Keeping pseudomonas under control
Steven Booth of Guardian Water Treatment discusses the problem of pseudomonas and a best-practice approach to tackling it.
Poor water hygiene is commonly talked about in terms of legionella and the risk it poses to human health. Apart from causing illness in people, bacteria-filled water also creates corrosion and system fouling, compromising the efficient running and lifespan of closed-circuit water systems in particular. While legionella is still present in these waters, the main ‘fouling’ culprit is pseudomonas.
Pseudomonas poses a particular risk to the efficient running of closed-circuit water systems. While less well-known than legionella, if left untreated and not considered at the construction stage, pseudomonas can pave the way for fouled systems that can be hard to rectify once the bacteria has taken hold.
Any stagnant or slow-flowing water will give pseudomonas the perfect conditions to grow and multiply, leading to biofilm formation and other problems such as corrosion and the rise of sulphate-reducing bacteria. These bacteria metabolise naturally occurring sulphate in the water to produce sulphide under clumps of bacteria, depolarising the metal surface, resulting in localised pitting corrosion and eventual perforation.
In general, bacterial build-up in the water, especially in stagnant areas, can create a microbial biofilm layer on pipe and heat-exchanger surfaces, causing a reduction in efficiency and flow restrictions.
One of the key exacerbators of pseudomonas build-up in large commercial projects is dead legs in the water system. The risk of bacterial growth in dead legs is well documented for heating and cold-water systems, but the same problem also occurs in closed systems or if certain areas of the system are not circulated regularly.
Often included to make way for future expansion, these potentially vast areas of stationary water present the ideal conditions for bacteria to grow. Once bacterial growth is present on a grand scale it can be very difficult to get back under control. While removing ‘dead legs’ altogether may not be practical, we suggest that designers include a bypass or flow around the dead end to combat the problem and implement a valve-exercising regime to create full circulation.
Pre-commissioning cleaning is a standard requirement of all new closed water installations under the latest BSRIA guidelines. This approach is far easier and more cost effective than cleaning up existing systems that have been left to the mercy of bacteria. Remediation cleaning is possible, but it will never render a system ‘as new’.
There is a range of pre-commissioning cleaning techniques.
• System dynamic flushing removes debris from the system to reduce the potential for blockages by maximising flow rates through main risers and sub branches.
• A full biocide wash to remove bio-films and bacteria from the system.
• Chemical cleaning. Following commissioning, movement of the pipe-work due to thermal expansion or contraction may cause contaminants, such as iron oxide (rust), to be released into the fluid stream. Chemical cleaning loosens surface deposits so they can be removed from the system to create a stable surface within the pipework, which if maintained by an ongoing water-treatment regime will inhibit further corrosion.
• System inhibiting. On completion of the chemical clean, the system is dosed with a suitable corrosion inhibitor and biocide to ensure ongoing protection from water, metal corrosion and bio-fouling.
• Back flushing. All terminal units should have been isolated during the pre-flushing procedures. The purpose of back-flushing is to remove any small debris that has collected within these terminals.
To maintain water system cleanliness, there are non-chemical alternatives to chemical dosing, which will also reduce maintenance requirements. Photo-catalytic water purifiers greatly reduce bacterial levels in the water, using a specific frequency of light and photo-catalytic surfaces to create free radicals that break down harmful micro-organisms and other pollutants in water. The radicals are short lived and exist for only a few milliseconds, which means they have no possibility of leaving the reaction chamber.
Our own trials of the Wallenius AOT (anti-oxidation technology photo-catalytic water purifier — pictured), can produce a log 5 reduction (100 000 times) in legionella bacteria levels in the water passing through it, killing 99.999% of micro-organisms, including pseudomonas. We have several example projects where we have eliminated 100% of detectable pseudomonas that have entered a system from the mains water.
It’s a much repeated phrase, but one that rings true time and time again: prevention is better than cure, and to ensure the on-going hygiene of closed circuit water systems the right steps must be taken during its construction. Preventing biofilm from forming in the first place is essential; once biofilms start to form, the bacteria that reside within them can be thousands of times more resistant to biocides than those found free floating in the water, so stopping this film will pay dividends to the building owners and facilities managers responsible for maintaining these systems.
Steven Booth is associate director with Guardian Water Treatment.