7.3 Disinfection: Chlorine Strengths, Contact Times and Safe Disposal

7.3 Disinfection: Chlorine Strengths, Contact Times and Safe Disposal

Why disinfection has its own unit

The 2023 National Water Hygiene e-Learning programme gives disinfection a unit of its own, and for good reason. Almost every job on the clean water network ends with chlorine somewhere: a spray bottle for fittings and cut ends, a tablet dose for a new main, a solution for tools that have been somewhere they should not. Card-holders are not expected to design a disinfection procedure, but they are expected to understand why each step exists, to use the strengths their water undertaker specifies, and to recognise when something has gone wrong.

Two principles frame the whole topic:

• Disinfection is a back-up, not a licence. Water UK's technical guidance describes chlorine as an effective disinfectant for most bacterial and viral pathogens. It does not reliably inactivate Cryptosporidium oocysts (Section 2.2), and it cannot remove mud, swarf or oil. Keeping contamination out of the pipe always comes first; disinfection deals with what hygienic working cannot completely prevent.
• Strength and contact time work together. A disinfectant needs enough concentration and enough time in contact with every wetted surface. A weak solution, a rushed contact time or a surface the solution never reached can each defeat the process.

At the treatment works, the Water Supply (Water Quality) Regulations 2016 require all drinking water to be disinfected effectively before it is supplied, and Water UK's Principles of Water Supply Hygiene treat any failure of disinfection as serious. Out on the network, the job is to make sure that nothing you add to the system undoes that work.

The numbers in Water UK's guidance

Water UK's technical guidance notes (TGNs) set the industry baseline that each undertaker builds into its own code of practice. Your company's procedure always takes precedence, but these are the figures you will meet:

Notice the pattern. Small surfaces that can be reached directly, such as fittings and cut ends, get a strong solution for a short time. A long pipe that must be disinfected from the inside gets a weaker solution for much longer. A significant drop in chlorine during a fill-and-stand contact period is itself a warning: it can mean the pipe is dirty or otherwise contaminated.

Making up and handling chlorine solutions

Water UK's TGN 13 sets out how chlorine solutions should be prepared:

• Use only approved products. Chemicals must be approved under regulation 31 of the Water Supply (Water Quality) Regulations 2016 or the equivalent rules in the devolved administrations. The common ones are sodium hypochlorite solution (about 10 to 15% available chlorine), calcium hypochlorite tablets (about 33 to 35%) and isocyanurate tablets.
• Respect the expiry date. Chlorine products lose strength with age and poor storage, so they must not be used after their stated expiry date. Diluted solutions, such as spray mixtures, should be used within the time your procedure states.
• Use dedicated, clearly marked containers. Never store chlorine solutions in a container that held anything else; Water UK specifically warns against containers that held petroleum products. Storage areas need bunding to contain spills.
• Protect yourself. Concentrated solutions, powders and tablets are hazardous, so follow the health and safety controls and wear the protective equipment your employer specifies.

Undertakers give field staff a ready-reckoner table so nobody has to improvise. As a worked example from that kind of table: 10% sodium hypochlorite contains about 100 mg of available chlorine in every millilitre. To make 10 litres of the 1,000 mg/l spray solution you therefore need about 100 ml, because 100 ml supplies 10,000 mg and 10,000 mg spread through 10 litres is 1,000 mg/l. To make 100 litres at 50 mg/l you need about 50 ml. Never guess with a "glug" from the drum: too weak fails to disinfect, and too strong wastes chemical, harms the environment when discharged and can leave the water tasting of chlorine.

Disposing of chlorinated water

The water used to disinfect a main or a reservoir does not simply disappear. Water UK's TGN 14 explains why it needs care: chlorine is toxic to fish and other aquatic life, and strong solutions can also upset the biological processes at a sewage treatment works.

• Watercourses: chlorinated water and chlorine solutions must not be discharged to a stream, river or ditch without prior consent from the Environment Agency, and surface water drains or land drains that lead to a watercourse need the same care.
• Foul sewers: discharge to the foul drainage system is possible after a risk assessment confirms that the receiving sewage treatment works will not be affected.
• Dechlorination: where necessary, chlorinated water is neutralised before disposal, usually with sodium thiosulphate or sodium bisulphite. As a guide, 10 g of sodium thiosulphate crystals neutralises 1,000 litres of water containing 5 mg/l of free chlorine.

The same care applies after a mains repair: Water UK's repair guidance expects enough neutralising agent to be added where flushing water could reach a watercourse.

Site scenario: spraying fittings on a burst repair

Your gang is completing a cut-out repair on a depressurised main beside a brook. Before anything goes in, you check that the hypochlorite in the van is within date and make up a fresh 1,000 mg/l spray in a clearly labelled, dedicated sprayer, using the ready-reckoner. Every fitting, the replacement pipe and both cut ends are sprayed so that all surfaces are wetted. After the repair the main is flushed at the nearest downstream hydrant, and because the hydrant drains towards the brook, the flushing water is dechlorinated before it is released. The supervisor then arranges the downstream sample. Each step has a reason, and none of them depends on how small the repair looks.

Common candidate traps

• "Chlorine kills everything." It does not reliably kill Cryptosporidium, and it cannot remove physical dirt. Keeping contamination out comes first.
• "A stronger mix is always safer." Strengths are specified for a reason. Over-strong solutions waste chemical, harm aquatic life and can taint the water, while weak ones fail to disinfect.
• "Any old bottle in the van will do." Out-of-date chlorine loses strength, and containers that held other products can contaminate the solution.
• "Flushing water can go down the nearest gully." A road gully may discharge straight to a watercourse, so consent, a suitable discharge point and dechlorination must be considered first.