usually divided into two major groups: point source wastewater and non-point source wastewater. Point source wastewater includes wastewaters that enter natural waters (such as lakes, rivers, and oceans) from defined locations. The most common point sources are sanitary sewers and storm drains. Non-point source wastewater is wastewater that is not connected to a specific source. This includes runoff (water that drains away) from agriculture and urban (city) areas, and acidic waters from mines. In many ways, point source wastewater is much easier to manage because its source and the pollutants it contains are known. Non-point source wastewater, on the other hand, is both hard to identify and treat.
One of the largest sources of wastewater is that which comes from homes and industries. These wastewaters all flow into sanitary sewers, which direct them into sewage treatment plants. Wastewaters from homes contain human waste, food, soaps, and detergents. They also contain pathogens, which are organisms that can cause diseases. Industrial wastewaters contain toxic (poisonous) pollutants, which can endanger human health and harm other organisms. These include pesticides, polychlorinated biphenyls (PCBs,) and heavy metals like lead, mercury, and nickel. These metals are generally toxic to plant and animal life. The goal of sewage treatment is to remove all of these pollutants from the wastewater so that it can be returned to natural waters.
Sewage treatment involves three stages: primary treatment, secondary treatment, and tertiary treatment. Primary treatment physically separates solids and liquids. The wastewater passes through a grating that strains out large particles. The remaining water is left to stand in a tank, where smaller sediments (particles of sand, clay, and other materials) settle to the bottom. These sediments are called sludge. At this point, the liquid part of the wastewater still contains many pollutants and is not safe for exposure to humans or the environment.
In the second step, called secondary treatment, the liquid part of the wastewater passes through a trickling filter or an aeration tank. A trickling filter is a set of pipes with small holes in it that dribbles water over a bed of stones or corrugated plastic. Bacteria in the stones or plastic absorb pollutants from the water and break them down into substances that are not harmful. An aeration tank is a tank that contains bacteria that break down pollutants. The liquid part of the wastewater from primary treatment is pumped into the tank and mixed with the bacteria. Air is bubbled through the tank to help the bacteria grow. As bacteria accumulate, they settle to the bottom of the tank and form sludge. The sludge is removed from the bottom of the tank and buried in landfills.
After secondary treatment, the water is generally free from the majority of pathogens and heavy metals. It still contains high concentrations of nitrate and phosphate, minerals that can over-stimulate the growth of algae and plants in natural waters, which can ultimately cause them and the surrounding organisms to die. Tertiary treatment removes these nutrients from the wastewater. One method of tertiary treatment involves using biological, chemical, and physical processes to remove these nutrients. Another method is to pass the water through a wetland or lagoon (shallow body of water cut off from a larger body).
In most cities in the United States, the sewers that carry storm waters are routed through sewage treatment plants. Much of the runoff from storms contains fertilizers, oils, and other chemicals that should be removed from the water before it enters lakes, rivers, and oceans. When there are very heavy rainfalls, however, the sewage treatment plants can become overwhelmed by the volume of water entering the facility. At these times, sewage and wastewater from storms may be dumped directly into natural water bodies. Many cities have programs underway to separate the storm sewers from sanitary sewers, but these projects are very costly and time consuming.
Agricultural runoff occurs when rain falls to the ground and then runs through agricultural fields or livestock-raising farms. The rainwater can accumulate fertilizer, oils, and animal wastes before it runs into rivers, lakes, and oceans. These materials pollute natural waters and can cause fish to die, contaminate drinking water, and speed up the rate of sedimentation (particles settling to the bottom of a waterway) in lakes and streams. In the summer of 1995, runoff from hog farms in North Carolina caused the rapid growth of the algae Pfisteria. This algae released toxins that affected the nervous system of fish as well as humans in the area.
In an attempt to manage agricultural runoff, the Office of Wastewater Management (OWM) of the U.S. Environmental Protection Agency has designated farms as Animal Feeding Operations (AFOS). As of 1998, nearly half a million AFOS had been identified. By designating AFOS, the OWM can regulate the disposal of animal waste products. This moves a large portion of agricultural runoff from the non-point source category to the point source category, and allows for better management of agricultural pollutants.
Acid mine drainage
In places where coal is mined, the mineral pyrite is a waste material. A series of complex reactions between pyrite, oxygen, and water result in acid mine drainage. Acid mine drainage is wastewater that is extremely acidic and contains high concentrations of heavy metals. Acid mine drainage is one of the major sources of stream pollution in the Appalachian mountain region. Acid mine drainage has severely damaged more than half the streams in Pennsylvania and West Virginia. There are at least 200,000 abandoned mines throughout the United States that produce acid mine drainage.
Acid mine drainage can be treated using chemical treatments that decrease the acidity of the water, and allow the heavy metals to precipitate (separate from the water). This type of treatment is often very expensive. Another way to treat acid mine drainage is by passing it through a lagoon or wetlands, which removes heavy metals and decreases the acidity of the water. Acid mine drainage is also treated by passing it through a channel of limestone (a rock that is very alkaline), which also neutralizes the acidity of the water.
When rain falls on natural lands such as forests and meadows, some of it soaks into the soil and then slowly makes its way to rivers, lakes, and oceans. In cities, much of the land is paved with cement and asphalt, and water is unable to sink into the ground. Instead, it quickly moves to storm drains and then into natural waterways. This great volume of water causes much erosion (wearing away of the land) and sedimentation. In addition, as the rainwater runs over paved surfaces, it gathers oil and grease from cars, fertilizers and pesticides from gardening, pathogens form animal wastes, road salts, and heavy metals. These are dumped directly into natural waters with urban wastewater. Runoff from urban areas is the largest source of pollution in estuaries (the wide part of a river where it nears the sea) and the third largest source of pollution in lakes.
Controlling urban runoff is extremely difficult because its sources are hard to identify. The Environmental Protection Agency works to influence developers to take into account urban runoff when planning new buildings. Some ideas to minimize runoff include adding vegetation and drainage areas to new construction sites. Some cities have instituted sewer-stenciling programs that remind people that rainwater flows directly into natural waters. Gas stations have also been targeted as businesses that can help control car oils and grease. Schools have also developed programs to teach students about urban runoff and non-point source wastewater.
Juli Berwald, Ph.D.
For More Information
Cunningham, William P. and Barbara Woodworth Saigo. Environmental Science: A Global Concern. Boston, WCB/McGraw-Hill, 1999.
Raven, Peter H, Linda R. Berg and George B. Johnson. Environment, Second Edition. Orlando, FL: Saunders College Publishing, 1998.
"Agricultural Runoff Management." Wisconsin Department of Natural Resources.http://www.dnr.state.wi.us/org/water/wm/nps/animal.htm (accessed on August 24, 2004).
"Managing Urban Runoff." United States Environmental Protection Agency.http://www.epa.gov/OWOW/NPS/facts/point7.htm (accessed on August 24, 2004).
"The Science of Acid Mine Drainage and Passive Treatment." Pennsylvania Department of Environmental Protection.http://www.dep.state.pa.us/dep/deputate/minres/bamr/amd/science_of_amd.htm (accessed on August 24, 2004).
"Wastewater Primer." United States Environmental Protection Agency.http://www.epa.gov/owm/primer.pdf (accessed on August 24, 2004).