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General questions about the chlorination process

Ban Qasim
related country: Malaysia
related country: Malaysia

What is the chlorination process ?

How do you measure chlorine in water, how do you test it, and what are the required materials and equipment to use in lab?

How do you do calculation for breaking point chlorination?

4 Answers


Hi Ban,

The most common approach for measuring chlorine in water is with the chemical reagent called DPD, which changes color in response to the free or total chlorine concentration (depending on the type of DPD used). You can read this color change with a simple color wheel (e.g. Hach color wheel), or use a more expensive electronic colorimeter (Hach pocket II). There are also very inexpensive test strips that can measure both free and total (combined) chlorine. This one from ITS is even approved by the US EPA.

Breakpoint chlorination is a bit more complex and requires careful monitoring of the influent and effluent, as well as the pH and nitrogen compounds. You could start by looking into manuals used in water treatment, such as White's Handbook.

Good luck! John


Thank you John. Can Hach help me with providing water treatment manuals, and how can I access White's Handbook? Can I follow EPA standards as I am a postgraduate student in Malaysia and I could not find any standards here or manuals. I just need a good manual to follow the instructions on how to do tests, collecting samples and what to do exactly at the lab, as well as the appropriate dose of chlorine and ammonia to add, and how to control PH of water to get the chlorination breaking point. What manual and standards would you recommend Sir? If you could get back to me I would be very grateful. Thank you

Ban Qasim gravatar imageBan Qasim ( 2016-01-07 17:05:46 )

Hi Ban, I can imagine some of these resources might be hard to find online. You might take a look at online courses in water treatment (such as this one at MIT). Look for the sections on Disinfection.

Following EPA standards can be confusing because the documents are designed around regulatory compliance, rather than the design and operation of treatment processes. For what you are interested in, I would look for a good water treatment engineering book (such as Water and Wastewater Engineering). This will cover all the steps of the treatment process, including disinfection, pH control, and monitoring. I would love it if this sort of material was more freely available. Perhaps some others are aware of more free resources?

johnf gravatar imagejohnf ( 2016-01-11 03:30:11 )

Dear John, thank you for your help and support. But regarding the Hach System and DPD method, I am confused about what type of HACH to use and how to define the right one as there are so many models. Yes, this Hach Wheel color is available but there are other types like spectrophotometer HACH DR2800 and DPD method as well as the Standard APHA ASTM 2004. Can you provide me any information if you know anything about it and what appropriate software to use for calculation and analyzing? I would be grateful to you and I look forward hearing from you. Thank you again.

Ban Qasim gravatar imageBan Qasim ( 2016-01-11 09:16:29 )


Dear Sir, can you help with more information regarding water disinfection because I am confused with the methodology to test break point chlorination, method, materials, equipment? How about DPD method? In addition, where can I find the procedure and how to apply it for 3 types of water, ground water, rain water and hill water - if you could help me with this Sir I would be so grateful, I look forward hearing from you, thank you again.

Ban Qasim gravatar imageBan Qasim ( 2016-01-12 15:59:59 )

Chlorination is a chemical disinfection process. Chlorine is added to the water to be treated as either gaseous chlorine, or more commonly in emergency/development programmes as a solution made from some form of hypochlorite, either sodium or calcium. When added to the water the chlorine forms hydochlorous acid and chloride ions. In fact a succession of equilibrium reactions occur. In addition to forming acid and ions the chlorine will react with any metal ions present in the water and any nitogen based compounds in the water such as ammonia, to form various chloramine compounds. The chloramines are known as combined chlorine, whereas the acid and chloride ions are known as free chlorine.

As the chlorine is added to the water, the residual chlorine concentration will decrease with dose as the reactions that form chloramines occur. Once these reactions have occurred the concentration of free chlorine will increase with the dose. The point at which the chlorine residual concentration begins to increase in proportion to the dose added is the break point.

Due to the complexity of the chemical equilibria it is not possible to calculate the dose of chlorine required for the break point to be reached from first principles. This must be determined empirically through the use of jar tests. In jar tests fixed masses of chlorine are added sequentially to known volumes of raw water, and the chlorine demand is determined by constructing a dose-residual curve.

Chlorine residual is measured by clorimetry, ion sensitive electrodes, and colour comparators. In most field settings simple colour comparators are used. DPD 1 tablets are used to measure the free chlorine. While DPD 2 & 3 tablets are used to measure combined chlorine as chloramines.

For chlorination to be an effective disinfectant the raw water must have a maximum turbidity of <5NTU. The water must also be free from organics, as the chlorine will react with these to form trihalomethanes (THM’s). The formation of THM’s imparts a strong chlorine taste and odour to the water, which results in it being unpalatable. THM’s are thought to be carcinogens, and chronic exposure has been linked with bowel cancer. The EU has set maximum acceptable concentration limits for THM’s. Hence unless the raw water source is a protected groundwater source, some form of pre-treatment will generally be required for effective chlorination, generally involving filtration, coagulation and flocculation or both.

Contact time required for complete disinfection depends on temperature and the pH of the water. It is normally assumed that a 0.5mg/l free residual chlorine after 30 minute contact time indicates complete disinfection. It is important to ensure full mixing of the hypochlorite solution with the raw water, so the solution should be added under turbulent mixing conditions, for example at the inlet to a tank.

Hope this helps.


Dear Sir, Thank you a lot for the valuable information that you provide me with here. But I have inquiries regarding the break point chlorination and the controlling dose of chlorine as you mentioned in your third paragraph that due to the complexity of equilibrium of chemicals it is not possible to control the dose of chlorine in water and it must be done through jar test. Can you please provide me more information about that test how to do it, the procedure and materials with equipment required at lab and how to measure the break point chlorination. I mean how to do the calculations, which formulas or software use to do the calculation to get that curve of breaking point chlorination if you could please help me with that I would be grateful to you. Thank you again.

Ban Qasim gravatar imageBan Qasim ( 2016-01-14 01:25:54 )

Please use following link

https://watsanmissionassistant.wikisp... Regards

John Cody

John Cody gravatar imageJohn Cody ( 2016-01-14 07:36:08 )

Dear Sir, thanks a lot for your help and support with all valuable information that you are providing me here. I have one more enquiries regarding a chlorine test by Hach DR 2800 Spectrophotometer. Is it the proper one to test chlorine in water and also to find breaking point chlorination? I would be grateful to you. I look forward hearing from you.

Ban Qasim gravatar imageBan Qasim ( 2016-01-15 13:33:22 )

Hi Ban

Can you have a look at

The kit you specified in your mail would do the job, as would a pool tester from Palintest or Wagtech

Obviously the type of equipment that you choose will depend on cost an the purpose of your specific application.

I get the impression from your mails that you have a specific question that I am not answering? If you specify what the purpose of the information that you are looking for is then perhaps I could be more specific.




Dear Sir, thank you again for replying and your cooperation, yes you are correct, the specific things that I am confused about are listed down below: 1. Through my research work I must observe the role of break point chlorination in 3 types of water (i. Ground water ii. Rain water iii. Hill water). 2. The water is in raw samples and I have to add chlorine to it step by step and increase dose to observe the reaction of chlorine with raw water, the chlorine will interact with organic and non organic materials in water and the factors that affecting the chlorine are : Temperature , PH , NH3 , and contact time to reach break point chlorination, and as I understand I should calculate Total and Free Chlorine to get Chlorine residual which is important for water disinfection. However my confusion is regarding the methodology that I should use and follow. I ...(more)

Ban Qasim gravatar imageBan Qasim ( 2016-01-23 09:28:25 )