Microbes in sewage treatment — NEET & JEE Questions

Question 1 · difficulty L1 · recall

What does BOD measure in sewage treatment?

• A. The total amount of dissolved salt
• B. Oxygen used during microbial decomposition
• C. The number of viruses only
• D. Methane concentration in treated drinking water after complete purification

Question 2 · difficulty L2 · understanding

What is the main purpose of primary sewage treatment?

• A. Methane production only
• B. Complete biological oxidation of dissolved organics
• C. Physical removal of large and suspended particles
• D. Sterilisation of all water

Question 3 · difficulty L2 · understanding

What are flocs in secondary sewage treatment?

• A. Methane bubbles coated with suspended oil and mineral particles
• B. Large mineral crystals
• C. Clusters of red blood cells
• D. Bacterial-fungal microbial aggregates

Question 4 · difficulty L2 · understanding

During secondary treatment of sewage, the primary effluent is passed into aeration tanks where microbes grow vigorously as masses of bacteria associated with fungal filaments forming a mesh. These masses are called

• A. flocs
• B. sludge
• C. biogas
• D. activated carbon

Question 5 · difficulty L3 · understanding

Which of the following is put into Anaerobic sludge digester for further sewage treatment?

• A. Effluents of primary treatment
• B. Primary sludge
• C. Floating debris
• D. Activated sludge

Question 6 · difficulty L3 · application

Why is air pumped into aeration tanks during secondary sewage treatment?

• A. To supply oxygen to aerobic decomposers
• B. To encourage obligate anaerobic methanogens to multiply aerobically
• C. To precipitate every dissolved salt
• D. To stop microbial respiration

Question 7 · difficulty L3 · application

Two water samples are tested: sample A has a BOD of 5 mg/L and sample B a BOD of 300 mg/L. Which conclusion is correct?

• A. A is more polluted, because a low BOD means little oxygen
• B. B is more polluted, because BOD measures the oxygen that bacteria would consume in oxidising the organic matter in the water; a higher BOD means more biodegradable organic matter
• C. Both are equally clean; BOD measures temperature
• D. A is more polluted because bacteria have already used up its oxygen

Question 8 · difficulty L3 · application

After the effluent from the aeration tank is passed into a settling tank, the bacterial flocs sediment. A small part of this sediment is pumped back into the aeration tank. Why?

• A. To increase the BOD of the tank
• B. To produce biogas in the aeration tank
• C. To serve as an inoculum of active microbes for the next batch of effluent
• D. To filter out floating debris

Question 9 · difficulty L4 · analysis

After secondary treatment, part of the activated sludge is returned to the aeration tank. Why?

• A. To increase the amount of untreated organic waste entering the tank
• B. To seed the tank with active microbes
• C. To eliminate oxygen from the tank
• D. To stop floc formation

Question 10 · difficulty L4 · application

The major part of the activated sludge is pumped into large tanks where other kinds of bacteria digest the bacteria and fungi of the sludge. Which option correctly names the tank, the conditions in it, and the mixture of gases produced?

• A. Aeration tank; aerobic conditions; oxygen and nitrogen
• B. Primary settling tank; aerobic conditions; carbon dioxide only
• C. Sludge digester; aerobic conditions; methane only
• D. Anaerobic sludge digester; anaerobic conditions; a mixture of methane, hydrogen sulphide and carbon dioxide (biogas)