When Emergency Chlorination Is Required
Emergency chlorination becomes necessary when testing reveals unacceptable bacterial levels in your water system. A positive legionella result above the actionable threshold of 100 CFU/litre requires immediate remedial action, and chlorination is the fastest, most effective response.
Other situations demanding emergency chlorination include visible contamination events (sewage ingress, flooding), discovery of dead animals or significant debris in storage tanks, and any incident where water supply safety has been compromised. Buildings reopening after extended vacancy — common in London's commercial property market — may also require chlorination before the water system is safe for use.
Chlorination Standards and Concentrations
Professional chlorination follows the procedures set out in BS 8558. The chlorine concentration and contact time must be carefully controlled to ensure complete disinfection without damaging the system. The table below shows the standard protocols.
| Protocol | Concentration | Contact Time | Typical Use |
|---|---|---|---|
| Standard disinfection | 50 mg/l | 1 hour minimum | Post-contamination, commissioning |
| Extended disinfection | 20 mg/l | 2 hours minimum | Larger systems, planned maintenance |
| Hyperchlorination | 50 mg/l | 4+ hours | Severe contamination, biofilm removal |
| Post-treatment target | < 0.5 mg/l residual | After flushing | Before return to service |
The Step-by-Step Process
Our engineers follow a rigorous process for every chlorination, whether emergency or planned. Understanding each stage helps you plan for the disruption and know what to expect.
- •1. Assessment — engineer inspects the system, identifies affected sections, and determines the appropriate chlorination protocol
- •2. Isolation — affected system sections are isolated and drained if necessary
- •3. Dosing — sodium hypochlorite is introduced at the calculated concentration using calibrated dosing equipment
- •4. Distribution — chlorinated water is pushed through every pipe, outlet, and dead leg to ensure complete coverage
- •5. Contact time — the required contact period is maintained with regular monitoring of chlorine residuals
- •6. Flushing — the system is thoroughly flushed with clean water until residual chlorine drops below 0.5 mg/l
- •7. Sampling — water samples are taken from multiple points and sent to a UKAS-accredited laboratory
- •8. Certification — a chlorination certificate is issued documenting the treatment, concentrations, and test results
Timeline and Disruption
The table below gives realistic timelines for different system sizes. During treatment, the affected water supply will be unavailable. For occupied buildings, we can often treat sections sequentially to maintain partial supply.
| System Size | Typical Duration | Water Unavailable |
|---|---|---|
| Small (single unit / flat) | 3 – 4 hours | 3 – 4 hours |
| Medium (residential block) | 4 – 8 hours | 4 – 8 hours (or sectional) |
| Large (commercial building) | 8 – 16 hours | Can be phased by zone |
| Very large (hospital / estate) | 1 – 3 days | Phased by building / riser |
After Chlorination: What Happens Next
After chlorination and flushing, water samples are sent for laboratory analysis. Results typically take 7–10 working days. Until satisfactory results are confirmed, enhanced monitoring measures should remain in place — this means daily temperature checks and close observation for any recurrence of symptoms.
Crucially, chlorination treats the symptom but not necessarily the cause. Your risk assessment should be reviewed and updated to identify and address the root cause of contamination. Without addressing underlying issues — dead legs, poor temperature control, dirty tanks — contamination will likely recur.