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Method Statement 3.6 - Disinfection of Mains

Document Information

Field Value
Document Reference MS 3.6
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 Disinfection of 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

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

Procedure:

  • 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

Equipment 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)

Procedure:

  • Rinse two 20 mm glass phials with water to be sampled. Fill one to the 20mm mark with sample water. Wiping away any excess on the outside of the cell with a clean tissue and place into the LH cell of the Comparator.
  • Open the foil wrappers tablet dpd no 1 and add two No1 tablets to the second glass phial.
  • Fill the second glass phial to the 20mm mark with the water to be sampled and gently mix using the tablet crusher to dissolve the tablets.
  • Allow standing for 5 minutes for maximum colour development. Take a reading by rotating the disc until it matches the colour of the glass phial containing the two no1 tablets this reading is the free chlorine residual reading evident in the top right corner of the comparator, in milligrams per litre (parts per million).
  • Two more tablets dpd no 3s are added to the phial crushed and stirred wait a further 5 minutes and take a second reading this is the chlorine total residual reading.
  • The free and total chlorine level should be representative of the supply main.

Bacteriological Sampling of new mains

In the absence of any procedures from the adopting water company, the following procedure shall be followed:

  • In date sample bottles are to be used along with the local water authority paperwork.
  • After the main has been flushed and tested for low chlorine content (equal to the supply main), wearing disposable gloves first disinfect the sample tap and equipment remove the sample bottle lid (ensuring the lid cannot be contaminated by drips, splashes or dust) and place the sample bottle (type as required by the water authority laboratory) in the centre of a steady stream of water. Care shall be taken to ensure that nothing touches the open neck of the bottle i.e. skin, clothing.
  • Once the sample bottle is full the cap shall be secured into place taking care not to touch the inside of the bottle and the sample identification sheet completed the sample bottle should be placed in a clean cool box.
  • Each sample shall be logged in at the collection point ensuring that each bottle is labelled with sufficient information. Date/Time, chlorine residual free and total readings and the exact sample location and address.
  • If the main fails the test, the Team Leader shall follow instructions recommended by the laboratory to re-sterilise and flush the main. The sample must reach the laboratories within 6 hours
  • The sample results shall be sent to the nominated manager for action.
  • The team leader shall ensure that the sample numbers are identified on each section of main sampled.