Solved examples for class 7 science : Chapter 18
I. Multiple Choice Questions (MCQs).
Q.1. The percentage of total water stored in the oceans, is
Answer (c) 97.4%
Q.2. STP stands for
a) solid treatment plant
b) secondary treatment plant
c) sewage treatment plant
d) none of these
Answer (c) sewage treatment plant
Q.3. A liquid waste that has dissolved and suspended impurities in it, is
d) none of these
Answer: (b) sewage
Q.4. The water (Prevention and Control of Pollution) Act was enacted in the year
Answer: (b) 1974
Q.5. In the sedimentation of sewage, all the solids settle down and form primary
Answer: (c) sludge
II. Fill in the blanks by selecting appropriate words from the given list:
List : manhole, garbages, Disinfection, rivers, life
Q.1. Water provides a suitable medium for several ______________ processes to take place in living organisms.
Q.2. The place where two or more sewers are joint, a _____________ is located.
Q.3. In our country, a large amount of sewage is released directly into the ______________ leading to water pollution.
Q.4. Sewage water is strained to remove ______________ using a manual or automatic bar screen.
Q.5. ______________ is done to reduce the number of microorganisms.
III. Match the items of Column A with those of Column B.
Answer: 1- e, 2- a, 3- d, 4- b , 5- c
IV. Write ‘T’ for the true and ‘F’ for the false statements.
Q.1. The wastewater should be used in such a way that water does not go waste.
Q.2. Waste is added into water in different ways and from different sources.
Q.3. The wastes that are generated by individual households, are called contaminants.
Q.4. In our country, there are enough sewage treatment plants to treat large quantities of domestic and industrial effluents.
Q.5. Dried sludge is used as manure, which makes the soil fertile.
Answer: 1- T, 2-T, 3-F, 4-F, 5-T
V. Very Short Answer Questions.
Q.1. What percentage of water does our body contain?
Ans: About 70% of our body contains water.
Q.2. Give the composition of biogas.
Answer: Biogas is a mixture of methane, hydrogen, carbon dioxide and Hydrogen sulphide.
Q.3. What do you understand by the tertiary treatment of water?
Answer: Tertiary treatment of water is a chemical process which includes the use
of chemicals to remove nitrogen and phosphorus from the water.
Q.4. Define eutrophication.
Answer: The process by which a body of water becomes enriched in dissolved of nitrogen and phosphorus is known as eutrophication. This causes overgrowth of weeds and algae that deplete the oxygen contents of the water body.
Q.5. Why is aeration of sludge important?
Answer: The aeration of sludge important because this speeds up the process of breakdown of organic material.
VI. Short Answer Questions.
Q.1. Define wastewater in your own words.
Answer: We use water in our homes, schools, offices and other places where we spend our time. The water that becomes dirty due to human activities such as washing utensils and clothes, flushing toilets, throwing garbage in water, etc., is called wastewater.
Q.2. Why is cleaning of dirty water necessary?
Answer: About 70% of our body contains water. It is an important natural resource. This wonderful liquid on the earth has covered nearly three-fourth of total surface of the earth. It provides a suitable medium for several life processes to take place in living organisms. Unfortunately, clean water is not available to all. More than one billion of human beings have no access to safe drinking water. It causes many water related diseases and may cause death. So, cleaning of dirty water is necessary.
Q.3. Name the kinds of sewage contaminants with examples.
Answer: Sewage contaminants can be classified as:
(1) Organic Impurities – Human and animal faeces, urine, pesticides, weedicides, fruit and vegetable wastes.
(2) Inorganic Impurities – Phosphates, Nitrates, Metals.
(3) Microbes – Bacteria that can cause cholera, dysentery, typhoid, etc.
(4) Nutrients – Nitrogen, Sulphur and Phophorus.
Q.4. What do you mean by clean water? What are the factors responsible for shortage of clean water?
Answer: The water which is free from contaminants is known as clean water.
Some of the factors responsible for increasing the scarcity of freshwater are:
• Global warming
• Increasing population
• Industrial development
• Mismanagement of governmental policies, etc.
Q.5. What do you understand by sewage and sewer system? Explain them briefly.
Answer: Sewage is liquid waste that has dissolved and suspended impurities in it. These impurities are called contaminants.
Sewage is made up of – o Wastewater from factories, homes, schools, etc. o Rainwater mixed with dirt and dust of the ground.
A network of pipelines, used to carry sewage is known as sewer system.
Q.6. What are the public places and how can we contribute to maintain sanitation in public places? Explain briefly.
Answer: Places like railway stations, bus depot, airport, hospitals, tourist places, etc. are known as public places.
We can contribute in maintaining sanitation in the following ways:
• Do not throw anything on the surroundings, use the dustbins placed at every public place.
• If there is no dustbin at any public place, carry the Litter home and throw it in the dustbin.
• There should be sufficient toilet facilities for the maximum number of people.
• There must be a washbasin with clean water and soap.
• There must be clean water supply.
• The information about health and hygiene should be available at public places.
VII. Long Answer Questions.
Q.1. What do you mean by sewage treatment? How is the sewage treated in a sewage treatment plant?
Answer: The polluted or wastewater is treated to remove all the impurities is known as sewage treatment.
Treatment of sewage is done by the sewage treatment plants (STPs) which is also known as wastewater treatment plant to make it less polluting. Wastewater treatment involves physical, chemical, and biological processes, which removes physical materials (rags, cans, polythene bag, etc.), chemical materials (nitrate, phosphate, etc.) and biological materials (excreta, urea as urine, etc.) that pollute water. Sewage treatment involves three stages;
i Primary treatment
ii Secondary treatment and
iii Tertiary treatment.
Primary treatment includes the removal of material that can be collected easily from the wastewater such as sand, gravel, human wastes, floating materials such as empty cartons, chocolate wrappers, plastic bag, polythene bag, plastic box, rags, etc. Sewage water is strained to remove garbages using a manual or automatic bar screen. Water is allowed to pass through a tank which remove grit and sand.
Secondary treatment is a biological process to degrade the biological content of the sewage such as human waste, food remains, detergents, etc. Organic material present in the sludge is broken down by allowing bacteria to act upon them. The sludge can also be activated by blowing air into it, called aeration. This speeds up the process of breakdown of organic material.
Tertiary treatment is a chemical process. It includes the use of chemicals to remove nitrogen and phosphorus from the water.
Q.2. What are sanitation and water-borne diseases? How does polluted water cause diseases?
Answer: Sanitation refers to the provision and facilities for public health(conditions) related to clean drinking water and proper disposal of wastes.
It includes all measures that promote:
• Proper disposal of human and animal wastes (solid as well as liquid wastes). It also includes disposal of hazardous wastes from hospitals, industries and other sources.
• Use and proper maintenance of toilets.
• Avoiding of open defecation.
Water-borne diseases: The diseases caused by drinking polluted water or by eating food prepared with polluted water containing viruses , bacteria, protozoa or many other parasites are called water-borne diseases. Some common water-borne diseases are typhoid, cholera, diarrhoea, dysentery etc.
Polluted water is unfit for human consumption. When people dump garbage into rivers, the water becomes dark and foul smelling. It becomes the breeding place for many germs. This leads to the spread of water-borne diseases such as jaundice, typhoid dysentery, meningitis, hepatitis, cholera, diarrhoea etc.
Q.3. What do you mean by alternative arrangement for sewage disposal? Explain in brief.
Answer: Alternative arrangement for sewage disposal
• By “on-site human waste disposal technology” the human excreta can be disposed along with the production of biogas which is used as a fuel for domestic purpose.
• In an aeroplane vacuum toilets are used in which the waste and a small amount of blue sanitizing liquids are sucked into a septic holding tank.
• A clean environment promotes good health, we all have a role to play in keeping the environment clean and healthy.
• In vermi-processing toilet, human excreta is decomposed by earthworms. The operation of the toilet is very simple and hygienic. The human excreta is completely converted to vermi cakes. Vermi cake is very good manure for the soil in present conditions.