Welcome to the Chapter 18 - Wastewater Story, Class 7 Science NCERT Solutions page. Here, we provide detailed question answers for Chapter 18 - Wastewater Story. The page is designed to help students gain a thorough understanding of the concepts related to natural resources, their classification, and sustainable development.
Our solutions explain each answer in a simple and comprehensive way, making it easier for students to grasp key topics Wastewater Story and excel in their exams. By going through these Wastewater Story question answers, you can strengthen your foundation and improve your performance in Class 7 Science. Whether you’re revising or preparing for tests, this chapter-wise guide will serve as an invaluable resource.
(a) Contaminants
Cleaning of water is a process of eliminating pollutants before it enters a water or aquatic body or is reused. This wastewater treatment process is commonly known as sewage treatment. It takes place in various stages.
(b) Sewage
Sewage is a waste. Most of the dirty water having dissolved and suspended impurities or contaminants in it. The impurities present in sewage are known as contaminants or pollutants.
(c) Dung
Cow dung, which is dark brown in colour is mainly used as manure. If the dung is not recycled into the soil by species like earthworms and dung beetles, cow dung can dry out remain on the pasture, creating an area of grazing land which is unpalatable to animals.
(d) Plastic and sludge
The waste water is passed into a sedimentation tank and allowed to stand there for a while. Most of the solid organic matter settles down on the bottom of the slope of sedimentation tank in the form of a sludge; the solid part of sewage is called sludge.
The open drains system creates the most unhygienic and unsanitary conditions in its surroundings. Open dirty water drains are breeding places for flies, mosquitoes and other disease causing organisms and bacterias. We can become an active citizen and approach the municipality or we can say Gram panchayat and insist that the open dirty water drains in our area be covered as soon as possible. The covering of open drains will make the environment healthy which will be free from disease causing organisms. If the sewage of a particular house leaks from the pipes, spills on the road and makes the neighbourhood dirty, we should ask the house owner to be more considerate about other’s health and get the leakage of sewage rectified.
Across:
3. Liquid waste product- Sewage
4. Solid waste extracted in sewage treatment- Sludge
6. A word related to hygiene- Sanitation
8. Waste matter discharged from human body- Excreta
Down: 1. Used water- waste water
2. A pipe carrying sewage- Sewer
5. Microorganisms which causes cholera- Bacteria
7. A chemical to disinfect water- Ozone
(ii); (b) and (c); It is used to disinfect water and it absorbs ultraviolet rays; this statement is correct.
Ozone is a powerful oxidising agent which, when dissolved in water, produces a broad spectrum biocide that destroys all bacteria, viruses etc. Ozone water treatment is being used commercially for the treatment of potable water. It absorbs ultraviolet rays or radiations, present in the stratosphere, ozone is created primarily by ultraviolet radiation. Ozone is extremely valuable since it absorbs a range of ultraviolet energy which is very harmful for the earth and our body; and by absorbing these radiations it protects from the damage that could happen to our earth.
Sewage: The waste water and faeces (human excretion) from homes and other buildings which is carried into the sewers is called sewage. Sewage includes household wastewater from kitchen sink, washing machine, washbasin, bathroom shower, bath tub and faeces from the toilet. Sewage is also produced and released by shops, offices, schools, hospitals, factories, industries and all other places where humans work or live. Sewage also includes rainwater that has run down the streets and roads during rains carrying harmful substances with it which also spreads various diseases along it, and flows into sewer pipes. Sometimes, sewage contains harmful and poisonous chemicals that are being discharged by factories and industries. Most of the dirty water contains dissolved and suspended impurities in it which are known as contaminants or pollutants.
Waste cooking oils and fats should not be released in the kitchen sink to go down the drain because cooking oils and facts can block the drainage pipes or system. On the other hand, in an open drains, the fats blocks the soil pores and reduces its effectiveness in filtering water. The waste oils and fats should always be thrown in the dustbin. The oil travelling into waterways can cause water’s oxygen level to drop which can also affects the wildlife living in it. Cooking oils are commonly liquid at room temperature but can get solid at the low temperature if the oils contains fat such as coconut oil, palm oil and palm kernel oil. The oil extracted from plants sources are olives, soybean oil, palm oil, musturd oil, peanut oil and other vegetables oil etc.
We need water for all our as usual activities. Each and every process is in connection to the water that we use. Now a days, we waste water too much. Water conservation is not being done properly. So water is clarified from the wastewater (also known as sewage) generated by various activities like washing, laundries and etc.
Clarified water from wastewater:
Wastewater is collected and passed through bar screens where plastic bags, sticks, cans, napkins etc. are removed. This water then enters a grit and a sand removal tank.
The speed of the water is decreased so that the sand particles get settled at the bottom of the tank. After this, the water is allowed to settle in a large tank which is tilted towards the centre.
Solid wastes are settled and removal by the scraper and which is known as clarified water. Air is pumped in this water to help aerobic bacteria to grow.
The bacteria consume the wastes by microbial action. These microbes settle down after several hours and the water at the top is removed.
This water is discharged into rivers and oceans or is put to other uses. Nature cleans it up further. Sometimes it may be necessary to disinfect water with chemicals like chlorine and ozone before releasing it into the distribution system.
The waste water (or sewage) is then passed into a sedimentation tank and allowed to stand there for a while. Most of the solid organic matter settles down on the slopping bottom of the sedimentation tank in the form of a sludge. Thus, the solid part of sewage is called sludge.
The sludge is taken out from the bottom of first sedimentation tank and put into a large, closed tank called digester tank. In the digester tank, many types of anaerobic bacteria decompose the organic matter present in sludge to produce biogas (also called sewage gas). The anaerobic bacteria do not need oxygen of air to carry out the process of decomposition. That is why the digester tank is a closed tank. The biogas produced here can be used as a fuel directly or it can be used to generate electricity. The digested sludge left after the removal of biogas is taken out, dried and used as a manure. The wastewater treatment gives us two useful products: biogas and sludge. The biogas is used as a fuel whereas sludge (or rather ‘digested sludge’) is used as a manure. The use of dried sludge as a manure returns the nutrients to the soil.
Majority of people in our country still do not contains sewage facilities for the safe disposal of human excreta. Due to this, a very large number of our people defecate in the open in fields, near railway tracks, on dry river beds and many a time directly in water (in ponds, lakes and rivers etc.) The untreated human excreta is a health hazards because it can cause a large number of diseases and also cause soil pollution and water pollution which can lead to various diseases. Since, the human excreta is carried along by rainwater, both in lakes and rivers as well as groundwater get polluted with it. The river water and groundwater are the sources for performing various purposes for many people.
The drinking water contaminated with untreated human excreta becomes the most common route for the spreading the diseases.
It may be necessary to disinfect the treated water with chemicals like chlorine or ozone before releasing it into the distribution system. The purpose of disinfection in the treatment of wastewater is to eliminate completely the harmful micro- organisms in the water to be discharged back into the environment. The treated wastewater can also be passed through layers of sand (called sand filters). Sand filtration removes most of the remaining suspended matter from the treated wastewater. It has been suggested that we should plant Eucalyptus trees all along the sewage ponds. This is because eucalyptus trees absorb all the surplus
wastewater from the sewage ponds rapidly and release pure water vapour into the atmosphere (through transpiration).
A place where wastewater from houses and other building like industrial area is brought for processing is called wastewater treatment plant. Waste-water treatment plant can be written as WWTP. The wastewater treatment plant is also called sewage treatment plant. In a wastewater treatment plant, bar screen removes large solid components from water. The wastewater is allowed to pass through bar screen so that large solid components like rags, napkins, sticks, cans, plastic bags, polythene etc. present in wastewater can then be removed. These bar screens can then be used as a primary filtration method in the waste water treatment plants.
The bar screen is a kind of mechanical filter and is used in the waste water treatment plant to remove the bigger and larger impurities or contaminants that leads to pollution like plastics or used cups, etc. After removal of larger impurities, it becomes easy to treat the water to get rid of the smaller impurities or impurities mixed in water with the help of chemical treatment.
A large number of diseases are caused due to poor sanitation and contaminated drinking water. Poor sanitation leads to the growth of harmful microbes, flies and mosquitoes which are the source of spreading many diseases like cholera, typhoid, polio, meningitis, hepatitis and dysentery. So it’s better to have good sanitation and proper hygiene. Sanitation refers to public health conditions related to clean drinking water and adequate treatment and disposal of human wastes and sewage. Preventing human contact with faeces is part of sanitation, as is hand washing with soap.
Relationship between Sanitation and Diseases:
1. In our country, a large number of people even today do not have sewage facilities and thus defecate in open fields, railway, tracks, etc. The untreated human excreta thus pollute soil and water sources including the underground water. When this contaminated water is used for drinking, it can cause diseases such as cholera, typhoid, hepatitis, dysentery, etc. which may even lead to death. Therefore, the lack of sanitation resulting in the drinking of contaminated water can cause health problems.
2. If sanitation is there, no disease will occur; but if sanitation is not there various types of diseases will occur and spread. So sanitation should be kept to avoid diseases.
