 Greetings students. My name is Mayuru Valle. I am an assistant professor in civil department WIT Sallapur. Today we are going to discuss the topic Oxygen SAG analysis. So what are the learning outcomes? At the end of this session, students will be able to understand Oxygen SAG analysis in the stream. So first of all, what is Oxygen SAG analysis? To go into the details, first we have to analyze what are the parameters which are included in it. Oxygen SAG or Oxygen deficit in the stream at any point of time during the self purification process is basically the difference between the saturation dissolved oxygen that is DO content and actual DO content at the time. So in simple words, what we can see? Oxygen deficit that is capital D is equal to saturation DO minus actual DO. We have to note that the normal saturation DO value for freshwater depends upon the temperature and its value which varies from 14.62 mg per litre at 0 degree Celsius to 7.63 mg per litre at 30 degree Celsius. So as the temperature increases, saturation DO decreases. So to understand the Oxygen SAG analysis, first we have to understand what is the rate at which DO is decreasing and what is the rate at which BOD is decreasing. If you see at the first, this part is the part where the point of disposal in the stream is happening. While adding the liquid waste or the treated effluent into the stream, there will be not the change of DO happens. But the value of DO is very much higher. But there will be a sudden change is going to occur after the disposal point. It means after a few distance. So this point is noted as DO that is initial DO. So as the stream flows, the disposed effluent is going to be dispersed and slowly slowly the DO is going to decrease. Why it is so? Because whatever the microorganisms which are present in the stream, they are going to use this DO and automatically they are going to disperse or degrade whatever the organic matter which is present in the stream. So the DO will slowly slowly decrease to one point where all the microorganisms have been killed and all the organic matter is also going to be killed up by microorganisms. So automatically till this point, the DO will going to be lessened down. This point is known as critical point where the least amount of DO content will be present in the stream. Now slowly slowly as there is no one to accept or to absorb the oxygen which is coming from the atmosphere into the stream. So automatically DO is started to increase. So you can sleep with respect to time that DO is started to increase. So this red line is showing us the oxygen sag analysis. Now talking about the BOD, you can see there is a sudden increase of BOD happening where at the point of disposal. Because large amount of organic matter is coming inside the stream at the disposal point. As they are slowly slowly eaten up by microorganisms, so slowly slowly BOD exerted is going to decrease. So as the X that is distance is going to increase, the DO is slowly slowly going to decrease. And after the critical point that is XC, the DO is started to increase. And this sudden increase of BOD will be our L0 that is expected BOD. Now in the following we also have to analyze how the oxygen which is present in the stream is going to be degraded and how it is going to be slowed down. And also how the oxygen is going to be increased. For that we have to use separate curves. We have to understand separate processes. So when the pollution load is discharged into the stream, the DO content of the stream goes on depleting. This depletion of DO is known as deoxygenation. The variation or depletion of DO content of the stream with respect to time is depicted by deoxygenation curve. Though the DO content of the stream is gradually consumed due to BOD load, atmosphere supplies oxygen continuously to the water through the process of re-aeration or re-oxygenation. And it is also depicted by re-oxygenation curve. So you can see there are two curves. The green curve is showing the re-oxygenation curves. Where whatever the oxygen which is present in the atmosphere is continuously indulged or getting introduced into the stream. So automatically DO is slowly slowly slowly increasing from the point of disposal. So here you can see in the graph the x-axis is the time and y-axis is the DO content. So by that we can understand as the time passes DO in the water due to addition of oxygen by the atmosphere is going to increase. But still we had not talked about how much the DO is going to be taken up by microorganisms. Only one parameter is being seen in the re-oxygenation curve. For eating up of organic matter by the microorganisms they need oxygen. So whatever the oxygen or DO which is present in the stream will be taken up by microorganisms. And slowly slowly the DO is being used up. So you can see the blue curve that is the de-oxygenation curve. Where if the oxygen is present at the higher stage that is our saturation DO is slowly slowly eaten up by microorganisms. And is made to come down till the least value. So addition of these two value is going to happen simultaneously and parallely in the stream. Oxygen is coming from the atmosphere into the stream and this oxygen is going to used up by microorganisms also. So addition of these two curves is going to give us the curve of oxygen sag analysis. So till the critical point what we can say the de-oxygenation curve is very much higher in preferential to eat the DO. And after the critical point what we can say addition of DO is very much higher than taking up of oxygen by microorganisms. To represent or to analyze the same curve and to find out what is the DO content at every stage when the flow of stream is going to happen with respect to distance. To do that we have Steter-Felps equation where DT is equal to KL0 divided by R minus K in bracket 10 raise to minus KT minus 10 raise to minus RT bracket complete plus D0 into 10 raise to minus RT is the formula. Where DT is the DO deficit at any time T, T is in the days. And L0 is the ultimate BOD which has been found out by addition of the organic waste or treated effluent at the point of disposal. K is the BOD reaction constant to the base 10. R is the reoxygenation constant to the base 10. So now talking more on reoxygenation constant that is R it also varies with respect to temperature and it can be expressed by the equation that is R T is equal to R 20 into 1.024 raise to T minus 20. Where T is in temperature in degree Celsius R T is reoxygenation constant at temperature T that means varying temperature and R 20 is the reoxygenation curve at temperature 20 degree Celsius. So as if you can see varying flow of water is always gives out varying reoxygenation constant at 20 degree Celsius. So if you see small points and black waters have reoxygenation constant of 0.04 to 0.1. Talking about the next that is sluggish streams and large rakes have the reoxygenation constant that is capital R of 0.1 to 0.15. If I increase the velocity now that is large stream of slow velocity is having a capital R of 0.15 to 0.2. Now again I will increase the velocity now that is large streams of normal velocity that is having the capital R value of 0.2 to 0.3. Now again I will increase the velocity that is swift streams which is having the capital R of 0.3 to 0.5. Now I will increase more velocity now and at the highest value of the stream that is rapid and waterfalls which is having rapid or turbulent velocity that is having the capital R of greater than 0.5. So it means as the flow or as the velocity of the stream increases addition of oxygen from the atmosphere to the stream increases and automatically reoxygenation constant also increases. Now let us have few review questions first one deoxygenation curve depend on organic matter remaining in the stream, time, temperature or all of the above. Atmospheric re-arition is directly proportional to deodeficit till critical point whether this statement is true or false. Reoxygenation curve depends on depth of water in a stream, temperature, velocity of flow in the stream or oxygen deficit below saturation DO. So let us have their answers deoxygenation curve depends on all the above that is organic matter remaining in the stream, time and temperature. Atmospheric re-aration is directly proportional to deodeficit till the critical point this statement is true. Reoxygenation curve depends on the depth of water in the stream, temperature, velocity of flow in the stream. So all of these are the right answers. So for this video I have used this reference. Thank you.