1. 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.

2. 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

3. 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

4. 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.

5. 1. Point of outfall of sewage (organic matter e.g. urea,
Q11 (a)
Sequence of events
1. Point of outfall of sewage
(organic matter e.g. urea,
protein, dietary fibre +
bacteria)
 Organic pollution

6. 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