- With reference to fresh water treatment:
(a) describe the process for superchlorinating the fresh water system, stating relevant periods, times, and chlorine levels;(6)
(b) explain what is meant by copper staining and the treatment required to prevent it.(4)
(a) Process of Superchlorination (6 marks)
Superchlorination is a method of disinfecting the entire fresh water system when contamination is suspected.
- Raise chlorine level – chlorine is added until the residual free chlorine concentration in the system is 50 ppm (mg/L).
- Contact period – the system is left standing for 24 hours to allow thorough sterilisation of all tanks, pipelines, and fittings.
- Flushing – after 24 hours, the system is completely drained and flushed with fresh water until chlorine concentration is reduced to normal safe levels (0.2–0.5 ppm for potable water).
- Testing – chlorine residual is checked before the water is declared fit for consumption.
(b) Copper Staining (4 marks)
- Meaning: Copper staining is the appearance of blue/green stains on sanitary fittings, sinks, and baths caused by dissolved copper in the water supply.
- Cause: Water that is slightly acidic or aggressive corrodes copper pipework, releasing copper ions into the water.
- Treatment/Prevention:
- Add hydrated lime or similar alkaline chemicals to raise the pH and reduce acidity.
- Maintain water in a non-corrosive state by ensuring proper alkalinity.