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| Legionella and Building Services |
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At best this device highlights the need to significantly reduce the health risks from what is believed to be the single largest source of Legionnaires’ disease – conventional showers of the mixer valve type. However, independent scientific trials have shown this device is unsuitable for the control of legionella, and as such L8, paragraph 167, describes they should not be considered for this purpose. The drain down valve is a simple device which drains stagnant water from the pipe work fitted between a fixed shower head and a mixer valve when not in use. The device is connected to the lower part of the mixer valve and closes off any draining effect when the shower is operated. Simple analysis - Typical parts of conventional showering systems 1. Shower head spray plates act as coarse filters which collect debris and legionella when in use and allow partial drainage after use; often they are located in warm room conditions, and have been show to support bio films harboring legionella colonies. The first 50ml sample taken from conventional shower systems is often found to be the most contaminated sample taken from the whole domestic water system. 2. The blended water pipe, connected between the mixer valve and shower head, is often located within warm rooms or even warmer service voids; stagnant water at these temperatures is suitable for legionella multiplication. 3. Stagnant water found within mixer valves often provides the second highest contaminated samples, probably due to an array of complicated internal parts of mixed materials, which can trap particulate and provide a plentiful supply of nutrient. 4. The pipe work connected between the common supplies and mixer valves are often referred to as dead legs, and are often located in warm rooms or service voids. When showers are not in use, stagnant water within this pipe work has been shown to provide conditions suitable for legionella multiplication – especially the hot water dead leg. Consider the drain down valve operating in the conventional showering system described above 1. A fixed shower head spray plate is normally located below the highest point of the blended water pipe (swan neck) and the drain down valve has little if any effect. Shower heads normally partially drain through the spray plate apertures or through additional conduits, however, after use residual water remains and lines the internal aspects of shower heads which can support legionella and biofilm growth. 2. Most of the water is drained from the blended water pipe after use, however, residual water remains and lines the internal aspects of the pipe. Biofilms often line the internal aspects of pipe work and fittings; legionella has been shown to proliferate in moist bio film where there is a plentiful supply of nutrients. Hydraulic shocks in systems can dislodge biofilm and should be avoided whenever possible. As this pipework is mainly empty after use, the next flow of water through is more likely to shear off the outer layers of unstable biofilms; contaminated biofilm not retained by the spray plate will pass into the showering space and potentially infect the operator. 3. Water is partially drained from the mixer valve by the drain down valve – see point 2. 4. The drain down valve design is not designed to drain water from contaminated dead legs. Conclusions The drain down valve appears to fail to control any part of a typical conventional showering system; it may even cause concern for possibly increasing the risks to health. However, very important lessons have been learned from this device and it became quite clear around 1990 that engineers, microbiologists and researchers needed to improve their communication in the battle against LD. As a result of better communication and intensive scientific research, we now have two alternative showering controls in place
It is worth remembering that a scientific discovery can only be realised through the fullness of time, without which it may at first appear to off set one problem but only later is it realised it can exaggerate another. Article by: |