Water is treated to make it safe to drink and to use for other purposes, such as to spray on agricultural plants. Water that contains domestic and industrial waste is often required to be treated to lessen or remove the contaminants prior to the discharge of the water into a river, lake, or ocean.
Some industrial processes require water that is free of impurities and microorganisms. One example is the water used in the manufacture of pharmaceuticals. The preparation of a medicine using contaminated water could be disastrous for the patient.
The need for treatment of drinking water is becoming more urgent, even in developed countries. The increasing populations of developing countries are encroaching more on previously undisturbed watersheds. As the watershed quality deteriorates, the ability of the watershed to naturally purify the water flowing through it is lessened. As well, the increasing use of chemicals is contaminating groundwater. Watersheds that were pristine only a few decades ago are now under threat.
The treatment of water for drinking is also referred to as water purification. Purification typically involves several steps. These are designed to remove objects from the water, particularly if the water is from a surface source like a river or a lake, and also to treat the water to minimize the risk from microorganisms.
The physical removal of objects like sticks and leaves is the first step in drinking water treatment. The water is filtered and then passed into a settling tank. As the name implies, the tank allows sand and grit to settle out on the bottom. Even smaller material is next removed in a step called coagulation. Here, a chemical called alum is added. The alum forms globs that attach to bacteria, silt, and other materials. The globs subsequently sink to the bottom of the holding tank.
Water can then be treated in several ways. It can be pumped through a filter that has much smaller holes in it than the filter designed to remove large objects. The holes or pores of the filter are so small that particles as small as viruses, bacteria, and protozoa cannot pass through to the other side of the filter. The filtration is intended to mimic the movement of water down from the surface through the soil and rock layers.
Filtration has become an important way to clear water of protozoa such as Cryptosporidium and Giardia. These organisms, which are typical residents of wild animals like the beaver, are resistant to the traditional chemical treatment of water. Chemical treatment utilizes chlorine to kill susceptible microorganisms. The process of killing the microbes is referred to as disinfection.
Chlorination disinfection has the advantage that a residual amount of the chemical remains in the water as the water passes through the pipelines on its way to the tap. This property, and the efficiency of killing by chlorine and chlorine-containing compounds, has made chlorination the most popular drinking water treatment method for over 50 years. However, the method is not without drawbacks. In particular, chlorine by-products can form in the presence of organic material. These byproducts, which are known as trihalomethanes, have been linked to health problems in humans. There is concern that the long-term ingestion of trihalomethanes can be harmful to health. Increasingly, the use of chlorine dioxide, which does not form trihalomethanes, or alternatives to chlorination, either alone or as secondary treatment that permit the chlorine concentration to be lowered, are being used.
Other means of disinfection that are becoming increasingly popular include the use of ozone and ultraviolet light. Home-based ultraviolet systems that sterilize the water just prior to the tap are becoming popular.
Wastewater treatment includes domestic and industrial waters. Domestic water commonly includes water flushed down toilets and the "gray" water from bathing and dish washing. Industrial water is water that has been used in production processes. Such water can contain chemicals that are toxic or foul smelling. Processing of domestic and industrial wastewater is necessary to remove the noxious compounds and microorganisms, or reduce the amounts of these items to acceptable levels, before the water is discharged into another body of water. Increasingly, the treatment of wastewater is a legal requirement.
Like the treatment of drinking water, wastewater treatment is a multi-stage process. Initially, a pre-treatment step filters out or grinds up objects such as sticks, rags and bottles that would clog equipment further on in the process.
The primary treatment step allows materials to either settle to the bottom or, in the case of liquids such as grease or oil that do not mix with water, to float to the surface. The surface waste is skimmed off. The clarified water passes on to the secondary treatment.
Secondary treatment uses microorganisms to digest organic material in the water. This can be done in one of three ways. The first method is called the fixed film system. This was developed in the mid-nineteenth century. The film is a film of microorganisms that has grown on rocks, sand, or plastic. In the case of a film on a flat support such as a plastic sheet, as in a typical domestic septic field, the wastewater can be flowed over the microbial film. As the water slowly passes over the film, the bacteria in the film digest the impurities in the water. Alternatively, the fixed film can be positioned on an arm, which can slowly sweep through the wastewater.
A third version of secondary treatment is called the suspended film. Microorganisms are suspended in the wastewater. Over time, the microbes clump together and settle out as sludge. The sludge can then be removed. Some of the sludge is added back to the wastewater to keep the digestion process going. This cycle can be repeated on the same volume of water, in order to digest most of the impurities.
The sludge that is collected can be subsequently used as compost, or can be digested by the bacteria, which produce methane that can be collected for use as a fuel and power source.
A forth version of secondary treatment is a lagoon. Wastewater is added to a lagoon and the sewage is degraded over the next few months. The algae and bacteria that are normal residents of the lagoon will use compounds such as phosphorus and nitrogen as food sources. Bacteria will produce carbon dioxide that is used by algae. The resulting algal activity produces oxygen that stimulates growth of the bacteria. This cycle of microbiological activity can continue until the organic matter in the water is consumed.
The final treatment step removes or neutralizes bacteria and other microorganisms. This step involves the use of a disinfectant like chlorine, or the use of filters, ozone, or ultraviolet light. Neutralization of the disinfectant chemical might be necessary prior to the flow of the treated water into a river, stream, lake, or other body of water. For example, chlorine can be removed by a reaction with sulfur dioxide.
Within the past several decades, the use of treatments that rely on the presence of living material such as plants to treat wastewater has become more popular. These systems, which are known as "living machines," can produce water that meets the requirements of purity for drinking water.
American Water Works Association. Water Quality and Treatment. 5th ed. Denver: American Water Works Association, 1999.
Droste, R. L. Theory and Practice of Water and Wastewater Treatment. New York: John Wiley & Sons, 1996.
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