Self Purification of streams and oxygen sag curve

 Self Purification of streams & Oxygen Sag Curve

Self-Purification of Streams & Oxygen Sag Curve

Introduction:

⇒ The main ultimate disposal of waste water can only be in to land or in to the water.

⇒ Majorly when the water are used for the ultimate disposal, the waste water will be provides a treatment process to prevent any problem to the aquatic life inside the water.

⇒ Basically the treatment process consist of the removing of dissolved solids and suspended solids, through different treatment equipment's.

⇒ Maximum engineers will design a treatment plant to do primary and secondary treatment process, but a complete treatment i.e. 100 percent treatment of the pollution load is not possible and never aimed in any of the waste treatment plant.

⇒ In any treatment plant, major work is to remove the dangerous chemicals or any solids, present inside the waste water, after removing those waste from the water then those water will be dispose into the stream and from there stream will be self purified.

Self Purification of stream: 

⇒ When sewage is discharged into a natural body of water, the receiving water will gets polluted, but this condition are not remain same forever, because of natural forces of purification go on acting upon the pollution elements and bring back the water into original position, this complete process is called as self-purification or self-purification of stream..

⇒ if the wetted surface of the river bed is large, a large portion of the organic load is also removed from the main stream by adsorption process.

⇒ At the same time the aerobic micro-organisms, which utilizes the organic pollutants as the source of their food and energy, grows till the food supply is adequate for them, and thus organic matter will be stabilize under aerobic conditions.

⇒ The removal of the organic matter are done by

• Settling and Adsorption
• Microbiological Activities 

Factors Affecting Self-Purification:

• 1.Dilution
• 2.Current
• 3.Temperature
• 4.Sunlight
• 5.Rate of Oxidation

1.Dilution:

⇒ When required dilution of water is available in the receiving water body or stream, where the waste water will be discharged, the dissolved oxygen(DO) level in the receiving stream may not reach to zero or the critical DO due to the availability of sufficient DO initially in the river or stream water before receiving discharged waste water.

2.Current:

⇒ when there is strong water current then the discharged water will be thoroughly mixed with the stream or river water preventing deposition of the waste solids.

⇒ In some small currents the solids from the waste water will get deposited at the bed following by the decomposition and reduction in the Dissolved Oxygen(DO) levels.

• Must Read: Steps to Install Staad Pro Software

3.Temperature:

⇒ The quantity of Dissolved Oxygen(DO) available in the stream water will be more in cold temperature when compared with the hot temperature.

⇒ The activity of the microorganisms is more at the higher temperatures, hence the self purification will; take less time in hot temperatures rather than in winter.

4.Sunlight:

⇒ By using sunlight, algae produces oxygen due to photosynthesis process.

⇒ Majorly this sunlight helps in purification of streams by adding the oxygen through the photosynthesis process.

5.Rate of Oxidation:

⇒ Due to the oxidation of organic matter, which is discharged in the river or stream the DO depletion occurs.

⇒ This rate is faster in higher temperatures and low at lower temperatures.

⇒ Rate of oxidation of organic matter depends on the chemical composition of organic matter.

Oxygen Sag Analysis:

⇒ Oxygen deficit or oxygen sag curve in the stream or river at any point of time during the self purification process is the difference between the saturation dissolved oxygen(SDO) and actual dissolved oxygen(ADO) at that time, is termed as Oxygen sag curve.

⇒ Oxygen deficit (D) = Saturation DO - Actual DO

⇒ The saturation dissolved oxygen value for the fresh water depends upon the temperature and total dissolved salts present in it and its value varies from 14.62 mg/l at 0*C to 7.63 mg/l at 30*C, and the lower dissolved oxygen at higher temperatures.

⇒ The dissolved oxygen in the stream may not be are saturation level and there may be the initial oxygen deficit.

⇒ In this stage, the effluent with initial BOD(Biochemical Oxygen Demand) load is discharges in to stream or river, the dissolved oxygen concentration of the stream starts depleting an the oxygen deficit increases.

⇒ The variation of the oxygen deficit with the distance from the stream and hence with the time of the flow from point of pollution is decreased by using the Oxygen Sag Curve.

⇒ The major point in the sag analysis is the point o minimum dissolved oxygen i.e. maximum deficit.

⇒ The maximum or critical deficit occurs at the inflection point of the oxygen sag curve.

Oxygen Sag Curve

Deoxygenation and Reoxygenation:

⇒ When the waste water is discharges into the stream the dissolved oxygen level in the stream goes on depleting.

⇒ This depleting of the dissolved oxygen content is called as deoxygenation. and the rate of deoxygenation depends upon the amount of organic matter to get oxidized at any time as well as the temperature at which this reaction occurs.

⇒ The ordinate below the deoxygenation curve, which indicates the oxygen remaining in the river or natural stream after fulfilling the BOD and the oxidizing matter.

⇒ When the dissolved oxygen content of the stream is consumed gradually due to the BOD, the atmosphere supplies oxygen continuously to the water through the process of re-aeration or reoxygenation and these all are continuous process.

The rate of reoxygeneation depends upon the following factor:

⇒ Velocity of the flow in the stream; which is less for stagnant water.

⇒ Depth of the water in the stream; which is more for the shallow depth.

⇒ Oxygen deficit below the saturation dissolved oxygen, since the rate depends on the difference between saturation concentration of dissolved oxygen and existing concentration of dissolved oxygen.

⇒ Temperature of the water in stream, the solubility of the oxygen is less in higher temperature and also saturation concentration is lower in higher temperature.

Mathematical analysis of oxygen sag curve i.e. Streeter-Phelps Equation:

⇒ If you want complete derivation comment or contact us

⇒ At last after the completion of the derivation of Streeter Phelps Equation, the final equation got is as follows:

tc = [1/KR'-KD'].Log10 [(KR'/KD'){(KD' Lo - KR' Do + KD' Do/KD' . Lo)}]

Conclusion:

⇒ Detailed information about Self purification of streams is given above read it carefully and note it.

⇒ With the self purification the Oxygen Sag Curve theory has also given and the derivation is not provided but the equation is provided.