Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | ocTober 2014 | 33 In the know Technically speaking: water treatment T he introduction of the revised Bathing Water Directive, (rBWD) legislation in 2006 renewed in- terest in measures to attain the highest BW classification "Excellent". Bathing beaches throughout the European Union are required to comply with the rBWD by 2015 and in 2008 new Regu- lations were made in the UK to bring this into effect. The conventional solution is to provide stormwater detention tanks to limit the volume and frequency of untreated wet weather discharges. The resulting storage volumes can oŠen be tens of thousands of cubic metres. Raised infiltration following rainfall may prevent such large volumes being emptied before the next event. With- out increasing the treatment capacity of the works, or reducing the infiltra- tion, a storage option alone may not be viable. The hunt is on Conventional solutions are expensive and take time, and deadlines for BW compliance are oŠen tight. So we need to discover other ways of delivering the same environmental outcome more quickly, cost effectively, and sustain- ably. An approach recently considered is to disinfect stormwater using ultravi- olet (UV) irradiation, before discharg- ing it to the receiving water. UV irradiation is the most used technology in the UK for disinfecting final effluent discharging to bathing UV disinfection of wastewater & stormwater collaborative approach delivers new way to plan and permit wastewater and stormwater disinfection and shellfish waters. The Environment Agency's approach to permitting UV disinfection systems in England and Wales is described in EPR7.01. It has developed over the past 15-20 years, but is substantially unchanged since its introduction in the early 1990s. There was no guidance on permitting stormwater discharges. Following extensive negotiations with the Envi- ronment Agency, in 2009, Welsh Water commissioned a settled stormwater disinfection plant at Cog Moors STW, Cardiff. Permitting this new activity followed an approach similar to that used for final effluents over the past 20 years. A new approach Following the success of the Cog Moors plant, the Environment Agency and the water industry, supported and advised by MWH, have together developed a new way to plan and permit wastewater and stormwater dis- infection. In addition to extending the current guidance to include stormwa- ter disinfection, other key drivers for this move were to accommodate recent developments in technology and inter- national design practices. The industry views UV disinfection as being a high power/carbon process and there is sig- nificant drive to become more efficient and reduce the power consumed in operating these facilities. Although a recently completed UKWIR project (13/WW/17/15) identi- fied that typically there was scope for improving power consumption, particularly on multi-channel instal- lations, a key concern was the lack of certainty of what the existing target dose value (e.g. MAD) could achieve on a site-specific basis. For continuous discharges the Agency has committed to moving to permit a Validated Dose. Validation using biodosimetry is based on the use of standard microbes to test the log reduction achieved by a given UV system under controlled conditions. This approach follows international best practice that was originally devel- oped for potable water applications, has become accepted practice as part of recycled water and water reuse schemes in countries such as the USA and Australia and has been extended to wastewater discharges globally. The key advantages brought by this approach are: • Data collection allows site spe- cific targeting of the appropriate UV dose. The validation process links the target UV dose directly to the microbial reduction per- formance achieved for that site, and results in a high level of confidence that the disinfection objectives will be achieved, while minimising power use through suitable control strategies; • As the performance of the UV reactor is tested over a range of inlet conditions (e.g. UV trans- mittance, flow) during validation, a reactor can be applied to any wastewater type, provide it fits within the range of parameters tested. It is therefore not neces- sary to make any distinction between stormwater and final ef- fluent; the same methodology is used for the design and selection of a reactor system in each case. • Moving to a UV system that has been designed to meet a target dose validated by biodosimetry provides a level playing field be- tween the manufacturers; if two reactors are designed to deliver the same validated dose, they will achieve the same disinfec- tion performance. Importantly, the method in EPR7.01 remains unchanged; permitting is still based on selecting a target UV dose. The main difference is how the target UV dose is determined and the need for validation of the reactor performance. The Environment Agency will be per- mitting the process, rather than specify- ChrisTy WhiTe PrinciPal Process engineer MWH global rob MCTAggArT senior PrinciPal consulTanT MWH global PeTer LoUghrAn senior Process engineer MWH global ►

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