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Sewage Treatment

Septic Tanks

Not all homes and businesses are connected to municipal sewage systems. Some are too remote, or in towns too small for sewage systems and treatment plants. In such cases, septic systems must be used.

A septic system consists of a septic tank, a drain field or leach field, and associated piping. Gray water from washing and black water from household toilets runs through water-tight sewage pipe to the septic tank. Anaerobic decay takes place in the septic tank, primarily in a layer of floating scum on top of the sewage. An outlet pipe leads to the drain field. The sewage undergoes final processing in the drain field, including filtration and aerobic decay.

When sewage reaches the septic tank, solids settle out of it. Anaerobic bacteria, yeast, fungi, and actinomycetes break down the biosolids, producing methane and hydrogen sulfide. Fine solids, grease and oils form a layer of scum on the surface of the liquid, insulating the anaerobic community from any air in the tank.

There are numerous septic tank designs. The primary requirements are that the tank be watertight, that it have inspection/cleaning ports, and that it be large enough to contain three to five days worth of sewage from the household. This ensures that the anaerobic creatures are able to process the sewage prior to its release to the drain field, and that the tank does not fill up and/or overflow; a rather revolting prospect. This outflow pipe is normally at a lower level than the inflow pipe and at the far end of the tank from the inflow pipe, to ensure that only processed sewage is released. Many septic tank designs include baffles or multiple chambers to force the black water through maximum processing prior to release to drain field.

Aerobic decay of the sewage takes place in the drain field. The outflow from the septic tank, called effluent, still contains pathogens. Effluent travels through a network of pipes set in gravel several feet below ground. The sections of pipe are slightly separated at the joints, allowing the liquid to seep out. The soil and gravel of the drain field filter the effluent and expose it aerobic bacteria, fungi, and protozoa that feed on the organic material, converting it to soluble nutrients. The liquid eventually either percolates down to the water table or returns to the surface via evaporation or transpiration by plants.

Roughly 4 ft (1.2 m) of soil are needed to process effluent, although authorities differ on the exact number, which varies with the makeup of the drain field soil. In other words, effluent passed through a couple yards of soil is pure enough to drink. To ensure a significant margin of safety, a drain field must be from 50-400 ft (15.2-121.9 m) from the nearest water supply, depending on the soil and the number of people served by the aquifer.

Some areas use incineration for the disposal of sludge. Earlier incinerators proved very expensive to operate and for this reason many of the plants were abandoned. Many grass-roots organizations also disapprove of incinerators because of health reasons. Incinerators release carcinogenic (cancer-causing) and toxic chemicals from their smoke stacks, including heavy metals (such as arsenic, lead, cadmium, mercury, chromium and beryllium); acid gases, including hydrogen fluoride; partially-burned organic material such as polyvinyl chloride (PVC), herbicide residues and wood preservatives; other organic chemicals, including polycyclic aromatic hydrocarbons (PAHs); and dioxins and furans. One recent analysis identified 192 volatile organic compounds being emitted by a solid waste incinerator. In more recent years, a new form of incinerator has been developed based on the use of a fluidized bed which is proving more successful.

Under the Clean Water Act (CWA), sewage treatment plants and factories must obtain pollution permits, or legally binding agreements, that limit the volumes and types of pollution discharged into the nation's lakes and rivers. These permits form the basis of virtually all water-pollution tracking and reduction, as well as enforcement of water pollution laws. They must be renewed at least every five years, and with each new permit the amount of polluted discharge allowed is to be lowered toward the eventual goal of zero pollution. At the beginning of 2000, The Friends of the Earth (FOE) and Environmental Working Group conducted a review of the publicly available water-pollution records from the 50 states and the District of Columbia. The FOE rated states on a pass-fail basis. The grade assigned to each state was based on the percentage of expired permits as of the start of this year. States with more than 10% of their permits expired were failed based on a 10% maximum permit backlog set by the United States Environmental Protection Agency (EPA). They found that 44 states and the District of Columbia failed their criterion.

The average person produces roughly 60 gal (227 l) of sewage daily, including both black and grey water. Municipal treatment plants and septic systems use mechanical and biological treatment methods to process out most of the pathogens and oxygen-consuming organisms. Toxic wastes are more difficult to remove, and are present in significant volumes in largely untreated stormwater runoff. In particular, industrial effluent presents environmental and health risks. It falls to us as citizens to be responsible in our use and disposal of these substances, which eventually find their way back into the environment.



Alth, M., and C. Alth. Constructing and Maintaining Your Well and Septic System. Blue Ridge Summit, PA: Tab Books, 1984.

Cheremisinoff, P. Biomanagement of Wastewater and Waste. Englewood Cliffs, NJ: Prentice-Hall, 1994.

Escritt, L. Sewerage and Sewage Treatment. New York: Wiley, 1984.

Kristin Lewotsky


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Active sludge

—Sewage that has been aerated in a tank and has developed powerful organic oxidation capabilities.


—Requiring or in the presence of oxygen.

Algal ponds

—A variation on the active sludge method in which aeration is performed by algae photosynthesis.


—Describes biological processes that take place in the absence of oxygen.



Black water

—Sewage that contains biosolids, e.g. water from the toilet.

Drain field

—Underground layer of soil and gravel where aerobic decay of septic tank effluent takes place.


—Liquid that flows from a septic tank or sedimentation tank; pathogens—bacteria and viruses capable of causing disease.

Grey water

—Sewage that does not contain biosolids, e.g. water from the kitchen sink or the shower.

Leach field

—Drain field.

Percolating filters

—A sewage treatment system in which effluent is trickled over gravel beds and efficiently purified by bacteria.

Septic tank

—Tank in which anaerobic decay of sewage takes place.


—Piping and collection system for sewage.

Additional topics

Science EncyclopediaScience & Philosophy: Semiotics to SmeltingSewage Treatment - Separation Of Liquid And Biosolids, Biomanagement Of Effluent, Urban Stormwater Runoff, Septic Tanks