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expert reaction to reports of high levels of chlorine in Derbyshire water

The water company Severn Trent has advised thousands of customers in Derbyshire and Leicestershire not to use its water following the detection of high levels of chorine.


Jo Parker, chartered engineer and Director of Watershed Associates, said:

“Potable water disinfection uses chlorine and is carried out all water treatment plants. The chlorine is a strong oxidant and is used to ensure that any pathogens in the water too small to be taken out by the filtration process are rendered ineffective. Normally a level of chlorine higher than is suitable for drinking water is added via a chlorine solution. The water is then left in contact with the chlorine for sufficient time to allow it to work. This part of the process is called super-chlorination. The level of chlorine is then brought down to a level such that the water can be drunk. This is usually by adding a further chemical solution. This process is termed de-chlorination.

“These processes are generally carried out automatically with sensors measuring the amount of chlorine in the water when it is dosed, at the end of the ‘contact time’ and after de-chlorination. These sensors are linked to the pumps which add the chlorine solution and the de-chlorination solution so that the levels are constantly being adjusted to ensure the right amount of water is added. There are special circuits which shut down the water output if the levels of chlorine are outside agreed ranges, so that there should be no danger of un-disinfected water getting to the public as this could compromise public health.

“In this case there is no information as to what exactly went wrong. The most likely event I can think of is that the de-chlorination process failed in some way. However, I do not know the Castle Donnington system myself so cannot judge. I also have no information about the exact levels of chlorine in the water.”


Sue Pennison, Principal Inspector, Drinking Water Inspectorate, said:

“There are a number of ways of disinfecting water with chlorine, most commonly by the use of chlorine gas or the addition of sodium hypochlorite to water during the treatment process, or by additional booster chlorination in the distribution network. Additionally, mains pipework is flushed with water containing elevated chlorine which is then disposed of prior to refilling the main and putting it back into service.

“Over chlorination can be resolved by flushing the pipework through with water with the usual level of chlorine in it. Chlorine can cause skin and eye irritation and a burning sensation, but it is not carcinogenic and irritation should be transient. However anyone concerned should see their GP.”


Declared interests

Jo Parker: I am managing a research project of which STW are a partner. I am also managing research projects for UKWIR, of which STW is a member. Finally I am a past chair of the Pipeline Industries Guild for the Midlands Branch and still on that committee and worked alongside STW employees.

Sue Pennsion: No conflicts of interest

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