WATER SAFETY PROCEDURES


so, because twice-weekly flushing, six-weekly disinfections, and local disinfections, are incapable of providing control (Fig 4). These examples show that flushing can be either effective or not, depending upon the specific circumstances. A further important example to remember is that if samples from hot water systems are producing positive Legionella counts, then if the root cause is the failure of subordinate or tertiary loop circulation (which in many instances are not being monitored), flushing is unlikely to have any beneficial impact on the stagnation in the return pipework, and is therefore not expected to be an effective control measure, even in the short term.


Difference between ‘purging’ and ‘flushing’


What is the difference between purging and flushing? Armed with the knowledge that infrequently used outlets should be flushed, a keen member of the trades team may find an outlet which has been unused for some time, and be tempted to immediately ‘throw the taps open’, and flush away madly. This is indeed crazy, because now that person, and anyone in the vicinity, will more than likely be breathing in water containing large numbers of legionellae. This does not follow the COSHH approach, which, as described previously, favours elimination over control. Firstly, the outlet appears to be unused, not infrequently used. The intention should therefore be to remove it, not flush it. However, whether it is to be retained or removed, there will be a need to remove the stagnant water, and the process required is purging, not flushing. The objective is to remove the stagnant water from the outlet fitting and its supply pipework, without creating an aerosol. This may be as simple as connecting a hose with a backflow prevention device and directing the discharged water into a drain or other location without allowing droplet/aerosol dispersal. Once the purging process has been completed, which might also involve local disinfection, the outlet would be safe to be used or flushed. Routine flushing should present no more risk to the person carrying out this task than to the user of the outlet.


Disruption of bioflim


What happens to the ecology of a water system when flushing is undertaken? While the objective of flushing may be to remove stagnant water, an inevitable consequence is that the biofilm of the system will also be disrupted. This may be perceived as a further benefit of flushing, but one could be considered to be ‘playing with fire’. The short-term effect of flushing can be to strip biofilm, releasing Legionella into the water, and


3.0 2.5 2.0 1.5 1.0 0.5 0


Flushing started


Flushing ended


Time Figure 3. The effect of flushing on Legionella count (based on Makin and Hart12


distributing the biofilm around the system, whereupon it can be deposited elsewhere. Biofilm formation by Legionella pneumophila is caused by the growth of planktonic (‘free swimming’) rather than sessile (‘immobile’) bacteria.13


Legionella


initially adheres to surfaces, but detaches over time, the detachment correlating with flow rate. The persistence of Legionella in biofilms depends on the species of other microorganisms in the biofilm. For example, it persists in biofilms formed by Empedobacter breve or Microbacterium species, but not in biofilms produced by Klebsiella pneumonia or other environmental bacteria, which suggests that specific interactions between bacteria influence adherence. A study14


of 50 strains of 38 Legionella


species has found that Legionella pneumophila has the greatest ability to form biofilms. The quantity, rate of formation, adherence stability, and structure of Legionella pneumophila biofilms depended on nutrients, temperature, and surface material. Biofilms formed at 37˚C and 42˚C were mycelia mat-like, and were composed of filamentous cells, while at 25˚C, cells were rod-shaped. The filamentous cells were found to be multinucleate, and lacked septa. These cells rapidly gave rise to a large number of short rods in a fresh liquid culture at 37˚C. The filamentation of Legionella could be a strategic regulated response, to rapidly increase its population, rather than an accidental response to stress. These experiments were in a nutrient-rich environment. In a nutrient-poor environment, one would expect less biofilm to be formed. The Legionella pneumophila strategy may be to compete with fast growers in the environment, by anchoring itself at a location transiently flooded with nutrients, increasing rapidly in biomass, while being able to access the nutrients by eliminating the process of septum formation, and then proliferating and


dispersing in great numbers when conditions are appropriate.


Flow-rate dependent The formation of biofilms and concentration of bacteria in the water of copper and polyethylene pipework systems are dependent upon flow rate and nutrients,15


).


both of which are affected


by flushing. Rapid changes in water flow rate resuspend biofilms and sediments which increase the bacterial counts in water. The issue of biofilm growth in pipes, and the factors which affect it, are not just of importance to building operators; they are equally relevant to water undertakers. The United States Environmental Protection Agency has published guidance on the subject of the control of biofilm growth in drinking water distribution systems.16


Water


Discolouration is caused, and EPS are mobilised during flushing, in a wide range of conditions. It has been demonstrated that biofilm formation can be controlled in public water supply networks using a combination of chlorination and flow control.18


industry research has shown that the composition of biofilms in terms of the number of bacterial cells and extracellular polymer substances (EPS) are affected by flow rate and the variation in flow rate.17


How can flushing go wrong? Flushing can flatten storage temperatures unless it stops when the correct hot water temperature is detected at the outlet. I suspect the guidance did not anticipate such a large number of infrequently used outlets. I have demonstrated to clients using BMS plots that flushing flattens calorifier temperatures. It can also ‘drag’ stagnant water around the system. If these two effects happen concurrently, the opportunity for pasteurising the Legionella in the calorifiers would be lost. This is illustrated by the potential scenarios in Figures 1 and 2. There are other potential risks with


November 2020 Health Estate Journal 59


Legionella log10


cfu/mL–1


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