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system for the removal and disposal of chiefly liquid wastes and of rainwater, which are collectively called sewage. The average person in the industrialized world produces between 60 and 140 gallons of sewage per day.

Types of Sewage Disposal Systems

Domestic sewage, produced in urban residences, institutions, and businesses, is usually collected by pipes and conduits called sanitary sewers, which lead to a central discharge point. In rural residences domestic sewage is often collected in a septic tankseptic tank,
underground sedimentation tank in which sewage is retained for a short period while it is decomposed and purified by bacterial action. The organic matter in the sewage settles to the bottom of the tank, a film forms excluding atmospheric oxygen, and anaerobic
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 on the property. Industrial wastes, which consist of liquids produced in manufacturing processes, are sometimes collected in sanitary sewers, but the nature of many industrial wastes may make it dangerous or difficult to do so. Often industries dispose of their own wastes. Storm sewage, which comes from rain and groundwater, is collected either in a storm sewer or, with domestic sewage and industrial wastes, in what is called a combined sewer.

Sewer pipe must be strong enough to withstand the structural stresses to which it is subjected by being buried in the ground. In addition, the pipe itself and the joints between sections of pipe must be capable of withstanding at least moderate water pressure without significant leakage of sewage into the environment. Materials used for sewer pipe include plastics, vitrified clay, cast iron and steel, corrugated iron, concrete, and asbestos cement. Although usually circular, pipes are also made egg-shaped or semi-elliptical so that suspended solids do not accumulate even at a relatively low rate of flow, about 2 ft (.6 m) per second. Sewer pipes are usually inclined downward toward the central collection point so that sewage will flow to it naturally, although pumping stations may be required.

Sewage is eventually discharged into underground or surface watercourses that naturally drain an area. In past centuries, the dilution produced by discharging sewage into large bodies of water was considered sufficient to render harmless any toxic substances contained in it. However, the volume of sewage is now so great that dilution is no longer considered an adequate safeguard.

Sewage Treatment

The biochemical processes that take place in water bodies have also been relied on to neutralize sewage. Aerobic, or oxygen-requiring, bacteria feed on the organic material in sewage, decomposing it. However, this process uses the oxygen that is dissolved in water. Often the concentration of organic waste is so great that the biochemical oxygen demand (BOD) depletes the water's oxygen supply, killing fish and plants. In order to avoid these problems, it is now recognized that all sewage except unmixed storm sewage must be treated before it is discharged.

Sewage treatment is classified as primary, secondary, or tertiary, depending on the degree to which the effluent is purified. Primary treatment is removal of floating and suspended solids. Secondary treatment uses biological methods such as digestion. Complete, or tertiary, treatment removes all but a negligible portion of bacterial and organic matter. Industrial wastes are treated by a number of methods, depending on the specific nature of the waste. Increasingly, governments are forcing industries to process effluents either chemically or mechanically, or both ways, so that harmful substances are removed.

Domestic sewage must be treated to produce discharge water that is free of odors, suspended solids, and objectionable bacteria. (Coliform bacteria, which inhabit the lower intestines of mammals, while not pathogenic of themselves, are taken as an index of contamination of watercourses.) In rural areas, sewage can be stored in a holding tank, e.g., a septic tank; naturally occurring anaerobic bacteria can decompose the solids, which then settle to the bottom. While suitable for small systems, this method has several disadvantages. First, anaerobic decomposition produces noxious gaseous effluents, and it is fairly slow. Second, harmful bacteria may still be present in the liquid effluent.

In large urban systems, a combination of processes must be used. Decomposition can be speeded by forcing air through the mass so that aerobic bacteria can be used. This oxidation process is typically combined with filtration, either in sand or in granular activated carbon, and with several hours of aeration. The liquid can then be discharged, often after being disinfected with chlorine. The liquid may be also treated by microfiltration, reverse osmosis, and hydrogen peroxide and ultraviolet light to produce very clean water that can be reused. Another method of removing solids is to allow the liquid to stand in large tanks until the solids fall out and form a sediment, but the process is slow and requires the accumulation of large volumes of liquid.

Once solids are removed, they are treated in one of several ways. Most often they are removed in a semiliquid mass referred to as sludge. Sludge may be transferred to tanks where it is digested by aerobic or anaerobic bacteria. Gaseous byproducts of this digestion are collected for use as fuels. After digestion, solids may be dried and enriched with plant nutrients for use as fertilizer. In other cases, with or without digestion, they may be dried and incinerated at 1200 to 1400 degrees F (650 to 760 degrees C). In other cases solids are buried in landfills or dumped far at sea, although environmental objections to such dumping has led to its drastic curtailment.

See pollutionpollution,
contamination of the environment as a result of human activities. The term pollution refers primarily to the fouling of air, water, and land by wastes (see air pollution; water pollution; solid waste).
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Historical Sewage Systems

One of the earliest known sewers was the Cloaca Maxima in Rome, built (c.6th cent. B.C.) to drain the site of the Forum. Although London's drainage system dates from the 13th cent., the discharge of offensive waste into it was illegal until 1815. The Parisian sewers were constructed before the 16th cent., but by 1893 fewer than 5% of the city's houses were connected to the system. As early as 1701, Boston had drains. Generally, however, systematic sewage disposal was not widely introduced until the mid-19th cent.


See D. Sundstrom and H. E. Klei, Wastewater Treatment (1979); J. R. Holmes, Practical Waste Management (1983); W. W. Eckenfelder, Industrial Water Pollution Control (1989).



the set of engineered structures, equipment, and sanitation measures that provide for the collection and removal of polluted waste waters beyond the limits of population centers and industrial enterprises, along with the treatment and decontamination of the waters before use or release into bodies of water.

A distinction is made between interior and exterior sewerage. The interior system is used to receive effluents at their point of formation and deliver them from a building to the exterior sewerage network. The elements of the interior system include a building’s sanitary equipment, drain pipes, standpipes, and drainage outlets. The exterior system, which is designed to transport the effluents beyond the limits of population centers and industrial enterprises, consists of main pipelines (gravity-fed and pressurized), pumping stations, and sewage treatment facilities.

The sewerage system provides for the combined or separate drainage and removal of three categories of waste water: household, industrial, and rain. Both systems have been widely used in urban planning. In the combined system (see Figure 1) all three categories of waste water are removed through a common network of pipes and canals beyond the limits of the population center. In the separate system (see Figure 2), rainwater and sufficiently clean industrial waters are removed through one network of pipes and conduits and household and polluted industrial wastes are removed through another (by one or more sewer networks). The separate sewerage system may be comprehensive or partial.

Figure 1. Combined sewerage system: (1) sewage conduits, (2) sewage mains, (3) storm sewers, (4) pumping station, (5) treatment facilities with discharge outlet

A technically and economically sound design tor an accepted sewerage system that takes account of local conditions and development prospects of the sewerage project (city, settlement, industrial, or residential area) is called a sewer plan. Every sewer plan may be implemented by a variety of technical procedures, in regard to the routing and depth of placement of networks and sewers, the number of pumping stations, the number and location of treatment facilities, the necessary degree of treatment, and the sequence of construction.

Figure 2. Separate sewerage system: (1) and (2) household network, (3) and (4) rainwater network, (5) pumping station, (6) treatment facilities

Depending on the nature of the terrain, the entire area of a population center to be provided with sewerage is conventionally divided into sewerage basins, that is, areas bounded by watersheds. In each basin the waste waters are collected in one or more sewers through the underground pipes of the street network. The waste waters move through these sewers by force of gravity. When the main is especially deep the network is divided into several districts with normal pipeline depth. The waste waters are sent from these district networks to a district pumping station, from which they are delivered through a pressurized pipeline to higher, gravity-fed mains. Pumping stations are also built for delivering sewage waters directly to the treatment facilities, from which the purified waters are released through outfall sewers into bodies of water. (See Figure 3 for an example of the general design and main facilities of a modern sewerage system in a population center.)

Figure 3. General plan and basic sewerage facilities of a population center: (1) boundaries of sewerage basins, (2) street network and sewage conduits, (3) district pumping station, (4) pressurized waterlines, (5) industrial enterprises, (6) sewage main, (7) main pumping station, (8) out-of-town main, (9) treatment facilities, (10) outfall sewer; (113-123) contour lines marking elevations; (A) and (B) points at which pressure lines enter gravitation network

Waste water has been removed from population centers by pipe since antiquity. Excavations in Egypt have uncovered sewage canals built 2, 500 years B.C., and similar structures are known to have existed even earlier in India. The Cloaca Maxima was built in Rome in the sixth century B.C. and is still partially in use today. However, these structures required enormous expenditures of labor and materials and were provided only for palaces, temples, and the public baths. During the feudal age, and particularly during the period of capitalist development that followed, the increasing density of population led to a deterioration in urban sanitation. The growing frequency of epidemics made it essential to build water pipes and, later, sewerage systems as well. This development was also dictated by the development of industry and an increase in the volume of industrial waste waters.

Intensive construction of sewerage systems in Europe did not begin until the 19th century. In Russia, the first underground canals for removing polluted water were built in the 11th-14th centuries (Novgorod and the Moscow Kremlin), but it was not until the early 19th century (in St. Petersburg and Moscow) that they were built on a significant scale. In prerevolutionary Russia the 18 largest cities had sewage disposal systems.

In the USSR, simultaneous with the growth of cities and settlements, public services and amenities have been provided, including the construction of centralized water and sewerage systems. Standard designs have been developed and are in use for a large number of sewage disposal works. These designs significantly reduce the expenditure of labor and the building time of the systems. Industrial construction methods are widely used (in particular, the use of heading in laying sewers and of prefabricated elements for sewage disposal installations). By 1980, the Soviet Union plans to add more than 270, 000 km to the existing sewer networks and to increase the capacity of sewage treatment facilities to 90 million cu m per day. The volume of industrial waste waters treated daily will reach 120 million cu m.


Kanalizatsiia. Edited by A. I. Zhukov. Moscow, 1969.



(civil engineering)


The works required to collect, treat, and dispose of sewage, including the sewer system, pumping stations, and treating works.


1. an arrangement of sewers
2. the removal of surface water or sewage by means of sewers
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