Consider the following statements regarding biological treatment systems:

Question

Consider the following statements regarding biological treatment systems:

I. Trickling filters are attached-growth systems in which wastewater trickles over a media bed.

II. Sequencing Batch Reactor (SBR) is a suspended-growth system that operates in time-phased cycles in a single tank.

III. Anaerobic filters are used only for treating sewage and not for industrial effluents.

Which of the statements given above is/are correct?

1. A. I and II only (Correct answer)
2. B. I and III only
3. C. II and III only
4. D. I, II and III

Correct Answer

Option A — I and II only

Detailed Solution & Explanation

The correct answer is I and II only.

Key Points

• Statement I is correct. A trickling filter is an attached-growth system: wastewater is distributed over a bed of stone or plastic media on which a biofilm grows, and treatment occurs as the liquid trickles past.
• Statement II is correct. The SBR is a suspended-growth system that performs fill, react, settle and decant sequentially in a single tank.
• Statement III is incorrect, which is what decides the question — it misdescribes the third system named.

Additional Information

• The two families are distinguished by where the biomass lives: fixed to a surface (trickling filters, RBCs, biological aerated filters) or suspended in the mixed liquor (activated sludge, oxidation ditches, SBRs, aerated lagoons).
• Trickling filters are classified by hydraulic and organic loading as low-rate, high-rate and roughing filters; low-rate units achieve nitrification, high-rate units need recirculation.
• Their characteristic nuisance is ponding, when excess biofilm blocks the media voids, and **filter flies (*Psychoda*)** breeding in the wet media.
• A trickling filter still requires a secondary clarifier downstream to catch sloughed biofilm — attached growth removes the need for return sludge, not for clarification.