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TERM-II, DATE: November 16, 2021, CLASS-7, SCIENCE, CH-18: WASTEWATER STORY, WORKSHEET-6, SUB-TOPIC: Important Notes, Introduction, • We all need clean water however, not all can access the same., • Hence, it is necessary not only to preserve water but clean the used water before dumping it into the, seas or any other water body., • We should clean the water and use it., • This is why the United Nations proclaimed the period between 2005 and 2015 as the International, decade for action on ‘water for life’., • World Water Day: 22 March, Waste water, • The water that has been used and is not fit for usage again is called Wastewater. It is dirty water from, the laundry, toilets, sinks and drains., • Rich in lather, mixed with oil, black– brown water that goes down the drains from sinks, showers,, toilets, laundries is dirty. It is called wastewater., What do you mean by cleaning of water?, It is a process in which the pollutants from the water are removed before the water reaches a water body or, before it is used again. This process is also called Sewage Treatment., Sewage, • Sewage is wastewater released by homes, industries, hospitals, offices and other users., • It also includes rainwater that has run down the street during a storm or heavy rain., • The water that washes off roads and rooftops carries harmful substances with it., • Sewage is a liquid waste., • Most of it is water, which has dissolved and suspended impurities., • These impurities present in the sewage are called Contaminants as they contaminate the water., Sewers, • Large and small pipes that are installed in buildings to carry sewage water are sewers., Sewerage, • A network or transport system consisting of sewers is called Sewerage., • It carries sewage water from the point where it is produced to the point where it is disposed, (treatment plant)., • It consists of manholes that are located at every 50 to 60 metres., • The manholes are installed at a point where two or more sewers intersect or change their directions., • Every locality has as a system that allows removal of waste from the houses and industries., Waste Water Treatment Plant (WWTP)

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Treatment of wastewater involves physical, chemical, and biological processes, which remove physical,, chemical and biological matter that contaminates the wastewater., 1. Separation of Large Objects from Sewage Water,  The wastewater is first passed through bar screens which remove big objects from the water, like plastic bottles, napkins, rags, cans and sticks., 2. Separation of Sand and Grit,  The wastewater is then sent to a grit and sand removal tank. The speed of the water is, decreased at this step. This allows the sand, dirt and pebbles to settle down., 3. Removal of Solids,  The water is then allowed to settle in a large tank which is sloped towards the middle. Solids, like faeces settle at the bottom and are removed with a scraper. This is the sludge.,  The sludge is then separated from the water with the help of a Scrapper.,  Substances like oil and grease are removed with the help of a Skimmer as they float above, the water. In this step clarified water is obtained.,  The sludge so obtained is passed into a tank where anaerobic bacteria can decompose it and, produce biogas. The biogas is then used as a fuel., 4. Suspended Sludge Removal/Aeration,  Air is pumped into the clarified water to help aerobic bacteria to grow. Bacteria consume human, waste, food waste, soaps and other unwanted matter still remaining in clarified water.,  After several hours, the suspended microbes settle at the bottom of the tank as activated sludge. The, water is then removed from the top.,  This treated water contains fewer amounts of impurities and suspended matter. This water is safe to, dump into a water body or under the ground. Hence, it is then discharged into a sea, river or, underground. Nature further cleans it up.,  The activated sludge is about 97% water. The water is removed by sand drying beds or machines., Dried sludge is used as manure, returning organic matter and nutrients to the soil., Chemical Treatment of Wastewater, • Sometimes harmful chemicals can be present in the water even after treating them physically and, biologically., • These chemicals are therefore removed by using disinfectants such as chlorine and Ozone., • Often chlorine gas or ozone gas is introduced in this water which purifies it chemically.