Environmental Engineering

Water Demand, Treatment & BOD

Per-capita demand, sedimentation, disinfection and the first-order BOD equation — the public-health core.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Town water demand scales with population and per-capita rate. In a sedimentation tank, a particle is removed if its settling velocity exceeds the surface overflow rate. BOD measures biodegradable organic load and decays first-order with time. Chlorine demand is the dose consumed before a residual remains.

The Core Formula Matrix

Average demand:

Q = \frac{population \times lpcd}{1 0 ^{6}} MLD

BOD exerted:

y_{t} = L_{0} (1 - 1 0^{- k t})

(base-10 rate

k

).

Chlorine demand: applied

-

residual.

Surface overflow rate:

v_{0} = \frac{Q}{A _{plan}}

; removal if

v_{s} \geq v_{0}

The ‘IIT Traps’

⚠
Base-10 vs base-e BOD. $k$ values differ by a factor of $2.303$ ; match the equation form.
⚠
Overflow rate depends on plan area only, not tank depth.
⚠
Hardness is reported as CaCO₃ equivalent.

📚 Standard references

Environmental Engineering Vol. I (Water Supply) & Vol. II (Wastewater) — S.K. Garg

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · NAT

A town of population

100, 000

has a per-capita demand of

135 lpcd

. The average daily demand is _____ MLD.

Q2MEDIUM1 Mark · NAT

5 mg/L

of chlorine is applied and the residual is

0.5 mg/L

, the chlorine demand is _____ mg/L.

Q3HARD2 Marks · NAT

The ultimate BOD of a waste is

300 mg/L

with

k = 0.1 day^{- 1}

(base 10). The BOD exerted in

5

days is _____ mg/L.