Notes of chapter: Wastewater Story are presented below. Indepth notes along with worksheets and NCERT Solutions for Class 7.
(1) Water which is rich in lather, mixed with oil, black- brown water that goes down the drains from sinks, showers, toilets, laundries, is known as wastewater.
(2) Clean water is our life line. It is the basic need of human being.
Activity 1
(3) It has been reported that more than one billion of humans being have no access to safe drinking water.
(4) Reasons of increasing scarcity of fresh water are given below:
(5) World water day is celebrated on 22nd March.
(6) The General Assembly of the United Nations proclaimed the period 2005 – 2015 as the International Decade for action on ` Water for life’, aiming to reduce by half the number of people who do not have access to safe drinking water.
(7) Cleaning of water is a process of removing pollutants before it enters a water body or is reused. It is also known as “Sewage Treatment”.
(8) Sewage is a liquid waste, comes from homes, industries, hospitals, offices, and other users.
(9) Contaminants are suspended impurities dissolved in wastewater or sewage.
(10) Sewage is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease causing bacteria and other microbes.
Organic impurities –
Human faeces, animal waste, oil, urea (urine), pesticides, herbicides, fruit and vegetable waste, etc.
Inorganic impurities-
Nitrates, Phosphates, metals.
Nutrients-
Phosphorus and Nitrogen
Bacteria-
Such as which causes cholera and typhoid.
Other microbes-
Such as which causes dysentery.
(11) Activity 2
Contaminant Survey
| S.No. | Type of sewage | Point of origin | Substances which contaminate | Any other remark |
| 1. | Sullage water | Kitchen | Soap, detergent, bleach dirt, etc. | Can safely recycled within 24 hours |
| 2. | Foul waste | Toilets | Faeces, urea | Should be treated before releasing in to the environment |
| 3. | Trade waste | Industrial and commercial organisations | Metals, oils, petrol, many chemicals | Should take permission before releasing it in sewage |
(12) The network of big and small pipes is called sewers.
(13) Sewerage is like a transport system that carries sewage from the point of being produced to the point of disposal, i.e., treatment plant.
(14) Manholes are located at every 50m to 60m in the sewerage, at the junction of two or more sewers and at points where there is a change in direction.
(15) Treatment of polluted water
Activity 3
(i) Fill a large glass jar ¾ full of water. Add some dirty organic matter such as grass pieces or orange peels, a small amount of detergent, and a few drops of an ink or any colour.
(ii)Cap the jar, shake it well and let the mixture stand in the Sun for two days.
(iii)After two days, shake the mixture and pour a small sample into test tube. Label this test tube “Before treatment: Sample 1”.
(iv) Use an aerator to bubble air through the sample in the glass jar.
(v)The next day when aeration is complete, pour another sample into a second test tube. Label it as “After aeration: Sample 2”.
(vi)Fold a piece of filter paper to form a cone. Wet the paper with tap water and then insert the cone in a funnel.
(vii)Place layers of sand, fine gravel and finally medium gravel in the funnel.
(viii) Pour the remaining aerated liquid through the filter into the beakers. Filter it a few times till you get clear water.
(ix)Pour a sample of the filtered water into a third test tube labeled “Filtered; Sample 3”.
(x) Pour another sample of the filtered water into a fourth test tube. Add a small piece of a chlorine tablet. Mix well until the water is clear. Label the test tube “Chlorinated; Sample 4”.
(xi) Observe the samples carefully in all the test tube.
Answers of questions related to activity 3
(a) What changes did you observe in the appearance of the liquid after aeration?
Ans- No change is observed in the appearance of the liquid after aeration.
(b) Did aeration change the odour?
Ans-Yes, aeration lessens the odour.
(c) What was removed by the sand filter?
Ans- Organic matter was removed from the filter.
(d) Did chlorine remove the colour?
Ans– Yes, Chorine works as bleaching agent. So , chlorine remove the colour.
(e)Did chlorine have an odour? Was it worse than that of the wastewater?
Ans-Yes, chlorine has an odour. But, it is not worse than that of the wastewater.
(16) Wastewater treatment involves removal of physical, chemical, and biological matter that contaminates the wastewater.
(17) Functioning of wastewater treatment plant-
(i) Wastewater is passed through bar screens to remove large objects ( eg- rags, sticks , plastics, napkins etc.).
(ii)From bar screen water goes to a grit and sand removal tank, where speed of wastewater keep slow to settle down the sand, grit and pebbles.
(iii)The water is than allowed to settle in a large tank which is sloped towards the middle. Sludge (Solids like faeces) settle at the bottom and are removed with a scraper. Floatable solids (oil and grease) are removed by skimmer. The water than obtained is called clarified water.
The sludge is transferred to a separate tank where it is decomposed by the anaerobic bacteria. The biogas produced in the process can be used as fuel or can be used to produce electricity.
(iv)Clarified water is pumped to help aerobic bacteria to grow. These bacteria consumes unwanted matter (human waste, soaps, etc) still remain in the water.
After several hours, the suspended microbes settle at the bottom of the tank. The water is than removed from the top. Dried sludge is used as manure, returning organic matter and nutrients to the soil.
The treated water is discharged in to sea, river or into ground. Sometimes, it may be necessary to disinfect water with chemicals (chlorine and ozone) before releasing it into the distribution system.
(18) Better house keeping practices-
(i) Do not throw oil and fats in the drain. They can harden and block the pipes.
(ii) Do not throw fats in open drain because they can clog the pores of soils and can reduce the filtration of water.
(iii)Do not throw chemicals (paints, solvents, insecticides, motor oil, medicines) in to the drain because they can kill microbes that help purify water.
(iv)Always use dustbin to throw used tealeaves, solid food remains, soft toys, cotton, sanitary towels, etc. because these waste can choke the drains. They do not allow free flow of oxygen. It can hamper the degradation process.
(19) Better sanitation habits-
(i) Always drink clean water.
(ii) Always use toilets. Do not defecate in the open because untreated human excreta are a health hazard. It may cause water pollution and soil pollution. Both the surface water and ground water get polluted. Ground water is a source of water for wells, tubewells etc. So, it becomes source of many water borne diseases like cholera, typhoid, polio, meningitis, hepatitis and dysentery.
(iii)Low cost onsite sewage disposal systems (septic tanks, chemical toilets, composting pits) can be used for the places where sewage is not available.
(iv)Hygienic onsite human waste disposal technology can be used. These toilets do not require scavenging. Excreta from the toilets flow through covered drains into a biogas plant.
(v) Always throw waste generated in public places in dustbins, otherwise epidemics could break out.
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