# DBT JRF 2022 — Question 145

Exam: DBT JRF 2022 · Section: B · Topic: Environmental Biotechnology › Sewage Treatment Processes · Difficulty: Medium

## Question

Which one of the following method can be used to treat crude sewage and nitrify secondary effluent?

## Options

- **A.** Duck weed ponds
- **B.** Reed beds
- **C.** Algal fish ponds
- **D.** Water hyacinth ponds

## Answer

Option B

## Explanation

Reed beds for sewage treatment and nitrification: Constructed wetlands using macrophytes (especially Phragmites australis, common reed) treat wastewater through: • Physical: sedimentation, filtration through root/gravel matrix. • Chemical: adsorption, photodegradation. • Biological: complex microbial communities exploiting oxygen gradients. Zone structure around reed roots: • Aerobic rhizosphere: O₂ released by roots → nitrification ( $\text{NH}_4^+ \rightarrow \text{NO}_2^- \rightarrow \text{NO}_3^-$ ) by Nitrosomonas and Nitrobacter. • Anaerobic zones: denitrification ( $\text{NO}_3^- \rightarrow \text{N}_2$ ), anaerobic degradation. Capabilities: • Can treat CRUDE SEWAGE (primary effluent) — handles high BOD loads. • Can NITRIFY secondary effluent — converting residual ammonium to nitrate. • Remove BOD, suspended solids, pathogens, some heavy metals. Other options: • Duckweed/algal/water hyacinth ponds: used for secondary treatment but primarily as stabilisation ponds; less effective for crude sewage. Answer: Reed beds (Phragmites-based constructed wetlands) can treat crude sewage AND nitrify secondary effluent through their diverse aerobic/anaerobic microbial zones.

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