What is pipe chlorination and pressure testing?
Pipe chlorination and pressure testing are the two commissioning techniques carried out at the end of a new water main installation, a service connection, or a pipe rehabilitation job. Chlorination disinfects the inside of the pipe so the water leaving it is fit to drink. Pressure testing confirms the pipe holds its rated pressure without losing water at joints, welds, or fittings. On a UK water main, both have to pass before the water company will allow the main to be connected to the live network.
Although the two techniques measure different things, they are almost always delivered together as a single commissioning workflow. Contractors running self-lay water schemes, main-laying jobs, and slip-line rehabilitations treat them as one package because the adopting water company inspects one pack of evidence at the end: pressure test logs, chlorination certificates, bacteriological sample results, and as-laid drawings, submitted together.
How pipe chlorination works
Pipe chlorination is the process of filling a new or rehabilitated water main with a dosed chlorine solution, holding it for a set contact time, flushing it out, and then sampling the water to confirm that the pipe is free of bacterial contamination before it goes into service. The dose, contact time, and sampling regime for new mains and new service connections are set by the water industry’s Principles of Water Supply Hygiene TGN 2, with individual water companies layering their own requirements on top.
When chlorination is required
Chlorination is required any time a water main or service pipe is laid new, repaired in a way that exposes the internal surface, or taken out of service for long enough that biofilm or bacteria could establish. The common triggers on a utility civils job are a new water main laid under a self-lay scheme or main-laying contract, a slip-lined or refurbished main coming back into service, and a new service connection above the water company’s threshold diameter or length. Most UK water companies require chlorination on service pipes 50 mm internal diameter and above or longer than 50 metres.
Dose, contact time, and sampling
The default disinfection method in TGN 2 is a fill-and-stand approach. The main is filled with potable water dosed to 20 mg/L free chlorine, held for a minimum of 16 hours, then flushed until the chlorine residual matches the incoming supply. An alternative spray-disinfection method uses 1,000 mg/L applied to the internal surface. After flushing, water samples are taken at appropriate points along the pipe and sent to a UKAS-accredited laboratory for microbiological and chemical analysis. A pass requires zero E. coli, zero coliforms, turbidity below 2 NTU, pH between 6.5 and 9.5, and no detectable taste or odour. Certificate validity varies by water company, typically between 14 and 30 days from the date of the satisfactory sample.
How pressure testing works
Pressure testing a water main is a hydrostatic test: the pipe is filled with water, air is bled out, and the internal pressure is raised to a defined test pressure above the main’s normal operating pressure. The pressure is then held for a set period, with a calibrated data logger recording either pressure decay (for non-viscoelastic materials) or the creep-analysis curve (for polyethylene). The test confirms that every joint, weld, and fitting in the test section holds under load without losing water beyond the permitted allowance.
Material-specific test methods
The UK has two current test methods for hydrostatic pressure testing, and the choice depends on the pipe material. Polyethylene and PE barrier mains exhibit viscoelastic creep, meaning they stretch slightly under pressure before stabilising, so a standard water-loss test gives a false fail. For PE mains the current UK specification is WIS 4-01-03 Issue 1 (March 2024), published by the Water UK Standards Board, which replaced the earlier IGN 4-01-03 for polyethylene. For ductile iron, steel, and glass-reinforced polyester (GRP), the water-loss method in Section 11 of BS EN 805 applies.
What a pass looks like
A pressure test pass depends on the method, but the principle is the same. For PE mains under WIS 4-01-03, the test compares the slope of the pressure decay curve at defined intervals and passes if the ratios stay within specified tolerances and air content is below the permitted limit. For non-viscoelastic mains under BS EN 805, the water added to maintain pressure over the test period must stay below the allowable loss rate for the pipe material, diameter, and length. In both cases the test pressure is set at 1.5 times the main’s maximum design pressure, per the Water Supply (Water Fittings) Regulations 1999 and the adopting water company’s specification.
When chlorination and pressure testing are required
The requirements for chlorination and pressure testing are set in UK law by the Water Supply (Water Fittings) Regulations 1999, with drinking water quality enforced by the Drinking Water Inspectorate under the Water Supply (Water Quality) Regulations 2016. Every new water fitting has to withstand an internal pressure of at least one and a half times the maximum pressure to which it will be subjected in service, and every new installation has to be disinfected before it is connected to the network. In practice the individual water companies add their own specifications on top, covering diameter and length triggers, sampling regimes, certificate validity, and the format in which evidence has to be submitted.
The common trigger points on a utility civils scheme are: a new water main laid for adoption by a water company, a new service connection above the company’s diameter or length threshold, a slip-lined or rehabilitated main returning to service, a main returning to service after a prolonged shutdown, and any pipework where contamination is known or suspected. Developers working with Anglian Water and the other regional water companies will find their specific triggers and evidence requirements set out in the water company’s developer guidance.
The commissioning workflow
Chlorination and pressure testing are rarely the starting point of a job. They are the sign-off steps at the end of a longer workflow, and on a utility civils scheme they usually sit inside one of three pieces of work. On a self-lay water scheme, commissioning is the handover step that lets the water company adopt the new main and connect it to the live network. On a main-laying contract, commissioning happens after the new trunk or distribution main has been laid and before the company signs it into the adopted network. On a repair and maintenance job involving slip lining or pipe bursting, commissioning re-certifies a rehabilitated main as fit to carry potable water once the structural work is complete.
A new water connection above the relevant diameter or length threshold carries the same requirement. Whichever service the commissioning work sits inside, the evidence pack handed to the water company is the same: pressure test data logs, chlorination certificate, bacteriological and chemical sample results from a UKAS-accredited laboratory, as-laid drawings, and confirmation that the pipe was capped after disinfection. For water companies operating across the East and South East, the commissioning pack is part of a wider body of work we deliver as water infrastructure services.
Chlorination and pressure testing in practice
McFadden Utilities commissions new and rehabilitated water mains across Hertfordshire, London, Surrey, Essex, Cambridgeshire, and Berkshire. Chlorination and pressure testing sit at the end of every self-lay water scheme, main laying job, and slip-line rehabilitation we deliver, with sign-off to the adopting water company's specification forming part of the turnkey package.
Kit and crews
Fully data-logged pressure test equipment running the current UK test methods: WIS 4-01-03 creep analysis for polyethylene mains, and the BS EN 805 Section 11 water-loss method for ductile iron, steel, and GRP. Commissioning kit is stainless-steel and food-grade hose, kept dedicated to potable water work to maintain supply integrity. Every operative on a commissioning job holds the EUSR National Water Hygiene Card as a minimum. Bacteriological and chemical analysis is partnered out to a UKAS-accredited laboratory so the sample results the water company receives are independently accredited.
Typical delivery
Commissioning runs in sequence at the end of a main-laying or rehabilitation job: swab and flush, pressure test to 1.5 times the maximum design pressure, chlorinate to the TGN 2 dose, hold for contact time, de-chlorinate and discharge, flush, sample, then hand the certificates and data logs to the water company. Recent examples include commissioning of slip-lined 180 mm PE mains across the M4 at Southall Bridge for Affinity Water, pressure testing and chlorination on 70 m of 250 mm main slip-lined under concrete silos at Wintersells Cement Works, and commissioning of a new 200 mm main with flow meter and booster chamber at Swanley for IWS.