Technically Speaking: UV disinfection of wastewater and stormwater
Collaborative approach delivers new way to plan and permit wastewater and stormwater disinfection
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.
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 substantial amounts of groundwater infiltration in sewers as a consequence of an abnormally 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 UWWTD. 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 permitting 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.
UV disinfection of stormwater at Scarborough forms part of Yorkshire Water’s £110m investment programme in bathing water along the Yorkshire coast aimed at achieving the rBWD ‘Excellent’ standard. An optioneering phase followed on from detailed catchment investigations, a marine modelling exercise and pilot scale testing. After 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 identified 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 station to the existing Scarborough wastewater treatment works where 9,300m3 of new storm tank capacity is being constructed . After settlement, 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 bathing season.
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 conventional solution was to construct 6,000 m3 of additional stormwater storage. However, this had a number of issues including minimal land availability; significant carbon and capital construction costs; 3rd party issues and the significant 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.
By Christy White, Principal Process Engineer, MWH Global, Rob McTaggart, Senior Principal Consultant, MWH Global, and Peter Loughran, Senior Process Engineer, MWH Global
The introduction of the revised Bathing Water Directive, (rBWD) legislation in 2006 renewed interest 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 Regulations 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 often be tens of thousands of cubic metres. Raised infiltration following rainfall may prevent such large volumes being emptied before the next event. Without increasing the treatment capacity of the works, or reducing the infiltration, 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 often tight. So we need to discover other ways of delivering the same environmental outcome more quickly, cost effectively, and sustainably. An approach recently considered is to disinfect stormwater using ultraviolet (UV) irradiation, before discharging it to the receiving water.
UV irradiation is the most used technology in the UK for disinfecting final effluent discharging to bathing 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 Environment 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 disinfection. In addition to extending the current guidance to include stormwater disinfection, other key drivers for this move were to accommodate recent developments in technology and international design practices. The industry views UV disinfection as being a high power/carbon process and there is significant drive to become more efficient and reduce the power consumed in operating these facilities.
Although a recently completed UKWIR project (13/WW/17/15) identified that typically there was scope for improving power consumption, particularly on multi-channel installations, 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 developed 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 specific targeting of the appropriate UV dose. The validation process links the target UV dose directly to the microbial reduction performance 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 transmittance, 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 necessary to make any distinction between stormwater and final effluent; 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 between the manufacturers; if two reactors are designed to deliver the same validated dose, they will achieve the same disinfection 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 permitting the process, rather than specifying 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 Directive (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 sufficient to simply mitigate the polluting effects of the discharges. The Environment 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.
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