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

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

WASTEWATER TREATMENT

Water containing human waste and excreta is generally termed "wastewater." Usually, wastewater consists of 99.9 percent water and 0.1 percent waste. In the United States, each state has a law that requires the disposal of human waste in a sanitary manner. Treatment of wastewater is required to prevent the pollution of surface waters, the pollution of groundwater, and to prevent pathogenic and microbial contamination from the use of excreta as fertilizer. Also, wastewater should be disposed of in a sanitary manner to make it inaccessible to insects that transmit disease.

Wastewater treatment consists of physical, chemical, and biological processes—either aerobic or anaerobic. The aerobic process is used most frequently. In the activated sludge process, air has to be forced into the liquid in a tank that is used to maintain aerobic microbial activity and to prevent odor. Additionally, temperature and pH must be maintained for the microbial activity.

In a municipal system the flow moves as follows: from sanitary sewer to screening and grinding process, to primary clarification, to activated sludge or trickling filter, to secondary clarification, to chlorine treatment, and finally to a water body such as a river or stream. Wastewater from the home enters a domestic or sanitary sewer—a system of pipes that collect the wastewater. The waste is then transported to a wastewater treatment plant. As it enters the plant, it flows through a bar screen, which strains out large materials. It then continues into a grit basin or chamber, where the water is slowed down enough to allow heavy or dense particles to settle out. These particles are then removed and taken to a landfill. The materials that do not settle out are ground up to prepare them to be digested by microorganisms in the treatment plant.

The wastewater then enters the primary clarifier, which allows materials to settle out. The flow of water through the clarifier is slow, allowing large amounts of suspended solids to settle at the bottom in the form of sludge. The sludge is then scraped and pumped away to allow the process to continue.

From the primary clarifier, the wastewater enters activated sludge tanks or trickling filters. Trickling filters are large areas of biological decomposition consisting of rocks that host biological organisms on their surfaces. These organisms metabolize most of the suspended solids that did not settle in the primary clarifier. The buildup on these rocks eventually sloughs off. The activated sludge tank is also used to remove waste from the wastewater. In this process, water from the primary clarifier is pumped into an aeration tank and combined with a mixture rich in bacterial growth. Pure oxygen is pumped through, allowing the decomposition of the organic materials in the wastewater. The remaining water is moved from the top of the tank, leaving sludge at the bottom.

Water from the trickling filter moves to a secondary clarifier, which settles any remaining suspended solids. The solids are then pumped into a digester, while the effluent is chlorinated and released back into a water channel, river, or stream.

Mark G. Robson

(see also: Chlorination; Sewage System; Water Quality; Water Treatment )

Bibliography

Koren, H., and Bisesi, M. (1995). Handbook of Environmental Health and Safety, 3rd edition, Vol. II. Boca Raton, FL: Lewis Publishers.

Morgan, M. (1997). Environmental Health. Madison, WI: Brown & Benchmark.

Nadakavukaren, A. (2000). Our Global Environment, 5th edition. Prospect Heights, IL: Waveland Press.