Stage of treatment Coliform bacteria / number per 100 cm3 Chlorine concentration / mg dm-3 River intake 48 000 0.0 After filtration 3 200 0.0 After chlorina...
Question 1 Report
| Stage of treatment | Coliform bacteria / number per 100 cm3 | Chlorine concentration / mg dm-3 |
|---|---|---|
| River intake | 48 000 | 0.0 |
| After filtration | 3 200 | 0.0 |
| After chlorination | 4 | 0.8 |
The table below shows water-quality measurements at three stages in a small town's treatment works. The tests were made after flooding had increased the amount of sewage-contaminated water entering the river. Coliform bacteria are used as an indicator that water may contain pathogens causing disease.
| Stage of treatment | Coliform bacteria / number per 100 cm3 | Chlorine concentration / mg dm-3 |
|---|---|---|
| River intake | 48 000 | 0.0 |
| After filtration | 3 200 | 0.0 |
| After chlorination | 4 | 0.8 |
(a) State the stage at which the greatest decrease in the number of coliform bacteria occurred. [1]
(b) Calculate the decrease in coliform bacteria per 100 cm3 between the river intake and after filtration. [2]
(c) Describe what the results show about filtration and chlorination. [3]
(d) Explain why treated water should still be tested before it is supplied to people. [5]
Answer Details
(a) The greatest decrease occurred during chlorination, between filtration and chlorination. [1 mark]
(b) \[48\,000-3\,200=44\,800\] The decrease is 44 800 coliform bacteria per 100 cm3. [2 marks]
(c) Filtration reduced coliform bacteria from 48 000 to 3 200 per 100 cm3, but did not remove all bacteria. Chlorination then produced a much larger further reduction, to 4 per 100 cm3. [3 marks]
(d) Water must still be tested because equipment or chlorine dose may fail or change, and contamination of source water can vary after rain or sewage leaks. Tests check that pathogen-indicator numbers are acceptably low and can detect contamination before water reaches homes, helping prevent water-borne outbreaks. [5 marks]
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