High levels of bacteria growth can be very difficult and exceedingly costly to resolve retrospectively.
In most cases this problem is a factor of system design and installation standards in the original installation.
Pipe work design and materials of construction
The use of the correct materials of construction are very important. ABS and PVC pipe systems are very effective with an excellent range of fittings being available for the ultimate system construction.
Materials are one aspect, but all of this can be wasted if the installer is not aware and trained in all aspects of purified water system design and construction. Within the pipe work installation all joints should be crevice free, (workmanship). No dead spaces, (deadlegs) should be longer than 4 internal pipe diameters. (Workmanship and design).
The purified water system should achieve a design velocity of 1.5 to 2 meters per second, (design).
It is possible to install a complete purified water system in 316L grade stainless steel. Once again this is only as good as the installation and construction standards. Really by this we mean crevice free and orbit ally welded as well as the factors described above.
No matter what material is used crevice free construction and zero deadlegs are imperative to prevent hide out areas where bacteria can grow.
Inevitably within the purified water system there will be one or more system water tanks. These tanks should be of the correct material of construction, which will in most cases be polypropylene or 316L stainless steel.
Tanks must be manufactured with crevice free interiors and have a sealed lid.
For integrity the tank will be vented and this vent will also be filtered to 0.01 micron to prevent the ingress of air borne bacteria.
If the tank is part of a recirculation system it should include a 360 / 180 degrees upwards spray ball through which will pass the returned water.
Tank level controls should be of the “non-intrusive” type. Hydrostatic level controls with diaphragm seal are best for crevice free reliable operation.
Bacteria strains such as pseudomonas will multiply at temperatures as low as 20 degrees Centigrade. It is therefore good reliable practice to maintain the system purified water to 18 degrees Centigrade.
It should be remembered that devices such as pumps and UV sterilisers will pass heat energy into the purified water.
A stainless steel plate heat exchanger and air cooled chiller are an effective method of removing excess heat.
Flow rates, ( velocities)
In purified water system ring mains we look to achieve a velocity of 1.5 to 2 meters per second. This is a factor of pipe work and pump size.
Ultra Violet Sterilisation
UV sterilisers are effective devises in the prevention of bacteria growth.
The UV unit needs to be of 316L stainless steel construction with a correctly sized 254 nanometre per second UV arc lamp. Many UV arc lamps have an effective “kill” life of 9000 hours. The lamp should be replaced at this time
There are many UV units available of different budgets. A unit should be selected which includes a UV intensity monitor to enable the user to monitor the UV lamp performance.
Even with the best designed and constructed purified water system it is still advisable to disinfect every six months.
This disinfection is completed by recirculating a disinfecting chemical around the pipe work and tanks. Disinfecting chemicals used are normally Sodium hypochlorite, hydrogen peroxide or perecetic acid.
Disinfection procedures can be complex and should not be undertaken by anyone who has not had adequate training. In addition to health and safety issues it is also important that the disinfection has been effective in achieving the end result. Learn more about the prevention of bacteria with this great article - The Prevention And Removal Of Bacteria In Purified Water Systems.