Disinfection is the major means of guaranteeing the microbiological quality of drinking water. However some organisms (e.g. Cryptosporidium) are immune to chlorine at concentrations normally used for drinking water disinfection. Many water supply authorities in Africa do not disinfect the water supplied to consumers. This is most likely as a result of lack of technical capacity, high cost and lack operation and maintenance programmes.
What is Chlorine?
Chlorine is a relatively cost effective and readily available chemical that, when dissolved in clear water in sufficient quantities, will destroy most disease causing organisms without being a danger to people. If enough chlorine is added, there will be some left in the water after all the organisms have been destroyed; this is called free chlorine residual. Free chlorine will remain in water until it is either lost through dissipation or used destroying new contaminants. Therefore if water is tested and found that there is still some free chlorine left, it proves that most potential disease causing in the water have been destroyed and that the water should be safe to drink. The testing of free chlorine is called measuring the chlorine residual.
Chlorine is available in a gas or liquid forms (called hypochlorites). The decision to use either chlorine gas or hypochlorites should be based on several factors as:
- The quantity of water to be treated
- The cost and availability of chemicals
- The equipment needed for its application and'
- The skill required for operation and control
Chlorine gas feed equipment is more expensive, more difficult to operate and maintain, and more dangerous than solution type hypochlorinators. On the other hand, chlorine gas is generally less expensive than hypochlorite containing an equivalent amount of available chlorine. Chlorine gas can be stored for longer periods without deteriorating.
The effectiveness of disinfection depends on:
- The nature and concentration of the disinfecting agent
- The type of micro-organisms present
- Contact time (the length of time the disinfectant is available for inactivation)
- Satisfactory mixing of disinfectant and target micro-organisms
- The degree to which the micro-organisms are protected by adsorption (e.g. high turbidity waters) and attachment to surface of pipes or fittings
- The level of competing inorganic and organic reactants
- Temperature and pH
In addition to the above, the following important points should be considered:
Chlorine needs at least half an hour contact time with water to disinfect it. The best time to disinfect is after any other treatment process, and before storage and use.
Chlorine should never be applied before slow sand filtration or any other biological process as chlorine will kill off the bacteria which assist treatment, making the treatment ineffective.
Chlorine should never be added in a solid form directly to a water supply, as it will not mix and dissolve.
Disinfection is only one defence against disease. Every effort should be made to protect water sources from contamination, and to prevent subsequent contamination during collection and storage.
The correct procedure for applying a disinfectant to water should be strictly adhered to, and water supplies should be monitored regularly to ensure that they are free from bacteria.
The chlorine dose required to disinfect a supply will increase if the water is very turbid. In such circumstances, it is best to treat the water to reduce turbidity before chlorination.
Turbidity, the concentration of the disinfectant, contact time and pH can all be monitored continuously, and can provide a useful indication of microbiological quality control.
How Does Chlorine Work?
When chlorine is added, it purifies the water by destroying the cell structure of organisms, thereby killing them. The process only works if the chlorine comes into direct contact with the organisms. If the water contains silt, the bacteria can hide inside it and not be reached by the chlorine. Chlorine takes time to kill all the organisms. In water above about 18 degrees Celcius 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. It is normal to add chlorine as it enters a storage tank or a long delivery pipeline to give chemical time to react with the water before it reaches the consumer. (WHO, 2004) stated that to ensure the absence of most micro-organisms, the guidelines recommend that the following conditions of disinfection with chlorine be met:
- Free residual chlorine: 0.5mg/l
- Contact time: 30 minutes
- pH: < 8.0
- Turbidity: < 5 NTU
Testing for Free Chlorine Residual
The most common place to use chlorine as disinfection is in a piped water supply. The chlorine residual is usually tested at the following points:
- Just after the chlorine has been added to the water to check that the chlorination process is working.
- At the outlet of the consumer nearest to the chlorination point to check that residual chlorine levels are within acceptable levels (between 0.5 and 0.2 mg/l).
- At the furthest points in the network where residual chlorine levels are likely to be at the lowest. If chlorine levels are found to be below 0.2mg/l it might be necessary to add more chlorine at an intermediate point in the network.
The amount of chlorine residual in water changes, depending on the time of the day. This is because the water stays in the system for longer at night, when the demand is lower, and so there is more opportunity for the water to be contaminated which will use up the residual chlorine.
The most common test is the diethyl paraphenylene diamine (DPD) indicator test, using a comparator. This test is the quickest and simplest method for testing chlorine residual. With this test, a tablet is added to a sample of water, colouring it red. The strength of colour is measured against standard colours on a chart to determine the chlorine concentration. The stronger the colour, the higher the concentration of chlorine is in the water.