New Pipework Chlorination: BS 8558 Procedure, Sodium Hypochlorite Dose Calculation, Flushing and Bacteriological Sampling

Summary

Chlorination of new water systems is not optional — it is a legal requirement under the Water Supply (Water Fittings) Regulations 1999 and Water Industry Act 1991. Any new pipework, tank, or water system installed in a building must be disinfected before use to eliminate bacteria introduced during installation. Construction sites are particularly vulnerable: open pipe ends exposed to the air, debris from cutting tools, and contaminated hands can all introduce pathogens.

BS 8558:2015 (Guide to the design, installation, testing and maintenance of services supplying water for domestic use within buildings and their curtilages — Complementary guidance to BS EN 806) is the key reference standard. It specifies the chlorination procedure, chlorine residual targets, contact times, and sampling requirements. Water companies and building control authorities may request the chlorination certificate and bacteriological results as part of project sign-off — particularly on new build, healthcare, schools, and HMO projects.

For tradespeople, the key skills are: calculating the correct dose of sodium hypochlorite for the system volume, executing the chlorination safely (sodium hypochlorite is corrosive and dangerous to mix with other chemicals), and arranging accredited laboratory testing of the final samples. A failed bacteriological test requires re-chlorination and re-testing — understanding what causes contamination helps prevent it.

Key Facts

• BS 8558:2015 — the UK standard for water system installation and disinfection; references BS EN 806-4 for testing procedures
• Target chlorine concentration — 50mg/L (50 ppm) free chlorine during the disinfection period
• Minimum contact time — 1 hour at 50mg/L; or alternatively 30 minutes at 100mg/L (higher risk of corrosion to copper)
• Chlorine residual after flushing — must be reduced to below 0.5mg/L before sampling or commissioning
• Sodium hypochlorite — typical trade/domestic product is 4–6% available chlorine; swimming pool grade may be 10–14%
• Dose calculation — volume of system (litres) × target concentration (mg/L) ÷ % available chlorine (as decimal) = volume of hypochlorite to add
• Bacteriological standards — E. coli: 0 per 100mL; Total Coliforms: 0 per 100mL; Colony Count (22°C): acceptable guidance value ≤100 per mL
• Sampling — samples taken in sterile bottles from the furthest point and a mid-point of the system; sent to UKAS-accredited laboratory within 6 hours
• Material compatibility — sodium hypochlorite at 50mg/L is compatible with copper, plastic, and stainless; do not use at higher concentrations on galvanised steel
• Temperature during chlorination — system water should be below 25°C; higher temperatures accelerate chlorine decay
• Storage tanks — clean tanks, inspect for debris and damage before filling and chlorinating
• WRAS-approved materials — all pipework, fittings, solder flux, and jointing compounds must be WRAS-approved for potable water contact before chlorination
• Records — issue a Disinfection Certificate recording: date, volumes, concentration used, contact time, flush duration, and sample results

Quick Reference Table

Formula: Volume (L) × 50 ÷ (% concentration × 10,000) = hypochlorite volume in litres. Example: 500L × 50 ÷ (5 × 10,000) = 0.5 litres.

Detailed Guidance

Step 1: Preparation Before Chlorination

Before chlorination begins, the system must be physically clean:

1. Flush with mains water — flush all pipework with clean mains water to remove debris, swarf, flux residue, and pipe-end caps. Continue until water runs clear from all outlets.
2. Inspect storage tanks — remove any debris, check for damage, clean the interior with clean water. Do not use detergents.
3. Check all joints — perform a visual inspection. Any leaking joint must be repaired before chlorination — leaks during the contact period allow chlorinated water to escape and prevent the required concentration being maintained.
4. Isolate dead ends — ensure all branches, spurs, and dead legs are included in the chlorination. A missed dead leg that isn't flushed can harbour bacteria and cross-contaminate the system after commissioning.

Step 2: Calculating the Sodium Hypochlorite Dose

Step 2a: Estimate system volume

For design drawings: calculate from pipe diameter and length.

• 15mm copper: approximately 0.13 litres per metre
• 22mm copper: approximately 0.32 litres per metre
• 28mm copper: approximately 0.54 litres per metre
• 35mm copper: approximately 0.84 litres per metre
• Storage tank: use actual tank volume

For unknown systems: fill the system from a known volume (e.g., measure fill volume from a metered supply).

Step 2b: Calculate dose

Using 5% sodium hypochlorite (check label — products vary):

Volume of hypochlorite (mL) = System volume (L) × 50 ÷ 50,000 × 1000

Simplified: for 5% product, dose = system volume in litres × 1mL per litre (to achieve 50mg/L)

Step 2c: Verify concentration after dosing

Use a DPD free chlorine test kit (pool test kit) to verify the chlorine concentration at the furthest point from the dose point. If residual is below 40mg/L, add additional hypochlorite and re-test. Do not start the contact time until the correct concentration is verified throughout the system.

Step 3: Dosing the System

1. Mix the calculated volume of sodium hypochlorite into a small volume of mains water (do not add water to neat hypochlorite — always add hypochlorite to water)
2. Introduce the solution to the system at a suitable entry point — typically via the cold water storage tank, header tank, or a purpose-fitted dosing point
3. Open all valves, taps, and outlets in turn to draw the chlorinated water through the entire system
4. Verify chlorine concentration at multiple points before starting the contact time
5. Close all outlets and allow the system to stand for the contact time (minimum 1 hour at 50mg/L)

Safety:

• Sodium hypochlorite is corrosive — wear gloves and eye protection
• Do not mix with acids (including some descaling products) — chlorine gas is released
• Ensure adequate ventilation in enclosed spaces
• Warn all occupants that the system is out of service

Step 4: Flushing After Chlorination

After the contact time:

1. Open all outlets systematically and flush until the free chlorine residual drops to below 0.5mg/L at every outlet
2. Test with the same DPD test kit — do not rely on smell alone; chlorine is perceptible below 0.5mg/L
3. Dispose of chlorinated flush water safely — do not discharge to watercourses; drain to the foul sewer where permitted. High-concentration rinse water may require notification to the local water company
4. Once all outlets show below 0.5mg/L, the system is ready for sampling

Step 5: Bacteriological Sampling

Bacteriological sampling must be carried out by or on behalf of the client:

1. Sample points: minimum of the most distant outlet and one mid-system outlet; for large systems, one sample per 100 metres of main
2. Use sterile sample bottles (supplied by the laboratory with sodium thiosulphate to neutralise residual chlorine)
3. Sterilise the outlet before sampling — ideally with a clean flame or 70% alcohol wipe; do not wipe after sterilising
4. Run the outlet for 30 seconds to clear the local dead leg
5. Fill the bottle without touching the inside
6. Label immediately with location, date, and time
7. Transport to a UKAS-accredited water testing laboratory within 6 hours (or refrigerate and deliver within 24 hours — check with the lab)

Acceptable results:

• E. coli: 0 per 100mL
• Total Coliforms: 0 per 100mL
• Colony Count at 22°C: ≤100 per mL (guidance value; not an absolute failure threshold)

If E. coli or Total Coliforms are detected, the system must be re-chlorinated and re-sampled. Investigate the cause: likely a contaminated dead leg, a tank that was not chlorinated, or sample collection error.

Disinfection Certificate

On completion issue a Disinfection Certificate recording:

• Project details and address
• Date of chlorination
• System volume
• Product used (name, concentration, batch number)
• Volume of hypochlorite added
• Chlorine residual verified during contact period
• Contact duration
• Flush duration and final residual
• Sample point locations
• Laboratory reference and results

Retain a copy; provide original to the client. The Water Supply (Water Fittings) Regulations 1999 require records to be kept.

Frequently Asked Questions

Does domestic new build always need chlorination and sampling?

Yes, BS 8558 and the Water Fittings Regulations apply to all new pipework serving potable water points. However, for small domestic extensions (e.g., a single bathroom or kitchen extension on an existing supply), many water companies accept a signed declaration that the system was chlorinated without requiring full laboratory sampling — contact the local water company to confirm their current policy. For new builds and commercial projects, laboratory sampling is always required.

What if the laboratory result fails?

Re-chlorinate the full system, re-flush, and re-sample. Before re-chlorinating, investigate the source of contamination: check all dead ends were included, inspect tank cleanliness, review sample collection procedure. A positive E. coli result suggests faecal contamination — re-inspect the site for cross-contamination with drainage.

Can I use pool chlorine tablets instead of liquid sodium hypochlorite?

No. Chlorine tablets (typically trichloroisocyanuric acid or calcium hypochlorite) dissolve slowly, make concentration calculation difficult, and can leave residues. Always use liquid sodium hypochlorite for pipework chlorination. The concentration is known, dosing is precise, and flushing is reliable.

Is chlorination needed for hot water cylinders?

Yes, if the cylinder is new or has been open to the air during installation. Unvented cylinders should be flushed and filled with chlorinated water, contact time observed, then flushed before commissioning. The entire cold feed to the cylinder and all associated distribution pipework should be included in the same chlorination exercise.

Regulations & Standards

• BS 8558:2015 — Guide to design, installation, testing and maintenance of services supplying water for domestic use within buildings; primary chlorination standard

• BS EN 806-4 — Specifications for installations inside buildings conveying water for human consumption; commissioning and testing

• Water Supply (Water Fittings) Regulations 1999 — statutory requirement for water system hygiene and disinfection

• Water Industry Act 1991 — statutory framework for water supply quality

• Regulation (EU) 98/83/EC (retained in UK law as The Water Supply (Water Quality) Regulations 2016) — microbiological standards for drinking water

• WRAS Water Regulations Guide — technical guidance on water regulations compliance

• WRAS Water Regulations Advisory Scheme — Water Fittings Regulations guidance

• BS 8558:2015 — BSI standard for water system installation and disinfection

• Drinking Water Inspectorate (DWI) — standards for drinking water quality

• UKAS Accredited Laboratories — find an accredited water testing laboratory

• legionella management — Legionella risk assessment and water temperature management

• hot water systems — unvented and vented hot water system design

• cold water storage — cold water storage tank design and hygiene

• pipe sizing — domestic water pipe sizing and flow calculations