Bacteria can penetrate many systems and are often introduced by tiny insects which are more adept at gaining entry. These bacteria, once in the system, are not easily dislodged. Inside the system the bacteria gravitate towards the surfaces of the water tanks and pipes, where nutrient levels of adsorbed organic molecules are at their richest. Owing to their small size, the bacteria could very easily be swept away in the water flowing through the system. To prevent this happening, they secrete a polysaccharide glue-like substance that fixes them to the surfaces and enables them to pick out nutrients carried along in the water. This polysaccharide matrix is what makes up the biofilm - surrounding each bacterium in layers of slime and creating a miniature "static pond" for it, safe from disruptive water currents. The biofilm also acts as a barrier to biocides like chlorine, which react with its surface layers while letting the protected bacteria population grow unchecked, until they break out of the biofilm to cause disease and infection, such as an outbreak ofLegionnaires Disease.
Problems Associated with Biofilms
AESTHETIC QUALITY:
The build up of general bacteria and biofilms in water systems is a major source of taste and odour problems. Water stored for even a short period can develop a musty odour and taste. Biofilms can also cause turbidity and colour in the water. Chlorine can be a major source of taste and odour problems long after the system has been disinfected and flushed. Also, evidence shows that chlorine generally has no effect in removing biofilms from a system.
BACTERIA:
Once a year chlorination does not control biofilms in the distribution system. Where biofilms are present on heat exchange surfaces, efficiency is greatly reduced, and energy costs increase. Biofilms can trap Legionella and provide nutrients for Legionella growth.
SCALE:
Scale is a major problem in both hot and cold water systems, dripping taps can deposit scale in the tap, and with high ambient room temperatures, provide an ideal growth medium for LP. In hot systems, scale can trap Legionella and biofilms. This provides a perfect growth medium which disinfectants cannot penetrate. Scale deposits colonised by Legionella can continuously recontaminate a system, even after disinfection. Biofilms are a source of nutrients for LP and can lead to taste and odour problems from the products of their metabolism. Scale is a major cause of inefficiency of hot water systems; scale on heat exchange surfaces dramatically reduces the heat transfer efficiency and promotes corrosion in the calorifiers and pipework. Descaling of a hot water system is time consuming and
expensive. Water softeners can reduce scale, but there is growing concern over the increase, to sometimes high levels, of sodium in the water.
DEADLEGS:
Deadlegs are parts of the pipework system that have been closed off and are no longer used. We can also include sections of pipework which are only used occasionally, or sections in a building wing or extension that is closed for long periods. Deadlegs are ideal growth areas for bacteria and LP. The water stagnates and can become heavily contaminated. In sections which are closed off, water can seep back, carrying high levels of bacteria to colonise the main system. In sections which are used infrequently, or are closed down for long periods, the same heavy contamination can occur, with potentially worse risks to users, as the water will, at some point, be used when the area which the system supplies is reoccupied.