BOD at Higher Level: At HL, BOD is one of the measures combined into a water quality index, and a legal limit for sewage works. Your examples differ from SL: the Selwyn (Waikirikiri) River below New Zealand's dairy farms, the EU's 25 mg per litre limit and a cider factory on the River Frome.
Practise this as you read
- Outline one full method for measuring the impact of organic matter.
- Calculate BOD and explain what it can and cannot show.
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How much oxygen the waste will eat: Biochemical oxygen demand (BOD) is a measure of how much dissolved oxygen microorganisms need to decompose the organic material in water.
The points to remember
- BOD = the dissolved oxygen that microorganisms need to decompose the organic matter in water.
- Units: mg of oxygen per litre of sample, used over 5 days at 20 °C.
- It is an indirect measure of organic matter: it measures the oxygen used, not the waste itself.
- High BOD = lots of organic matter (sewage, manure, food waste) = oxygen falls fast.
- Low oxygen kills fish and insects that need it: mayfly larvae and trout go first.
- Clean rivers have a low BOD, a few mg per litre; raw sewage has a BOD of hundreds.
Remember it as: More rotting waste, more hungry bacteria, less oxygen for fish.
Real example: since 1991 the European Union's rules on urban waste water say treated sewage must have a BOD of no more than 25 mg of oxygen per litre over 5 days at 20 °C.
Indirect, not direct: BOD does not measure the organic matter itself; it measures the oxygen used to break it down. That is why it is called an indirect measure.
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You measure oxygen twice, five days apart, and incubate the sample in between. The steps below are shown on the Selwyn (Waikirikiri) River, on New Zealand's Canterbury Plains, below dairy farms.
The points to remember
- Take water samples at several points along the river: above, at and below the source.
- Fill two sealed bottles from each sample: measure dissolved oxygen in one now.
- Measure oxygen with an oxygen probe, a test kit or a titration (Winkler method).
- Keep the other bottle sealed, in the dark, at 20 °C, for 5 days.
- Measure its oxygen again: BOD = oxygen at the start − oxygen after 5 days.
- Repeat at intervals (e.g. each month) and compare sites and dates.
Sample
Bottles filled above the dairy farms and at three points below.
Measure now
Winkler titration on the first bottle from each site.
Incubate
Second bottle sealed, in the dark at 20 °C, for 5 days.
Repeat
Measure again; repeat every month to compare dry and wet seasons.
Why dark, sealed, 20 °C and 5 days
- Dark: algae cannot photosynthesise, so no oxygen is added; the fall is only what was used.
- Sealed: no oxygen gets in from the air.
- 20 °C and 5 days: everyone uses the same conditions, so results can be compared.
Calculating BOD is a subtraction. Comparing sites then shows where organic waste enters a river.
How to calculate it
- Write the formula first: BOD = DO at the start − DO after 5 days.
- Use the same units throughout: mg of oxygen per litre (mg/L).
- A bigger fall in oxygen = a higher BOD = more organic matter.
- Along a river, BOD peaks at the source and falls downstream as the waste is broken down and diluted.
Worked example: Below the dairy farms on the Selwyn, the oxygen fell from 10.4 mg/L to 6.2 mg/L in 5 days. BOD = 10.4 − 6.2 = 4.2 mg/L, against 1.5 mg/L above the farms: organic matter from manure is entering.
Reading it: BOD jumps from 2 to 24 mg/L at the outfall, so organic waste enters there. It falls to 3 mg/L 6 km downstream, as decomposers use up the waste: the river is recovering.
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BOD is useful, but it has limits. A question may ask for one disadvantage: give a real limit of the method, not a vague complaint.
Disadvantages of BOD
- Slow: results take at least 5 days.
- Measures only biodegradable organic pollution: metals, plastics and pathogens are missed.
- Toxins in the sample can kill the microorganisms, so the BOD reads too low.
- Indirect: it does not name the pollutant or show the damage to wildlife.
- A snapshot: it shows only the moment the sample was taken.
These do NOT score: 'It is not accurate', 'it does not identify the source of the pollution' and 'it is expensive' are not accepted. Say what BOD cannot measure, or why its result can be wrong or late.
BOD
- Indirect: oxygen used by decomposers
- A number that compares sites
- Misses toxins, metals, pathogens
- Takes 5 days
Biotic index
- Indirect: the animals living there
- Shows the effect on wildlife
- Shows weeks of conditions
- Needs skill to identify animals
Real example: on the Selwyn, nitrate from the dairy farms is a major problem, but BOD does not measure nitrate at all, so a low BOD there does not mean clean water.
How this comes up: Paper 2: outline one method to measure the impact of a build-up of dead organic matter in water [4]. Choose ONE method (BOD here) and give four steps.
Every autumn, leaves and fruit waste from a cider factory build up in the River Frome in Somerset, England.
Outline one method that can be used to measure the impact of a build-up of dead organic matter in the River Frome.
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