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Micro-organisms thrive in the presence of water and nutrients. The movement of water creates an unstable environment for survival and, consequently, micro-organisms have evolved in a manner that allows them to adhere to substrates with great tenacity. When they find a suitable surface they attach themselves and initiate rapid multiplication, with new cells adhering to each other as well as extending laterally across the surface. This creates a "woven matrix colony" in which other organisms can grow and develop - the woven matrix is often referred to as "slime".
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There are a variety of nutrient sources within water systems including scale, surface corrosion products and other trapped organic and inorganic matter. The movement of the water ensures that biofilm colonies receive a constant supply of nutrients. When this is combined with specific conditions of temperature, often found in and around calorifiers, heat exchangers etc, the perfect conditions exist for rapid microbial growth. The population will predominantly consist of aerobic and anaerobic bacteria but there will also be fungal and viral species which, in turn, support predatory organisms such as protozoa and metazoan which ingest bacterial cells as a food source.
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Biofilm forms
an ideal habitat for pathogenic bacteria, such as legionella,
to grow and proliferate. An absolute link has been established
between the presence of amoebae (protozoans) and high
levels of legionella species in water systems. Amoebae
usually ingest bacterial cells for food but, when an
amoeba ingests a legionella bacterium the legionella
colonises the amoeba and proliferates within it. This
occurs until the expansion in numbers can no longer
be contained, the amoeba bursts and huge numbers of
legionella are released into the water. These high numbers
of legionella may then be transported to outlets and
disseminated into aerosol sized droplets. Inhalation
of these aerosols leads to Legionnaires Disease. |
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The control of biofilm within water systems is, therefore, essential and as climate conditions change cold water systems are increasingly reaching temperatures conducive to biofilm growth and pathogen proliferation.
Biofilms are costly - they have an impact on the energy requirements of a system, cause corrosion and increase frictional drag - their presence poses a constant threat to health.
Energy Costs - biofilm is 85-95% water and
acts as an insulating layer of stagnant water. As water has
on fifth the thermal conductivity of carbonate scale, a 1mm
layer of biofilm has the insulating properties of 5mm of scale.
This results in greatly reduced heat exchange efficiencies.
Corrosion - anaerobic sulphate reducing bacteria
can form and live in biofilm. These bacteria produce sulphuric
acid which causes rapid corrosion of pipes. Anaerobic iron
oxidising bacteria can cause serious corrosion to pipes, leading
to high maintenance costs and loss of production.
Frictional Drag - as a biofilm develops in
a pipe it reduces the internal diameter. This causes frictional
drag, pressure drops and a need for increased pumping power.
This results in reduced efficiency and increased costs.
Using Chlorine Dioxide as a part of a water hygiene management programme will eliminate biofilm and maintain a safe, efficient and cost effective water system.
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