Disinfection Flashcards
What poses the greatest risk to potable water quality?
Sewage and animal waste
These contaminants can carry diseases.
What is the purpose of disinfection in water treatment?
To kill or inactivate pathogenic micro-organisms
What is the most widely used disinfectant for large municipal water supplies?
Chlorine (Cl2)
What are the common sources of chlorine used in water disinfection?
- Liquefied chlorine gas
- Sodium hypochlorite solution (NaOCl)
What does chlorine form when it dissolves in water?
Hypochlorous acid (HOCl) and hydrochloric acid (HCl)
What is the chemical reaction when chlorine dissolves in water?
Cl2 + H2O ↔ HOCl + HCl
What can hypochlorous acid (HOCl) dissociate into?
Hydrogen (H+) and hypochlorite (OCl-)
What is a more powerful disinfectant, hypochlorous acid or hypochlorite?
Hypochlorous acid
What pH range is recommended to favor hypochlorous acid formation?
Between 5 and 6
What does the World Health Organisation recommend for effective disinfection of drinking water?
- pH preferably less than 8.0
- Contact time greater than 30 minutes
- Free chlorine residual of 0.2 to 0.5 mg/L
What risk is associated with chlorine reacting with organic matter in water?
Formation of organic halogen compounds such as trihalomethanes
What are the potential issues with excessive chlorine in water?
- Undesirable taste and odours
- Corrosion risk
What is formed when ammonia (NH3) is added to chlorinated water?
Chloramines (NH2Cl)
What is a disadvantage of chloramines compared to chlorine?
Chloramines are not as effective in disinfecting water
What are the advantages of chlorine treatment?
- Low initial cost
- Low running costs
- Simplicity of handling
- Protection against re-infection
What are the disadvantages of chlorine treatment?
- Gives an off-taste that requires carbon filtration
- Some organisms are resistant to chlorine
- Formation of organic halogen compounds
What does the World Health Organisation provide guidelines for?
Drinking water quality
What are legal potable water standards based on in the UK?
European Drinking Water Directive 1998 and national standards
Fill in the blank: The __________ can be found online by searching for USA potable water standards.
potable water standards
