L8 is the Health and Safety Executive document that gives recommendations for the control of Legionnaire’s Disease. It recommends that domestic water tanks are inspected annually and if found to be dirty should be cleaned and then disinfected.
The following method covers the procedure for disinfecting the tank and where appropriate the associated down services. Whether or not the down services are to be disinfected will depend on the clients wishes and whether it is practical e.g. it will not be possible to disinfect the down services in a hotel which is in constant use. The standard requires the maintenance of either 50 ppm of Chlorine for at least 1 hour or 20 ppm for at least 2 hours.
If after reading the method statement you decide you would prefer an expert team to come in and perform the disinfection please contact us. The products mentioned in this article are available as a Tank Cleaning Bundle available in our web shop.
HEALTH AND SAFETY
Review the generic risk assessment for the procedure and ensure that all the necessary control measures are in place and the correct PPE is worn when handling chemicals.
PROCEDURE
Prior to disinfection the tank should be physically cleaned. Depending on its condition this may simply involve hosing out any accumulated deposits or scraping, sand blasting and wire brushing to prepare the surfaces for painting. A tank should always be inspected prior to disinfection to ensure that it has been adequately cleaned. The disinfection procedure is detailed below.
- Electrically isolate any system booster, and/or circulating pumps.
- Ensure that any hot water system to be chemically disinfected is operating at a temperature of less than 25ºc, and that the primary, and where appropriate, secondary heat source have been isolated. If the system temperature is above the aforementioned value, then outlets should be opened and water drawn off until the temperature reaches the required value.
- Isolate tank by closing inlet and outlet valve.
- Drain tank with submersible pump. In the case of rooftop tanks, ensure that the drain is of adequate size and not prone to leaks within the building. When the water level in each tank has dropped to approximately 50% of normal operating level, open the inlet valve(s) fully and flush the rising main for at least five minutes. This should cause any sediment in the main to be washed into the tank and prevent problems with sediment and discoloured water after the chlorination is completed.
- Wash down internal walls and base with a 10 % FEEDCHLOR solution. Depending on the nature of deposits present, brushes, sponges, or scouring pads maybe used to aid removal of staining and accumulation of scale.
- Vacuum clean base of tank such that all traces if sediment are removed.
- Exit tank and add sufficient FEEDCHLOR to ensure that when the tank is refilled at least 65ppm of chlorine is introduced i.e. at least 0.65 litre of FEEDCHLOR per 1000 litre of tank capacity.
- Refill tank and verify that the required free chlorine level has been attained. In the case of larger tanks mixing with a submersible pump maybe necessary in order to obtain a homogeneous disinfection solution within the tank. If the free chlorine reserve is inadequate, then a further addition of FEEDCHLOR should be made.
- Open tank outlet valves and reinstate electrical supply to any booster pumps. It is particularly important to re-activate secondary hot water pumps, as failure to do so may mean that chlorinated water does not enter the return circuit.
- Open outlet farthest from tank and draw off water until free chlorine is detected. Verify with either chlorine test strips or using a high range chlorine comparator kit.
- Working progressively back towards the tank, open each outlet in turn and check that chlorinated water is present using chlorine test strips.
- When chlorinated water has been drawn through to all outlets, establish that the correct free chlorine reserve has been attained at each outlet and in the tank. It is often necessary to make a further addition of FEEDCHLOR to the tank, owing to the dilution effect caused when water is drawn through to the outlets.
- Record free chlorine residuals on the job sheet and monitor at 15 minute intervals thereafter. Should the free chlorine reserve fall below the minimum level then the process will have to be repeated or the contact period lengthened accordingly.
- Record all further analysis results on the job sheet.
- When the pre-determined contact period has elapsed it will be necessary to neutralise the free chlorine in the tank with DECHLOR.
- Add the necessary quantity of DECHLOR to the tank. In the case of large tanks it maybe necessary to mix the tank contents with a submersible pump.
- Electrically isolate any booster or circulating pumps. Pump out tank(s) to foul sewer and rinse.
- Refill tank and reinstate electrical supply to any booster/circulating pumps.
- Flush the entire system until the free chlorine in the water exiting each outlet is equal to that in the incoming main.
- Remove all warning notices and isolations from hot water heat sources.
DECHLORINATION OF TANKS
Where it is necessary to remove the excess chlorine from a volume of water prior to discharge to sewer, this can be achieved using DECHLOR
The DECHLOR should be added at the rate of 11ml per ppm of Cl2 per m3 of water.
Example To remove 30 ppm of free residual chlorine in a 80m3 tank requires:-
11 x 30 x 80
= 26,400 ml
= 26.4 litres of DECHLOR
N.B. It is important not to over dechlorinate since this will absorb any free chlorine residual in the incoming main.
COMPLETING JOB SHEETS/RECORD KEEPING/REPORTING
This part of the method is relevant to Feedwater support services staff and has been excluded.
PROTECTING THE ENVIRONMENT
- Locate and use the foul drain (sewer)
- Take care to avoid chemical spillage’
- Find out site contact for emergencies
- Always neutralise chlorine / chlorine dioxide residuals with Dechlor before draining
- Never allow effluent to enter surface water without express permission
- Remove empty drums and any rubbish generated by the works from site and ensure that these are disposed of via a licensed waste disposal contractor. It maybe possible to arrange for re-cycling of empty drums.
To visit our shop click here or on the product product images within the page, if you are looking for a professional service to perform a tank clean and disinfection see our services page here: /disinfection-chlorination-cleaning/ and please get in contact to see what we can do for you.
