WATER HYGIENE AND SAFETY – SPECIAL REPORT


Table 1. What length of pipe is emptied by flushing, per minute? Tap


Pipe


flow rate diameter (L/min) (mm)


4 4 6 6


15


22 15


22 Πr2 (m2)


0.000177 0.00038 0.000177 0.00038


outlets which are in normal use. The HSE9 and Department of Health guidance documents5,6


length Does flushing work?


Volume 1 minute 2 minute per metre flush pipe flush pipe (litres)


length 0.177


0.380 0.177


0.380


(metres) (metres) 23 11


45 21


34 16


68 32


only recommend flushing of


infrequently used outlets. Flushing is an inferior substitute for normal use. both in terms of frequency of use of the outlet, and volume flushed. The preference in order of priority should be ‘use it, lose it, or flush it’. That is, outlets should preferably be frequently used. If that is not possible, then they should be removed (together with associated pipework). If it is necessary to retain the outlet, it should be flushed.


Problematic wording


The wording ‘or as indicated by the risk assessment’ is problematic, because it requires judgement on behalf of the risk assessor. One would need to consider which factors the risk assessor would judge in order to arrive at a frequency different from that in guidance. Vulnerability might be one such factor, or known system defects such as capped-off pipework in a concrete floor. Such factors might justify an increased frequency, but the actual frequency recommended might only be proven by monitoring the effects of such flushing on Legionella counts. It is known that, in the case of untreated evaporative cooling towers10 baths,11


and spa outbreaks of Legionnaires’ disease


have occurred within two and four days of start-up, respectively. This would seem to lend credence to the idea of twice-weekly flushing, i.e. flushing every few days, rather than weekly, albeit I have not seen evidence to suggest that domestic cold and hot water systems have caused outbreaks quite so quickly.


Although the question refers only to taps and showers, there should be flushing of all parts of the water systems which might give rise to stagnation – for example, WCs and other deadlegs, such as tank drain pipework, cul-de-sac expansion vessels, etc. The relevant summary in HTM 04 (2016) Part B5 is shown in Table 2.


POU filter use


What about flushing when POU filters are in place? On the face of it, if Legionella cannot pass through a POU filter, one could argue that there is little point flushing the outlet with the filter fitted, assuming that the outlet is infrequently used. However, HTM 046


recommends


exactly that, unless the manufacturer advises otherwise. Presumably, the benefit of flushing an outlet even though a POU is present is that it draws fresh water containing disinfectant (either courtesy of the water undertaker, or from secondary biocide application) to the filter and through it.


Table 2. Flushing Requirements Described in HTM 04 Part B.5 Service


Action to take


Infrequently used outlets


Consideration should be given to removing infrequently used showers, taps and any


the pipework supplying a more frequently used upstream fitting), but preferably by removing the feeding ‘T’. Infrequently used equipment within a water system (i.e. not used for a period equal to or greater than seven days) should be included on the flushing regime. Flush the outlets until the temperature at the outlet stabilises and is comparable to supply water and purge to drain. Regularly use the outlets to minimise the risk from microbial growth in the peripheral parts of the water system; sustain and log this procedure once started.


Pressurisation Where practical, flush through and purge to drain. Where removable, bladders or vessels and expansion vessels


10 Health Estate Journal December 2020 Monthly–six


diaphragms should be changed according to the manufacturer’s guidelines, or as indicated monthly, as by the risk assessment.


indicated by the risk assessment


The answer to this question depends on what is meant by ‘work’. Obviously, flushing removes water from the system, and one can see that water is leaving the tap, shower, etc. If that water contains Legionella, then it is easy to believe that it also removes Legionella from the water system. However, if by ‘work’ one means ‘Does flushing eliminate the root cause of Legionella contamination?’, then more often than not the answer would be ‘no’. Flushing is a process which must have realistic objectives. In the world before point-of-use (POU) filters, it may have formed a part of the immediate response to finding Legionella, which is to protect the water user, which typically involved restricting access to particular outlets, carrying out an initial ‘purge’ of contaminated water, and disinfection, followed by regular flushing until the root cause could be eliminated and re-sampling proved its effectiveness. In many cases, the arrival of POU filters changed the approach to this initial response. Rather than limiting access and purge/flushing, the process became ‘fit a POU filter and maintain access to the outlet’.


Infrequently used outlets The difficulty is that sometimes the root cause cannot be readily removed. The typical example here is infrequently used outlets, notably showers. There is evidence dating back to at least 199012


that regular


flushing at an appropriate frequency can control Legionella. This is illustrated in Figure 3. Another example of a root cause which cannot be easily eliminated in my experience is a hospital building which has capped off hot and cold water pipes (‘blind ends’) which are connected to the live water systems, These blind ends are within and beneath the concrete floor slab. The building will soon be demolished, and a new hospital built. In the meantime, POU filters are in place, and are likely to remain


Frequency Weekly, or as


associated equipment that uses water. If removed, any redundant supply pipework should indicated by the be cut back as far as possible to a common supply (e.g. to the recirculating pipework or


risk assessment


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