Sewage Treatment and Disposal
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.
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.
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 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.