Distribution Operation Maintenance Strategy. Swabbing Mains

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1 Purpose of? To provide Guidance, Knowledge and Understanding of the Water Supply Procedures and work instruction, to be used when Swabbing and Flushing Mains after Installing, Commissioning New Mains, or as part of Mains re-habilitation programme. Note: This toolbox Talk is not exhaustive and you must refer to Scottish Water s Water Supply Hygiene Code of Practice or other DOMS documentation i.e. DOM-WN-PRC Work Instruction Commissioning / Decommissioning Water Mains & DOM-WN-PRC MAINS SWABBING Why? To safeguard the quality of water to customers and to demonstrate that Scottish Water and their partners apply due diligence to the distribution network. Swabbing and Flushing Mains is an important process for Installing and Commissioning any new main or Re-habilitation work on existing mains as part water distribution network for maintaining the quality of water supplied. The main must be swabbed and flushed to remove any material which may have entered into the main during installation or Re-habilitation of the main. Who will be affected? All Staff who undertake the installation of new mains or Re-Habilitation of existing mains on the water distribution network. When do I need to Swab the main? Swabbing and Flushing Mains is required as per Scottish Water s HCOP appendix 1 and DOMS for (a) All New Water Mains Installations. (Prior to Pressure testing stage) (b) Any Rehabilitated Water Mains (incl. replacement by pipe bursting/slip lining, swagging or relining). (c) Swabbing as part of the Full Disinfection process using tablet 20mg/l or 50mg/l instead of dosing units on new or existing mains. All the activities above must have swabbing and flushing undertaken to prevent any actual contamination of the new main or existing water network. Swabbing Method Statement All new mains installed must be pressure tested to ensure that there are no leaks on the section of main to be disinfected which may allow ingress of contaminated material into the main or escape of highly chlorinated water. The main must be swabbed and flushed to remove any material which may have entered into the main during installation etc. Water for swabbing, testing and disinfection of any new and or existing water mains and structures shall be taken from existing supplies wherever possible. NOTE temporary pumps may be needed to be made available if the pressure and or the flow is inadequate. The arrangements used to take water from the existing network to fill, swab, and test or flush the new main must be sufficiently protected to ensure that no backflow or back siphoning to the network can take place. This will require the fitting of a double check valve assembly if a direct connection is being used. Prior to any swabbing approval should be sought from Scottish Water for feed water to be made available for the planned swabbing operation. Page 1 of 5

2 All debris and dirt shall be removed using a single new clean hard grade foam swab for the mains less than 200mm in diameter. For larger mains two foam swabs in tandem shall be used. In both cases the swabs shall be soaked in a 20mg/litre Hypo-chlorite solution before insertion and propelled through the main at a low speed using mains pressure ideally the swab velocity should be between.8 and 1.5 m/sec. Swabbing shall be continued until the flushing water runs clear and that no erosion, silting or contamination occurs in the receiving water course into which it is discharged. The swabbing must be repeated as many times as necessary until the water in front of the swab is clear with no particulate material present (a turbidity tube/meter will aid inspection). The final swab should not have any ingrained material present or be discoloured. This process shall be carried out in one sequence of operations on any length of the main. The smell of the swab should also be noted if any unusual odour is present the swabbing should be repeated until no unusual odour is apparent. This is particularly important for ductile iron mains. Any unusual odour should be reported to the public health team. Any debris that is expelled should be reported for feedback to the contractor / company pipe layers and operation staff. Pushing a swab through the main by water pressure is not appropriate for water mains above 200mm (8 ) diameter. This results in too much water passing the swab and having to be pumped out of the next exit excavation with the potential for re-contamination. The maximum length that should be swabbed and disinfected using this method, in one pass, is, 200m unless agreed by Scottish Water. Also, more importantly from a disinfection point of view, water passing the swab is continuously diluting the 20mg/l chlorine leaving insufficient time for adequate disinfection and or possible contaminations of a water course. Swabbing shall generally be carried out in separate lengths not greater than 1km for pipe diameters under 200mm unless otherwise approved by Scottish Water Staff. All swabs must be recovered and accounted for from the retrieval point on the main being swabbed and immediately resealed, ensuring that no deleterious material enters the main. It is important to ensure that the complete swab or all its components part are recovered after swabbing operation to avoid problems on particularly existing mains with choking of metres, ball valve, ferrules connection etc. at a later date. In the event of a swab failing to emerge from the main, the swab shall be located and removed together with any obstruction. The swab is removed and the main resealed as soon as the possible after removal ensuring that no deleterious material enters the main. it is important to ensure that the complete swab or all its components part are recovered after swabbing operation to avoid problems on particularly existing mains with choking of meter s, ball valve, ferrules connection etc at a later date. Page 2 of 5

3 Foam Swabs In general, swabs shall be soft absorbent hard backed foam, medium grade with a minimum diameter of 25mm greater than the nominal internal diameter of the pipe, up to and including 200mm pipe work and a minimum of 50mm greater for pipe work of nominal internal diameter greater than 200mm In general swab size used should be the next size up from the main to be flushed. E.g. 100mm main minimum 125mm swab up to a maximum of 150mm swab 150mm main minimum 175mm swab 250mm main minimum 300mm swab Swabs shall only be used once and then disposed of appropriately. Swabs should be kept stored in sealed plastic bags prior to use, prior to insertion the swab should be soaked in 20mg/l hypochlorite solution and sealed in a plastic bag for transport or until ready to use. Key points of 1 Water for swabbing, shall be taken from existing supplies wherever possible. 2 The filling of mains for testing swabbing etc should only be done through 25mm hydrant standpipes fitted with double check valve. 3 Close all connections to the main being swabbed. 4 Swabs should be kept stored in sealed plastic bags prior to use, prior to insertion the swab should be soaked in 20mg/l hypochlorite solution and sealed in a plastic bag for transport or until ready to use. 5 Swab insertion points should be thoroughly disinfected using 250mg/l solution and the insertion point resealed immediately. 6 The correct type/ size of swabs are used for the main being swabbed. 7 Swab speed, Ideally the swab velocity should be between 0.8 and 1.5 meters/second 8 The swabbing must be repeated as many times as necessary until the water in front of the swab is clear with no particulate, material is present. 9 All swabs are accounted for and removed from the main. 10 When the swabbing process is complete: Disinfect & Sample Main as per HCOP. This TOOLBOX TALK and has been designed to provide information and guidance. It is NOT a substitute for information held in DOMS documents that can be viewed on the Intranet. IT IS ESSENTIAL THAT REFERENCE IS MADE TO THESE DOCUMENTS TO OBTAIN FULL GUIDANCE. Page 3 of 5

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