Which of the following statements about the secondary treatment of sewage is INCORRECT?
- AOxidation of organic matter occurs in trickling filters under aerobic conditions.
- BActivated sludge is biologically active.
- CPrimary treatment of sewage is not essential for efficient working of contact beds.Correct
- DSecondary treatment methods are broadly divided into filtration and activated sludge process.
Solution
Concept
Biological filters such as contact beds and trickling filters need primary treatment (sedimentation) before them. Otherwise suspended solids clog the filter media.
Step-by-step solution
1. Check A: Trickling filters oxidise organic matter by aerobic bacteria. This is correct. 2. Check B: Activated sludge has active microorganisms. This is correct. 3. Check C: Contact beds need settled sewage. Without primary treatment, solids clog the beds. So this statement is incorrect. 4. Check D: Secondary treatment is broadly divided into filtration (contact beds, trickling filters, sand filters) and the activated sludge process. This is correct. 5. The incorrect statement is C.
Explanation
Contact beds and trickling filters work well only when settleable solids are first removed in primary treatment. Raw sewage would quickly clog the filter media and reduce efficiency. So statement C is incorrect.
Why the other options are wrong
A: Correct, trickling filters work under aerobic conditions. B: Correct, activated sludge has living microorganisms. D: Correct, this is the standard broad classification of secondary treatment.
Shortcut
Any biological filter needs primary sedimentation first, so a statement saying it is 'not essential' is wrong.
Common mistake
Thinking contact beds can take raw sewage because they work by biological action.
Correct answer: C) Primary treatment of sewage is not essential for efficient working of contact beds.