Describe the purification of water on a small scale.

Purification of Water on a Small Scale

• Boiling is the most reliable household method — a rolling boil for 5 to 10 minutes is sufficient
• Its advantage is that it kills everything — bacteria, viruses, and the cysts and ova that chlorine does not touch
• Its disadvantages — it requires fuel, which is expensive and itself a source of indoor air pollution; it changes the taste by driving off dissolved air, so people dislike it; it does not remove turbidity or chemicals; and, critically, it leaves NO residual protection, so the water is recontaminated as soon as it is stored or handled carelessly
• That last point is the practical key to all household treatment — safe storage matters as much as treatment, in a narrow-mouthed covered vessel with a tap or a long-handled dipper, so that hands never enter the water
• Chemical disinfection:
• Bleaching powder (chlorinated lime) — contains about 33% available chlorine when fresh, but loses it rapidly on exposure to air, light and moisture, which is why it must be stored in a dark, airtight container and tested before reliance
• The horrocks apparatus is used to determine the dose of bleaching powder required for a given well, and is a classic examination item
• Chlorine tablets (halazone) — convenient but expensive for routine use
• Sodium hypochlorite solution — supplied under the national programme, typically one drop per litre of clear water, with 30 minutes of contact
• The essential conditions for any chlorination to work — the water must be clear; the correct dose must be used; and a contact time of at least 30 minutes must be allowed before drinking
• Iodine — effective and useful for travellers and emergencies, but not for prolonged use, in pregnancy or in thyroid disease
• Potassium permanganate is not recommended, being unreliable, and it colours the water and alters the taste
• Physical methods:
• Filtration through household filters — ceramic (Pasteur-Chamberland, Berkefeld) candle filters, which remove bacteria and turbidity but not viruses, and must be scrubbed and boiled weekly, since the candle itself becomes a culture medium if neglected
• Activated carbon improves taste and odour and removes some chemicals but does not disinfect
• Modern domestic units — ultraviolet units, which are effective against bacteria and viruses but require clear water and a reliable electricity supply, and leave no residual; and reverse osmosis, which removes dissolved salts, fluoride, arsenic and nitrate as well as organisms
• A caution about reverse osmosis is worth making — it wastes a large volume of reject water, removes beneficial minerals, and is unnecessary where the water is microbiologically the problem rather than chemically so; it is widely over-sold
• Solar disinfection (SODIS) — clear water in a transparent plastic bottle exposed to full sunlight for about 6 hours, working by ultraviolet radiation together with heat; it is free and suited to sunny climates, but needs clear water and a full day of sun
• Multi-barrier approaches such as flocculant-disinfectant sachets, which coagulate turbidity and disinfect in one step and are therefore useful in emergencies where the water is muddy
• Disinfection of a well is a standard practical question — estimate the volume of water in the well; calculate the bleaching powder required using the Horrocks apparatus or an assumed dose; make a paste, dilute it, allow the lime to settle and use the supernatant; distribute it through the well by agitation with a bucket; and allow at least one hour of contact before use
• The principle that should conclude the answer — no household method is a substitute for a safe piped supply; they are interim, depend on daily correct use, and place the burden on the household rather than on the system
• Boiling leaves no residual protection, so safe storage matters as much as the treatment itself
• Store in a narrow-mouthed covered vessel with a tap, so that hands never enter the water
• Reverse osmosis is widely over-sold, wasting water and stripping minerals where the problem is microbiological
• Chlorination needs clear water and contact time, at least thirty minutes before the water is drunk
• Bleaching powder loses chlorine rapidly, so it must be stored dark and airtight and tested before reliance
• The Horrocks apparatus finds the dose for a given well, which is a classic practical item
• Scrub and boil a ceramic candle weekly, since the candle itself becomes a culture medium if neglected
• Activated carbon improves taste but does not disinfect, which is a distinction consumers rarely appreciate
• Ultraviolet units need clear water and power, and leave no residual protection afterwards
• Solar disinfection needs six hours of full sun and clear water in a transparent bottle, and costs nothing
• Avoid iodine in pregnancy and thyroid disease, and do not use it for prolonged periods
• Potassium permanganate is not recommended, being unreliable and altering colour and taste
• Household methods are interim, not a substitute for a safe piped supply, and place the burden on the family