 Hello friends, today we are going to learn about zones of pollution in the stream. Myself, Mahirubalai, I am an assistant professor in WIT, Solapur. So, by learning this session, we students are going to understand about various zones of water pollution in the stream. First, let us talk about the water pollution which is occurring in the stream. Mostly, the domestic sewage and the industrial wastewater which is coming into the stream. In the stream means it can be a river, it can be a connecting canal or whatever, which is the main connection between the city or the village to the wastewater treatment plant. And this connection is also helping the collection of water for the water purification and sending back to the city after purification of that water. So, let us have few discussion about various zones of pollution in the stream. First one is our clear water where the water is not too much polluted. Here you can see this is the pipe which is adding up the wastewater. So, this zone here you can see the fish or the animals, the fishery is present or what we can say the lives are present. Here you can see the two terms which are highly important for us to understand various zones. First one is dissolved oxygen, which is our DO. And another one is BOD, that is biochemical oxygenation demand. Here you can see the DO is highly present in the water. It is above 6 ppm, mostly it is taken as 8 ppm. And BOD is very much low. Why it is so? Because the food is not available and the demand is very much lower. Because no wastewater is present, it means no food, no bacteria and so very much less BOD. Fine, the DO is very much highly present. What does it mean? It is used by fishery also and mostly the wastewater is not present. So, the food is also present in the abandoned. That is what this DO is telling us. Fine, so let us study the first zone. Our first zone is the zone where the wastewater is added. This zone is containing a highly presence of food. What is our food here? Our wastewater, which is added into the water. Here you can see the fisheries are also present. They are not getting killed. Now the start of growth of bacteria is happening. And automatically when the bacteria started to grow up, they will take the food. That is our wastewater and they start to eat that food. Fine, with the help of DO. That is why the DO percentage is started to going down. And here you can see as the wastewater is added, the BOD jumps up. And it simultaneously grows down. Why it is so? Because as the bacteria is taking the DO and automatically the requirement of food is started to decrease. That is why the BOD also started to getting decreased. So here you can say in the red it is also going on decreasing. Fine, such kind of a zone, we will call it as zone of degradation. In the zone of degradation, it is mostly present below the outfall of the sewer. Which we had seen in the picture. Now here in this zone the water is mostly dark. It is turbid with the sludge at the bottom. Fine, the DO will start to reduce and it will reduce up to 40% of the saturation level. What is our saturation level? The level of DO in the clearer zone, which we had seen earlier. Where the 8 ppm is written. It is our saturation level. It means highest percentage of DO is present in that stream. That is our DO of saturation level. So in the zone of degradation, the DO will reduce up to 40% of saturation level. CO2 content will start to increase because after eating up that food the microorganisms will start to throw out carbon dioxide gases. The re-aeration is started with a slower rate and it will be very much slower than de-oxygenation. It means what is meant by re-aeration? The addition of oxygen into the stream will decrease rather than the de-oxygenation. It means oxygen is highly taken up by organisms rather than addition of oxygen from the atmosphere. Fine, the conditions are favorable for aquatic life. Whatever all these things happening it will not affect highly to the aquatic life. Anaerobic decomposition takes place in that zone. It means whether the oxygen is present or not highly facultative bacterias or microorganisms will start to grow and eat that food and it will happen by anaerobic decomposition process. Fine, so let's see the next zone. Our next zone is the zone where all the fish or the aquatic life is absent because here in the earlier zone that is zone of degradation highly increase of bacterias are there. So what they will do? They will be present in that next zone or in that stream and they will eat up all that food. Fine, and at that condition the DO level is very much lower because they had taken all the oxygen from that. So here you can see in this zone the DO level is very much lower. It is almost zero. Fine, the BOD is still decreasing. It means the growth of bacterias is still happening and the decomposition is still happening. So we can call this zone as zone of active decomposition. Fine, so in this zone there are also various properties which are very much important to study. So let's study its properties. In this zone the water is mostly greyish and darker than the previous zone. Fine, the DO concentration falls to zero. Methane, H2S gas, CO2 gas and N2 gas present because of anaerobic decomposition. And because of this hazardous gases also the aquatic life is not present. And with the absent of oxygen automatically our fishery life will start to decrease and it will end in this zone. Fine, and here we had discussed the fish life is absent due to various types of bacterias are present. And at the end of this zone the DO again started to rise to 40% to the saturation. What does it mean? There will be a point or what we can say a peak point will come where the growth of bacterias will be there. But automatically as from the first zone the decomposition of food is continuously happening or the decomposition of waste is continuously happening. So that the time will come the food will the food the presence of food will decrease and it will go on to the zero. It means no food will be present in the stream. At that time what will happen? The microorganisms will start to eat themselves and the presence of microorganisms will decrease. And when this happens the requirement of oxygen is also going to be decreased and the presence of oxygen in the stream will increase. And automatically the DO content of stream will automatically increase. So the limit of zone is where when the DO will be 40% of the saturation. And at the end of this zone the aquatic life starts to reappear. Why it is happening? Because the presence of oxygen is present in that stream. Let's talk about the next zone. Our next zone is the zone where the DO will start to increase at a higher level and the BOD will start to decrease at a higher level. Why? Because here the food is not present. So automatically whatever the oxygen which is absorbed by the stream from the atmosphere it is automatically going to store in the stream. And the microorganisms which are remaining from this zone will start to eat again themselves and they will be removed off fully in this zone. So here the DO is in the recovery stage and also the stream is getting again a proper previous life like this. It is not fully like clearer water but it is in the stage where it is recovering like an earlier part. So such type of zone we call it as zone of recovery. In this property the recovery starts stabilization of organic matter takes place. It means whatever the microorganisms which are remaining they will eat themselves and become stabilized or what we can say no bacteria will be there so stabilization will be there. Again the BOD falls and DO content increase as we had said earlier. In this zone nitrates, sulphates and carbonates are formed because as the organic matter is stabilized they will form various structures in terms of nitrates or sulphates and carbonates. So at the end of this zone the entire aquatic life will again fully repair. Fine. What is the last zone? It is a zone what we call it as a zone of clearer water. It is the same water which is present before the adding up of wastewater. Here the DO is again reached 8 ppm and the BODs again come to the lowest stage which was earlier present before adding up the wastewater. So we are calling it the zone of clearer water. So what are its properties? It is clearer and attractive in appearance which was like earlier. The DO adjusts to saturation level which we had talked about. The oxygen balance will be there which was present earlier. The recovery is fully complete. It means no organisms are present and it is fully recovered. Some pathogenic microorganisms may be present but it is mostly in the industrial areas but not the streams which are nearby domestic areas. So let's sum up all these things in a proper table. If you see here the main important part here in the bottom table we had discussed with respect to the properties. So if you want to learn and sum up in a very short way you can pause this video and you can draw this table and easily learn and buy hard. But I want to focus more important part is our DO level. If you see that in the zone of degradation the DO will decrease to 40% and in the active decomposition it will reach 0%. The zone of active decomposition will be there till the DO will increase to 40% and in the recovery the DO will increase up to the saturation level and the zone of clearance it will reach again up to the saturation level. So let us have some quick questions. When a BOD of wastewater increases then what will happen? Second, rate of aeration is independent of DO content of wastewater true or false? A dissolved oxygen content is less than 40% in which zone? So let us have its answers. So when the BOD of wastewater increases rate of oxygen deficit increases which we had talked earlier the rate of aeration is independent of DO content of wastewater it is highly false. Third, dissolved oxygen content is less than 40% only in zone of active decomposition. So these are the references I have studied in it. Thank you.