In the UK we tend to take the provision of clean drinking water in our homes and businesses for granted… but what goes on behind the scenes to make this happen? In this guide the water hygiene specialists at Water Treatment Services take a look at how and why we clean our drinking water to make it safe, looking in particular at the process of water chlorination, its history, why we chlorinate water and what’s involved.

Disinfecting drinking water to keep people safe

Water from rivers, lakes, reservoirs, wells, boreholes and other natural sources may contain microorganisms.

Not all of these microscopic bugs, which are for the most part invisible to the naked eye are dangerous to humans.

However, some of them, known as pathogens, can make us seriously ill if we allow them to infect us.

If pathogens are present in a water supply or distribution system and are drunk, they can start to multiply in the guts of humans, potentially causing serious waterborne disease which could include Cholera, Typhoid, and Dysentery.

We therefore need to take steps to treat our drinking water to kill off these microorganisms which might be present and could cause us harm.

Treating drinking water using chlorination

There are lots of different ways of treating drinking water including sedimentation, filtration and coagulation.

Perhaps the best known of all the treatment processes is chlorination, and using this method helps to clean our drinking water making it safe for everyone to drink.

The idea of water chlorination was first proposed in Sweden during the 1740s.

The chlorination of water is a tried and tested treatment technique that has been around for over a century and it’s still commonly used today.

Chlorination is a method of disinfecting water using a chlorine based chemical, or another substance containing chlorine to treat the water source.

The history of water chlorination

The idea of water chlorination was first proposed in Sweden during the 1740s.

Back then, scientists were not aware that water contained microscopic organisms, and thought that smells from water caused disease.

By the 1830s, chlorine was being used to remove the smell from water, and as a consequence killing the microorganisms too.

By 1890, when the link between something in the water itself and disease had been made, the UK began routinely chlorinating drinking water.

The practice quickly spread to the USA and Canada at the start of the 20^th century.

Even now, chlorination is still used extensively to treat drinking water

In the 21^st century, chlorination remains the main way of treating and disinfecting water worldwide.

Why chlorinate water?

Over the years, there have been numerous studies about the effectiveness of chlorine as a way of disinfecting water.

One of the main benefits of chlorine as a water disinfectant is that it is very effective against bacteria and viruses, however, some types of protozoan cysts are immune to the bug killing effects of chlorine.

Protozoan cysts are not a major concern in all water sources.

If there is not an issue with this type of contamination, then chlorination water can be highly effective.

The chlorination of water is very cheap, and is also proven to be very effective in wiping out most of the other possible reasons for water contamination.

In emergency situations, such as a breakdown in a filtration system or an accidental mixing of treated and raw water, chlorination can be used to treat a large number of pathogens very quickly.

How does water chlorination work?

Chlorine works by damaging the cell membrane which surrounds a microorganism.

When the membrane is damaged, the chlorine then gets into the cell and stops both respiration and DNA activity.

Without both of these processes, the cell is unable to survive and so dies off.

When should water be chlorinated?

Another benefit of chlorination is that it can be carried out at any point in the water treatment process.

Separate doses of chlorine might be added at many different stages through the process to ensure the water is treated effectively.

Pre-chlorination

Pre-chlorination refers to the process of adding chlorine to water as soon as it enters the treatment plant.

Chlorine is usually added straight into the raw water or into a mixing vessel, which makes sure that the chlorine is evenly spread throughout the body of water being treated.

Adding chlorine at this stage gets rid of any algae or other living organisms in the water so that this doesn’t cause issues at later stage.

If water is pre-chlorinated at this stage it can stop smells and strange tastes in the water.

It can also prevent contamination in the water system itself as there are no biological elements left to grow on pipe surfaces, water tanks or in filters.

Adding chlorine can also cause a chemical reaction to oxidise unwanted minerals such as manganese, iron and hydrogen suphides which can then be easily filtered out.

Pre-filter chlorination

Another common stage for disinfection is before the water is filtered but after it has gone through the sedimentation process.

Disinfection at this stage helps to control any further growth or organisms, and also helps to remove dissolved solids.

Final stage chlorination

A final option is to make chlorination the last step in the water treatment process.

This is what happens in most water treatment plants in the UK.

Chlorination at this final stage also disinfects the water, and keeps it disinfected as it travels through the distribution network to our homes and businesses.

It is more effective to chlorinate water which has already been filtered as lower concentrations and shorter contact times are required.

In simple terms, this means that as the water has already been through several processes to clean it up, you can use less chlorine for a shorter time than at other stages in the process.

What about residual chlorine levels?

Adding chlorine at any stage in the treatment process will result in both hypochlorous acid and hypochlorite ions forming in the water.

It is these compounds which perform the disinfecting action.

The more effective of the two compounds is hypochlorous acid.

The levels of both types of compound is largely determined by the acidity or alkalinity of the water before the chlorine is added.

If the water pH tends towards the acidic range, then there will be more hypochlorous acid available.

Free chlorine is formed by the combination of hypochlorous acid and hypochlorite ions.

Free chlorine is a better disinfectant than other forms of the element, and also will more readily react with other chemicals.

What is combined chlorine?

As a result of chlorine reacting with ammonia, combined chlorine might be formed.

This isn’t as effective at disinfecting water, as it has a lower oxidation potential.

Because of this, ammonia is usually added towards the end of the water treatment process.

How much chlorine is needed to disinfect water?

The quantity of chlorine needed to clean up water will depend on what sort of impurities have been found.

Some impurities require large amounts of chlorine to ensure there is enough present to deal with all the impurities.

The quantity of chlorine needed to deal with these impurities is known as the chlorine demand.

Another way of thinking of it is the amount of chlorine needed before free chlorine starts to be produced… and this is known as the breakpoint.

Continuing to add chlorine after the breakpoint will increase the levels of free chlorine present.

Another factor to take into account is the level of ammonium present in the water.

Breakpoint chlorination will not take place until all the ammonium in the water has also reacted with the chlorine.

Anything between 10 and 15 times more chlorine than ammonium is needed to reach breakpoint.

A common issue with smaller water treatment plants is not adding enough chlorine in proportion to ammonium, resulting in ineffective disinfection which can have serious consequences.

Specialist water treatment and disinfection solutions

Water Treatment Services offer a comprehensive range of water treatment solutions for the disinfection and sterilisation of water systems in commercial and industrial applications. Our experts can help you keep water systems safe, optimise costs and operational efficiencies, achieve water and energy savings, and increase plant reliability.

With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained technicians and in-field engineers we can offer professional, cost effective water treatment and engineering solutions throughout the UK and Internationally.