Method Statement 3.6.1 - Super Chlorination of Water Mains¶

Document Information

Field	Value
Document Reference	MS 3.6.1
Issue Number	2
Issue Date	1 June 2026
Next Review	1 June 2027
Prepared By	Sean Ashton
Approved By	Aaron Mason, Director

Task No	Task	PPE to be worn	Plant required	Reference is made to the Team Pack risk assessment
3.6.1	Super Chlorination of Water Mains	As defined by on-site risk assessment.	Works vehicle and hand tools. Disinfection and swabbing equipment. Pumps. Bower for disposal	N/A

Hazards¶

Manual Handling
Stored Energy in the Pipe
Defective Plant and Equipment
Towing Techniques
Incorrect Loading
Insufficient Space

Environmental Impacts¶

Noise
Dust
Light From Night working

Customer Impacts¶

Noise
Dust
Access

Training Required¶

As Per specific contract training matrix

Additional Plant and Equipment¶

Winch/Tractor Winch
Pipe Bursting/Slip Lining Rig
Mechanical Excavator

Permits Required¶

As Per specific contract specifications and Client requirements.

COSHH Assessment Required¶

2 Stroke Oil
Diesel/Petrol
Line Marker Spray
Lubricating Spray

Sequence of Operations and Control Methods¶

This Method Statement outlines procedures to be undertaken in order to disinfect apparatus and water mains. It will include requirements for swabbing, dosing and the disposal of chlorinated water and also covers the bacteriological sampling of new water mains.

Pre-Chlorination Procedure¶

Yard set up¶

The dedicated pre-chlorination yard will be established and enclosed with fencing panels. The enclosure will be locked when unmanned.
When the yard has been set up, it must be inspected by the Client where required before being put into operation.
Discharge consent for the yard will be obtained from the Environment Agency.

General requirements are:

Existing supply of potable water Clean hard standing
External lockable cage for chlorine storage with relevant signing for corrosive materials
All fittings to be stored off the ground
External lockable cage for chemicals

Pipe Delivery and Storage and recording¶

Coils will be delivered to the yard direct from the manufacturer.
Coils will be visually inspected for defects (e.g. scoring; kinks; damaged packaging etc.)
Where defects are observed the manufacturers are notified and the coil, if unloaded, put in quarantine marked area away from usable pipe works for collection / disposal.
All pipe materials and coils to be used on the contract must be stored in an approved manner in accordance with the manufacturer's recommendations.
End caps must be kept on all coils in the storage yard
The secure area should be subdivided into three appropriately signed areas:
- Area One for untreated pipes (storage area).
- Area Two for pipes undergoing treatment where each coil will be given a unique identifying number recorded in the Quality Control Register.
- Area three for treated pipes. Labelled with an in date and successful sample result with a sample pass date sticker attached to both ends of the coil.

The Quality Control Register¶

Used to record:

All unique pipe numbers
Chlorination date
De-chlorination date
Sampling dates including sample results pass /failed. Date
Dispatched to site

The Quality Control Register 2010 example:

Pipe number	Chlorination Date	De-Chlorination Date	Pipe labeled [Tick Box]	Sample date	Sample result Pass/Fail	Sample run out date	Dewatered and moved to site date	Operative Name
0001

Disinfecting solution for spraying fittings¶

The solution is prepared by mixing one part chlorine or one tablet (sodium hypochlorite 15% solution) to ten parts of water. The pre-chlorination technician shall wear protective gloves and goggles at all time when handling chlorine tablets or hypochlorite solution
The solution will be added to a spray bottle labelled with an adhesive sticker indicating that it contains 1000ppm of Hypochlorite solution.
A fresh solution shall be made up daily.
Consider wind direction when spraying

Pre-Chlorination Process¶

The coils for pre-chlorination are transferred from the storage area to the treatment area.
Flange adaptors are sprayed with the Hypochlorite solution a swab is placed into the pipe undergoing treatment, end the pipe and set the length of the pipe extracted and then the second flange adapter is fitted to the pipe end and the pipe is chlorinated to a minimum of 20ppm dependant on supply water quality and allowed to stand for 24 hours. The concentration of chlorine is checked with a Palintest digital chlorimeter via a sample point fitted to the test plate on the flange adapter.
The calibration of the chlorimeter will be checked weekly and the results recorded on the calibration register.
After the standing period of a minimum of 24 hours the coil is flushed with mains water and the chlorinated water nullified by Sodium Thio Sulphate the flushing is deemed to be complete when the water at the outlet end of the pipe is the same free and total residual readings as the supplying main.
Regular comparator readings are taken to ensure that the chlorine level of the discharged water from the de-chlorination unit stays below 0.02ppm.
After flushing the coil is brought to mains pressure and sealed and allowed to stand for a further 24hrs.

Bacteriological Sampling¶

After the standing period bacteriological samples will be taken by the Client.
The sample point on the coils will be sprayed with the disinfecting solution.
The operative will take the sample and complete the entry in the pre chlorination register.
The sample container will be marked with the coil reference number/date and passed to the Clients laboratory for testing.

Storage and Dispatch¶

If and when the sample test is successful the coil must be used within 7 days of chlorination, if a sample test fails the entire procedure is to be repeated.
If the 7 day period is exceeded the coil must be reflushed and will then last a further 7 days. Following the second seven days the pipe is deemed to have gone 'out of date' and must not be used before it is re-chlorinated.
It is usual to complete the above operation for a number of pipes at the same time by interlinking the coils with clean 1" hoses. DISCONNECT prior to sampling as one coil failure will jeopardise all connected coil results.
It is usual to chlorinate extra coils to accommodate any coil failures, under no circumstances are coils to leave the yard without a test pass.
Immediately prior to transportation to site all water must be purged from the coil by removing the end plate inserting a foam swab soaked in disinfecting solution (1 part chlorine to ten parts water) replacement of the end plate having first sprayed the fitting with the disinfection solution remove the extreme end plate and transmit the swab through the length of the pipe coil to purge it of water using a compressor fitted with an in line oil filter and via the 25mm inlet on the 1st end plate.

Pipe end Label¶

Pipe number ................
swab fill chlorinate date..................
De-chlorinate date ......................
Sample date ..............................
Sample pass date ........................

Chlorination Method of Works¶

Planning¶

Chlorinating and the de-chlorination of Water Mains is a requirement implemented to reduce the risk of contamination into the Clients networks and water courses. Chlorination alone should not be solely relied upon to prevent contamination. It is essential to incorporate good working practices, operational precautions, sampling and hygiene awareness. Both the client and contractors should understand and appreciate the need to maintain water quality standards. Only personnel who have a valid Energy & Utility Skills Register (EUSR) National Water Hygiene Card will be permitted to come into contact with live water mains providing they are medically well and without infection.

Prior to any works commencing the following procedures must take place:

A site specific risk assessment must be conducted and briefed to all operatives, site management & visitors associated with the planned works.
The method statement must be reviewed and briefed to all operatives as above.
Operatives must be adequately trained for the planned works.
Arrange a meeting with the Client's Network Liaison Office to plan the valve operations necessary for obtaining potable water supplies.

Pressure Testing¶

Pressure testing follows pipeline construction, sometimes mains are tested in sections. Only portable mains water shall be used for testing new mains. Until new mains are disinfected and approved.
The general planning and preparation of pressure testing can be found within the WRc guideline IGN (information guideline note provided by Water.org.uk)

Swabbing & Chlorination¶

The following procedures follow on from a successful pressure test result:

Swabbing and flushing of new main¶

All new mains must be swabbed and/or flushed prior to disinfection/chlorination. The provision of adequate swabbing locations and retrieval points shall be considered on site if not already considered during the design process. It is possible that large diameter mains may be swabbed manually.
The slightly over sized, soft foam swab must firstly be soaked in Hypochlorite Solution prior to insertion into the new main. The pipeline must then be recharged ensuring that the swab has not moved from its insertion location.
Prior to chlorination process all pipe work components, equipment and tools must be cleaned before use or assembly. Any materials, components and equipment that could potentially cause contamination must be disinfected using Hypochlorite Solution. If immersion is not practical then a spray solution shall be used to disinfect. After disinfecting is complete all equipment must be rinsed or flushed with mains water to prevent any excessive corrosion.
No new main shall be connected into the network supply until the water from the designated sampling point has met water quality criteria including turbidity and coliform tests.

Laboratory Criteria:¶

Mandatory Laboratory pass or fail applies
Turbidity shall not exceed 4 NTU (Nephelometric Turbidity Units)
If sample is unusually discoloured it shall be flushed until acceptable. If sample discolouring is severe main must be re-flushed/sampled.
Not more than 10(colony) coliform organisms shall be detectable in a sample after 48 hours incubation.
If an unacceptable level of coliform is detected in a repaired main sample a repeat sample must be taken. If repeat samples also fail the main must be flushed and re-chlorinated with immediate effect.

Soaking of main in chlorine solution¶

All new mains must be chlorinated to a minimum of 20ppm of chlorine and then left to soak for a minimum period of 24 hrs. Only then can the main be flushed with mains water to achieve chlorine residual equal to incoming mains water.
In order to ensure correct chlorine levels are introduced into the new main a dosing unit can be used utilising a Vernon Morris dosing system or similar. This system is connected to new mains via hoses. The required Sodium Hypochlorite solution is set and a pulse pump injects at the required rate.
All manufactures instructions and guidance notes must be read and understood before using the dosing system to dose new water mains. A sample point on the outlet of the dosing system ensures adequate levels of dosing are achieved. Sampling guidance for chlorine residuals and testing must be read and adhered to.

Disposal of chlorinated water¶

Estuaries – De-chlorinated water is disposed into surface water sewers which discharge into estuaries or the sea then there is no requirement for further de-chlorination. A hazardous atmosphere must be avoided within the sewers.
Foul sewers - Foul or combined sewers can be utilised for discharging following de-chlorination although care must be taken to ensure the rate of discharge does not affect the operations of storm sewer overflow.
Watercourses - Where disinfected water is discharged into watercourses it is mandatory that a chlorine concentration in excess of 0.02ppm must not be received 50m downstream from the discharge location. Discharge to the head of a watercourse must be avoided as these areas are normally spawning pools. Discharge into ditches can help soak up chlorine providing the above limit of chlorine is not exceeded.
Large volume disposal - The Environmental Agency should be contacted when disposing of large volumes of chlorinated water. The Environmental Agency will provide discharge consent for proposed discharge locations where feasible.

De-Chlorination¶

Where disposing of chlorinated water is not acceptable then de-chlorination must be applied. This may be carried out using Sodium Thio-sulphate (liquid or tablet) or sodium bisulphite.
1ml of 1.8% Sodium Thiosulphate solution will remove 5mg/l (ppm) of chlorine/litre.
Or
18grams of Sodium Thiosulphate will remove 5mg/l (ppm) of chlorine/m3 of water.
De-chlorination is carried out using a de-chlorination unit. In order to de-chlorinate disinfected mains de-chlorination tablets known as 'D-Chlor' tablets should be used or liquid sodium thio sulphate.
D-Chlor tablets are inserted into the de-chlorination unit, the unit must be kept topped up with D-Chlor tablets at all times until water leaving the 'De-Chlor' unit satisfies Water Quality criteria as discussed previously whereas Liquid sodium thio sulphate is injected into the outflow.

Sampling¶

After the chlorinating, de-chlorination and flushing processes are completed a sample must be taken. All new mains are required to be analysed for chlorine residual, coliforms, turbidity, taste, odour and appearance.

Sample procedures must be strictly adhered to and all instructions and guidance associated with sampling kits must be read and understood.

Commissioning¶

Checklist to be completed by Supervisor, Agent or Project Manager

Alternative Method - Testing Water for Testing, Swabbing and Disinfection¶

Water for testing, swabbing and disinfection shall be taken from the existing water network.

On completion of the pressure test a foam swab shall be passed through the main for final cleansing a sufficient number of times to achieve clear wash water.
Swabs shall only be used once and then disposed of appropriately.
Check whether the water company or client requires the swab recovery to be witnessed.
The disinfection of mains shall be carried out in accordance with the adopting water companies Disinfection Code of Practice.

Disposal of water from cleansing, testing or disinfection¶

Water used shall be made safe by de-chlorinating prior to disposal using sodium thio-sulphate.
Facilities shall be provided for the removal and disposal of water when required. Discharge to Sewers shall require the appropriate consent.
Inform Principle Contractor of hazards associated with chlorination and explain the de-chlorination process.

Testing of water samples to determine High chlorine level¶

Equipment required to measure high total chlorine:

Lovibond 2000 TK10 Comparator
20 mm phials
Comparator discs (low range and high range) capable of reading low 0.02ppm to 5ppm and high 5 to 200ppm

Process:

Rinse two 20mm phials with water to be sampled. Fill one to the 20mm mark with sample water
Wipe away any excess water on the outside of the phial with a clean tissue. Leave a few drops of sample water in the other phial
Place the full glass phial into left hand side of the comparator
Lightly crush 1 chlorine high range tablet and 1 acid tablet (use crush stick provided)
Fill the phial to the 20mm mark with the water to be sampled and gently mix to dissolve the tablets.
Let stand for 5 minutes for maximum colour development. Wipe away excess water from the outside of the cell with a clean tissue.
Place the cell in the right hand side of the comparator, face a bright light (north) and match the colours. The HR disc reading, evident in the top right hand corner of the comparator, represents the total chlorine residual in milligrams per litre (ppm Parts per million)
The total chlorine level in the pipe being sterilised should be not less than 20ppm (a medium yellow)

To measure low total chlorine the following equipment is required:

Lovibond 2000 TK Comparator
20mm cells
Comparator discs 3/40 E (covers total chlorine levels) 0.02-0.3ppm or 3\40F (covers total chlorine 0.2-0.8ppm)