Water & Wastewater Treatment

34 | october 2014 | WWt | www.wwtonline.co.uk In the know Technically speaking: water treatment Chichester In 2012, the environment Agency agreed to the use of UV disinfection of settled stormwater discharges as an interim medium-term solution to the prolonged discharges into chichester harbour from the storm tanks at chichester WWtW. Here the prolonged spills are caused by substan- tial amounts of groundwater infiltration in sewers as a consequence of an abnor- mally high groundwater table in the area. the environment Agency requires Southern Water to progressively reduce the infiltration and minimise spills over time in line with the standards of the UW- WtD. conventional control measures would result in the construction of significantly larger storm storage to limit spills to the environment, but once full would typically not empty for long periods. All parties, Southern Water, the environment Agency and the contractor (4Delivery) worked together to resolve the challenges around permit- ting designing constructing and commissioning the UV plant in less than a year. the new UV plant disinfects all spills from the existing storm tanks up to a maximum flow rate of 300l/s. the Storm UV plant has been in operation since March 31st 2014. Scarborough UV disinfection of stormwater at Scarborough forms part of Yorkshire Water's £110m in- vestment programme in bath- ing water along the Yorkshire coast aimed at achieving the rbWD 'excellent' standard. An optioneering phase followed on from detailed catchment investigations, a marine mod- elling exercise and pilot scale testing. A er discussions with key stakeholders including the environment Agency, a combination of storage at two locations and UV disinfection of settled storm sewage at a third location was identi- fied as the best solution to achieve 'excellent'. Here, the environment Agency was able to approve the long term use of storm UV disinfection because it was provided over and above the minimum standards required by the UWWtD. the scheme aims to provide an enhanced level of service by achieving the 'excellent' rather than the required 'Good' bathing water quality objective. the disinfection element of the scheme requires storm flows of 850L/s to be transferred from a sea front pumping sta- tion to the existing Scarbor- ough wastewater treatment works where 9,300m3 of new storm tank capacity is being constructed . A er settle- ment, flows will pass through the UV irradiation system before discharging by gravity through a long sea outfall. the system was in operation for the start of the 2014 bath- ing season. Millom to improve the quality of the Duddon estuary and meet Shellfish Waters and Bathing Waters Directives, untreated spills from the two discharge locations in the Millom area need to be reduced. the con- ventional solution was to con- struct 6,000 m3 of additional stormwater storage. However, this had a number of issues including minimal land avail- ability; significant carbon and capital construction costs; 3rd party issues and the signifi- cant impact on the treatment works of returning such large stormwater volumes. In contrast, a solution which increases flows to treatment and disinfects the settled stormwater discharges cost £1.6 million less and has significantly lower whole life carbon emissions. there was agreement to permit end of pipe standards with a commitment to review these in future. the new UV irradiation plant has been be in operation since autumn 2013. ing end of pipe standards, so it will need independent evidence supporting the UV dose and reactor selection. It is important to note here that the Urban Wastewater Treatment Direc- tive (UWWTD) requires sewer systems to be maintained to meet minimum standards of sewer capacity and performance. This was highlighted by the recent ECJ judgment that found the London Thames Tideway and Whitburn systems were in breach of the Directive. Therefore, it is not suf- ficient to simply mitigate the polluting effects of the discharges. The Environ- ment Agency cannot accept storm disinfection as a long term alternative to maintaining an adequate network. Far from being a "standard solution", the Agency will need to consider on a case-by-case basis the acceptability of disinfection of intermittent storm discharges for providing either interim or supplementary solutions to UWWTD and other Directives. Acknowledgements: the authors, christy White, rob Mctaggart, and Peter Loughran, would like to thank the envi- ronment Agency, Southern Water, United Utilities and Yorkshire Water for their support in producing this article. In action Here we summarise three recent cases where the environment Agency approved UV disinfection of intermittent settled stormwater discharges. In each case the inclusion of stormwater disinfection provides a more sustainable way of meeting microbial targets than storage alone would. the other key common factor in all of these projects is that on-site sampling and analysis work was used to determine the disinfection requirements, dose–response relationships and design envelopes on a site specific basis, coupled with the selection of UV reactors where the performance has been independently validated using biodosimetry. AGRU Pipe and fittings from IPS HDPE Extensive range of pipe and fittings from 20mm to 1400mm. Electrofusion fittings up to 500mm. 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