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Reason: 2 Marks
Example: 1 Mark
Open drain system serves as a breeding place for flies, mosquitoes, and organisms which cause diseases. Hence open drainage is never opted for when planning for a town.
Example:
Chandigarh being a planned city doesn't have any open drainage system. Instead,the city has a well planned underground network of pipes for disposal of sewerage. The system has been designed according to the natural slope of the city for sewage to flow under gravity.

Answer: Option C. -> 22nd March      
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C
World water day is celebrated on ${22}^{nd}$March every year to create awareness about water conservation and its significance.

Answer: Option B. -> False
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B
Vermi-processing toilet is a type of toilet in which human excreta is treated by earthworms. The human excreta is converted into vermi-cakes which are rich in nutrients and can be used as manure for soil. They benefit the soil as they do not contain any harmful chemicals which present in pesticides.
Also, they consume less amount of water than normal household toilets.

Answer: Option B. -> Phosphates
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B
Organic impurities generated from organic material such as food, garden, etc, in sewage include animal waste, urine, oil, vegetable and fruit waste, faeces and herbicides. Inorganic impurities (all impurities excluding organic) include phosphates, nitrates and metals.

Answer: Option B. -> Cholera
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B
Water-borne diseases are any illness caused by drinking contaminated water. Cholera and typhoid are water borne diseases.
Cholera is an infectious andfatal bacterial disease of the small intestine.It is generally acquired from infected water supplies and causes severe vomiting and diarrhea.
Smallpox, chicken pox, fever are not water borne diseases.

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Each point: 1 Mark
1. Clear and transparent water is not always fit for drinking.
2. Water might appear clean, but it may contain some disease-causing micro-organisms and other dissolved impurities. Hence, it is advised to purify water before drinking.
3. Purification can be done by water purifying systems or by boiling the water.

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Naming: 1 Mark
Commercial value: 1 Mark
By-products of wastewater treatment are sludge and biogas. Sludge is produced whensolids wastes like faeces settle at the bottom of the tank during sedimentation. When this sludge is decomposed by anaerobic bacteria, biogas is produced.
Sludge contains nitrogen, phosphorus and organic matter and is used as agricultural manure. Biogas is low-cost energy utilized by homes. Hence both have high commercial utility.

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Definition: 1 Mark
Explanation: 2 Marks
Sewage is a liquid waste 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.
Sewage water contains harmful substances. It is a complex mixture, containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease-causing bacteria and other microbes. So, if the untreated sewage is discharged into rivers and seas, it will pollute the water resources, affect aquatic life. The contaminants of sewage degenerate the water quality to such an extent that they become the home for dreaded diseases like cholera, typhoid, polio, meningitis. malaria and dengue.

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Explanation and working: 5 Marks
Sewage treatment plants or wastewater treatment plants are large plants where wastewater is cleaned before being sent to the nearest water bodies or being reused. The sewage treatment involves physical, chemical and biological processes to remove impurities from the wastewater.
Physical Process
1. Filtration:The wastewater is passed through bar screen. Large objectslike rags, sticks, plastic bags, cans, etc. are removed in this process.
2. Grit and sand removal:The wastewater is slowly passed through the grit and sand removal tank. Grit, sand, and pebbles settle down at the bottom.
3. Sedimentation:The wastewater is then sent to the sedimentation tanks. Solidslike faeces settle at the bottom. Floatable impuritieslike oil and grease float on the surface. A scraper removes the faeces from the bottom of the tank. The impurity thus collected is called sludgewhich is sent to the sludge tank. The sludge can be used to produce biogas or to produce manure. A skimmer removes the floatable impurities. Now, the water is called clarified water.
Biological Process
4. Aeration:Air is pumped into the clarified water so that bacteria can proliferate. Bacteria consume the human waste. It decomposesfood waste, soap and other unwanted materials in the water. The microbes settle down at the bottom after several hours. Water is then removed from the top. This water is fit for irrigation and can be used for that purpose.
Chemical Process
5. Chlorination:Water purified through aeration is not fit for human consumption. It needs to be treated with chlorine. For this, bleaching powder is added to the water. The chlorine kills the rest of the germs. After chlorination, the water is fit for drinking.

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Advantages: 0.5 Mark each
The various advantages of vermi-processing toilets are given below:
i) low water usage.
ii) safe for processing human wastes.
iii) Operationof the toilet is very simple and hygienic.
iv) Human excreta is completelyconverted into vermi-cakes, a resource much needed for the soil.