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All of the chemicals added to water before usage for humans

Hayley Finnegan made this Environmental Information Regulations request to Severn Trent Plc
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Dear Severn Trent Plc,


Yours faithfully,

Hayley Finnegan

CustomerEIR, Severn Trent Plc

ST Classification: UNMARKED

Dear Hayley Finnegan

Thank you for your recent request for information, I'm contacting you to provide an update on your enquiry.

Our response is still going through the governance and assurance processes therefore, I'm giving notice that we require a 20 day extension under the Environmental Information Regulations 2004 act.

Thank you for your patience in this matter, we will be in touch with further information in due course.

Reporting & Compliance Team
Group Compliance & Assurance
General Counsel
Email:[email address]

show quoted sections

McClansburgh, Jo-Avon, Severn Trent Plc

ST Classification: UNMARKED

Dear Hayley Finnegan

Thank you for your recent request for information, please find below our
response to your enquiry.

Chemicals used in water treatment


In the UK our drinking water is of the highest standard amongst the best
in Europe, as such we are heavily regulated by the Drinking Water
Inspectorate (DWI), whose overall governance is to protect public health
and ensure water quality is acceptable to customers. This is met through
ensuring that drinking water is ‘wholesome’ and thus defined in the law by
a very strict set of standards that were originally set in Europe as part
of the Drinking Water Directive 1998 together with national standards set
to maintain the high quality of water already achieved.


In order for water companies to comply with these standards and provide
‘wholesome’ drinking water the raw water must go through a vigorous
treatment cycle before being distributed to our customers. Part of this
treatment includes the addition of chemicals which are properly managed by
the water companies and all chemicals added to the water must comply with
Regulation 31 (Application and introduction of substances), of the Water
Supply (Water Quality) Regulations 2016 as amended
[1]https://www.dwi.gov.uk/water-companies/l... .


The chemicals that we add to the water will vary dependent on the raw
water quality and the treatment process will remove certain chemicals
before being distributed to our customers. Therefore, the focus of this
response is primarily based on chemicals that are added to the final
water. The chemicals in question fall into two categories;


 1. Addition of chemicals used in the treatment process, disinfection —
these chemicals are intentionally added.
 2. Substances that are added to the water to prevent leaching from
materials used in distribution or plumbing, or that arise from the
corrosion products of pipes.

Chemicals that are added to the final water for the purpose of
disinfection are chlorine gas and sodium hypochlorite. Chlorine is the
most used primary disinfection that removes and/or inactivates harmful
microorganisms (bacteria, viruses, and protozoa), that are still present
in water following upstream treatment. Chlorine is also used to ensure
residual chlorine is maintained within the distribution system. Monitoring
of the chlorine residual is frequently monitored within the distribution
through random sampling programs at customers properties, designed in
compliance with the Water Supply (Water Quality) Regulation 2016 as


In conjunction with the disinfection process at various of our water
treatment works we carry out the process of de-chlorination. The purpose
of this process is to ensure to reduce the chlorine residual leaving the
works to ensure the taste and odour of the supplied water is acceptable to
customers, maintain stable chlorine residuals within the distribution
system and to minimise any other factor that could affect the water
quality in the distribution system. In order to achieve this sodium
bisulphite or sulphur dioxide (gas) are added to the final water.


To ensure that water quality is of the highest quality in the distribution
system, disinfection must never be compromised, to do so would be a breach
in Regulation 26.


Orthophosphoric acid is added to the final water at the majority of our
sites with the sole purpose to coat the lead pipework within the
distribution system; to prevent any leaching into the drinking water.


Hexafluorosilicic acid or sodium silico fluoride is added to the final
water in certain parts of our distribution system to increase fluoride for
the purpose of improving dental health; areas to dose fluoride are
pre-determined by the local health authorities.


The pH of the water has a significant impact on the clarification and
disinfection processes within water treatment therefore chemicals are
added to the water for pH adjustment. Dependent on the source water but
either caustic soda, calcium hydroxide, sodium carbonate, calcium
carbonate (lime) or sodium hydroxide are added after disinfection before
being distributed into supply. This ensures that pH meets the required
final water standards and makes certain that water is less aggressive
within the distribution system.


The following table summarises all the chemicals mentioned above that are
added to the final water before being distributed to our customers; please
refer to Table 1.


Table 1: Summary of chemicals


|Chemical |Process requirement|Achievement |
|Chlorine gas |Disinfection |Removal of harmful |
| | |microrganisms still present |
|-----------------------|  |in the water and maintain |
|Sodium hypochlorite | |chlorine residual in the |
| | |distribution system |
|sodium bisulphite |De-chlorination |Maintain stable chlorine |
|-----------------------| |residual in distribution |
|Sulphur dioxide (gas) | |system |
|Orthophosphoric acid |Phosphate dosing |Reduce the risk of lead |
| | |within the distribution |
| | |system |
|Hexafluorosilicic acid |Fluoride dosing |Improvement to dental health|
|-----------------------| | |
|sodium silico fluoride | | |
|caustic soda |pH correction |ensures we meet the standard|
| | |of supplying water between |
| | |pH 6.5 and 9.5, prevents our|
| | |water being aggressive and |
| | |dissolving our distribution |
| | |assets and customers |
| | |pipework , prevents water |
| | |constituents from |
| | |precipitating out and |
| | |causing aesthetic issues. |


Kind Regards

Reporting & Compliance Team
Group Compliance & Assurance
General Counsel
Email:[email address]






Severn Trent Plc (registered number 2366619) and Severn Trent Water
Limited (registered number 2366686) (together the "Companies") are both
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1. https://www.dwi.gov.uk/water-companies/l...

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