Bacteria commonly found in drinking water create conditions which enable other - potentially harmful - bacteria to thrive, says new research by engineers from the University of Sheffield.

The research, published in the latest issue of Water Science and Technology:Water Supply, points the way to more sophisticated and targeted methods of ensuring our drinking water remains safe to drink, while still reducing the need for chemical treatments and identifying potential hazards more quickly.

The research team, from the University of Sheffield's Faculty of Engineering, studied four bacteria found in the city's drinking water to see which combinations were more likely to produce a 'biofilm'. Biofilms are layers of bacteria which form on the inner surfaces of water pipes.

"Biofilms can form on all water pipes and as these are usually non-harmful bacteria, they don't present a problem," explained lead researcher, Professor Catherine Biggs. "However, biofilms can also be a safe place for harmful bacteria such as Escherichia coli or Legionella to hide. If the bacterial growth is too heavy, it can break off into the water flow, which at best can make water discoloured or taste unpleasant and at worst can release more dangerous bacteria. Our research looks at what conditions enable biofilms to grow, so we can find ways to control the bacteria in our water supply more effectively."

Funded by the Engineering and Physical Sciences Research Council, the research isolated four bacteria from water taken from a domestic tap: two were widely found in drinking water everywhere, one was less common and one was unique to Sheffield. The researchers mixed the bacteria in different combinations and found that, in isolation, none of them produced a biofilm. However, when any of the bacteria were combined with one of the common forms, called Methylobacterium, they formed a biofilm within 72 hours.

"Our findings show that this bacterium is acting as a bridge, enabling other bacteria to attach to surfaces and produce a biofilm and it's likely that it's not the only one that plays this role," says Professor Biggs. "This means it should be possible to control or even prevent the creation of biofilms in the water supply by targeting these particular bacteria, potentially reducing the need for high dosage chemical treatments."

Domestic water supplies in the UK are regularly tested for levels of bacteria and, if these are too high, water is treated with greater concentrations of chlorine or pipe networks are flushed through to clear the problem. However, the standard tests look for indicator organisms rather than the individual types which are present. Testing methods being developed by the Sheffield team - as used in this research - involve DNA analysis to identify the specific types of bacteria present.

Professor Biggs continued:

"The way we currently maintain clean water supplies is a little like using antibiotics without knowing what infection we're treating."

"Although it's effective, it requires extensive use of chemicals or can put water supplies out of use to consumers for a period of time. Current testing methods also take time to produce results, while the bacteria are cultured from the samples taken.

"The DNA testing we're developing will provide a fast and more sophisticated alternative, allowing water companies to fine tune their responses to the exact bacteria they find in the water system."

Ultraviolet light is highly effective treatment to remove bacteria

Improved detection techniques help illuminate the complex biocide resistant defences and ecosystems deployed by microbial biofilms in drinking water supplies. Fortunately they also help water technologists overcome these microbial threats to public health by improving the effectiveness of disinfection technologies such as UV and chlorine. Such improvements in detection and treatment are helping water utility operators meet their responsibility to protect public health by eliminating pathogenic microbes from the drinking water network. Rising public expectations demand that we implement water treatment approaches which minimise both the potential for pathogenic biofilm build-up and the presence of toxic chemicals in the water system.

One of the most effective techniques for pathogen removal is ultra violet light (UV), especially when it is combined with pre-treatment to remove particulates which can cause 'shadows' that shelter pathogens from the UV light.

UV and chlorine each inactivate microorganisms by different mechanisms. UV destroys DNA and is increasingly used with very low levels of residual chlorine. In this way UV provides a more sustainable alternative to conventional steps of high dose chlorination followed by storage to allow the chlorine to work followed by de-chlorination.  The combined UV+chlorine process thereby minimises chlorine usage, it also minimises the formation of toxic chlorinated by-products such as trihalomethanes (THMs) and minimises the risk of chlorine-resistant microorganisms such as biofilm fragments and potentially lethal Cryptosporidium pathogens arriving at the customer's tap.

Breakthrough in innovative UV technology

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The improved efficiency is due to its unique chamber coating which reflects over 99% of UV light compared with less than 30% reflectance in traditional stainless steel UV chambers. A higher degree of reflection also provides higher UV intensity and uniformity throughout the chamber, making it harder for any organism to avoid treatment. The NeoTech system is designed to be modular and scalable, offering treatment capacities from 20 M3 to 21,000 M3 of water each day. It also provides the extreme UV dose required for effective photolysis and advanced oxidation to remove toxic organics from drinking water and recycled water.

Filtration followed by UV disinfection combined with the required low level of residual chlorine offers the ideal combination for safe, environmentally friendly drinking water.

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