Jeff Altemeier, age 13, of Henderson, Nev., for his question:
HOW DOES A SEWERAGE SYSTEM WORK?
Sewage is the waste matter produced by man. Most sewage in the United States is treated in some way before it goes into waterways, oceans or leaching fields.
In most cities and towns sewage from homes and other buildings flows into a public sewage system. About 80 percent of the nation's sewage comes from industrial sources.
In a public sewerage system, the largest sewers, called interceptors, carry the sewage to a wastewater treatment plant. Sewage treatment in most cities involves two main steps: primary and secondary. Some require a third step called tertiary.
Primary treatment removes the heaviest solid materials from sewage. At a treatment plant, sewage first passes through a screen that traps the largest pieces of matter. It then flows through a grit chamber, where heavy inorganic matter, such as sand, settles. The liquid next flows into a large primary sedimentation tank.
Many of the suspended solids sink to the bottom of the primary sedimentation tank and form a muddy material called sludge. Grease floats to the surface where it can be removed by a process called skimming. And the effluent, or semiclear liquid, is released into waterways.
Primary treatment removes about half the suspended solids and bacteria in sewage.
Secondary treatment removes from 85 to 90 percent of the solids and oxygen consuming wastes remaining in sewage after it has undergone primary treatment.
In the activated sludge process of secondary treatment, effluent from the primary sedimentation tank flows into a second tank called an aeration tank. Air is injected into this tank in a bubbling action. Sludge containing useful bacteria changes the organic matter.
In the trickling filtration process, the tanks are filled with crushed rocks. As sewage is distributed over the rocks, it comes in contact with slime that contains a useful bacteria that changes organic material in the sewage into less harmful substances.
Sludge resulting from primary and secondary treatment is pumped into a sludge digestion tank. In this tank, bacteria break the sludge down into less harmful substances and at the same time, produce methane gas, a useful fuel. The digested sludge may be dried for use as fertilizer or burned.
Tertiary treatment is used after primary and secondary treatment to produce purer effluent. There are various methods of tertiary treatment that include chemical treatment, microscopic screening, radiation treatment and discharging the effluent into lagoons.
Tertiary treatment makes effluent safer to discharge into waterways and safer for industry to use. Few treatment plants in the United States, however, use ail three methods.
Many rural areas are not served by public sewers. In such areas, most homeowners use septic tanks to treat their sewage.
Sewage flows into a septic tank where the solids sink to the bottom as sludge. Effluent then flows from the tank into a leaching field, a system of pipes with open joints that allows sewage effluent to be gradually distributed into the soil. Soil bacteria then destroy the remaining organic material in the effluent. The sludge is pumped out periodically and taken to sewage treatment plants.