Wastewater Treatment Technologies
Wastewater Treatment Technologies
Domestic and industrial activities produce billions of gallons of wastewater every year. This contains a wide range of impurities, ranging from human waste and bits of food, to detergents and industrial chemicals. If discharged straight into the environment, these quantities of wastewater would soon overwhelm and poison ecosystems, and create a serious human health hazard. Therefore, wastewater must be treated before it is allowed to rejoin the water supply.
Most wastewater is collected by the sewage system. Some industrial wastewater may be treated first to remove specific chemical pollutants. Afterward, the wastewater is filtered to remove solids and treated by chemical or biological processes to reduce levels of other pollutants to the safe limits specified by local environmental legislation. Pollution incidents arise when wastewater treatment is inadequate, or non-existent. There is an ongoing need for lower-cost treatment methods to meet the needs of developing countries.
Historical Background and Scientific Foundations
Sewage is the name usually given to the wastewater that is discharged from homes, offices, and industrial premises into the public sewerage system. Often, the terms sewage and wastewater are used interchangeably. In a home or office, wastewater is everything that goes down the drain, so it comprises water from the bath, shower, toilet, and washbasin, as well as that from the sink, washing machine, and dishwasher. Much industrial wastewater has been used for washing or cooling in various processes.
Wastewater contains human waste, with accompanying microbes, some of which are pathogenic (disease-causing), detergents, oil, food particles, grease, and industrial chemicals of various kinds. Wastewater is about 99.94% water and it looks cloudy because of its suspended solid content. Storm water is an additional type of wastewater arising from rain or melting snow, which runs along the ground. Storm water issuing from industrial premises, car parks, streets, and rooftops may contain oil, tars, metals, pesticides, fertilizers, and various other undesirable substances. Wastewater needs to be treated to remove these various impurities, or at least reduce them down to acceptable levels, before it is discharged back into the environment. Raw sewage, as untreated wastewater is often called, poisons fish and other aquatic organisms, and may cause human disease, as it contains pathogenic microorganisms. It also looks unsightly in waters used for recreational purposes.
Wastewater enters the sewage system, which is a series of pipes and pumps that carries it away from its place of origin to a facility where it can be treated to clean it up. Sewage systems date back to the nineteenth century, when the role that contaminated water plays in disease was first realized. Before that, people would just discard wastewater in the street.
At the sewage works, preliminary treatment consists of using a coarse screen on the incoming wastewater to remove larger particles such as rags, stones, and sand. It then undergoes a stage called primary treatment, where it is left to settle in a tank so that solids either sink to the bottom or float on the top of the wastewater. These bottom and top layers are skimmed off to form a sludge, which undergoes a drying treatment before being shipped as a fertilizer, which will return organic matter to the soil.
The clarified wastewater emerging from primary treatment is known as effluent. It then undergoes a biological treatment stage, known as secondary treatment, in which sewage microorganisms break down the organic matter in the effluent. There are various technologies for secondary treatment. In fixed film technology, microorganisms are grown on a surface such as rock, plastic, or sand where they create a film. The effluent then flows past the microbial film. One fixed film system is the trickling filter bed, which is a bed of stones coated with microbial film over which effluent drips via a series of perforated pipes or an overhead sprinkler.
Another system includes the suspended film system for secondary treatment in which microbes are suspended within the effluent in a tank and aerated. This system is sometimes also known as activated sludge. The third type of secondary treatment is the lagoon. This consists of a shallow basin in which the effluent is held for several months to allow natural microbial degradation approaches to work on it. Finally, the effluent undergoes chlorine or ultraviolet treatment to ensure that pathogenic organisms have been reduced to a safe level. Generally, the amount of pathogens in sewage is assessed using the count of coliform bacteria present
WORDS TO KNOW
COLIFORM: Bacteria present in the environment and in the feces of all warm-blooded animals and humans; useful for measuring water quality.
EFFLUENT: Wastewater from which solids have been removed.
SLUDGE: Semisolid material formed as a result of wastewater treatment or industrial processes.
STORM WATER: Water that flows over the ground when it rains or when snow melts.
WASTEWATER: Water that carries away the waste products of personal, municipal, and industrial operations.
within a sample. If chlorine treatment is used, then a neutralizing chemical will need to be added afterward, because chlorine can have an adverse impact on aquatic ecosystems once the treated wastewater is finally discharged.
Wastewater often contains high levels of nitrogen and phosphorus from human waste and detergents. Both are nutrients and, as such, enrich surface water when they are discharged. This enrichment, known as eutrophication, encourages the growth of algae and decomposer microorganisms to the detriment of other organisms, such as fish, and may upset the local ecosystem. Therefore, nitrogen and phosphorus may be removed from wastewater by adding chemicals that will precipitate them out.
Impacts and Issues
Standardized sewage treatment is usually provided for under environmental legislation such as the U.S. Clean Water Act of 1972. Those discharging wastewater into any kind of surface water need a permit and must adhere to limits of certain pollutants that have been laid down in law.
Setting up a sewer infrastructure can be costly and there is a need for lower-cost technologies for the developing world, so that clean water is available to all. One option is effluent sewerage, which is a mixture of a septic tank and a full sewerage system. The septic tank is a container used to allow sewage to settle and be digested by bacteria. It is intended for small-scale wastewater treatment. In effluent sewerage, a tank is used by each dwelling and its effluent is pumped to a central facility. Because only liquids are being treated here, smaller pipes, pumps, and treatment beds can be used, which lowers the cost.
IN CONTEXT: WASTEWATER TREATMENT STANDARDS
Wastewater treatment is usually subject to local and national standards of operational performance and quality in order to ensure that the treated water is of sufficient quality so as to pose no threat to aquatic life or settlements downstream that draw the water for drinking.
Chlorination remains the standard method for the final treatment of wastewater. However, the use of the other systems is becoming more popular. Ozone treatment is popular in Europe, and membrane-based or ultraviolet treatments are increasingly used as a supplement to chlorination.
Another approach is to use the natural remedial action of natural or artificial wetlands to treat wastewater. For instance, poplar plantations in India have been shown to be capable of carrying out secondary treatment on wastewater at a cost of a third, or less, of conventional mechanical treatment. Such constructed wetlands are now being used in American cities and in developing countries. Another low-tech option is to use duckweed on the surface of lagoons to purify wastewater, at a cost of one tenth of a conventional system. The duckweed can then be harvested and used as high-protein feed or as a fuel.
Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.
Ohio State University. “Wastewater Treatment Principles and Regulations.” http://ohioline.osu.edu/aex-fact/0768.html (accessed March 25, 2008).
World Health Organization. “Water, Sanitation, and Health: Wastewater Use.” http://www.who.int/water_sanitation_health/wastewater/en/ (accessed March 10, 2008).