Environmental Microbiology

Nitrogen Cycle & Biological N-Removal

Nitrification, denitrification and the microbial nitrogen transformations exploited in wastewater treatment.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Microorganisms drive the nitrogen cycle. Nitrification is the aerobic, autotrophic oxidation of ammonia → nitrite (*Nitrosomonas*) → nitrate (*Nitrobacter*); it consumes oxygen and alkalinity. Denitrification is the anoxic reduction of nitrate to nitrogen gas by heterotrophs, recovering some alkalinity. Engineered nutrient removal sequences these two steps.

The Core Formula Matrix

Nitrification:

NH_{4}^{+} N i t r oso m o na s NO_{2}^{-} N i t r o ba c t er NO_{3}^{-}

(aerobic, autotrophic).

Oxygen demand: ≈ 4.57 g O₂ per g of NH₄⁺-N oxidised to nitrate.

Alkalinity consumed: ≈ 7.14 g as CaCO₃ per g N nitrified.

Denitrification:

NO_{3}^{-} \to N_{2}

(anoxic, heterotrophic) — recovers ≈ 3.57 g alkalinity/g N.

The ‘IIT Traps’

⚠
Nitrifiers are autotrophs (use CO₂ as carbon source) and slow-growing — they govern sludge age.
⚠
Nitrification consumes alkalinity; denitrification recovers about half of it.
⚠
Denitrification needs anoxic (not anaerobic) conditions and an organic carbon source.

📚 Standard references

Wastewater Engineering: Treatment and Resource Recovery — Metcalf & Eddy · Biological Nutrient Removal

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

Nitrification is the microbial conversion of:

Q2MEDIUM1 Mark · MCQ

Denitrification, which removes nitrogen as N₂ gas, requires:

Q3HARD2 Marks · MCQ

Compared with heterotrophs, nitrifying bacteria in an activated-sludge plant are: