Class 7 Science Notes
Chapter 18 – Wastewater Story
The water rich in lather, mixed with oil and other pollutants that goes down the drains from sinks, showers, toilets, laundries is dirty. It is called wastewater.
The water which is unfit for human consumption, becomes the source of many water-borne diseases which ultimately lead to lots of human life.
It is estimated that one billion human beings do not get safe drinking water. So, realising the urgency we celebrate 22nd March as World Water Day to bring awareness amongst people for safe water, fit for human consumption.
On the World Water Day, i.e., 22 March, 2005, the General Assembly of the United Nations proclaimed the period 2005-2015 as the International Decade for action on Water for Life. All efforts made during this decade aimed to reduce by half the number of people who do not have access to safe drinking water.
Sewage
It is wastewater released by homes, industries agricultural fields and other human activities. It also includes rainwater that has run down the street during a storm or heavy rain and it is a liquid waste.
Most of its water has dissolved and suspended impurities called contaminants.
Composition of Sewage
The following components make the sewage:
(i) The organic impurities present in Sewage are human faeces, animal wastes (like animal dung), urea (as urine), oil, fruits and vegetable wastes, pesticides, herbicides, etc.
(ii) The inorganic impurities present in sewage are nitrates, phosphates and metals.
(iii) The nutrients present in sewage are nitrogen and phosphorus.
(iv) The bacteria present in sewage include those bacteria which cause water-borne diseases such as cholera and typhoid.
(v) The other microbes present in sewage are Protozoa which cause a water-borne disease called dysentery.
Sewerage System
In a house (a public building) generally, there are two sets of pipes, i.e., one set of pipes brings clean drinking water into the house and the other set of pipes takes away wastewater (sewage) from houses. For proper sanitation a well maintained sewage system is required.
The pipes which carry away wastewater or sewage from houses and other buildings are buried under the ground.
An underground pipe which carries away dirty drainage water and waste matter is called sewer.
The provision of drainage at a place by laying sewers under the ground is called sewerage. Sewerage is an underground network of inter connected pipes called sewers that carries the sewage from the place where it is produced to the sewage treatment plants, where it is processed.
Manholes
A manhole is a covered vertical hole in the ground, pavement or road, above the underground sewer pipeline through which a worker can go down upto the sewer pipes for inspection, cleaning, etc.
Manholes are provided at every 50-60 m distance in the main sewer pipeline. Manholes are also provided at the junction of two or more sewers and at points where there is a change in the direction of sewer line.
Treatment of Polluted Water
Activity 1: To treat polluted water and make it clean.
Collect few items like funnel, glass jar, filter paper, funnel stand, soil, glass pieces, orange peels, detergent, ink strainer and beaker.
1. First pollute the water by adding dirty organic matter, kitchen waste, etc. and keep it in jar in sun for 2 days. If available, we may collect the dirty water with organic impurities.
2. Shake the content of jar well and pour it through strainer to remove the large pieces like orange peel, etc.
3. Allow the polluted water to stand in the beaker for some time. The solid soil present in the polluted water will settle down at the bottom of the beaker.
4. Transfer the polluted water from the tap of this beaker into another beaker by decantation. Pass air into this polluted water for several hours by using an aerator. This process is called aeration.
5. When the aeration is complete, we will find that the foul smell from polluted water has almost disappeared.
Filter the aerated water through the layer\of sand, fine gravel and medium gravel arranged on a filter paper in a funnel. Collect the clean water as a filtrate in a beaker, kept below the funnel.
6. Add a small piece of chlorine tablet to the clean water collected in the beaker. Mix well until the water is clear and colourless. This is the clean and clear safe water obtained by the treatment of polluted water.
With this activity, we can obtain clear usable odourless water out from polluted water. It contains the smell of chlorine only but chlorine kills all the germs present in the water and makes it fit for drinking.
The following observations can be made of the different samples of water.
|
Sample |
State |
Colour |
Transparency |
Smell |
|
A |
Raw sample (before treatment) |
Greyish |
Almost opaque |
Foul smell |
|
B |
After aeration |
Greyish |
Almost opaque |
Unpleasant smell |
|
C |
After filtration |
Almost colourless |
Almost transparent |
No smell |
|
D |
After chlorination |
Colourless |
Transparent |
Odourless |
Wastewater Treatment Plant (WWTP)
A place where wastewater or sewage from houses and other buildings is brought for processing is called wastewater treatment plant.
Treatment of wastewater involves physical, biological and chemical processes depending on, the nature of contaminates.
1. Physical Process (Screening)
The bar screens first remove the large rubbish objects like rags, sticks, cans, polybags, napkins, sanitary towels, etc., from the wastewater.
After passing through the bar screen, wastewater is taken to a tank called grit and sand removal tank. In this, sand and grit settle down slowly at the bottom of the tank as water passes slowly through this tank. The settled sand and impurities are removed from the tanks from time to time.
2. Biological Process
The first sedimentation tank is sloped towards the centre. Solid like faeces settle at bottom and is called sludge which is removed by scraper.
Oils and grease float at the 'surface of water are removed by skimmer. The biogas produced (by anaerobic bacteria) in the process can be used to produce electricity. Here, water gets cleared of oil, grease, etc. and we get clarified water which is sent to aeration tank now.
In the aeration tank, the watery waste already contains bacteria (aerobic) in it. The compressed air bubbles are passed through this waste to provide O2 to the bacteria to increase bacterial activity which ultimately digests human waste, food waste, soaps and other unwanted and harmful matter still remaining in the wastewater leaving behind fairly pure water.
3. Chemical Process
The water after aeration tank is allowed to stand in second sedimentation tank. Here, the microbes present get settled at the bottom at the tank in the form of activated sludge which is about 97% water. At this stage, water has very low level of organic matter suspended matter and does not contain much harmful things. It is safe for human consumption but is disinfected by chlorine or ozone before distributing it.
Better House Keeping Practices
We must minimise and manage waste at our houses before its disposal in the following manner:
(i) Cooking oil and fats should not be thrown down the drain. They can harden and block the pipes. In an open drain the fats clog the soil pores reducing its effectiveness in filtering water. Throw oil and fats in the dustbin.
(ii) Used tealeaves, solid food remains, soft toys, cotton, sanitary towels, etc., should also be thrown in the dustbin. These wastes choke the drains. They do not allow free flow of oxygen. This hampers the degradation process.
(iii) The chemicals like paints, solvents, insecticides, medicines and motor oils should not be thrown in drains as they kill helpful microbes which digest the organic waste.
Sanitation and Disease
Contaminated water and poor sanitation practices are the major causes of number of infectious diseases in our country. Safe sewage disposal facilities are still not available at many cities and villages in India. So, people go in open place and defecate. This causes the increase in insect-vector population which transmit diseases like cholera, typhoid, meningitis, etc.
Untreated human excreta are a health hazard which cause soil pollution and water pollution also. The river water and groundwater are sources of water for drinking for many people. So, the contaminated water can spread many diseases especially water-borne.
Vermi-processing Toilets
In the vermi-processing toilets, human excreta are treated by earthworms in a pit. The earthworms usually eat up all organic matter present in human excrete and tum it into compost.
These are low water use toilets for the safe processing of human.
Alternate Arrangement for Sewage Disposal
Low cost outside sewage disposal system have been developed to take care of places where there is no sewage system, e.g., rural' areas, isolated buildings. These are described below:
(i) Septic tanks: Septic tank is a low cost onsite sewage disposal system. Septic tanks are suitable where there is no sewerage made. These tanks need cleaning every four to six months.
A septic tank usually consists of a big, covered underground tank made of concrete having an inlet pipe at one end and on outlet pipe at the other end. The toilet seat is connected to the inlet pipe of the septic tank.
The human excreta from the toilet seat enters into the septic tank through the inlet pipe. The solid part of excreta keeps on collecting at the bottom of the septic tank in the form of a sludge whereas watery waste remains above it.
The anaerobic bacteria breakdown most of the solid organic matter present in human excreta due to which the volume of solid waste is reduced too much. The digested solid waste keeps on depositing at the bottom of septic tank. The watery waste is also cleaned by anaerobic bacteria. The excess water goes out of the septic tank through the outlet pipe and get absorbed in soil.
(ii) Composting pits: These are self-sustained human waste disposal units which is not connected to a sewer line or a septic tank. A composting toilet breaks down and dehydrate human waste to compost.
(iii) Chemical toilets: These toilets have limited storage capacity for human waste and need to be emptied periodically. These are the toilets which use chemically treated reservoir located just below the toilet seats. The chemicals reduce the foul smell coming out of human excrete and carry out partial disinfection of human waste.
Sanitation at Public Places
In our country, fairs are organised periodically. A large number of people participate in them. In the same way, railway stations, bus depots, airports, hospitals are very busy places. Thousands of people visit them daily which generate large amount of waste. It must be disposed of properly otherwise epidemics could break out.
The government has laid down certain standards of sanitation but unfortunately, they are not strictly enforced. We should not scatter litter anywhere. If there is no dustbin in sight, we should carry the litter at home and throw it in the dustbin.
Mahatma Gandhi said, 'No one needs to wait for anyone else to adopt a humane and enlightened course of action'.
Conclusion
We all have a role to play in keeping our environment clean and healthy. We must realise our responsibility in maintaining the water sources in a healthy state. Adopting good sanitation practices should be our way of life. As an agent of change your individual initiative will make a great difference. Influence others with your energy ideas and optimum. A lot can be done if people work together. There is great power in collective action.
As an active citizen, we have many responsibilities regarding sanitation. These can be listed as follows:
(i) To ensure that our surroundings are clean.
(ii) To ensure that the sewerage system in our house is properly managed.
(iii) If any leakage or any open drain in the sewerage system is present, then it should be reported to the municipality or the gram panchayats to ensure that the open drain must be covered properly and several air and water-borne diseases could be prevented.
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