CBSE – 7th Standard Science

Question Bank – Chapter 18 – Waste Water Story



Q1. What will happen if waste water is consumed?

Ans . Waste water contains many suspended impurities called contaminants which are the cause of major life threatening diseases in the world like cholera,  dysentery ,malaria etc.



Q2. What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.

Ans. Sewage is wastewater released by homes, industries, hospitals, offices and other users. It also includes rainwater, It has organic, inorganic impurities and bacteria which is harmful to discharge into rivers.


Q3. Why should oils and fats be not released in the drain? Explain.

Ans. 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.


Q4. Name some other things that should not be released to drain. Why?

Ans. Used tealeaves, solid food Chemicals like paints, solvents , insecticides, motor oil, medicines may kill microbes and do not allow free flow of oxygen that help purify water.


Q5. Describe the steps involved in getting clarified water from wastewater

Ans. Treatment of wastewater involves physical, chemical, and biological processes, which remove physical, chemical and biological matter that contaminates the wastewater


Q6 . Explain the function of bar screens in a wastewater treatment plant

Ans. Large objects like rags, sticks, cans, plastic packets, napkins are removed when wastewater passes through bar screen.


Q7. What is sludge? Explain how it is treated.

Ans. The wastewater is allowed to settle in a large tank which is sloped towards the middle. Solids like faeces settle at the bottom and are removed with scraper. This is the sludge, Aerobic Bacteria can clear the sludge.


Q8. What skimmer does with activated sludge, is it useful now?

Ans. A skimmer removes the floatable solids like oil and grease, Water so cleared is called clarified water and activated sludge is about 97% water this sludge is used as manure, returning organic matter and nutrients to the soil.


Q9. Untreated human excreta are a health hazard. Explain.

Ans. Untreated human excreta causes water pollution and soil pollution, groundwater is polluted which is a source of water for tubewells and rivers it becomes the most common route for water borne diseases like cholera, typhoid, polio, meningitis, hepatitis and dysentery, thus it is a health hazard.


Q10. Which plants are helpful to recycle treated water.

Ans. Eucalyptus trees all along sewage ponds, these trees absorb all surplus wastewater rapidly and release pure water vapor into the atmosphere.


Q11. Explain the relationship between open drains and disease.

Ans. Open drain system is a breeding place for flies, mosquitoes and organisms which cause diseases.


Q12. What are the alternative arrangements for sewage disposal?

 Ans. On site human waste disposal and Vermi-processing toilet are the alternates, where excreta flow through covered drains into a biogas plant and biogas produced is used as a source of energy or earthworms has been used for sewage disposal, human excreta is completely converted to vermi cakes, a resource much needed for soil.


Q13. Name two chemicals used to disinfect water.

Ans. Chlorine and ozone are used to disinfect water


Q.14 What is wastewater and what should be done with it?

Ans. Rich in lather, mixed with oil, black or brown water that goes down the drains from sinks, showers, toilets, laundries is called wastewater. It should be treated before release to rivers.



Q15. What is known as sewage treatment?

Ans. Cleaning of water is a process of removing pollutants before it enters a water body or is reused. This process of wastewater treatment is commonly known as “Sewage Treatment”.