CBSE Questions for Class 7 Biology Wastewater Story Quiz 3 - MCQExams.com

$$\textbf{Correct option: }$$ A

$$\textbf{Solution: }$$ 

$$\textbf{Option (A) }$$

$$\bullet$$The sewage effluent is pumped into an aeration or oxidation tank which contains sludge.

$$\bullet$$The flocs later sediment at the bottom of the aeration tank and are known as activated sludge. 

$$\bullet$$A part of activated sludge is pumped back into an aeration tank to act as inoculum while a major part of it is pumped into an anaerobic sludge digester for anaerobic microbial decomposition and production of biogas. 

$$\textbf{Option (B) }$$

$$\bullet$$Sewage water is determines as the waste water that has the maximum concentration of faeces, urine, and household waste.

$$\bullet$$The bacteria and fungi filaments are the categories of microbes naturally occurring in sewage water.

$$\bullet$$The bacteria multiply rapidly and along with the presence of fungus filaments form the large masses that are known as flocs.  

$$\textbf{Option (C) }$$

$$\bullet$$Potable water is the drinking water which does not contain any effluent or sewage components. 

Hence, the correct option is (B)

$$\textbf{Correct option: A}$$

$$\textbf{Solution:}$$

Option A:

$$\bullet$$ The water becomes polluted when harmful substances like toxic chemical substances, pesticides, weedicides, industrial chemicals, etc get mixed with it.

$$\bullet$$ This polluted water causes water pollution of several water sources like, lakes, ponds, rivers and many more.

$$\bullet$$ Polluted water has harmful effects on aquatic organisms directly and on humans when they eat sea food.

$$\bullet$$ These harmful substances get accumulated in their body and affects their health.

Hence the correct option is option A.

• Treatment of wastewater involves physical, chemical, and biological processes, which remove physical, chemical and biological matter that contaminates the wastewater. This treatment is done in Wastewater treatment Plant (WWTP).
  
• The treated wastewater from the WWTP has a very low level of organic material and suspended matter. It is discharged into a sea or a river.
  
• Thus, Treated waste water usually disposed in to water bodies.

• Ozone, or trioxygen, is an inorganic molecule with the chemical formula $$O_3$$. It is a pale blue gas with a distinctively pungent smell.
• Oxygen is essential for the breathing of living organisms. Hence statement (a) is incorrect.
• Ozone is a powerful oxidant, stronger than chlorine. Ozone is used to disinfect, deodorize, decolourize and decompose organic substances in water. Hence Statement (b) is correct.
  
• The ozone layer or ozone shield is a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet radiation. Hence statement (c) is correct.
  
• The proportion of ozone in the air is 0.000004%. Hence Statement (d) Is incorrect.
• So, the correct answer is '(b) and (c)'.