Wastewater Treatment

Introduction

Wastewater includes water used for domestic purposes, such as bathing, laundry, and dish washing, as well as the sewage-bearing water that is flushed down toilets. Also included is all the water used by commercial facilities and institutions. In most countries, the water that is discharged from homes, institutions, and commercial facilities is treated to remove compounds and microorganisms that could pollute the water into which the wastewater is discharged. Wastewater treatment technologies need to respond to the stresses created on treatment systems by environmental changes and population growth. Some of these challenges arise from weather extremes, with flooding created by heavy storms high on the list of problems for wastewater treatment systems.

Storms can overwhelm normal wastewater drainage systems and treatment facilities, causing contaminants to be dumped into the environment, including into freshwater reserves. In densely populated coastal regions, storms can send seawater into freshwater sources. When this happens, costly treatment is needed to make the water safe for human consumption. Stress is also placed on wastewater treatment technologies by growing urban populations in countries where industrialization is increasing, such as in China, and by overall population growth in developing countries where sanitation is already inadequate or nonexistent.

Historical Background and Scientific Foundations

Wastewater treatment technologies have been developed to prevent the spread of diseases from waterborne microbes. Treatment technologies start with the identification of what is in the wastewater, since the technologies used must be specific to the contaminants in the wastewater in order to make it safe for reuse or discharge into the local environment. Municipal waste is largely human waste consisting of organic matter, but industrial waste may contain hazardous chemicals and heavy metals that require special treatment.

Disinfection—using chlorine or iodine, ozone, or ultraviolet radiation—is the most widely used treatment technology for wastewater that contains organic matter. Filtration through sand or through membranes is usually combined with disinfection.

Industrial wastewater requires special treatments that are determined by the specific contaminants in the wastewater. Many types of metallic contaminants can be precipitated out of the wastewater with chemical treatments and then removed by filtration. The removal of highly toxic elements like arsenic and lead is especially important because most wastewater treatment facilities discharge the treated water into the local environment where the water becomes part of the normal hydrologic cycle. There also is a trend to reuse treated water in industrial or agricultural applications where there is a shortage of adequate freshwater resources.

Impacts and Issues

Global warming, and the growing populations that are contributing to global warming, create challenges for the handling and treatment of wastewater. The volume of wastewater being generated in cities with expanding populations stresses municipal systems. The increased frequency of severe storms with flooding in some regions only adds to the problem of wastewater contaminating freshwater resources.

Wastewater from municipal treatment plants or storm drains can overflow during periods of flooding, dumping contaminated water into the ecosystem and reducing the amount of available oxygen in the water—oxygen that is needed for healthy aquatic life. Even more serious is the fact that contaminated water also can spread diseases among human populations.

The issue of waterborne disease is particularly critical in developing countries where less than half of the rural population has access to any wastewater treatment. An even more serious issue is the fact that about 90% of sewage and 70% of industrial waste are discharged into the local water environments without treatment in developing countries. United Nations studies indicate that more than 2.2 million people, mostly in developing countries, die each year from waterborne diseases.

In rural regions everywhere, agricultural wastewater can have very high concentrations of nutrients that become pollutants due to their concentrations. The treatment of agricultural wastewater is a challenge not only in developing regions but in many regions of

developed nations as well. The increased demand for agricultural products by growing populations and the weather extremes created by climate changes exacerbate these wastewater treatment issues.

WORDS TO KNOW

HYDROLOGIC CYCLE: The process of evaporation, vertical and horizontal transport of vapor, condensation, precipitation, and the flow of water from continents to oceans. It is a major factor in determining climate through its influence on surface vegetation, the clouds, snow and ice, and soil moisture. The hydrologic cycle is responsible for 25 to 30% of the mid-latitudes' heat transport from the equatorial to polar regions.

MEMBRANE FILTRATION: The use of plastic sheets or tubes that have holes so small only very small molecules, like water molecules, will pass through them.

ORGANIC MATTER: Remains, residues, or waste products of any living organism.

OZONE: An almost colorless, gaseous form of oxygen with an odor similar to weak chlorine. A relatively unstable compound of three atoms of oxygen, ozone constitutes, on average, less than one part per million (ppm) of the gases in the atmosphere. (Peak ozone concentration in the stratosphere can get as high as 10 ppm.) Yet ozone in the stratosphere absorbs nearly all of the biologically damaging solar ultraviolet radiation before it reaches Earth's surface, where it can cause skin cancer, cataracts, and immune deficiencies, and can harm crops and aquatic ecosystems.

WATERBORNE DISEASE: Infectious disease or parasite that is transmitted by unclean water supplies.

Although wastewater treatment technologies are available to handle effectively the challenges introduced by climate changes and population growth, the costs associated with these technologies have made wastewater treatment inadequate or unavailable in many regions of the world.

See Also Floods; Ocean Circulation and Currents; Water Shortages.

BIBLIOGRAPHY

Books

Cohen, Joel E. How Many People Can the Earth Support? New York: W. W. Norton, 1995.

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. New York: Rodale Press, 2006.

Web Sites

Gleick, Peter H. “Dirty Water: Estimated Deaths from Water-Related Diseases 2000–2020.” Pacific Institute Research Report, August 15, 2002.<http://www.pacinst.org/reports/water_related_deaths/water_related_deaths_report.pdf> (accessed October 4, 2007).

Miriam C. Nagel