Why understanding the chlorination process is vital

As an Expert Witness one of the issues I come across regularly is disinfections with chlorine incorrectly carried out. This can lead to numerous problems such as an inability to disinfect a system or even loss of a legal case on grounds of failing to correctly follow specifications.

There is a lot of published work on carrying out a chlorination, so it is frustrating when a chlorination does not match the guidance it purports to follow.

Firstly, it is important to ensure that you use the correct specification and not one that has either been superseded or doesn’t exist in the first place. In the UK this means not saying that the clean is carried out to BS6760 (superseded) or ACoP L8 (L8 doesn’t contain a specification for chlorination).

Once you have identified the specification that you are using it is then important to follow it. The number of times it has been claimed that a particular process complies with a standard when in reality it doesn’t is saddening as the client doesn’t always know and relies on the company carrying out the clean to be “experts”.

Interestingly, HSG274 Part 2 states:

“2.135  The disinfection of a water system is normally based on chlorine being dosed at 50 ppm for a minimum contact period of one hour, at the end of which the concentration should not be less than 30 ppm free residual chlorine. However, lower concentrations and longer contact times are considered acceptable, as set out in BS 8558.”

BS8558 2015 states:

“ Exceptionally, disinfection of the water system may be undertaken: a) for both the hot and cold water system, as described in PD 855468; and b) for the hot water system only, by thermal disinfection procedures (see the HSE’s Approved Code of Practice L8, Legionnaires’ disease – The control of Legionella bacteria in water systems [26] and HSG274 Part 2 [30]).”

Sometimes then you must dig deep to find the correct “standard” (PD855468 is not a standard). PD855468 contains the “specification” to use therefore and this is where the fun starts.

Firstly, PD855468 repeats the level in HSG 274 Part 2:

“ The disinfection of a water system inside a building is normally based on chlorine being dosed at 50 mg/L (50 ppm) for a minimum contact period of one hour, at the end of which the concentration should be not less than 30 mg/L (30 ppm) (or 40% drop in concentration) free residual chlorine.”

It then starts to get interesting as it goes on to say:

“ Sodium hypochlorite solutions deteriorate over time and this deterioration is proportionate to the storage temperature. As a result, material that has exceeded its shelf life recommendations needs to be added in greater quantities to achieve the desired concentration of available chlorine. This can have the undesired effect of increasing the pH of the receiving water to the extent that the efficacy of the dosed chlorine is significantly reduced. 15% sodium hypochlorite used for the cleaning of equipment and plant should not therefore be older than three months after the date of manufacture.”

Stock rotation of sodium hypochlorite is therefore an issue and I very rarely, if ever, see the batch number or date of manufacture of the sodium hypochlorite on a chlorination certificate.

PD855468 goes on to state:

“ Chemical disinfection involves adding an agent such as chlorine or a similarly effective disinfectant, drawing it through to every outlet, then closing the outlets and allowing the agent to remain in contact for a suitable period (known as the contact time).”

The contact time is given as:

Contact time = 3000/free chlorine.  

Where contact time is minutes, free chlorine is mg/l.

This requires the correct test to be carried out; free chlorine (not total chlorine)

Further PD855468 states:

“It is generally accepted that in order to achieve effective disinfection a concentration of available free chlorine in the form of HOCl is 25 mg/L. This is achieved with a dose of 50 mg/L available free chlorine at a pH around 7.6. There is a consensus of opinion that concentrations of HOCl below 10 mg/L are less effective against microorganisms such as protozoa which can be refractory to disinfection if ineffectively applied. This is equivalent to a dose of 50 mg/L Cl at a pH in the region of 8.0 to 8.5.”

All straightforward so far but then comes the sting in the tail and this is the step that is rarely carried out but is critical to the whole process:

pH vs chlorine

This chart is describing the effect of pH on the addition of sodium hypochlorite (NaOCl) to water. Sodium hypochlorite dissociates into two components when added to water, Hypochlorous acid (HOCl) and hypochlorite (OCl) of which HOCL is the primary biocide. This is seen by the following equations.

When a hypochlorite is added to water, it also reacts to form hypochlorite ion and hydroxide:

These forms can exist together and the relative concentration of each depends on the pH of the solution and not on whether chlorine gas or bleach was added:

Combining the information above it means that at pH7.5 only half of the expected biocide (HOCl) is available so 50ppm expected is really only 25ppm. This is described in PD855486 as being acceptable when using 50ppm but when extending the time and reducing the chlorine level there is a danger you reach the level at which chlorine can no longer kill protozoa and it is within these protozoa that the bacteria Legionella grow and multiply.

Consequently, the importance of system water pH cannot be underestimated, and it is this vital piece of information and an amendment to the dosage rates that is so often missed on disinfection certificates.

So next time you read a disinfection certificate check for the following critical information:

  1. System water pH
  2. Appropriate amendment to dosage rate of sodium hypochlorite due to pH
  3. Contact time
  4. Free chlorine level at the start of the contact time
  5. Free chlorine level at the end of the contact time
  6. Addition of extra sodium hypochlorite or an extension of time to compensate for consumption of free chlorine during the process
  7. The correct specification quoted

Hopefully this will the reduce the number of occurrences where following a clean Legionella, or other bacteria, reoccur within a short space of time caused by a poor quality clean and disinfection.

Please be aware that there are other specifications available that are not discussed above.

If you would like to discuss the issues raised above with an expert in water treatment then contact Collaton Consultancy for advice and support. Email us on general@collatonconsultancy.com

Collaton Consultancy Limited are a specialist consultancy offering technical advice on Legionella, Pseudomonas and water treatment. We also act as Authorising Engineer (Water) in healthcare sites and as Expert Witnesses. We also offer both online and face to face training in a host of subjects.

More information can be found on the following websites:





Collaton Consultancy Limited

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