Chlorination

Chlorination of drinking water, a practical approach for emergincies : 

Chlorination of drinking water, a practical approach for emergincies By Osama Ali Maher

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Disinfectants Physical disinfectants are treatments like boiling or filtration applied to the water which is then safe to drink but they have no residual benefit. Some chemical disinfectants remain in the water after application and can protect against re-contamination.

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The Sun The suns rays contain ultraviolet radiation which has been used for disinfection of small community water supplies. Ultraviolet has been most effective when the radiation has been artificially generated but the system is expensive and the water must be of high clarity. The sun has more often been used to heat the water and provide a very limited degree of disinfection this way. Not a very effective method, used at household level, cheap, no residual effect.

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Boiling Boiling is a simple way of disinfecting water but uses a lot of fuel. Boiled water has an unpleasant taste unless allowed to re-aerate for several hours. Water should be brought to a rolling boil for two or three minutes or even longer if the water is not clear. Effective but with no residual effect, used at household level, high fuel requirement, useful for emergencies.

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Filtration May be considered a method of disinfection but differs in that germs are removed rather than inactivated. Removal of all germs by filtration may not be complete and therefore filtered water is often also disinfected. Slow sand filters are used in water treatment for large water supplies, sometimes as the only treatment, but they require a constant flow of water for maximum effectiveness.

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Small systems are also designed for household use. Effective if well designed and operated, suitable for large and small scale application, no residual effect, household systems may be installed relatively cheaply. Filtration- Cont.-

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Filtration May be considered a method of disinfection but differs in that germs are removed rather than inactivated. Removal of all germs by filtration may not be complete and therefore filtered water is often also disinfected. Slow sand filters are used in water treatment for large water supplies, sometimes as the only treatment, but they require a constant flow of water for maximum effectiveness.

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Iodine is similar to chlorine in its action and is more stable in storage. Its higher cost reduces its use in large scale water supplies. Its stability and availability in tablet form make it suitable for personal disinfection and for use in emergencies. Relatively expensive, effective, residual effect, useful for small scale personal use.

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Ozone Unstable gas only slightly soluble in water. As it is unstable it does not persist and therefore has no residual effect. Assists in removing tastes and odours. Ozone is manufactured on site and is only suitable for large treatment plants. More expensive than chlorine. Large scale use, no residual effect, expensive, efficient.

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Chlorine

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Chlorine is the most readily available and widely used chemical disinfectant for water supply. The aim of chlorination is the destruction of pathogens and the protection of the water supply. When chlorine is added, it purifies the water by destroying the cell structure of organisms, thereby killing them.

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To achieve this, a chlorine dose must be sufficient to: Meet the chlorine demand of the water, i.e. to oxidize the contaminants (including reacting with any organic or inorganic substances).

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Leave a residual to give protection against further contamination. This is achieved by ensuring a free residual of 0.2 – 0.5 mg/liter of chlorine in the disinfected water after a contact time of 30 minutes. The residual will inhibit any subsequent growth of organisms within the water supply system. Higher residuals may give an unpleasant taste and people may not drink the “safe water.”

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Chlorine takes time to kill all the organisms. In water above about 18 oC the chlorine should be in contact with the water for at least 30 minutes. If the water is colder then the contact time must be increased.

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A precondition for effective chlorination is that the turbidity of the water be low. In emergency water supply the aim is to have a turbidity of less than 5 NTU (Nephelometric Turbidity Units). Chlorination will function relatively effectively up to 20 NTU but steps should be taken to reduce turbidities as soon as possible.

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At high turbidity levels, large quantities of chlorine are needed to oxidize the organic matter present. This leaves a strong chlorine taste that may cause people to use other, possibly contaminated, sources of water for drinking. Furthermore, some pathogens inside particles of organic matter may survive the oxidizing effects

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Chlorine may be added to a water supply by: Dosing with a continuous flow of a 1% solution of chlorine. Adding chlorine tablets or powder directly to a tank of water (for emergency chlorination). Chlorine may be added to a water supply by:

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FORMS OF CHLORINE Chlorine gas is normally used in conventional water supply schemes of substantial size. Chlorine gas dosing equipment is expensive to install, complicated to operate and maintain, and it can be dangerous if not handled properly. Chlorine gas is unlikely to be used in an emergency water supply.

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High Test Hypochlorite (HTH) – calcium hypochlorite granules supplied in drums (70% available chlorine).

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• Sodium Hypochlorite – supplied in liquid form as Household disinfectant 5% to 15% available chlorine. Laundry bleaches 3% to 5% available chlorine Antiseptic solutions 1% to 2% available chlorine.

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Chlorine tablets – various relatively expensive types. small calcium hypochlorite tablets (60% to 70% available chlorine) used in tablet chlorinators. ‘swimming pool’ tablets containing trichloroisocyanuric acid. These tablets can be suspended in a tank with a purpose-made float to give a slow release of chlorine.

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HOW MUCH CHLORINE IS REQUIRED? Enough chlorine must be provided to meet the chlorine demand and to leave a free residual of 0.2 – 0.5 mg/l after a contact time of 30 minutes. There are two types of chlorine residuals – combined residuals and free residuals.

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Combined residual chlorine is the proportion of the original chlorine dose that combines with ammonia and organic nitrogen compounds to form stable but less effective disinfectants than free chlorine. Free residual chlorine is that part of the chlorine dose which remains after the chlorine demand has been fully satisfied.

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MAKING A 1% CHLORINE SOLUTION Preparation of 1 liter of 1% Chlorine solution

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Chlorine Doses for Common Volumes (assumes dosage of 5 mg/l) Chlorine Doses for Common Volumes (assumes dosage of 5 mg/l)

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HOW MUCH CHLORINE IS REQUIRED, - A practical approach! The actual dose of chlorine will depend on the condition of the water. It can be expected to be in the range of 1 – 5 mg/l. You can determine the actual dosage as follows: 1 ) Prepare a 1% chlorine solution 2) Take 4 non-metallic containers of a known volume (e.g., 20liter buckets)

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3) Fill the containers with equal amounts of water to be treated. 4) Add to each bucket a progressively greater dose of 1% solution with a syringe. For example, if we were using 20 liter volumes of water and we wanted to try doses of 1 mg/l, 2 mg/l, 3 mg/l and 4 mg/l, we would dose the containers as follows: 1st container: 2 ml 2nd container: 4 ml 3rd container: 6 ml 4th container: 8 ml

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5) Wait 30 minutes for chlorine to react then measure free chlorine. 6) Choose the sample which shows a free chlorine residual between 0.2 – 0.5 mg/l. You may have to run the test again with stronger or weaker doses depending on the results.

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The free residual chlorine levels of the water in the buckets, measured half an hour after adding 2, 4, 6 and 8 ml of the 1% chlorine solution respectively are as follows: 1st container: 0 mg/l 2nd container: 0.1 mg/l 3rd container: 0.4 mg/l 4th container: 1 mg/l Example: Chlorination of water in a 2,000 liter reservoir

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Solving for x yields: x = 600 ml of 1% solution The dosing rate chosen therefore will be that for the 3rd container (result between 0.2 and 0.5 mg/l). If it needs 6 ml of a 1% solution to chlorinate 20l of water at the correct dosage, then it needs 100 times as much to chlorinate 2,000 l. E.g.: 6 ml of 1% solution in a 20 litre container gave the desired residuale chlorine, How much is needed for 2000 litre? 600 ml Solving for x yields: x = 600 ml of 1% solution