IB ESS HL 4.4.4 Biochemical Oxygen Demand Bod Questions

a. Water samples could be taken at various points along the river;
b. Water could be tested for BOD by taking samples and measuring oxygen concentration at start / initial oxygen concentration is measured in the sample;
c. Incubate samples for ( 5 days at $20^{\circ} \mathrm{C}$ ) in the dark;
d. Measure reduction in oxygen concentration after 5 days;
e. Example of method e.g. dissolved oxygen can be measured using an oxygen probe/test kit/titration;
f. Testing could be repeated at different time intervals (and compared to previous results);

a. Takes minimum of 5 days to get results / long time lag for results;
b. Measures only (biodegradable) organic pollution (indirectly);
c. Not all water pollution impacts BOD;
d. Toxic substances can kill micro-organisms which affects results;
e. It is an indirect measure of pollution rather than measuring pollutants directly/damage caused to species;
f. Does not identify/measure actual/specific pollutant eg pathogens/heavy metals;
g. Snapshot in time / only represents the moment the sample was taken;

Marking guidance:

Do not accept "it is not accurate".
Do not accept "it does not identify the source of pollution".
Do not accept "it is expensive".