Introduction to Sewage

Sewage, the mixture of water and waste products carried off through a drainage system of underground pipes, or sewers. The waste products consist of both organic and inorganic matter, including human wastes, mineral salts, and garbage. In most large cities sewage also contains liquid wastes from various industrial processes.

The organic matter in sewage decomposes rapidly, giving off foul-smelling and hazardous gases. Disease-causing organisms are passed into sewage through the feces and urine of infected persons. The wastes from industrial processes are often toxic. The safe disposal of sewage is therefore essential to the health of a community.

The collection and treatment of sewage is one of the most important municipal services. In the United States, most cities and towns have sewer systems that carry sewage to a sewage treatment plant. At the plant the sewage is treated to destroy disease organisms and to remove substances that can cause harmful or other undesirable effects in the water. The treated sewage is then discharged into nearby streams, lakes, or coastal waters. In rural areas, the treated sewage is usually disposed of in the soil.

The disposal of untreated or inadequately treated sewage directly into a stream or body of water can result in serious water pollution. Disease organisms endanger water supplies and swimming areas. Various chemicals may poison the water, killing fish and other wildlife, while certain nutrients in the sewage can cause an excessive growth of aquatic plants. As wastes decompose, they can deplete the oxygen supply in the water, making it unfit for all forms of aquatic life.

The facilities for collecting, treating, and disposing of sewage are called a sewerage system. In cities and towns, sewerage systems are designed to serve the entire community. Sewage from individual buildings flows into collecting sewers, which carry the waste to a central plant for treatment and disposal. Sewers that only carry domestic sewage—that is, sewage from residential and commercial buildings—are called sanitary sewers. Storm sewers are designed specifically for carrying runoff from rain and melted snow. In many systems, both domestic sewage and runoff water are carried in combined sewers. Combined sewers are generally undesirable, however, because overflow caused by heavy rains often makes it necessary to discharge untreated sewage from the sewage treatment plant.

Sewers are usually made of clay, concrete, or plastic. Sewerage systems are usually built so that gravity will carry the waste through the pipes. Where this is not possible, the sewage must be pumped.

In rural areas and in communities not served by municipal sewerage systems, sewage is disposed of through the use of septic tanks or, less commonly, cesspools. The privy, or outdoor toilet, is commonly used for disposing of human wastes where there is no indoor plumbing.

Sewage Treatment and Disposal

Municipal Sewage Treatment Plants

Raw sewage entering the plant is first passed through screens to remove coarse debris. This material may be buried, burned, or ground in disintegrators and returned to the flow. After screening, the sewage flows slowly through a grit chamber, a shallow tank in which sand and other heavy particles settle to the bottom. This material is periodically removed and usually disposed of in a landfill. The sewage is then pumped into primary settling tanks, where much of the remaining solid material settles out. Chemicals are sometimes added to the sewage to help remove suspended particles and reduce foam caused by detergents. The settled material is called sludge.

The liquid portion is drawn off and is usually given further treatment. This treatment, called secondary treatment, removes most of the organic matter remaining in the sewage. The two most common methods of secondary treatment are the activated sludge process and the trickling filter process.

Activated Sludge Process

In this process the liquid from the primary settling tank passes into large, elongated tanks where it is mixed with sludge containing large numbers of bacteria. The bacteria decompose the organic matter, using it as food. Oxygen is dissolved in the mixture by bubbling either compressed air or pure oxygen gas through it, a process called aeration. Aeration provides the bacteria with the oxygen they need for respiration. The mixture is then transferred to settling tanks called final clarifiers. The activated sludge settles out and the clear liquid that remains is removed from the tank. The liquid is then sometimes disinfected with chlorine before it is discharged into a body of water. Some of the activated sludge that settles out in the final clarifiers is recycled to the aeration tanks.

Trickling Filter Process

In this process the liquid is sprayed over a filtering material, typically a bed of crushed rock. As the liquid seeps through the crushed rock, bacteria and other organisms growing on the rock surfaces decompose most of the organic matter. The products of decomposition are simple compounds, such as nitrates and sulfates, and humuslike matter. The mixture then flows into secondary settling tanks, where the solids settle out as sludge. Chlorine is sometimes added to the clear liquid to disinfect it. The liquid, which contains various inorganic compounds, is then discharged into a body of water.

Very fine particles and such substances as nitrogen and phosphorus compounds usually remain in the sewage water even after secondary treatment. Therefore, when the water discharged from a sewage treatment plant must be of especially high quality, it is given tertiary treatment. Methods of tertiary treatment include extended aeration and filtering using fine-meshed screens, sand, or activated charcoal.

The sludge formed in the various treatment stages contains both organic and inorganic solids. It flows by gravity or is pumped into large closed tanks. In the absence of air, certain types of bacteria in the sludge decompose much of the organic matter. This process is called sludge digestion. During this process, methane and other gases are produced. The gas mixture is usually used as fuel to provide heat and power for the sewage plant or sold commercially as fuel.

After digestion, the remaining sludge is often dried by air on large beds of sand or by various mechanical methods that use pressure to remove the water from the sludge. It is then buried, burned, or sold as a soil conditioner or filler for commercial fertilizers. In some areas, the sludge is dried by heat or incinerated to destroy the remaining organic substances before it is sold commercially.

Septic Tanks

Septic tanks are widely used on farms and in communities that do not have municipal sewage disposal systems. The septic tank, which is located underground, usually serves a single building. It is usually made of concrete or steel. The size of tank needed depends upon the number of occupants of the building.

Sewage from the building flows to the tank through the building sewer. Inside the tank, heavy solids settle out as sludge; grease and fine particles rise to form a scum. Bacteria in the sewage digest the organic substances both in the sludge and in the scum, converting them to liquids and gases. The remaining sludge accumulates in the tank and must be removed at intervals.

The liquid portion, usually containing some solids, and the gases are carried into the surrounding soil through a system of distribution pipes. The gases escape to the surface. Liquid and solid wastes are absorbed by the soil, where bacteria decompose the organic matter. In heavy soils, two sets of distribution pipes usually are used alternately to insure proper absorption.

Any disease-causing organisms in the sewage ordinarily do not survive long in either the tank or soil. The sewage wastes and organisms, however, may be carried for long distances through the soil where it is fractured or creviced. It is therefore essential that the tank and disposal pipes be located where there is no danger of contaminating underground water supplies.


Cesspools are sometimes used for sewage disposal on farms and other isolated areas. The cesspool is an underground pit lined with either brick or stone, without mortar. The sewage liquids seep out into the soil through the open spaces m the lining. Solids accumulate as in a septic tank and must be removed at intervals. In time the soil around the cesspool may become clogged with solids, causing overflow. When they are poorly covered, cesspools allow foul-smelling gases to escape and often become breeding places for mosquitoes. Cesspools are not recommended by most health authorities.


In ancient cities, covered channels or pipes were often used for removing human wastes from dwellings. Rome, for example, had a system of sewers for disposing of wastes and rainwater. Most ancient sewers fell into disrepair during the Middle Ages. Refuse and human wastes were commonly thrown into the streets. By the late 1700's, many large cities had sewers for removing storm water, but cesspools were usually used for sewage disposal. Both cesspools and privies were widely used in cities and towns.

Modern sewage disposal systems were introduced in the 19th century. Existing storm sewers were usually enlarged to carry both rainwater and wastes. In the late 19th century, either combined sewers or sanitary sewers were used for disposing of sewage and industrial wastes. The wastes were usually discharged into nearby bodies of water.

Municipal sewage treatment was introduced in the early 20th century, but many cities and industrial plants were slow to adopt it. As the population increased, some existing sewage plants became inadequate. In cities with combined sewers, heavy rains periodically overloaded otherwise adequate disposal facilities. By the 1970's, water pollution caused by the discharge of untreated or inadequately treated sewage had become a major problem. In the United States, legislation was passed in the 1970's to help control and prevent water pollution. The legislation set standards for the quality of treated sewage discharged into streams and lakes and established funding to assist in the improvement and construction of sewage treatment facilities. Additional legislation passed in the 198O's set standards for storm water discharges.