Severn Trent gets biological

With annual cost savings of £60,000, a comprehensive revamp of Severn Trent Water’s wastewater treatment site at Loughborough has proved a success. ITT Water & Wastewater’s refurbishment of the existing Activated Sludge Plant (ASP) and Final Settlement Tank (FST) Channel mixing plant has contributed to a 30% operational cost saving at the site as Severn Trent Water looks to more cost-effective and greener wastewater treatment technologies.

Serving the university town of Loughborough and the nearby village of Hathern, Loughborough WwTW has a catchment area of up to 68,000 people. New targets from the Environment Agency (EA) meant that Severn Trent Water had to look at more effective methods of treating wastewater at the site.

Removing phosphate from the effluent of a sewage works is an essential part of the Urban Wastewater Treatment Directive, and Severn Trent Water wanted to conduct a series of pilot tests to use Biological Nutrient Removal (BNR). Experts from ITT WWW were therefore called in alongside Severn Trent Water’s contractor, Nomenca, to assess the site for areas of improvement.

ITT WWW worked with Nomenca and Severn Trent Water to develop a 3D model of the layout of the plant, enabling all three parties to visualise any potential problems and map out the changes required to create an optimal overall design. To incorporate BNR at Loughborough, the site needed to be restructured and, with cost saving in mind, the team from ITT WWW set about specifying the most efficient processes to complement the treatment processes.

Logical choice

Dale Collison, project leader at Severn Trent Water, says: “We have an environmental obligation to reduce the amount of phosphorus in our final effluent and there are two main ways of doing this. We can dose the works with chemicals, or we can make our process work smarter and make better use of the naturally-occurring bacteria found in the effluent.

“This is not only better for the environment but saves us a lot of money, as we don’t have to buy as much chemical.”

For environmental and cost reasons, BNR seemed like the logical choice. Bacteria within the effluent survive on oxygen, but when the effluent reaches a part of the process that is starved of oxygen, the bacteria feed off the next best nutrients, including phosphorus. The process only works with strong sewage, however, which is not found at Loughborough, so the project needed an extra process to enable it to work effectively.

This process, Return Activated Sludge (RAS) Fermentation, takes a small amount of RAS out of the system for a few days to allow it to ferment. Once the fermented RAS is reintroduced, more phosphorus removal can take place.

As part of the comprehensive refurbishment, staff from ITT WWW and Nomenca addressed the issue of plant shutdown. To keep the ASP facility operational at all times, ITT WWW installed the new air-main pipework above the old air-main pipework using temporary valves and large diameter flexible crossover couplings. This allowed both sets of pipework to be coupled together, allowing the maximum degree of flexibility during removal and installation.

The removal and installation of diffusers for mixing in the FST Channels had to be conducted while effluent was still flowing through the channel, and ITT WWW had to remove the existing diffusers and retrofit lift-in tubular ones to cater for this particular requirement.

Storage space

Experts from ITT WWW’s specialist aeration division, Sanitaire, installed seven 9″ Silver Series II membrane diffusers on to a removable testing grid, which will enable both ITT WWW and Severn Trent Water to monitor the effects of fouling on the Sanitaire diffusers over a seven-year period while simultaneously assessing the site-specific performance.

ABS Centrifugal framework blowers were chosen as their whole life costing is less than the traditional positive displacement type. The air-main pipework feeding ASP lanes 1-4 was positioned at a convenient height to enable good access to the drop-leg valves and actuators from the gantries. This meant safer and easier operation and maintenance while there was no longer any need to work from a height in the tank.

Once low-level pipework was streamlined and redundant air receivers and electrostatic filters were removed, access was improved, and Severn Trent Water now has a large additional storage space at its disposal.

Duncan Leathley, UK sales manager for Sanitaire at ITT WWW, says: “The BNR process at Loughborough is widely recognised as market leading and, along with other site modifications, has led to a significant cost saving. ITT Water & Wastewater demonstrated their ability to offer turnkey solutions to a variety of unique problems as Severn Trent Water reported an anticipated wire to water efficiency

of up to 4kgO2/kWhr.”

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