Chlorination and Flushing Procedures

Chlorination disinfects new or repaired water mains before they are put into public supply service.
Chlorine is a toxic gas and a strong oxidiser; sodium hypochlorite solution is the most common dosing chemical.
Contact with concentrated sodium hypochlorite causes severe burns to skin, eyes, and respiratory passages.
Chlorine concentrations during main disinfection are far higher than normal drinking water levels.
Flushing removes chlorinated water and construction debris from the main before it enters supply.
Flushed water is heavily chlorinated and must not enter watercourses without dechlorination treatment.
COSHH 2002 requires risk assessment and controls for all workers handling chlorination chemicals.
The disinfection process involves filling, dosing, contact time, sampling, and final flushing stages.
Water sampling confirms the main is bacteriologically safe before it is connected to the live network.
Only trained operatives with the correct qualifications should carry out chlorination procedures.

Chemical burns	Concentrated sodium hypochlorite causes immediate severe burns to skin and eyes. Chlorine gas inhalation damages lungs.
Public health	Incorrect chlorination or inadequate flushing allows contaminated water to enter the public supply, causing illness outbreaks.
Environmental damage	Discharging heavily chlorinated flush water to watercourses kills fish and aquatic life, leading to prosecution.

Do

Don't

Wear chemical-resistant gloves, goggles, and RPE when handling sodium hypochlorite.
Follow the approved chlorination procedure for dosing concentration and contact time.
Dechlorinate flushing water before discharging it to watercourses or surface drains.
Take bacteriological samples and await satisfactory results before connecting to supply.
Store sodium hypochlorite in bunded, shaded areas away from incompatible chemicals.
Have emergency eyewash and skin drench facilities available at the dosing point.
Ensure only trained and qualified operatives carry out the chlorination process.
Record all dosing quantities, contact times, and sample results accurately.
Ventilate enclosed areas where chlorine gas may accumulate during dosing operations.
Dispose of waste chemicals and containers through an approved waste contractor.

DON'T handle sodium hypochlorite without chemical-resistant gloves and eye protection.
DON'T discharge chlorinated flush water to watercourses without dechlorination treatment.
DON'T connect the main to the live network before bacteriological results confirm safety.
DON'T mix sodium hypochlorite with acids; this releases toxic chlorine gas instantly.
DON'T store chlorination chemicals in direct sunlight or near heat sources.
DON'T skip the flushing stage; residual chlorine and debris must be removed.
DON'T carry out chlorination without the required training and competency qualifications.
DON'T ignore splashes of hypochlorite on skin; flush immediately with clean water.
DON'T enter enclosed spaces where chlorine gas may have accumulated without monitoring.
DON'T reuse contaminated dosing equipment without thorough cleaning and inspection.

See also: Water Main Installation Safety | Chlorine and Water Treatment Chemicals

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