Online decontamination
Online decontamination is a process referred to within the Legionella and heterotrophic micro-organisms sample control strategies
Follow this recommended process to decontaminate cooling water systems:
- Follow all relevant workplace health and safety procedures, including the use of personal protective equipment.
- Cease any chemical treatment. Isolate all electrical equipment except the water treatment pump.
- Add a low foaming chlorine-compatible biodispersant or low-foaming bromine-compatible biodispersant to the recirculating water.
- Disinfect the system by dosing the water with a biocide, either:
- a chlorine-based compound (with detergent properties) equivalent to at least 10mg/L of free chlorine for at least one hour, while maintaining the pH of the water between 7.0 and 7.6 OR
- a bromine-based compound (with detergent properties), equivalent to at least 20mg/L of free bromine for at least one hour, while maintaining the pH of the water between 7.0 and 8.5.
Add the disinfectant slowly (over five to ten minutes) to a turbulent zone of the tower basin. This promotes rapid dispersion. If excessive foaming occurs, use an antifoaming agent.
- Circulate the system for one hour. Measure the pH and free chlorine or bromine levels regularly (say every 15 minutes) and adjust as required. Keep a written record of levels and actions.
- Ensure the water is circulated through all parts of the system, including the standby condenser pump and chillers that may be off line.
- Switch off equipment and drain the cooling tower according to local council requirements. If it is not practicable to drain the cooling tower, use a very high bleed off rate during Step 4 while maintaining free disinfection concentration. Use a wet vacuum cleaner to easily remove waste from the basin floor.
- Refill with clean water and switch on the recirculating pump.
- Repeat Step 4. Switch off the recirculating pump and drain the cooling tower system according to local council requirements.
- Inspect the drift eliminators and clean, repair or replace if necessary. If you move the eliminators, check they are correctly replaced. Take precautions to minimise the release of aerosols during the cleaning operations.
- Brush and gently hose all surfaces to thoroughly clean the internal shell, fill and tower sump. Remove all debris. Take care not to damage the tower and accessories during this operation.
- Thoroughly internally clean all water filters, strainers, separators, water nozzles and fittings associated with the water distribution system. Pay particular attention to the tower fill and distribution trays.
- Re-assemble all components and hose them with clean water.
- Repeat Step 4. Switch off the recirculating pump and drain cooling tower system according to local council requirements.
- Refill with clean water and switch on the recirculating pump.
- Repeat Step 4. If the water is visually clean, repeat Step 13. Clean the water filters and strainers, then repeat Step 13. Repeat this sequence until the water quality is satisfactory.
- Immediately reinstate comprehensive effective water treatment including biocide(s), anti-corrosives and scale control.
- Keep a written record all actions.
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