Sewage Disposal. I INTRODUCTION Sewage Disposal, or wastewater disposal, various processes involved in the collection, treatment, and sanitary disposal of liquid and water-carried wastes from households and industrial plants. The issue of sewage disposal assumed increasing importance in the early 1970s as a result of the general concern expressed in the United States and worldwide about the wider problem of pollution of the human environment, the contamination of the atmosphere, rivers, lakes, oceans, and groundwater by domestic, municipal, agricultural, and industrial waste. See Air Pollution; Water Pollution. II HISTORY Methods of waste disposal date from ancient times, and sanitary sewers have been found in the ruins of the prehistoric cities of Crete and the ancient Assyrian cities. Storm-water sewers built by the Romans are still in service today. Although the primary function of these was drainage, the Roman practice of dumping refuse in the streets caused significant quantities of organic matter to be carried along with the rainwater runoff. Toward the end of the Middle Ages, below-ground privy vaults and, later, cesspools were developed. When these containers became full, sanitation workers removed the deposit at the owner's expense. The wastes were used as fertilizer at nearby farms or were dumped into watercourses or onto vacant land. A few centuries later, there was renewed construction of storm sewers, mostly in the form of open channels or street gutters. At first, disposing of any waste in these sewers was forbidden, but by the 19th century it was recognized that community health could be improved by discharging human waste into the storm sewers for rapid removal. Development of municipal water-supply systems and household plumbing brought about flush toilets and the beginning of modern sewer systems. Despite reservations that sanitary sewer systems wasted resources, posed health hazards, and were expensive, many cities built them; by 1910 there were about 25,000 miles of sewer lines in the United States. At the beginning of the 20th century, a few cities and industries began to recognize that the discharge of sewage directly into the streams caused health problems, and this led to the construction of sewage-treatment facilities. At about the same time, the septic tank was introduced as a means of treating domestic sewage from individual households both in suburban and rural areas. Because of the abundance of diluting water and the presence of sizable social and economic problems during the first half of the 20th century, few municipalities and industries provided wastewater treatment. During the 1950s and 1960s, the U.S. government encouraged the prevention of pollution by providing funds for the construction of municipal waste-treatment plants, water-pollution research, and technical training and assistance. New processes were developed to treat sewage, analyze wastewater, and evaluate the effects of pollution on the environment. In spite of these efforts, however, expanding population and industrial and economic growth caused the pollution and health difficulties to increase. In response to the need to make a coordinated effort to protect the environment, the National Environmental Policy Act (NEPA) was signed into law on January 1, 1970. In December of that year, a new independent body, the Environmental Protection Agency (EPA) was created to bring under one roof all of the pollution-control programs related to air, water, and solid wastes. In 1972 the Water Pollution Control Act Amendments expanded the role of the federal government in water pollution control and significantly increased federal funding for construction of waste-treatment works. Congress has also created regulatory mechanisms and established uniform effluent standards. III TRANSPORT OF WASTEWATER Wastewater is carried from its source to treatment facility pipe systems that are generally classified according to the type of wastewater flowing through them. If the system carries both domestic and storm-water sewage, it is called a combined system, and these usually serve the older sections of urban areas. As the cities expanded and began to provide treatment of sewage, sanitary sewage was separated from storm sewage by a separate pipe network. This arrangement is more efficient because it excludes the voluminous storm sewage from the treatment plant. It permits flexibility in the operation of the plant and prevents pollution caused by combined sewer overflow, which occurs when the sewer is not big enough to transport both household sewage and storm water. Another solution to the overflow problem has been adopted by Chicago, Milwaukee, and other U.S. cities to reduce costs: instead of building a separate household sewer network, large reservoirs, mostly underground, are built to store the combined sewer overflow, which is pumped back into the system when it is no longer overloaded. Households are usually connected to the sewer mains by clay, cast-iron, or polyvinyl chloride (PVC) pipes 8 to 10 cm (3 to 4 in) in diameter. Larger-diameter sewer mains can be located along the centerline of a street or alley about 1.8 m (about 6 ft) or more below the surface. The smaller pipes are usually made of clay, concrete, or asbestos cement, and the large pipes are generally of unlined or lined reinforced-concrete construction. Unlike the water-supply system, wastewater flows through sewer pipes by gravity rather than by pressure. The pipe must be sloped to permit the wastewater to flow at a velocity of at least 0.46 m per sec (1.5 ft per sec), because at lower velocities the solid material tends to settle in the pipe. Storm-water mains are similar to sanitary sewers except that they have a much larger diameter. Certain types of sewers, such as inverted siphons and pipes from pumping stations, flow under pressure, and are thus called force mains. Urban sewer mains generally discharge into interceptor sewers, which can then join to form a trunk line that discharges into the wastewater-treatment plant. Interceptors and trunk lines, generally made of brick or reinforced concrete, are sometimes large enough for a truck to pass through them. IV NATURE OF SEWAGE The origin, composition, and quantity of waste are related to existing life patterns. When waste matter enters water, the resulting product is called sewage or wastewater. A Origin and Quantity Wastewater originates mainly from domestic, industrial, groundwater, and meteorological sources, and these forms of wastewater are commonly referred to as domestic sewage, industrial waste, infiltration, and storm-water drainage, respectively. Domestic sewage results from people's day-to-day activities, such as bathing, body elimination, food preparation, and recreation, averaging about 227 liters (about 60 gallons) per person daily. The quantity and character of industrial wastewater is highly varied, depending on the type of industry, the management of its water usage, and the degree of treatment the wastewater receives before it is discharged. A steel mill, for example, might discharge anywhere from 5700 to 151,000 liters (about 1500 to 40,000 gallons) per ton of steel manufactured. Less water is needed if recycling is practiced. Infiltration occurs when sewer lines are placed below the water table or when rainfall percolates down to the depth of the pipe. It is undesirable because it imposes a greater load on the piping system and the treatment plant. The amount of storm-water drainage to be carried away depends on the amount of rainfall as well as on the runoff or yield of the watershed (see Drainage). A typical metropolitan area discharges a volume of wastewater equal to about 60 to 80 percent of its total daily water requirements, the rest being used for washing cars and watering lawns, and for manufacturing processes such as food canning and bottling. B Composition The composition of wastewater is analyzed using several physical, chemical, and biological measurements. The most common analyses include the measurements of solids, biochemical oxygen demand (BOD5), chemical oxygen demand (COD), and pH. The solid wastes include dissolved and suspended solids. Dissolved solids are the materials that will pass through a filter paper, and suspended solids are those that do not (see Filtration). The suspended solids are further divided into settleable and nonsettleable solids, depending on how many milligrams of the solids will settle out of 1 liter of wastewater in 1 hour. All these classes of solids can be divided into volatile or fixed solids, the volatile solids generally being organic materials and the fixed solids being inorganic or mineral matter. The concentration of organic matter is measured by the BOD5 and COD analyses. The BOD5 is the amount of oxygen used over a five-day period by microorganisms as they decompose the organic matter in sewage at a temperature of 20° C (68° F). Similarly, the COD is the amount of oxygen required to oxidize the organic matter by use of dichromate in an acid solution and to convert it to carbon dioxide and water. The value of COD is always higher than that of BOD 5 because many organic substances can be oxidized chemically but cannot oxidize biologically. Commonly, BOD5 is used to test the strength of untreated and treated municipal and biodegradable industrial wastewaters. COD is used to test the strength of wastewater that is either not biodegradable or contains compounds that inhibit activities of microorganisms. The pH analysis is a measure of the acidity of a wastewater sample (see Acids and Bases). Typical values of solids and BOD5 for domestic wastewater are given in the accompanying table. The organic matter in typical domestic sewage is approximately 50 percent carbohydrates, 40 percent protein, and 10 percent fat; the pH can range from 6.5 to 8.0. The composition of industrial waste cannot be readily characterized by a typical range of values because its makeup depends on the type of manufacturing process involved. The concentration of an industrial waste is usually placed in perspective by stating the number of people, or population equivalent (PE), that would be required to produce the same quantity of waste. PE is most commonly expressed in terms of BOD5. An average value of 0.077 kg (0.17 lb) 5-day, 20° C BOD per person per day is used for determination of the PE. The population equivalent of a slaughterhouse operation, for example, will range from 5 to 25 PE per animal. The composition of infiltration depends on the nature of the groundwater that seeps into the sewer. Storm-water sewage contains significant concentrations of bacteria, trace elements, oil, and organic chemicals. V WASTEWATER TREATMENT The processes involved in municipal wastewater treatment plants are usually classified as being part of primary, secondary, or tertiary treatment. A Primary Treatment The wastewater that enters a treatment plant contains debris that might clog or damage the pumps and machinery. Such materials are removed by screens or vertical bars, and the debris is burned or buried after manual or mechanical removal. The wastewater then passes through a comminutor (grinder), where leaves and other organic materials are reduced in size for efficient treatment and removal later. A1 Grit Chamber In the past, long and narrow channel-shaped settling tanks, known as grit chambers, were used to remove inorganic or mineral matter such as sand, silt, gravel, and cinders. These chambers were designed to permit inorganic particles 0.2 mm (0.008 in) or larger to settle at the bottom while the smaller particles and most of the organic solids that remain in suspension pass through. Today, spiral-flow aerated grit chambers with hopper bottoms, or clarifiers with mechanical scrapper arms, are most commonly used. The grit is removed and disposed of as sanitary landfill. Grit accumulation can range from 0.08 to 0.23 cu m (3 to 8 cu ft) per 3.8 million liters (about 1 million gallons) of wastewater. A2 Sedimentation With grit removed, the wastewater passes into a sedimentation tank, in which organic materials settle out and are drawn off for disposal. The process of sedimentation can remove about 20 to 40 percent of the BOD5 and 40 to 60 percent of the suspended solids. The rate of sedimentation is increased in some industrial waste-treatment stations by incorporating processes called chemical coagulation and flocculation in the sedimentation tank. Coagulation is the process of adding chemicals such as aluminum sulfate, ferric chloride, or polyelectrolytes to the wastewater; this causes the surface characteristics of the suspended solids to be altered so that they attach to one another and precipitate. Flocculation causes the suspended solids to coalesce. Coagulation and flocculation can remove more than 80 percent of suspended solids. A3 Flotation An alternative to sedimentation that is used in the treatment of some wastewaters is flotation, in which air is forced into the wastewater under pressures of 1.75 to 3.5 kg per sq cm (25 to 50 lb per sq in). The wastewater, supersaturated with air, is then discharged into an open tank; there the rising air bubbles cause the suspended solids to rise to the surface, where they are removed. Flotation can remove more than 75 percent of the suspended solids. A4 Digestion Digestion is a microbiological process that converts the chemically complex organic sludge to methane, carbon dioxide, and an inoffensive humuslike material. The reactions occur in a closed tank or digester that is anaerobic--that is, devoid of oxygen. The conversion takes place through a series of reactions. First the solid matter is made soluble by enzymes, then the substance is fermented by a group of acid-producing bacteria, reducing it to simple organic acids such as acetic acid. The organic acids are then converted to methane and carbon dioxide by bacteria. Thickened sludge is heated and added as continuously as possible to the digester, where it remains for 10 to 30 days and is decomposed. Digestion reduces organic matter by 45 to 60 percent. A5 Drying Digested sludge is placed on sand beds for air drying. Percolation into the sand and evaporation are the chief processes involved in the dewatering process. Air drying requires dry, relatively warm weather for greatest efficiency, and some plants have a greenhouselike structure to shelter the sand beds. Dried sludge in most cases is used as a soil conditioner; sometimes it is used as a fertilizer because of its 2 percent nitrogen and 1 percent phosphorus content. B Secondary Treatment Having removed 40 to 60 percent of the suspended solids and 20 to 40 percent of the BOD5 in primary treatment by physical means, the secondary treatment biologically reduces the organic material that remains in the liquid stream. Usually the microbial processes employed are aerobic--that is, the organisms function in the presence of dissolved oxygen. Secondary treatment actually involves harnessing and accelerating nature's process of waste disposal. Aerobic bacteria in the presence of oxygen convert organic matter to stable forms such as carbon dioxide, water, nitrates, and phosphates, as well as other organic materials. The production of new organic matter is an indirect result of biological treatment processes, and this matter must be removed before the wastewater is discharged into the receiving stream. Several alternative processes are also available in secondary treatment, including a trickling filter, activated sludge, and lagoons. B1 Trickling Filter In this process, a waste stream is distributed intermittently over a bed or column of some type of porous medium. A gelatinous film of microorganisms coats the medium and functions as the removal agent. The organic matter in the waste stream is absorbed by the microbial film and converted to carbon dioxide and water. The trickling-filter process, when preceded by sedimentation, can remove about 85 percent of the BOD5 entering the plant. B2 Activated Sludge This is an aerobic process in which gelatinous sludge particles are suspended in an aeration tank and supplied with oxygen. The activated-sludge particles, known as floc, are composed of millions of actively growing bacteria bound together by a gelatinous slime. Organic matter is absorbed by the floc and converted to aerobic products. The reduction of BOD5 fluctuates between 60 and 85 percent. An important companion unit in any plant using activated sludge or a trickling filter is the secondary clarifier, which separates bacteria from the liquid stream before discharge. B3 Stabilization Pond or Lagoon Another form of biological treatment is the stabilization pond or lagoon, which requires a large land area and thus is usually located in rural areas. Facultative lagoons, or those that function in mixed conditions, are the most common, being 0.6 to 1.5 m (2 to 5 ft) in depth, with a surface area of several acres. Anaerobic conditions prevail in the bottom region, where the solids are decomposed; the region near the surface is aerobic, allowing the oxidation of dissolved and colloidal organic matter (see Colloid). A reduction in BOD5 of 75 to 85 percent can be attained. C Advanced Wastewater Treatment If the receiving body of water requires a higher degree of treatment than the secondary process can provide, or if the final effluent is intended for reuse, advanced wastewater treatment is necessary. The term tertiary treatment is often used as a synonym for advanced treatment, but the two methods are not exactly the same. Tertiary, or third-stage, treatment is generally used to remove phosphorus, while advanced treatment might include additional steps to improve effluent quality by removing refractory pollutants. Processes are available to remove more than 99 percent of the suspended solids and BOD5. Dissolved solids are reduced by processes such as reverse osmosis and electrodialysis. Ammonia stripping, denitrification, and phosphate precipitation can remove nutrients. If the wastewater is to be reused, disinfection by ozone treatment is considered the most reliable method other than breakpoint chlorination. Application of these and other advanced waste-treatment methods is likely to become widespread in the future in view of new efforts to conserve water through reuse. See Absorption; Osmosis; Precipitation. D Liquid Disposal The ultimate disposal of the treated liquid stream is accomplished in several ways. Direct discharge into a receiving stream or lake is the most commonly practiced means of disposal. In areas of the United States that are faced with worsening shortages of water for both domestic and industrial use, municipalities and state and federal agencies are turning to reuse of appropriately treated wastewater for groundwater recharge, irrigation of nonedible crops, industrial processing, recreation, and other uses. Many reuse projects are located in California, Arizona, and Texas. The first large-scale wastewater-reclamation plant in the United States is the Denver Water Department's Potable Reuse Demonstration Plant. The one-million-gallonper-day plant was built to demonstrate the quality, reliability, and economic potential of reuse on a large scale. The quality and health-effects testing program, ended in 1993, after successfully meeting its goal of producing drinkable water from reclaimed water. The reused water was tested against the regular drinking water received by Denver residents and found to be equally drinkable. The treatment process involves conventional primary and secondary treatment followed by lime clarification to remove suspended organic compounds. During this process, an alkaline (high-pH) condition is created to improve the process. In the next step, recarbonation is used to bring the pH level to neutral. Then the water is filtered through multiple layers of sand and charcoal, and ammonia is removed by ionization. Pesticides and any other dissolved organic materials still present are absorbed by a granular, activated-carbon filter. Viruses and bacteria are then killed by ozonization. At this stage the water should be cleansed of all contaminants, but, for added reliability, second-stage carbon adsorption and reverse osmosis are used, and chlorine dioxide is added to attain the highest possible water standard. Similar reuse programs are underway in the southwestern United States, Saudi Arabia, and the Netherlands. E Septic Tank A sewage treatment process commonly used to treat domestic wastes is the septic tank: a concrete, cinder block or metal tank where the solids settle and the floatable materials rise. The partly clarified liquid stream flows from a submerged outlet into subsurface rock-filled trenches through which the wastewater can flow and percolate into the soil where it is oxidized aerobically. The floating matter and settled solids can be held from six months to several years, during which they are decomposed anaerobically. See also Solid Waste Disposal. Contributed By: Jerry Y.C. Huang Reviewed By: Gabor M. Karadi Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
