A brief treatment of sewage systems and modern sewage-treatment methods follows. For further discussion, see environmental works: Water-pollution control (environmental works).

Early civilizations often built drainage systems in urban areas to handle storm runoff. The Romans, especially, constructed elaborate systems that also drained wastewater from the public baths. During the European Middle Ages these systems fell into disrepair. As the populations of cities grew, disastrous epidemics of cholera and typhoid fever broke out, the result of ineffective segregation of sewage and drinking water. As the correlation between sewage and disease became apparent in the mid-19th century, steps were taken to treat wastewater. The concentration of population and the addition to sewage of manufacturing waste that occurred during the Industrial Revolution increased the need for effective sewage treatment.

Modern sewage systems fall under two categories: domestic and industrial sewers, and storm sewers. Sometimes a combined system provides only one network of pipes, sewer mains, and outfall sewers for all types of sewage. This type of system is less expensive to install in a district, but it has long been recognized that separate systems are best suited to modern metropolitan conditions. To avoid pollution, all waste water should pass through treatment plants, but it is uneconomical to build plants large enough to accept the enormously enlarged inflow from rainstorms and to treat sewage at the same time. In addition, a combined system must be made so large that it may not be able to provide adequate velocity for the dry-weather flow of the waste water alone. The preferred system provides separate sewers for human waste, which is then generally treated before discharge.

Sewer pipe is made of vitrified clay or concrete and its cross section is usually round. It is laid following street patterns, and access holes with metal covers are provided periodically for inspection and cleaning. Catch basins at street corners and along street gutters admit surface runoff of storm water and feed the storm sewers. Engineers determine the volume of sewage likely, the route of the system and the slope, or gradient, of the pipe to ensure an even flow by gravity that will not leave solids behind. In flat regions, pumping stations are sometimes needed. Under certain conditions, ventilating equipment is provided to remove corrosive gases.

Sewage treatment entails the removal of organic matter and is usually accomplished in two stages. In the first, or primary, stage sewage is first passed through large mesh screens to remove such large objects as wood, rags, and wire. It is then run through channels at a controlled velocity so that sand and ash grit is deposited on the bottom. After screening and grit removal, the sewage is passed into large tanks about 10 feet (3 metres) deep where many of the suspended solids (sludge) settle in a process called sedimentation. Two additional methods can supplement primary treatment. The Imhoff tank, developed by the German engineer Karl Imhoff, is a second compartment under the settling tank where solids are further decomposed by bacteria. Chemicals can also be added to the sewage to promote the coagulation of the finer suspended solids, so that they become heavy enough to settle in sedimentation.

The secondary treatment of sewage produces an effluent clean enough for discharge. The work of further purification is performed by microorganisms and bacterial slime, most commonly through the use of trickling and sand filters or the activated-sludge process. The organic matter remaining in sewage after solids have been removed is mainly in a dissolved state. The trickling filter is a bed of stones covered by a thin film of purifying slime through which sewage, sprayed from above, is allowed to trickle. It then runs onto a bed of sand that filters the water clean. The activated-sludge process utilizes sludge that has been allowed to breed microorganisms. The sludge is mixed with treated sewage and then aerated by jets of compressed air over a period of several hours; during this time the organic matter is oxidized by the microorganisms. Afterward the sewage is returned to settling tanks and then aerated a second time.

Sludge from both primary and secondary treatments is collected from the various tanks and hauled out to sea and dumped or buried in sanitary landfills. It may also be used, in liquid or dried-out form, as fertilizer. By placing it in digestion tanks heated to an optimal 95° F (35° C) it can be further decomposed to produce methane gas, which can be used to run the machinery of the treatment plant.