Comparing Biochemical Oxygen Demand (BOD) in Different Water Samples

Aim

To compare the Biochemical Oxygen Demand (BOD), an indicator of organic pollution, of different water samples, such as clean water and water containing added organic waste.

Variables

• Manipulated variable: the source or type of water sample tested, such as clean water compared with water that has organic matter or detergent added.
• Responding variable: the drop in dissolved oxygen (DO) level after a fixed incubation period, calculated as BOD = initial DO minus DO after incubation.
• Controlled variables: the incubation temperature, the incubation time (the standard method uses 5 days), the volume of each sample, the exclusion of light during incubation, and the dissolved-oxygen testing method used.

Materials and apparatus

• Water samples from different sources
• BOD bottles or other stoppered sample bottles
• A dissolved oxygen (DO) test kit or a DO meter
• An incubator, or a dark cupboard kept at a steady temperature
• A thermometer
• Labels

Procedure

1. Collect a water sample from each source and label the containers clearly.
2. Measure and record the initial dissolved oxygen level of each sample using the DO test kit or meter.
3. Fill and seal a BOD bottle with each sample, making sure no air bubbles are trapped inside.
4. Incubate all the sealed bottles in the dark at a fixed temperature, such as 20 degrees Celsius, for 5 days.
5. After 5 days, measure and record the dissolved oxygen level in each bottle again.
6. Calculate the BOD of each sample as the difference between its initial and final dissolved oxygen readings.
7. Compare the BOD values obtained for the different water samples.

Expected results

The water sample with more organic pollutants is expected to show a larger drop in dissolved oxygen, and therefore a higher BOD value, than the cleaner sample, as illustrated in Table 1.

Conclusion

A water sample with more organic pollutants has a higher BOD because the microorganisms in it use more dissolved oxygen to break down the additional organic matter over the incubation period. A higher BOD value therefore indicates greater organic pollution and lower water quality, while a lower BOD value indicates cleaner water.

Common mistakes

Paper 3-style questions

Question 1 (making a hypothesis). State a suitable hypothesis for this investigation.

Model answer. The water sample containing more organic waste has a higher BOD than the clean water sample. This links the manipulated variable, the type of water sample, to the responding variable, the BOD, which is the drop in dissolved oxygen over the incubation period.

Question 2 (controlling variables). Identify the manipulated and responding variables, and name one variable that must be controlled.

Model answer. The manipulated variable is the source or type of water sample; the responding variable is the BOD, calculated as initial dissolved oxygen minus dissolved oxygen after incubation. The incubation temperature and time must be controlled, so any difference in BOD is due to the water sample alone.

Question 3 (tabulating and calculating). Given the initial and final dissolved oxygen of each sample, show how to calculate and tabulate the BOD.

Model answer. Use BOD = initial DO − final DO. For the clean sample, 8.0 − 7.4 = 0.6 mg/dm3; for the polluted sample, 8.0 − 2.5 = 5.5 mg/dm3.

Tabulate columns for the sample, initial DO, DO after 5 days and BOD, all in mg/dm3, and display the BOD values as a bar chart because the sample is a categorical variable.

Question 4 (making an inference). The polluted sample has a much higher BOD. What can you infer about the microorganisms and the oxygen in it?

Model answer. You can infer that the polluted water contains more organic matter, so its microorganisms respire more and use up more dissolved oxygen in breaking it down. A high BOD therefore indicates greater organic pollution and less oxygen available for aquatic organisms.

Safety

• Wear gloves and wash your hands after handling water samples, as polluted or waste water may contain harmful microorganisms.
• Never taste any sample or draw it up by mouth; use a pipette filler, test kit or dissolved-oxygen meter instead.
• Follow the label when using the dissolved-oxygen test-kit reagents, avoid skin and eye contact, and rinse with plenty of water if any is spilt on you.
• Handle the glass BOD bottles carefully and clear up any broken glass safely.

Source:SRC-DSKP-EN

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