This technical troubleshooting guide looks at the control of biofilm in industrial cooling water systems. The guide examines the problems caused by bacteria and the growth of biofilm including system corrosion, reduced heat transfer efficiency, and the increased health risks from Legionella bacteria. It concludes by looking at bacteria hot spots and what measures can be taken to maintain control.
Biofilm growth in cooling water systems
Experts will all agree that one of the trickiest issues to deal with in any industrial cooling water system is the build-up of biofilm on internal surfaces, especially those associated with heat transfer processes.
The growth of biofim typically starts to develop in areas which are inaccessible or hard to reach by water treatment biocides. However, if left untreated the biofilm affecting these areas can grow, quickly developing into larger deposits which can cause damaging corrosion inside the system, create blockages, and provide the perfect environment for legionella and other bacteria to thrive. It’s also important to realise that if biofilms are allowed to become established inside a cooling water system, they can be very difficult to remove.
Biofilms aren’t just the preserve of badly-maintained cooling systems – they can develop in well-regulated water systems too. So, which factors make them hard to control, and what steps can be taken to tackle them?
What is biofilm?
Let’s start by looking at biofilm and what it is. A biofilm is a community of bacteria, it attaches itself to wetted surfaces and is protected by microbial secretions. Biofilms are everywhere, and whether you measure by weight or by number of organisms, they are the most plentiful lifeform on our planet.
Biofilms aren’t all bad news however; they have an important part to play in the planet’s ecology. They can be found living on many surfaces almost everywhere on Earth.
Where do biofilm flourish in cooling water systems?
Unfortunately, in most man-made water systems, biofilms are unavoidable. The only way of deterring them is by using extreme control measures, such as those used in the high tech worlds of sophisticated electronics and pharmaceutical industries.
In most organisations which use open cooling systems or use cooling towers, controlling the growth of biofilm is a particular challenge. This is because it’s easy for dust, debris and organic matter to enter the system, and this acts as food for the bacteria. Anyone managing a cooling water system has the job of “minimising” the effect biofilm has on heat transfer efficiencies, water flow, corrosion and disease risk, rather than pursuing a policy of complete elimination.
How biofilm helps to protect bacteria
The overwhelming majority of the world’s bacteria are found inside biofilms – it’s their preferred way of living. Bacteria living with other bacteria in a biofilm (sessile) have a better chance of survival than free-floating bacteria in water (planktonic).
All water, unless completely sterile, will have bacteria in it, and a high percentage of these bacteria won’t show up on conventional tests. Given the right environment, bacteria will start producing a sticky substance when they land on a surface, and in days, this can grow into a mature colony of bacteria and other microorganisms which starts to cause problems, especially inside engineered water systems such as chilled or cooling water systems.
Biofilms have a complex structure, and they are designed to shield the colony of bacteria from external attack. Biofilm can often adapt to whatever chemicals you add into the water system, and even remain after mechanical cleaning. If an environment becomes too hostile, biofilm can survive by releasing bacteria into the water, looking for another surface to colonise.
Controlling biofilm in cooling water systems
Most biofilms start to develop in parts of a cooling water system which can’t easily be reached by biocides. This could include areas of low water flow, inside dead-end pipes, on tower packing surfaces or under debris in the cooling tower basin. Cooling tower systems are known as being particularly prone to biofilm contamination, given their supply of dusts, nutrients, and often complex piping.
Once you have a problem with biofilm in your cooling system, it can often be tricky to deal with. The sticky secretions produced by the bacteria fix the colony firmly to internal surfaces, and form an outer shell which traditional biocides find hard to get through.
Historically, the most commonly used chemicals when attempting to control biofilms have been bromine or chlorine. However, these chemicals can only penetrate the outermost layers of the biofilm. Other chemicals can enter into the biofilm, but might only kill some types of bacteria and then stop working after a period. Over time, bacteria populations can change composition to favour the bacteria which can resist biocides and this can pose serious problems to the efficiency and safety of the system.
How to manage biofilm growth in cooling systems
Dealing with biofilms and bacteria in a cooling water system isn’t just a matter of adding a few chemicals in the hope that this will solve the problem. Managing this problem effectively and adapting your control programme over time requires a comprehensive approach. Companies must put together a detailed plan with a regular programme involving system maintenance, water sampling and ways of dealing with biofilm and bacteria when they are encountered. At many companies this is often too big and too complex a job to be managed in-house, and most organisations will work with a specialist water treatment expert such as Water Treatment Services to develop a cooling water treatment plan which is tailored to their site specific plant and equipment.
Cooling water system experts
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