Plain Sedimentation

Plain sedimentation is the quiescent settling or storage of water, such as would take place in a reservoir, lake, or basin, without the aid of chemicals, preferably for a month or longer, particularly if the source water is a sewage-polluted river water. This natural treatment results in the settling out of suspended solids; reduction of hardness, ammonia, lead, cadmium, and other heavy metals; breakdown of organic chemicals and fecal coliform; removal of color (due to the action of sunlight); and die-off of pathogenic microorganisms principally because of the unfavorable temperature, lack of suitable food, and sterilizing effect of sunlight. Certain microscopic organisms, such as protozoa, consume bacteria, thereby aiding in purification of the water. Experiments conducted by Sir Alexander Houston showed that polluted water stored for periods of 5 weeks at 32°F (0°C), 4 weeks at 41°F (5°C), 3 weeks at 50°F (10°C), or 2 weeks at 64.4°F (18°C) effected the elimination of practically all bacteria.43 A bacteria and virus removal of 80 to 90 percent can be expected after 10 to 30 days storage.44 Plain sedimentation, however, has some disadvantages that must be taken into consideration and controlled. The growth of microscopic organisms causing unpleasant tastes and odors is encouraged, and pollution by watershed surface wash, fertilizers, pesticides, recreational uses, birds and animals, sewage, and industrial wastes may occur unless steps are taken to prevent or reduce these possibilities. Although subsidence permits bacteria, including pathogens, to die off, it also permits bacteria to accumulate and grow in reservoir bottom mud under favorable conditions. In addition, iron and manganese may go into solution, carbon dioxide may increase, and hydrogen sulfide may be produced.

Presettling basins or upflow roughing filters are sometimes used to eliminate heavy turbidity or pollution and thus better prepare the water for treatment by coagulation, flocculation, settling, and filtration. Ordinarily, at least two basins are provided to permit one to be cleaned while the other is in use. A capacity sufficient to give a retention period of at least two or three days is desirable. When heavily polluted water is to be conditioned, provision can be made for preliminary coagulation at the point of entrance of the water into the basins followed by chlorination or other disinfection at the exit. Consideration must be given to the possible formation of trihalomethanes and their prevention.

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  • natsnet
    What is plain sedimentaion?
    3 years ago
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    What is plain sedimentation explain the factors affecting sedimentation?
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    What is plain sedimantion?
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    What is plain sedimentation in environmental engg?
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