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Hi students, you are welcome to 30 MGD sewage treatment plant of Delhi Jal board at Okhla. The treatment capacity of this plant is thirty million gallon per day. First of all clap yourself to become the part of Okhla STP today. Today we learn how raw sewage / waste water have being treated. Dear students Sewage treatment plant contains mainly three types of treatment processes that are primary treatment, secondary treatment & tertiary treatment. Let’s stats with primary treatment process. Primarily raw sewage has been collected in the inlet chamber of Sewage treatment plant. The incoming waste water carries various soluble and insoluble impurities like human waste, kitchen waste, sand, clay, polythenes, bottles, papers, clothes, tree leaves et cetera. So first of all, the waste water passes through different types of automatic or manual screens which separate the floating raw sewage waste depending upon their sizes, majorly the sizes of screen varies from 60 mm to 1 mm. Here at this plant we use 3 MM automatic fine bar screen so that all the floating particles larger than 3 mm size can be segregated from screen and with the help of belt conveyor all the floating waste dumped into a trolley standing below the conveyor outlet. Then these waste materials send to the waste to energy plant of Delhi Municipal Corporation for further management. After screen the raw sewage passes though grit chambers where the water retain for three to five minutes so that its waste get settled down in the bottom of the chamber by sedimentation process and with the help of mechanical moving scraper all the waste from the bottom pushed to a side cavity and from this cavity it lifted up with the help of screw conveyor. In this unit mainly particles sizes between 0.5 mm to 3 mm are separated from waste water. After this the waste water sent to primary settling tanks for settling down the suspended particles, as these particles are very small in size and very light in weight so they need more time to settle down. hence the water retain in PST up to 3 to 3.5 hours and again due to sedimentation process the waste particles get settle down in the bottom of the tank and with the help of mechanical moving scrapper all these settled particles pushed to a bottom cavity of this unit and after that collected in a deep sump called sludge sump and from the sludge sump these particles send to sludge digester with the help of pumps for its further treatment. Here the primary treatment is complete hope you understand clearly. Secondary Treatment is basically a biological treatment where soluble waste of water is being treated biologically. In this process a favorable conditions has been created in the aeration tank like level of oxygen and availability of food as required for the growth of aerobic bacteria in the tank. As the food is already available in the sewage water in soluble form so we have to provide only oxygen in the aeration tank for the growth of bacteria. So pressurize atmospheric air passes through the water in diffused form using membrane diffusers installed in the bottom of the aeration tanks so that air can mix properly in the entire aeration tank. As you are well aware that atmospheric air already contains twenty one percent of oxygen hence by this process the level of dissolved oxygen increases gradually in the aeration tank. Normally we maintained Dissolve Oxygen level from two ppm to four ppm as per the requirement of the process. The level of the oxygen in the aeration tank can be increases or decreases through adjustment of different RPM of air blower. In the aeration tank we have to maintain food and mass balance also known as FM Ratio. Here food stands for soluble waste and mass stands for quantity of bacteria. Please understand the importance of FM ratio. suppose if food increase as compare to bacteria then the food left un-consumable and this excess food remain in the water so we do not get good quality of treated water. Now in case of vise versa if bacteria increase as compare to food then all bacteria will not get proper food to survive and start dying and also start disturbing or killing other bacteria. Hence it is very important to maintain FM ratio in aeration tank. To check the FM ration routine samples checks has been done on daily basis in the lab. This process is also known aerobic process. Now after this process the water of aeration tank flows towards final settling tank similar to primary settling tank the only difference is that in primary settling tank we collect the primary sludge which send to digesters for biogas generation and sludge digestion. Whereas in secondary settling tank the aerobic bacteria get collected and then returned to the aeration tank for maintaining the desired level of bacteria in aeration tank and excess bacteria send to digester for further treatment with primary sludge. Generally five to ten present excess bacteria generated daily. Now the water separated from secondary tank from top of the tank and transferred to the chlorination contact tank for disinfection and after chlorination the treated effluent passes towards Yamuna River for further utilization of treated water for agriculture, irrigation or aquatic life. Here the secondary treatment completed hope you understand clearly. The sludge majorly collected through primary settling tank and minor through secondary settling tank send to sludge thickener for maintaining proper concentration as required for the optimum generation of biogas. Now from sludge thickener the concentrated sludge send to sludge digester for further treatment here the sludge is digested in absence of air so this process is also called anaerobic digester. Here due to anaerobic process the sludge bacteria get breakdown and formation of biogas take place in the digester and collected at the top of digester and from here the biogas transfer to biogas holders for storage. The Biogas contains following gases:- Methane 65% Carbon dioxide 25% Hydrogen Sulfide 5% Nitrogen, Hydrogen and other gases 5% The hydrogen sulfide is a corrosive & Toxic gas hence we remove this gas from biogas with the help of H2S scrubber Unit. Now after removing H2S the clean biogas utilized for power generation with the help of biogas engine. The Biogas engines are similar to generators. The only difference is that Biogas engine use methane as a fuel instead of petrol or diesel. The electricity generated by the gas engines are utilized inside the plant. Normally the gas engine fulfills the fifty percent demand of power requirement of the plant. Now the digested sludge from digester is supplied to sludge drying beds to become dry naturally and utilized as fertilizers at agricultural land. Parameters of treated effluent as achieved at plant are as follows:- T.S.S below 10 mgl B.O.D below 10 mgl C.O.D below 50 mgl Ammonia below 5 mgl Oil and Grease below 10 mgl Ph 6.5 to 9 Here the process of Sewage treatment plant completed hope you understand clearly. Delhi jal board wishing you best of luck for future. Thank you very much.