Environmental Engineering

Wastewater Treatment

Primary, secondary (biological) and tertiary treatment, detention time and BOD removal — cleaning sewage before discharge.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Wastewater is treated in stages: primary (physical settling of solids), secondary (biological oxidation of organics, e.g. activated sludge or trickling filters) and tertiary (nutrient/pathogen polishing). A sedimentation/aeration tank is sized by its detention time

t = V / Q

, and performance is reported as BOD removal efficiency.

The Core Formula Matrix

Detention time:

t = \frac{V}{Q}

BOD removal efficiency:

η = \frac{B O D _{in} - B O D _{o u t}}{B O D _{in}} \times 100%

Surface overflow rate:

v_{o} = \frac{Q}{A _{pl an}}

Activated sludge = aeration tank + secondary clarifier + sludge recycle.

The ‘IIT Traps’

⚠
Primary treatment removes settleable solids, not dissolved BOD — biological (secondary) treatment removes the organics.
⚠
**Detention time uses tank volume $V$ and flow $Q$ ** in consistent units (convert m³/day ↔ hours carefully).
⚠
Efficiency is based on influent BOD, so divide the removed amount by the inlet value, not the outlet.

📚 Standard references

Environmental Engineering Vol. II (Sewage Disposal & Air Pollution) — S.K. Garg
Wastewater Engineering: Treatment and Reuse — Metcalf & Eddy

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

The activated-sludge process is an example of:

Q2MEDIUM2 Marks · NAT

Influent BOD is

300 mg/L

and effluent BOD is

30 mg/L

. The BOD removal efficiency is _____ %.

Q3HARD2 Marks · NAT

A sedimentation tank of volume

1000 m^{3}

treats

2000 m^{3} / day

. Its detention time is _____ hours.