Q 6. How to measure BOD?
1. Measure the volume of a sample of
water in a dark bottle. Measure its
oxygen level by oxygen meter /
chemical method.
 2. Incubate the water at 20 oC in dark

to prevent photosynthesis for 5 days.
 3. Measure the oxygen level again.
 4. The difference between the 2 oxygen
level is BOD.

BOD (Biological oxgen demand)
-the amount of oxygen used (0.5) by
organism (0.5) in a fixed volume of
water(0.5) over a specified period of
time (~5 days).
 - indicate the amount of organic
matter and microorganisms
 indicate the degree of water
pollution
BOD high  degree of pollution high
BOD low  degree of pollution low 
clean water

Tubifex worms / blood worms
- can tolerate to low oxygen level
 - indicator species of polluted and
anoxic water
Mayfly nymph / dragonfly nymph
 - cannot tolerate to low oxygen level
 - indicator species of clean water with
high oxygen level

Q11
 (a)
Account for the changes in
level of

(i) dissolve oxygen

(ii) ammonium compounds

(iii) nitrate compounds

in the river water below the
source of pollution.
Q11 (a)
 Sequence
of events
1. Point of outfall of sewage
(organic matter e.g. urea,
protein, dietary fibre +
bacteria)
 Organic pollution
Q11 (a)


Sequence of events
decomposer (bacteria+fungi)
2. organic matter -------------------------
ammonium O2
3. Ammonium --------------- nitrite 
nitrate
ammonium
nitrate
4. O2  clean water species + polluted species
Nitrate  algae