Disinfecting Drinking Water – Chlorination and Chlorine Testing

Ensuring drinking water is safe for human consumption is of the highest priority across all drinking water providers. Water goes through multiple stages of processing in a water treatment plant before it is deemed safe for consumption. This article explores the water treatment process, with a specific focus on disinfection and how Palintest’s new Lumiso Expert photometer can ensure this process is managed efficiently.


How is Raw Water Made Safe for Consumption?

Water treatment varies depending on the type of water entering the water treatment plant. Some accepted technologies are adapted and combined to best suit the incoming source water and local standards; however, the process can usually be split into five main stages: pre-screening, flocculation, filtration, disinfection, and storage ready for use.

Pre-screening involves filtering out any large objects from the incoming water stream.

The flocculation stage follows pre-screening. Chemicals known as ‘flocculants’ are added to the water. These chemicals bind to small particles in the water and cause clumping to form larger, heavier masses which eventually sink to the bottom of the water where they can be removed as sediment.

Once the larger particles are removed, the water is filtered through multiple membranes to remove any smaller particles not removed during the previous stage. This stage also enables removal of other undesirable chemicals such as manganese.

Filtering does not remove all harmful microorganisms, and therefore the next stage of the treatment process is disinfection. This involves addition of a chemical disinfectant such as chlorine to the water to kill any remaining microorganisms. Enough disinfectant is added to maintain a residual level in the water to ensure safety during storage and distribution.

If the water is stored for long periods before distribution, then a secondary disinfection process may be required to increase the residual level back to an optimum amount for distribution.

What is Chlorination and Why is it Important?

Chemical disinfection is arguably one of the most important stages of the drinking water treatment process, as it ensures any harmful microorganisms are not present in the drinking water. Chemical disinfection also has the advantage of providing a residual to ensure any microorganisms which enter the water whilst in the distribution network are killed. This is something other forms of disinfection such as Ozone or UV light cannot achieve.

Specifically looking at chemical disinfection, chlorine is one of the most popular disinfectant options. Chlorination can be done using multiple chlorine-based chemicals including chlorine gas, calcium hypochlorite or sodium hypochlorite. Chlorination is often chosen over other chemical disinfectant options as it is a very effective disinfectant and available at a lower cost than some other chemicals.

When disinfecting with chlorine, monitoring chlorine concentration in the water sample is extremely important to ensure the correct amount is dosed for regulation compliance. The consequences of overdosing chlorine include the formation of potentially harmful disinfection by-products, as well as overspending on chemicals, whereas underdosing will lead to microbial contamination in the water supply which will have serious health effects if consumed.

Chlorination is usually the final step prior to storage in most treatment plants, where is it is used to disinfect the water and ensure a residual is maintained as it travels through the distribution network. However, it is also sometimes carried out at other stages within the disinfection process:

  • Pre-chlorination – This is when the raw water is chlorinated as soon as it enters the treatment works. Pre-chlorination is beneficial to remove any aquatic life/ algal contamination and oxidise contaminants such as iron and manganese ready for removal during the flocculation/ filtration stage.
  • After flocculation – Chlorination at this stage helps to eliminate physical issues such as odour, taste, and colour. It will also control algae growth and remove contaminants.
  • Post-treatment, in the distribution network – This is done to ensure high enough disinfectant residuals are maintained as the water travels through the distribution network.

What Can Be Used to Monitor Chlorine Levels in the Distribution Network?

Monitoring chlorine levels after chlorination and at points in the distribution network is vital to ensure the water has been effectively disinfected and a  residual remains as the water travels through the distribution network.

One of the most popular techniques for monitoring chlorine levels is using the DPD colorimetric method. This has been adopted as an industry standard for measuring chlorine concentration in water since it was originally pioneered by Dr Palin in the 1950s. Further information on the DPD method for determination of chlorine concentration can be found in our Measuring Chlorine using DPD article.

To ensure accuracy and precision using the DPD method, a reliable photometric platform is also required. Palintest’s Lumiso Expert offers a variety of DPD chlorine tests for free, total, and combined chlorine across multiple ranges. The Lumiso optical engine uses innovative photometric technology to ensure consistently accurate and reliable results which will ensure chlorination is controlled efficiently to prevent over or under-dosing. The Lumiso Expert photometer is also rated to IP67 for waterproofing and humidity and is impact tested to IK08 as part of EN61010. This is particularly important in drinking water monitoring where testing may be carried out in difficult conditions in the field. Finally, up to 1000 results are saved automatically including test data, user, labels, and notes making compliance and audits simple and pain-free.


If you are in need of an accurate method to ensure efficient chlorination monitoring, please contact our team.

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