Heating and Ventilating

Preventing Legionella infections in building services systems - including pipework systems - is a top health and safety priority. Des Dolan discusses how
Legionnaires disease is caused by a bacterium, legionella, that has always existed in water and remains inert at temperatures below 25 deg C. It proliferates in water circuits at temperatures fluctuating between 25 deg C and 45 deg C, meaning sanitary hot water and air conditioning circuits are most at risk from contamination.

Legionnaires disease is caused by inhalation and cannot be contracted from ingestion. Infection occurs when water mists containing Legionella bacteria are inhaled. These water mists can occur anywhere where water is taken from pipework systems, with showers, cooling towers, spas, high-pressure washers and humidifiers being among the most high-risk systems to spread the disease.

These pipework systems can be a major carrier of Legionella bacteria within buildings, therefore it is crucial that steps are taken to prevent growth of this bacteria within the pipe network.

There are a variety of factors that can assist the potential for bacteria growth including stagnant water, dead volumes / loops and temperatures from 25 deg C to 45 deg C. Within pipework systems, the Legionella bacteria develops and fixes in the presence of high concentrations of lime, magnesium, metallic residues and sludge, usually originating from corrosion and scaling. Within hot water and air conditioning circuits, bacteria can develop in pipework systems that are ageing, badly maintained, corroded and scaled.

Product selection has impact

While it is important that pipework systems are regularly maintained to ensure system safety, the initial product selection can also impact on the long-term health of the pipes. Some materials are more likely to assist bacteria growth than others so careful consideration should be given to pipework selection at the outset of a project.

Bacteria settle and develop in biofilm, which easily attaches itself to sharp or jagged parts of a pipe's surface, whether due to the quality of the extrusion of the new pipe, or to the corrosion/scale build up inside the used pipe, so a rougher surface will provide a stronger platform upon which to grow. This is due to the fact that rough surfaces have more surface area for bacterial attachment and growth and also provide more shielding from flow.

The smoothness of the selected pipework is therefore an important factor to consider; while the surface of some metallic pipes is rough, the smooth bore of plastic pipes ensures the ability for bacteria growth is limited. While copper has natural anti-microbial properties, as limescale builds on the used, corroded pipe surface, the biofilms then cling to the limescale and begin to colonise.

Once pipework systems are installed, it is good practice to protect chilled water and heating systems from biofilm development, and the resulting bacteria, growth by undertaking regular flushing and treatment processes from the outset. There are many effective methods that can be used to kill bacteria in pipework systems including flushing through systems, thermal shock and chlorine shock.

Flushing - This will limit bio-film thickness in water systems; shear forces caused by flushing will remove bio-film which extends out into the turbulent flow in the centre of a pipe. Plastic pipework should always be flushed out in accordance with BS5955 part 8.

Thermal Shock - To work effectively, a temperature of 70 deg C must be reached and maintained throughout the entire pipework system over a period of at least 10 minutes, as Legionella bacteria dies above 60 deg C. This solution is the most natural and is very effective for circuits in good condition.

Chlorine Shock - Another proven successful method is water disinfection. Undertake circuit chlorination by filling pipework and tanks with a chlorine mix at a concentration of 15mg per litre of cold water for 24 hours, or 50mg per litre for 12 hours, followed by draining and thorough rinsing of all pipework. Not all pipework is suitable for chlorine shock however and while C-PVC systems have exceptional resistance to chlorine, an ABS system would not be suitable for this method.

While preventative measures are effective, it is also important to ensure pipe safety as far as possible over the lifetime of a system.

The correct design and pipework material is the first area that needs addressing, once this is installed it is then vital to apply preventative measures on a continuous basis and undertake regular and thorough maintenance, sampling, cleaning and treatment. Ensuring there is no water stagnation and allowing good water circulation throughout the system is then an important ongoing check.

By their very nature plastic pipework systems can help combat bacteria growth; the material characteristics offer corrosion and limescale resistant properties, resulting in a smooth bore which can limit and slow the growth process.

If specifiers or consultants are looking for guidance regarding the safety of pipework, regulations and guidelines regarding Legionalla in water applications are constantly evolving.

Dealing with the risk

However, the HSE (Health and Safety Executive) has published HSC L8 (2000) Control of Legionella bacteria in waters. This is the approved Code of Practice and Guidance for the control of Legionella bacteria in water systems, setting out the statutory requirement for dealing with that risk and applies to all premises with a water system.

Proliferation of Legionella bacteria in water networks can become a major health risk, with 6,000 cases of Legionnaires disease diagnosed in Europe every year. The industry needs to be assessing the safety of current systems and carefully considering new specifications to ensure they meet the approved regulations and offer a safe and healthy solution over the lifetime of the system.

// The author is brand manager for Durapipe Building Services //