Process self purification of running streams, Explain oxygen sag curve

Last updated on May 15th, 2024 at 07:44 am

The various actions Involved in the self purification of running streams are physical-chemical and biological and may be explained as due to

1. Dilution: As the putrescible organic matter is discharged into the flowing water, it’s rapidly dispersed or diluted in it, the action resulting in diminishing the potential nuisance of sewage. This process is further accelerated by joining in with the mainstream of surface tributaries and underground streams.

2. Sedimentation: This helps by the separation of the settleable solids in sewage in the form of sludge deposits, Settleable solids are most stable and easily separated out.

self purification of running streams
                             self purification of running streams.

3. Oxidation As soon as the organic matter meets the water, it starts getting oxidized owing to the development of the oxidizing organisms in the water. This process goes on till the organic matter has been completely oxidized, the oxygen demand is then fully satisfied and the stream is said to have purified itself. It is therefore essential for the stream to have initially dissolved oxygen in it. Oxygen is also contributed through the dilution received from tributaries, through aeration by the action of wind or by the action of microscopic organisms.

4. Reduction: Reduction occurs due to the hydrolysis of organic matter either chemically or biologically. Anaerobic organisms liquefy splitting complex organic constituents of sewage with the evolution of odors and gases and in this way paving the way for stabilization by oxidization.

5. Sunlight: Sunlight is effective in self-purification through its stabilizing and bleaching effects on bacteria and through the biological action of certain microorganisms deriving the energy from the sun, converting themselves into food for other forms of life, absorbing carbon dioxide, and giving off an oxygen-a process known as photosynthesis.

Self Purification of Streams – Zones of Pollution | Waste Water Engineering

Explain oxygen sag curve

As the sewage is discharged into a body of water there is at first depletion of the D.O.content of the diluting water in order to meet the biological requirement (BOD) of the organic sewage.

This is termed as deoxygenation and closely follows the progress of BOD of polluted water.

For this reason, it is also sometimes referred as BOD reaction. The rate of deoxygenation action depends on.

  1. Volume and BOD of sewage
  2. Time available for decomposition and
  3. The temperature of the diluting water

Due to deoxygenation, the D.O. of water would have been rapidly consumed up, but for the fact that oxygen is also absorbed at the water surface from the atmosphere whenever tho Do content of the waterfalls below its saturation value Oxygen to also contributed by other factors including green plants under the influence of sunlight.

This supply of oxygen is termed as reaeration and its rate depends upon

1. depth of receiving water (rate is more in shallow depth)

2. condition of the body of water (rate is more in a running stream than in a quiescent pond)

3. saturation deficit or the deficit of DO below the saturation value.

Both deoxygenation and reaeration are occurring simultaneously in any polluted stream exposed to the air.

Their rates can be formulated into expressions and curves obtained therefrom.

In order to determine the amount of DO present at any Instant, the two can be combined together to give the DO or the oxygen curve.

Two distinct points of the oxygen sag curve are

1. The critical point or the point of least oxygen content, when the oxygen deficit shall be maximum
2. The point of inflexion, when the rate of recovery is maximum.

What are the different zones of pollution in a river stream?

A polluted stream undergoing self-purification has the following four distinct zones of pollution

1. Zone of degradation: This usually occurs below the outfall sewer when discharging its contents into the stream. The zone is characterized by the water becoming dark and turbid with the formation of sludge deposits at the bottom. D.O gets reduced to 40%. There is an increase in the carbon dioxide content reaeration occurs but is slower than deoxygenation. Conditions are favorable to the development of aquatic life, fungi at higher points and bacteria at lower points, breed small worms which work over and stabilize the sewage sludge.

2. Zone of active decomposition: This is marked by heavy pollution. It is characterized by the absence of DO, water is greyish and darker with active anaerobic organic decomposition accompanying and with the evolution of methane, hydrogen sulphide, carbon dioxide, and nitrogen bubbling to the surface_with masses of sludge forming black scum. Fish life is practically absent, fungi and bacteria disappear. As the organic decomposition slackens reaeration sets in and DO again rises to its original level.

3. Zone of recovery: In this zone the stream tries to recover its former appearance. Most of the organic matter has been settled as sludge, BOD falls and the DO content rises above 40%. Microscopic aquatic life reappears, water becomes clearer, fungi decrease and algae reappear. Mineralization is formed. active and the products such as nitrates, sulphates, and carbonates are formed.

4. Clear water zone: In this, the natural stream condition is restored, the DO is higher than the BOD. Oxygen balance (DO minus total BOD in the first stage) is attained and recovery is said to be complete. Water becomes attractive in
appearance. Some pathogenic organisms may however be present.

what are the different factors affecting self purification of water?

The following conditions affect the self purification of water

1. Dilution: When the sewage is mixed with a large volume of water the sewage always remain in aerobic conditions and DO always remain present in the water

2. Current: When there is no current the sewage matter deposits near the outfall causing formation of sludge bank and foul odors. In heavy currents, the sewage is thoroughly mixed up with the stream water preventing all nuisance.

3. Sedimentation: With slow current, the heavier solids settle in the stream bed and start anaerobic decomposition. The products of decomposition are again mixed with water by the current. If the dilution is sufficient the anaerobic the condition will not develop and the scouring tendency of streams during a flood will wash the deposits

4. Temperature: At low-temperature organisms activities are slow due to which rate of decomposition is slow and in warm temperature action is reverse Therefore in summer the streams will get self purified in less time than in winter. But the quantity of DO is more in cold water than in hot water.

5. Sunlight: The pathogens are killed if they are exposed to sunlight, therefore sunlight helps in self-purification. Algae also grow in sunlight causing the production of oxygen.

6. Oxidation. After mixing with water organic matter starts getting oxidized due to the development of the oxidizing organisms present in the water. This process prevails till the complete oxidation of the organic matter. The oxygen demand is satisfied by the aeration by wind or by the microscopic organisms and the stream becomes purified Reduction:

7. Reduction in streams occurs due to hydrolysis of the organic matter. biologically or chemically. Anaerobic organisms start splitting complex organic matter present in the sewage, the action produces gases and odour and the stabilization starts.

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