Chlorine Dioxide vs. Biofilms

WHAT IS A BIOFILM?

Biofilm is the natural habitat for bacteria in water systems and forms on any surface in contact with water.  You cannot necessarily see it, but surfaces feel slimy to the touch.  The bacteria migrate from the bulk water to a surface in a low flow or stagnant area of the system and attach by producing a slime layer.  This provides a secure habitat for many bacteria including harmful coliforms (E.  coli), Pseudomonas and Legionella. 

SO HOW DOES WATER BECOME CONTAMINATED?

Biofilm is essentially a layer of stagnant water on a surface in areas of low flow.  The biofilm grows out from the pipe wall until the flow is strong enough to detach clumps from the biofilm matrix.  These contain very high numbers of bacteria which are carried to other parts of the system for re-colonisation, or an outlet where it can enter food products, beverages, or contact with people. 

SOME PROBLEMS CAUSED BY BIOFILM

• Some bacteria living in the biofilm can produce acids that will cause extensive corrosion to tanks and pipes
• Biofilom growth can cause flow restrictions in pipes increasing pumping costs and reducing system efficiency
• Growth on heat exchanger surfaces will greatly reduce heat exchanger efficiency
• The effluent they produce can cause taint, taste and odour problems
• High TVC indicates extensive growth; sometimes you can see it in tanks; and surfaces feel 'slimy'

WHY IS CHLORINE DIOXIDE SO GOOD FOR BIOFILM CONTROL?

Traditional biocides like sodium hypochlorite cannot penetrate biofilm due to lack of flow through the biofilm, they only impinge on the surface layers killing them but leaving the underlying bacteria unaffected. 
Chlorine Dioxide is a true gas (like oxygen) and because it is a dissolved gas it can penetrate the slime layer protecting the bacteria in a biofilm.  Because it is a powerful oxidiser it 'dissolves' the slime layer and kills the exposed bacteria.  A study on biofilm control in domestic water systems carried out by the Building Services Research and Information Association proved that 0.1 ppm of Chlorine Dioxide was sufficient to kill biofilm in a pipe, and 0.4 ppm for biofilm attached to washers at outlets, and in mixer valves.  The Secretary of State for the Environment has approved Chlorine Dioxide for use in drinking water provided that the combined concentration of Chlorine Dioxide, chlorite and chlorate (by-products) does not exceed 0.5 ppm as Chlorine Dioxide.