Wastewater Treatment

Wastewater is simply water that has been used. It usually contains various pollutants, depending on what it was used for. It is classified into two major categories, by source:

1. Domestic or sanitary wastewater. This comes from residential sources including toilets, sinks, bathing, and laundry. It can contain body wastes containing intestinal disease organisms.
2. Industrial wastewater. This is discharged by manufacturing processes and commercial enterprises. Process wastewater can contain rinse waters including such things as residual acids, plating metals, and toxic chemicals.

Wastewater is treated to remove pollutants (contaminants). Wastewater treatment is a process to improve and purify the water, removing some or all of the contaminants, making it fit for reuse or discharge back to the environment. Discharge may be to surface water, such as rivers or the ocean, or to groundwater that lies beneath the land surface of the earth. Properly treating wastewater assures that acceptable overall water quality is maintained.

In many parts of the world, including in the United States, health problems and diseases have often been caused by discharging untreated or inadequately treated wastewater. Such discharges are called water pollution, and result in the spreading of disease, fish kills, and destruction of other forms of aquatic life. The pollution of water has a serious impact on all living creatures, and can negatively affect the use of water for drinking, household needs, recreation, fishing, transportation, and commerce.

Objectives and Evolution of Wastewater Treatment

We cannot allow wastewater to be disposed of in a manner dangerous to human health and lesser life forms or damaging to the natural environment. Our planet has the remarkable ability to heal itself, but there is a limit to what it can do, and we must make it our goal to always stay within safe bounds. That limit is not always clear to scientists, and we must always take the safe approach to avoid it.

Basic wastewater treatment facilities reduce organic and suspended solids to limit pollution to the environment. Advancement in needs and technology have necessitated the evolving of treatment processes that remove dissolved matter and toxic substances. Currently, the advancement of scientific knowledge and moral awareness has led to a reduction of discharges through pollution prevention and recycling, with the noble goal of zero discharge of pollutants.

Treatment technology includes physical, biological, and chemical methods. Residual substances removed or created by treatment processes must be dealt with and reused or disposed of in a safe way. The purified water is discharged to surface water or ground water. Residuals, called sludges or biosolids, may be reused by carefully controlled composting or land application. Sometimes they are incinerated.

Since early in history, people have dumped sewage into waterways, relying on natural purification by dilution and by natural bacterial breakdown. Population increases resulted in greater volume of domestic and industrial wastewater, requiring that we give nature a helping hand. Some so-called advancements in cities such as Boston involved collecting sewage in tanks and releasing it to the ocean only on the outgoing tide. Sludge was barged out to sea so as to not cause complaint.

Until the early 1970s, in the United States, treatment mostly consisted of removal of suspended and floating material, treatment of biodegradable organics, and elimination of pathogenic organisms by disinfection. Standards were not uniformly applied throughout the country.

In the early 1970s until about 1980, aesthetic and environmental concerns were considered. Treatment was at a higher level, and nutrients such as nitrogen and phosphorus were removed in many localities.

Since 1980, focus on health concerns related to toxics has driven the development of new treatment technology. Water-quality standards were established by states and the federal government and had to be met as treatment objectives. Not just direct human health but aquatic-life parameters were considered in developing the standards.

Wastewater Treatment Types

Rural unsewered areas, for the most part, use septic systems. In these, a large tank, known as the septic tank, settles out and stores solids, which are partially decomposed by naturally occurring anaerobic bacteria. The solids have to be pumped out and hauled by tank truck to be disposed of separately. They often go to municipal wastewater treatment plants, or are reused as fertilizer in closely regulated land-application programs. Liquid wastes are dispersed through perforated pipes into soil fields around the septic tank.

Most urban areas with sewers first used a process called primary treatment, which was later upgraded to secondary treatment. Some areas, where needed, employ advanced or tertiary treatment. Common treatment schemes are presented in the following paragraphs.

Primary Treatment. In primary treatment, floating and suspended solids are settled and removed from sewage.

Flow from the sewers enters a screen/bar rack to remove large, floating material such as rags and sticks.

It then flows through a grit chamber where heavier inorganics such as sand and small stones are removed.

Grit removal is usually followed by a sedimentation tank/clarifiers where inorganic and organic suspended solids are settled out.

To kill pathogenic bacteria, the final effluent from the treatment process is disinfected prior to discharge to a receiving water. Chlorine, in the form of a sodium hypochlorite solution, is normally used for disinfection. Since more chlorine is needed to provide adequate bacteria kills than would be safe for aquatic life in the stream, excess chlorine is removed by dechlorination. Alternate disinfection methods, such as ozone or ultraviolet light, are utilized by some treatment plants.

Sludge that settles to the bottom of the clarifier is pumped out and dewatered for use as fertilizer, disposed of in a landfill, or incinerated. Sludge that is free of heavy metals and other toxic contaminants is called Biosolids and can be safely and beneficially recycled as fertilizer, for example.

Secondary Treatment. Primary treatment provided a good start, but, with the exception of some ocean outfalls , it is inadequate to protect water quality as required by the Environmental Protection Agency (EPA).

With secondary treatment, the bacteria in sewage is used to further purify the sewage. Secondary treatment, a biological process, removes 85 percent or more of the organic matter in sewage compared with primary treatment, which removes about 50 percent.

The basic processes are variations of what is called the "activated sludge" process or "trickling filters," which provide a mechanism for bacteria, with air added for oxygen, to come in contact with the wastewater to purify it.

In the activated sludge process, flow from the sewer or primary clarifiers goes into an aeration tank, where compressed air is mixed with sludge that is recycled from secondary clarifiers which follow the aeration tanks. The recycled, or activated, sludge provides bacteria to consume the "food" provided by the new wastewater in the aeration tank, thus purifying it.

In a trickling filter the flow trickles over a bed of stones or synthetic media on which the purifying organisms grow and contact the wastewater, removing contaminants in the process. The flow, along with excess organisms that build up on the stones or media during the purification, then goes to a secondary clarifier. Air flows up through the media in the filters, to provide necessary oxygen for the bacteria organisms. Clarified effluent flows to the receiving water, typically a river or bog, after disinfection. Excess sludge is produced by the process and after collection from the bottom of the secondary clarifiers it is dewatered, sometimes after mixing with primary sludge, for use as fertilizer, disposed of in a landfill, or incinerated.

Advanced or Tertiary Treatment. As science advanced the knowledge of aquatic life mechanisms and human health effects, and the need for purer water was identified, technology developed to provide better treatment. Heavy metals, toxic chemicals and other pollutants can be removed from domestic and industrial wastewater to an increasing degree. Methods of advanced treatment include microfiltration, carbon adsorption, evaporation /distillation, and chemical precipitation.

Industrial Waste Treatment. Depending on the type of industry and the nature of its wastes, industries must utilize methods such as those used for advanced treatment of sewage to purify wastewater containing pollutants such as heavy metals and toxic chemicals before it can be discharged. Industries are permitted to discharge directly to receiving waters under the National Pollution Discharge Elimination System (NPDES) permit system or to municipal sewers under the Industrial Pretreatment Program. Pollution prevention programs are very effective in helping industries reduce discharged pollutants, by eliminating them at the source through recycling or through the substitution of safer materials. More and more industries are approaching or attaining zero discharge by cleaning and reusing their water over and over and over.

Combined Sewer Overflows

Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe. Most of the time, combined sewer systems transport all of their wastewater to a sewage treatment plant, where it is treated and then discharged to a water body. During periods of heavy rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers, or other water bodies. Some designs utilize an overflow at the treatment plant that diverts the excess flow to chlorination facilities for disinfection prior to discharge.

These overflows, called combined sewer overflows (CSOs), contain not only storm water but also untreated human and industrial waste, toxic materials, and debris. They are a major water pollution concern for the approximately 772 U.S. cities that have combined sewer systems.

CSO outfalls often result in violations of receiving stream-water quality standards and impairment to designated water uses. Violations can include aesthetics (including floatables, oil and grease, colors, and odor), solids, nutrients, harmful bacteria, metals, and reduced dissolved oxygen levels.

Historical and Regulatory Aspects

Environmental awareness and activism is not a present-day concept:

In the mid-1700s Benjamin Franklin and others petitioned the Pennsylvania Assembly to stop dumping waste and attempted to regulate waste disposal and water pollution. European countries were correlating sickness with lead and mercury in the late 1700s. In 1855, Chicago became the first U.S. city with a comprehensive sewer plan, and all U.S. towns with populations over 4,000 had city sewers by 1905.

In 1899 the Refuse Act prevented some obvious pollution of streams and placed the U.S. Army Corps of Engineers in charge of permits and regulation.

In 1914 U.S. government agencies began pollution surveys of streams and harbors. Reports filed by the early 1920s showed heavy damage from oil dumping, mine runoff, untreated sewage, and industrial wastes.

In 1924 the Oil Pollution Control Act prohibited discharge from any vessel within the three-mile limit, except by accident.

In 1948 the Federal Water Pollution Control Act and active House and Senate Public Works Committee in water pollution came about.

In 1956 Congress passed the Water Pollution Control Act, in 1961 the Clean Water Act, and in 1965 the Water Quality Act, setting standards for states.

In 1970 Congress and the president established the EPA.

In 1972 Congress passed the Federal Water Pollution Control Act (the "Clean Water Act").

In 1973 EPA issued the first NPDES permits.

In 1974 Congress passed the Safe Drinking Water Act.

The Clean Water Act of 1972. Said to be one of the most significant pieces of environmental regulations ever enacted, the federal Clean Water Act of 1972 was prompted by growing national concern for the environment in the late 1960s, fueled by such concerns as the burning Cuyahoga River in Ohio, an unfishable, unswimmable Potomac River, and a nearly dead Lake Erie.

National goals and objectives were established "to restore and maintain the chemical, physical, and biological integrity of the Nation's waters." There were two major goals:

1. Eliminate the discharge of all pollutants into navigable waters of the United States; and
2. Achieve an interim level of water quality that provides for the protection of fish, shellfish, and wildlife and recreation (the "fishable, swimmable" goal).

To help do this, the following were established:

A state grant program to support the construction of sewage treatment plants; the NPDES program, whose goal was to eliminate discharges to U.S. waters; and technological standards or discharge limits that had to be met, based on water-quality standards set by the states.

A minimum required percent removal of pollutants was added in 1985.

Secondary treatment was required, and limits were set for three major effluent parameters: biological oxygen demand, suspended solids, and pH.

The Water Quality Act of 1987 made several changes, addressing (1) excess toxic pollutants in some waters and (2) nonpoint source pollution . The construction grant program was phased out and replaced by financing projects with revolving fund, low-interest-rate loans. The amendments passed in 1987 also addressed storm-water controls and permits, regulation of toxics in sludge, and problems in estuaries. Penalties were added for permit violations. Also initiated were sludge-disposal regulations and funding for studies relative to nonpoint and toxic pollution sources.

The 1972 act has provided remarkable achievements, but there is still a long way to go. Forty percent of waters assessed by states still do not meet water-quality standards, mostly due to pollution from nonpoint sources. Other than from storm or combined storm sewer overflows, most of the remaining problem is not from pipes (point sources) but from sources such as farming and forestry runoff, construction sites, urban streets (storm water), automobiles, and atmospheric depositions, such as from power-plant air emissions (nonpoint sources). Current approaches to addressing nonpoint pollution include targeting and permitting by given watersheds and TMDL (total maximum daily load for a river stretch) assessments.

Many of the facilities funded by federal construction grants, which make up the wastewater collection and treatment infrastructure, are wearing out and are now undersized. Many, many dollars are needed to keep providing adequate treatment to maintain the status quo, let alone meet the needs of a growing populace.

Other Countries

Unfortunately, since the Industrial Revolution, most of Europe's rivers (not unlike in the United States) were utilized for transporting wastes to the sea, resulting in harm to human and aquatic health and causing coastal pollution. In earlier times, the rivers could handle the limited wastes discharged, through dilution and natural purification.

Significant progress has been made in treating the wastewater entering Europe's rivers, with measurable improvements in water quality. The agricultural sector (nonpoint pollution source) has not kept up, and nitrate levels are still high.

The fifteen-nation European Union's (EU) Urban Wastewater Treatment Directive has resulted in significant improvements in wastewater treatment capacity and methods. According to the European Environment Agency, increased treatment capacity has been realized in all EU countries except Sweden, Finland, and the Netherlands, where it is already efficient. The largest increase will be in southern Europe and Ireland. As a result, the EU's collection and treatment systems should be able to cope with all organic discharges from most member states by 2005. In Finland and Sweden most of the wastewater was being treated in tertiary plants in the 1980s.

see also Abatement; Biosolids; NPDES; Pollution Prevention; Water Pollution.

internet resources

environmental protection agency, office of water. (1993). "constructed wetlands for wastewater treatment and wildlife habitat." available from http://www.epa.gov/owow/wetlands/construct.

ohio state university extension, food, agricultural, and biological engineering. "wastewater treatment principles and regulations." available from http://ohioline.osu.edu/aex-fact/0768.html.

Raymond Cushman and George Carlson

The liquid and solid material removed from domestic septic tanks is called septage. Most septage is hauled to municipal sewage treatment facilities and most septage haulers must be licensed.

SETTLING POND

A settling pond, usually man-made, collects and slows water flow so that suspended solids (sediments) have time to precipitate or settle out of the water. Some applications of settling ponds include capturing runoff from farms (agricultural waste), construction projects (soil sediment) and mines (sediment and toxic waste). Settling ponds eventually fill and must be dredged to remain in operation. Polluted water from abandoned mines is diverted to settling ponds to remove solids such as iron oxide. When dredged, these sediments must be treated as contaminated waste. Pilot projects are underway to recapture iron oxide for use in paint pigments.

CONSTRUCTED WETLANDS

Constructed wetlands are wetlands that are specially built for the purpose of wastewater treatment and are utilized in place of naturally occurring wetlands. They provide a greater degree of wastewater treatment than natural wetlands, as their hydraulic loadings can be managed as required. Because these wetlands are constructed specifically for wastewater treatment, they should not be included in the jurisdictional group, which avoids the regulatory and environmental entanglement associated with natural wetlands. This is in accordance with Environmental Protection Agency regulations. The treatment process can be either aerobic or anaerobic , depending on whether the wetlands are constructed with an exposed water surface or one with subsurface flow. These wetlands can also be used to remove nitrogen, which is usually not removed during the standard wastewater treatment process. Nitrogen removal is accomplished by the growth of cattails and reeds, which utilize the highly nutrient wastewater and consequently remove nitrogen in the process. Sometimes the cattails and reeds must be harvested to complete the removal process.

Wastewater Treatment and Management

Waters that are used for drinking, manufacturing, farming, and other purposes are degraded in quality as a result of the introduction of contaminating constituents. Organic wastes, suspended solids, bacteria, nitrates, and phosphates are pollutants that commonly must be removed.

To make wastewater acceptable for reuse or for returning to the environment, the concentration of contaminants must be reduced to a nonharmful level, usually a standard prescribed by the U.S. Environmental Protection Agency. Furthermore, urban stormwaters flowing over lawns, rooftops, and paved surfaces are polluted by lawn chemicals, oil and gasoline spills on streets, plus other substances that become entrained in them as they make their way to a stream, river, or lake. These flows must also be subjected to some form of treatment to make them less harmful to the environment. Restoration of water quality is accomplished through the use of a variety of pollution control methods.

In urbanized areas, municipal wastewaters (mainly sewage) generally are conveyed to a point of treatment through sanitary sewers, whereas stormwaters are conveyed to their receiving bodies of water through storm drainage networks. In the past, cities sometimes used combined wastewater collection systems wherein a single sewerage network collected domestic wastewater, industrial wastes, and storm runoff water. But this configuration does not support the level of pollution control required today, and new systems of this type are no longer being built.

Sanitary sewers carry some level of flow during all hours of the day and night, whereas storm sewers flow mainly after periods of rainfall. During major storm events, the volumes of water carried by storm sewers are orders of magnitude greater than those carried by sanitary sewers. Wastewaters and stormwaters are subjected to treatment, but the types of treatment generally are quite different.

Wastewater Treatment Process

The task of designing and constructing facilities for treating wastewaters falls to environmental engineers. They employ a variety of engineered and natural systems to get the job done, using physical, chemical, biological, and sludge treatment methods.

The features of wastewater treatment systems are determined by (1) the nature of the municipal and industrial wastes that are conveyed to them by sewers, and (2) the amount of treatment required to preserve and/or improve the quality of the receiving bodies of water. Discharges from treatment plants usually are disposed by dilution in rivers, lakes, or estuaries . They also may be used for certain types of irrigation (such as golf courses), transported to lagoons where they are evaporated, or discharged through submarine (underwater) outfalls into the ocean. However, outflows from treatment works must meet effluent standards set by the U.S. Environmental Protection Agency to avoid polluting the bodies of water that receive them.

The categories of wastewater treatment are primary, secondary, and tertiary, or advanced. The minimum level of treatment required is usually secondary treatment, but some cities and industries are required to install tertiary or advanced wastewater treatment processes for removal of pollutants that are resistant to conventional treatment.

Stream classification documents, published by each state as required by the U.S. Clean Water Act of 1977, categorize surface waters according to their most beneficial present or future use, such as for drinking-water supplies, body-contact recreation, and so on. These publications also incorporate stream standards that establish maximum allowable pollutant concentrations for a given stream under defined flow conditions. Effluent standards under the Clean Water Act's National Pollutant Discharge Elimination System (NPDES) are used for regulatory purposes to achieve compliance with these stream standards. NPDES permits are issued to cities or other facilities that regulate the volume of discharge, contaminant concentrations, and timing of discharge so as to protect water quality in the receiving waterbody.

Conventional Treatment.

Conventional wastewater treatment consists of preliminary processes, primary settling to remove heavy solids and floatable materials, and secondary biological aeration to metabolize and flocculate colloidal and dissolved organics. Waste sludge drawn from these operations is thickened and processed for ultimate disposal, usually either land application or landfilling. Preliminary treatment processes include coarse screening, medium screening, shredding of solids, flow measuring, pumping, grit removal, and preaeration. Chlorination of raw wastewater sometimes is used for odor control and to improve settling characteristics of the solids.

Primary and Secondary Treatment.

Primary treatment involves sedimentation, and is the process by which about 30 to 50 percent of the suspended solid materials in raw wastewater are removed. Sedimentation must precede all biological filtration operations. The organic matter remaining after primary treatment is extracted by biological secondary treatment processes to meet effluent standards. Secondary treatment commonly is carried out using activated-sludge processes, trickling filters, or rotating biological contactors.

In the activated-sludge method, wastewater is fed continuously into an aerated tank where microorganisms break down the organics. The resulting microbial floc (activated sludge) is settled under quiescent (calm-water) conditions in a final clarifier and returned to an aeration tank. The plant effluent is clear supernatant from secondary settling.

Trickling filters and rotating biological contactors have media to support microbial films . These slime growths extract organic materials from wastewater as it trickles over the surfaces. Oxygen is supplied from air moving through voids (empty spaces) in the media. Excessive biological growth washes out and is collected in a secondary clarifier.

Tertiary Treatment.

Tertiary wastewater treatment is additional treatment that follows primary and secondary treatment processes. It is employed when primary and secondary treatment cannot accomplish all that is required. For example, phosphorus removal may be needed for wastewaters that are discharged to receiving waters that are likely to become eutrophic, or enriched with nutrients. (Cultural or human-enhanced eutrophication often is associated with nitrogen and phosphorous in effluent.) Water reclamation is achieved in varying degrees, but only a few large-scale plants are reclaiming water to near-pristine quality.

Sludge Processing and Disposal.

Primary sedimentation and secondary biological flocculation processes concentrate waste organics into a volume of sludge significantly less than the quantity of wastewater treated. But disposal of the accumulated waste sludge is a major economic factor in wastewater treatment. Methods for processing raw sludge include anaerobic (biological) digestion and mechanical dewatering by either belt-filter pressing or centrifugation. Conventional methods of disposal are application as a fertilizer or soil conditioner on agricultural land, landfilling in a dedicated disposal site, or codisposal with municipal solid waste.

Stormwater Treatment and Management

Stormwater treatment includes (1) storage in retention ponds where evaporation and seepage take place, and (2) diversion to natural or artificial wetlands , where pollutants are removed by vegetation and sedimentation and water is returned to the atmosphere by evapotranspiration. These methods take advantage of the ability of natural filtration and biological processes to aid in restoring water quality. Under certain circumstances, chemicals may also be introduced as treatment aids.

As noted above, the principal method used for stormwater treatment is storage wherein natural processes of sedimentation, evaporation, and nutrient removal take place. Because of the large volumes of water generated by storms, it usually is not practical to divert these waters to treatment plants such as those used to process municipal and industrial wastewaters. However, a number of devices can be inserted into stormwater systems to achieve various levels of removal of solids and other constitutents. These devices employ features of some of the components of wastewater treatment plants described previously.

see also Clean Water Act; Landfills: Impact on Groundwater; Pollution of Lakes and Streams; Pollution Sources: Point and Nonpoint; Runoff, Factors Affecting; Septic System Impacts.

Warren Viessman Jr.

Bibliography

Arms, Karen. Environmental Science. Philadelphia: Saunders College Publishing, 1990.

Cunningham, William P., and Barbara Woodworth Saigo. Environmental Science: A Global Concern, 5th ed. New York: Wm. C. Brown/McGraw-Hill, 1999.

Hammer, Mark J. Sr., and Mark J. Hammer Jr. Water and Wastewater Technology, 4th ed. Englewood Cliffs, NJ: Prentice Hall, 2001.

Loganathan, D., D. Kibler, and T. Grizzard. "Urban Stormwater Management." In Water Resources Handbook, ed. Larry Mays. New York: McGraw-Hill, 1996.

Makepeace, D. K., D. W. Smith, and S. J. Stanley. "Urban Stormwater Quality: Summary of Contaminant Data." Critical Reviews in Environmental Science and Technology 25 (1995):93–129.

ReVelle, Penelope, and Charles ReVelle. The Environment: Issues and Choices for Society, 3rd ed. Boston, MA: Jones and Bartlett Publishers, 1988.

Viessman, Warren Jr., and Mark J. Hammer. Water Supply and Pollution Control, 6th ed. Menlo Park, CA: Addison-Wesley, 1998.

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

Wastewater treatment

Wastewater often mixes with free-flowing water in rivers , streams, oceans, lakes , and other bodies of water. The addition of wastewater can radically alter the chemistry—and the ecological dynamics—of water bodies and hydrologic reservoirs. Wastewater includes the sewage-bearing water that is flushed down toilets as well as the water used to wash dishes and for bathing. Processing plants use water to wash raw material and in other stages of the wastewater treatment production process. The treatment of water that exits households, processing plants, and other institutions is a standard, even mandated, practice in many countries around the world. The purpose of the treatment is to remove compounds and microorganisms that could pollute the water to which the wastewater is discharged. Particularly with respect to microorganisms, the sewage entering a treatment plant contains extremely high numbers of bacteria, viruses, and protozoa that can cause disease if present in drinking water. Wastewater treatment lowers the numbers of such disease-causing microbes to levels that are deemed to be acceptable from a health standpoint. As well, organic matter, solids, and other pollutants that can add to stream load are removed.

Wastewater treatment is usually a multi-stage process. Typically, the first step is known as the preliminary treatment. This step removes or grinds up large material that would otherwise clog up the tanks and equipment further on in the treatment process. Large matter can be retained by screens or ground up by passage through a grinder. Examples of items that are removed at this stage are rags, sand , plastic objects, and sticks.

The next step is known as primary treatment. The wastewater is held for a period of time in a tank. Solids in the water settle out while grease, which does not mix with water, floats to the surface. Skimmers can pass along the top and bottom of the holding tank to remove the solids and the grease. The clarified water passes to the next treatment stage, which is known as secondary treatment.

During secondary treatment, the action of microorganisms is often utilized. There are three versions of secondary treatment. One version, which was developed in the mid-nineteenth century, is called the fixed film system. The fixed film in such a system is a film of microorganisms that has developed on a support such as rocks, sand, or plastic. If the film is in the form of a sheet, the wastewater can be overlaid on the fixed film. The domestic septic system represents such a type of fixed film. Alternatively, the sheets can be positioned on a rotating arm, which can slowly sweep the microbial films through the tank of wastewater. The microorganisms are able to extract organic and inorganic material from the wastewater to use as nutrients for growth and reproduction. As the microbial film thickens and matures, the metabolic activity of the film increases. In this way, much of the organic and inorganic load in the wastewater can be removed.

Another version of secondary treatment is called the suspended film. Instead of being fixed on a support, microorganisms are suspended in the wastewater. As the microbes acquire nutrients and grow, they form aggregates that settle out. The settled material is referred to as sludge. The sludge can be scraped up and removed. As well, some of the sludge is added back to the wastewater. This is analogous to inoculating growth media with microorganisms. The microbes in the sludge now have a source of nutrients to support more growth, which further depletes the wastewater of the organic waste. This cycle can be repeated a number of times on the same volume of water.

Sludge can be digested and the methane that has been formed by bacterial fermentation can be collected. Burning of the methane can be used to produce electricity . The sludge can also be dried and processed for use as compost.

A third version of secondary treatment utilizes a specially constructed lagoon. Wastewater is added to a lagoon and the sewage is naturally degraded over the course of a few months. The algae and bacteria in the lagoon consume nutrients such as phosphorus and nitrogen. Bacterial activity produces carbon dioxide . Algae can utilize this gas, and the resulting algal activity produces oxygen that fuels bacterial activity. A cycle of microbiological activity is established.

Bacteria and other microorganisms are removed from the wastewater during the last treatment step. Basically, the final treatment involves the addition of disinfectants, such as chlorine compounds or ozone , to the water, passage of the water past ultraviolet lamps, or passage of the water under pressure through membranes whose very small pore size impedes the passage of the microbes. In the case of ultraviolet irradiation, the wavelength of the lamplight is lethally disruptive to the genetic material of the microorganisms. In the case of disinfectants, neutralization of the high concentration of the chemical might be necessary prior to discharge of the treated water to a river, stream, lake, or other body of water. For example, chlorinated water can be treated with sulfur dioxide.

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.

Within the past several decades, the use of sequential treatments that rely on the presence of living material such as plants to treat wastewater by filtration or metabolic use of the pollutants has become more popular. These systems have been popularly dubbed "living machines." Restoration of wastewater to near drinking water quality is possible.

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.

See also Aquifer; Artesian; Drainage basins and drainage patterns; Drainage calculations and engineering; Hydrogeology; Stream capacity and competence

Wastewater treatment

Wastewater includes the sewage-bearing water that is flushed down toilets as well as the water used to wash dishes and for bathing. Processing plants use water to wash raw material and in other stages of the wastewater treatment production process. The treatment of water that exits households, processing plants and other institutions is a standard, even mandated, practice in many countries around the world. The purpose of the treatment if to remove compounds and microorganisms that could pollute the water to which the wastewater is discharged. Particularly with respect to microorganisms, the sewage entering a treatment plant contains extremely high numbers of bacteria , viruses , and protozoa that can cause disease if present in drinking water. Wastewater treatment lowers the numbers of such disease-causing microbes to levels that are deemed to be acceptable from a health standpoint. As well, organic matter, solids, and other pollutants are removed.

Wastewater treatment is typically a multi-stage process. Typically, the first step is known as the preliminary treatment. This step removes or grinds up large material that would otherwise clog up the tanks and equipment further on in the treatment process. Large matter can be retained by screens or ground up by passage through a grinder. Examples of items that are removed at this stage are rags, sand, plastic objects, and sticks.

The next step is known as primary treatment. The wastewater is held for a period of time in a tank. Solids in the water settle out while grease, which does not mix with water, floats to the surface. Skimmers can pass along the top and bottom of the holding tank to remove the solids and the grease. The clarified water passes to the next treatment stage, which is known as secondary treatment.

During secondary treatment, the action of microorganisms comes into play. There are three versions of secondary treatment. One version, which was developed in the mid-nineteenth century, is called the fixed film system. The fixed film in such a system is a film of microorganisms that has developed on a support such as rocks, sand, or plastic. If the film is in the form of a sheet, the wastewater can be overlaid on the fixed film. The domestic septic system represents such a type of fixed film. Alternatively, the sheets can be positioned on a rotating arm, which can slowly sweep the microbial films through the tank of wastewater. The microorganisms are able to extract organic and inorganic material from the wastewater to use as nutrients for growth and reproduction. As the microbial film thickens and matures, the metabolic activity of the film increases. In this way, much of the organic and inorganic load in the wastewater can be removed.

Another version of secondary treatment is called the suspended film. Instead of being fixed on a support, microorganisms are suspended in the wastewater. As the microbes acquire nutrients and grow, they form aggregates that settle out. The settled material is referred to as sludge. The sludge can be scrapped up and removed. As well, some of the sludge is added back to the wastewater. This is analogous to inoculating growth media with microorganisms. The microbes in the sludge now have a source of nutrients to support more growth, which further depletes the wastewater of the organic waste. This cycle can be repeated a number of times on the same volume of water.

Sludge can be digested and the methane that has been formed by bacterial fermentation can be collected. Burning of the methane can be used to produce electricity. The sludge can also be dried and processed for use as compost.

A third version of secondary treatment utilizes a specially constructed lagoon. Wastewater is added to a lagoon and the sewage is naturally degraded over the course of a few months. The algae and bacteria in the lagoon consume nutrients such as phosphorus and nitrogen. Bacterial activity produces carbon dioxide. Algae can utilize this gas, and the resulting algal activity produces oxygen that fuels bacterial activity. A cycle of microbiological activity is established.

Bacteria and other microorganisms are removed from the wastewater during the last treatment step. Basically, the final treatment involves the addition of disinfectants, such as chlorine compounds or ozone, to the water, passage of the water past ultraviolet lamps, or passage of the water under pressure through membranes whose very small pore size impedes the passage of the microbes. In the case of ultraviolet irradiation, the wavelength of the lamplight is lethally disruptive to the genetic material of the microorganisms. In the case of disinfectants, neutralization of the high concentration of the chemical might be necessary prior to discharge of the treated water to a river, stream, lake, or other body of water. For example, chlorinated water can be treated with sulfur dioxide.

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.

Within the past several decades, the use of sequential treatments that rely on the presence of living material such as plants to treat wastewater by filtration or metabolic use of the pollutants has become more popular. These systems have been popularly dubbed "living machines." Restoration of wastewater to near drinking water quality is possible.

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.

See also Biodegradable substances; Biofilm formation and dynamic behavior; Disinfection and disinfectants; Disposal of infectious microorganisms; Economic uses and benefits of microorganisms; Growth and growth media; Public health, current issues; Radiation mutagenesis; Water pollution and purification; Water quality