This polysaccharide layer, known as a biofilm, forms where medications or other additives are added to the water. It can be anywhere between 5-500 micrometres thick. In other words the thickness is unlikely to be much more than about 0.5mm even in the worst example. But it acts in a similar way to a culture medium by supporting the growth of various micro-organisms that attach themselves to the inside of the tube.

Expect the build-up of biofilm to be greatest at the end of the line. The flow rate there will be extremely low, so the water warms up more. This then means the biggest biofilm deposit and therefore the highest risk of contamination and infection.

The film will be harbouring a decidedly heterogenous population of organisms. On the outer surface, where more nutrients are to be found, there will be aerobic bacteria such as Pseudomonas. The level closest to the pipe has anaerobes bacteria such as sulphate-reducing Desulfovibrio. In between, we find aerophilic bacteria (Legionella and Campylobacter), air-tolerant anaerobes and also some nitrate reducers (not least of which would be E. coli).

Communities of symbiotic micro-organisms will develop. Different characteristics emerge for those microbes adhering to the pipe, compared with their 'free-floating' counterparts. A stem-like growth appears on the immobilised bacteria immediately after their adhesion, that allows them to create a matrix structure. They optimise their cell concentration and change outer membrane proteins to be more hydrophobic while also developing other protein elements which confer an increased resistance against stress factors.

Of course these bacteria are unpleasant and undesirable as direct health risks when they are consumed in the drinking water. Biofilms have another disadvantage: they can cause a form of microbiologically induced corrosion in metal pipes. The real culprits are their anaerobic bacteria, converting sulphate into sulphides that then corrode the iron of the pipe so the water becomes red. The sulphides also adversely affect the odour and taste of the water.

One result of the post-attachment change in bacterial cell structure described earlier is that the biofilm bacteria are now considerably more tolerant of classic biocides such as chlorine. But we cannot rely on chlorine in any case for the essential task of removing the film from a drinker line. It will not even penetrate past the surface, quite apart from the fact that chlorine is neutralised by contact with organic matter so it has less effect at the end of the line than at the start.

The most sure way to remove the biofilm is by oxidation. In our view this should be accomplished by using stabilised hydrogen peroxide. Our product CID 2000 combines the oxidant with organic acids, allowing the water to be sanitised and acidified simultaneously by continuous treatment. Where the clean-up is between batches in an all-in/all-out system, however, we find from experience that it is advisable to flush the main supply pipes once the film has been dissolved. Bring the dirt out physically through flushing and you know you will start the next batch with clean lines to the drinkers.

June 2006