What should system owners provide before chlorine disinfection?

Before chlorine disinfection begins, the system owner should provide the information, records and site arrangements needed to plan the treatment safely and demonstrate that it has been authorised. The contractor needs to understand how the water system is configured, how it has been managed, what condition it is in and what restrictions apply to the work.

Relevant records should include:

• the current legionella risk assessment and written scheme of control;
• the asset register, inspection records and maintenance history;
• recent temperature, disinfectant residual and other monitoring results;
• water test reports, previous treatment records and details of any known contamination incidents;
• records of alterations, extensions, unused sections or changes in occupancy and use; and
• details of outstanding defects, remedial works or areas that cannot currently be taken out of service.

This information helps establish whether chlorine disinfection is suitable and whether the proposed treatment can reach all parts of the system. A treatment may be ineffective if sections are isolated, poorly circulating, obstructed by scale or biofilm, or affected by valves and fittings that do not operate correctly.

The owner should also provide practical operating information that may not be clear from drawings alone. This includes normal operating temperatures, the usual flow direction, circulation arrangements, storage capacity, turnover patterns and any outlets or branches that are infrequently used. Details of pumps, dosing points, tanks, filters, softeners and other treatment equipment are important because they may need to be isolated, bypassed, protected or included within the disinfection plan.

Information about the system’s construction and materials is equally important. The contractor should be told about pipework materials, tank linings, seals, membranes, specialist components and connected equipment that could be damaged by the proposed chlorine concentration or contact period. Manufacturer instructions, previous chemical-treatment records and known compatibility limitations should be supplied where available. If the construction materials are uncertain, that uncertainty should be resolved before treatment rather than assumed away.

The system owner remains responsible for confirming the site’s management arrangements. Before work starts, there should be a named person with authority to approve the method, coordinate affected departments and make decisions if the system cannot be isolated as planned. The owner should identify relevant permit-to-work, isolation, lock-off, chemical handling and emergency procedures, together with any site-specific requirements for contractors.

Arrangements for the treated water must also be agreed in advance. Chlorinated water may require controlled discharge, neutralisation or other measures before it enters drainage or the environment. The owner should identify suitable discharge points, obtain any permissions required by the site or water authority, and confirm how neutralising chemicals will be selected, handled and recorded. Drainage capacity and the risk of discharge affecting other equipment should be considered as part of the method statement.

It is also useful to agree the treatment criteria before dosing takes place. The written plan should identify the sections to be treated, the intended chlorine concentration and contact period, the checks required during the work, and the evidence needed before the system is returned to use. The owner should confirm who will review the results, approve reinstatement and retain the records.

Where information is missing or conditions have changed, the disinfection should not proceed on assumptions. A competent water hygiene professional may need to inspect the installation, revise the method or recommend remedial work first. Providing complete records and clear site controls allows the treatment to be planned against the actual system rather than a standardised description of it.