Water & Wastewater Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/1040774
16 | NOVEMBER 2018 | WWT | www.wwtonline.co.uk • CHLORINE SAFETY W hile chlorine remains the universal choice for UK water companies, the form the chlorine takes varies. Chlorine gas was once the mainstay but it poses health and safety risks, leading to increased use of sodium hypochlorite – which contains chlorine in liquid form – as well as onsite electro-chlorination. Many treatment works are situated near residential areas, and the repercussions of a major unplanned chlorine gas release could be significant: used as a chemical weapon in the First World War and in Syria in recent years, it can have a fatal impact on the respiratory system. While deaths are unlikely in the case of a gas leak from a treatment works, particularly when safeguards are put in place to shut down the system if there is a release, it remains a serious threat to health, and those working onsite are at greatest risk. The larger sites must also contend with the dangers and expense of safely transporting and storing chlorine gas drums of up to 1,000kg, with the COMAH (Control of Major Accident Hazards) regulations understandably onerous. As a liquid, sodium hypochlorite does not pose the same issues. There are some drawbacks – it is only around 10 per cent chlorine, which means it must be stored in far greater quantities than chlorine gas, and its alkalinity means measures must be taken to prevent limescale build-up – but these can be weighed against its health and safety advantages and relative ease of use. Over recent years, several companies have started to move away from chlorine gas, particularly at their largest sites, but Yorkshire Water is in the process of becoming the first UK water and sewerage company to eliminate chlorine gas – as well as sulphur dioxide – from its water treatment processes entirely. "We had a re-evaluation of health and safety and it's driven quite a lot of changes throughout the business," Mark Broady, Yorkshire Water project manager, says. "Our CEO, Richard Flint, said we're going to be the first water and sewerage company to go completely away from gas treatments. "It's about removing the risk of having any releases that could affect our guys operating the plant or the public if they're in the area." Galliford Try is the main contractor on a programme to replace the chlorine gas with sodium hypochlorite – as well as sulphur dioxide with sodium bisulfite – at nine sites. Doncaster, York and Beverley are large treatment works that required the 1,000kg drums for chlorine gas; the remaining six sites relied on smaller cylinders of up to 87kg. At four of the sites, modular kiosks featuring everything required for treatment are being used to directly replace the existing systems. For the rest, Galliford Try is reutilising existing apparatus or buildings that were on site to install new dosing equipment, which requires the use of temporary dosing systems while the upgrades take place. Broady says there have been some "slight teething problems" with the modular kiosks in terms of ensuring compatibility with the other equipment on the sites, and the weather this summer led to supply and demand issues that held up work and mean the £14 million project is likely to take around two years. Where systems are up and running, though, staff are already seeing the benefits. Nigel Taylor, Galliford Try project manager, says: "Keld Head Water Treatment Works is running on a permanent sodium hypochlorite dosing system and it's running very, very well. It's less problematic than the gas because they don't get little leaks when they're changing things over. Those are planned releases, but you need to have several workers with the training and breathing apparatus to do it, so it takes time. With hypochlorite, you don't have any of that." Caption The Works: Drinking Water Treatment A safer dose: sodium hypochlorite treatment is being deployed at Yorkshire Water sites Modular kiosks are being used which directly replace the existing systems