Answer
1. General disinfection
With these rules you should not have problems in regard on bacteria or Legionella.
Therefore some general methods should be used.
UV radiation for disinfection
This method could be just used for one point in the system (mostly closed to incoming
point, means water meter). It could just make sure that the incoming fresh water will not bring any bacteria or Legionella into the system. It could not disinfect the entire system!
The UV radiation device must be designed according to the working temperature and volume flow (please see suppliers instructions).
It could be necessary to have more than one device in the system (for. e.g. Hospital,
laboratory,...)
Chemical disinfection
For chemical permanent disinfection mostly chlorine is used. For the general disinfection (long term) a maximum load of chlorine of not more than 0.100 mg/l should be used. Higher loads (in long term use) will effect the live time of the pipe system.
Thermal disinfection
According to DVGW worksheet W 551 (April 2004)
This worksheet is mainly written for the design and the usage of hot tap water systems.
There are described some general rules. For e.g.
- it must be possible that the hot water temperature could be 60°C at least once a
day (for big systems, means more than 400l)
- the water tank should be designed that the water will be heated up quickly (no
mixing in the tank between hot and cold water)
- if there are more than 3l in the hot water system, a circulation (hot water return)
should be used
- if circulation systems are used the temperature difference must not be more than
5K between hot water outlet of the tank and circulation return in tank
- in big facilities ( like hotel, hospitals, nursery homes, tank bigger than 400l, …)
the hot water temperature should be always 60°C in tank or heated up at least
once a day to 60°C. Accordingly it should be handled for small houses (one or two
family)
- the hot water tank should be not bigger than necessary
- not used parts of the system should be cut as close as possible to the main line
2. Short term disinfection
If your system is already contaminated (why ever) you have to disinfect the hole system by a method that should kill all bacteria and/or legionella. This could be also called a quick disinfection.
Please observe that a full documentation should be made and hand out to the customer/user of the system and maybe the responsible public authority.
Thermal disinfection
According to DVGW worksheet W 551 (April 2004)
- heat up the water temperature to 70°C, because with 70°C nearly all bacteria/Legionella and so on will be killed
- let the circulation pump run until inlet and outlet temperature on the hot water tank is (approx.) the same
- flush (fully opened valve) each hot water out let for at least 3 minutes with water temperature 70°C (at the outlet)
- make sure that nobody will hurt during this activity
Chemical disinfection
Also for the short term chemical disinfection mostly chlorine is used.
Before you start with the chemical disinfection you should “wash” the infected system at first. This should be done to wash out the bio film from the inner side of the tap water pipe. Because the bio film is often hiding the bacteria’s and Legionella. So if you don’t flush the pipes the chemical disinfection may not work proper.
The flushing should be done with an air/water mixture (please take care that it will be oil free air because we are working with the food stuff tap water). The flushing should not be done below 2 up to 3 m/s (you may use a buster pump to get this speed into the system). It is suggested to flush each valve for at least 1 minute.
After the flushing the system should be filled with the water chemical mixture (regarding the mixture please see below). The mixture should stay, under pressure, for some hours, maximum 12h, in the system.
After the residence time the system has to be flushed again to wash out the chemicals completely! This has to betested properly on each water outlet. Please obtain the local environmental protection rules.
Consider for some chemicals the responsible maximum concentration:
Chlorine bleaching (sodium hypochlorite NaOCl) max. 50 mg/l chlorine
Calcium hypochlorite (Ca (OCl)2) max. 50 mg/l chlorine
Chlorine dioxide (ClO2) max. 6 mg/l CIO2
If the water tanks should be disinfected separately they could be sprayed with a sodium hypochlorite mixture with 5g/l chloride. Therefore thy must be empty and dry. The chemical disinfection may effect the life time of the entire tap water system.
The safety rules for the work with high loads of chlorine must be always observed!
Please make sure that the local environmental protection rules will also be considered. Please check and follow also the chemical supplier rules!
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