Working together for clear water

Sampling water quality in an upland stream

Sampling water quality in an upland stream

23 February 2015 by Chris Freeman

Nobody wants peat in their tap water, and in some situations it can even threaten people's health. Getting rid of it is costly for water companies, and a warming climate will increase the problem. The more they know about these changes, the better they'll be able to cope.

As water runs off upland landscapes into reservoirs, it washes organic matter out of the soil. This dissolved organic carbon (DOC) is a major headache for water companies; it gives tap water a peaty tinge and sometimes creates strange smells and tastes. Usually this isn't dangerous, but few customers appreciate drinking brown water.

More seriously, DOC can cause microbes to breed in water supplies after they've been treated, carry pollutants like metals or pesticides or even react with the chlorine that's used as a disinfectant to form by-products like trihalomethanes (THM), which are suspected of being carcinogenic. All this means water companies have to pay a lot of money to remove carbon from their water before it can be sent to consumers, particularly when that water has flowed over peaty soil on its way to the reservoir.

The problem is only likely to increase. Average DOC levels in water running off upland catchments in north-west Europe and North America have been rising for decades. Climate change will make this trend worse, bringing more extreme weather that will wash more carbon out of the soil over short periods. Treatment facilities need to get better at dealing with sudden spikes of carbon, or we risk corresponding spikes of pollutants in our drinking water.

Professor Christopher Freeman and his team at Bangor University have been working with Dwr Cymru Welsh Water to understand the role of different water-treatment methods in removing DOC and other precursors of THM. They worked alongside the company's researchers to spend a year monitoring the output of a particular treatment works whose water contains a lot of DOC, because it comes from a reservoir that's fed by a large upland catchment of peatland, grassland and conifer forest.

Water treatment plantAmong other things, their study revealed that the way DOC was being removed at the treatment works makes the whole process more vulnerable to spikes of the chemical precursors of THM as the climate changes. The company has to maintain disinfection by-products at extremely low levels, both to comply with the law and to avoid endangering its customers' health, and Freeman's work shows a changing climate will put it at more risk of breaching these limits. The research also revealed seasonal patterns in THM risks - it turns out they are greater in spring and summer.

The scientists sampled the water throughout the treatment process, providing the first in-depth insight into the complex chemical interactions that take place as the carbon is removed. Dwr Cymru Welsh Water says Freeman's work, published in Science of the Total Environment, has been vital in helping it create a long-term plan to deal with the problem, forming an important element of its asset-management strategy.

"Knowledge that DOC levels are on an increasing trend allows us to plan strategically for the future," says Peter Perry, the firm's operations director. "Removal of DOC is the major treatment cost involved in producing potable water across the industry."

He explains that the insight into the trends in DOC and its effects that Freeman's research provided has let the company identify treatment works that are at risk of breaching limits on disinfection by-products, and to make better-informed decisions about what to do about the problem. Possible responses range from investing in better DOC treatment equipment, switching from chlorination to alternative disinfection techniques, or diluting with water from another source. If none of these are cost-effective, abandoning especially problematic works and piping water from elsewhere may even be necessary.

According to Perry, the research has been invaluable in helping the company understand the risks it faces. "Their findings are continuously helping to improve our knowledge and understanding of the complex interactions within our drinking water catchments, and provide a valuable insight into how they affect our ability to produce drinking water," he explains.

"NERC funding got us involved in a whole series of studies that have been of huge interest to us as environmental scientists," Freeman says. "But there's something incredibly rewarding about seeing your work prove of value to an end user like Dwr Cymru Welsh Water, and to realise it is helping their customers."

Professor Chris Freeman is a member of the School of Biological Sciences at Bangor University. The research was funded by the EU, by Dwr Cymru Welsh Water and by NERC.