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CoMag® System Provides Immediate Results

Innovative wastewater treatment upgrade to meet future population and phosphorus limit requirements

Customer Profile

Situated in Staffordshire in the UK, the Severn Trent Cannock wastewater treatment facility serves a population of 65,053. The plant had early obligation AMP7 requirements to meet for controlling phosphorus levels.



Severn Trent was faced with a projected population equivalent for its Cannock site in Staffordshire of 73,077 by 2033. That’s a 12.3% increase in population over a 12-year period, leading to an average daily flow of 226 l/s and flow to full treatment of 486 l/s. In addition, the plant needed to meet tightening phosphorus level requirements from 1 mg/l down to 0.3 mg/l by December 2021.

As is the case for many site constrained wastewater plants, expansion was not an option for Severn Trent, so these objectives had to be achieved within the confines of the existing operational site. This meant any technology solution had to deliver the operational specifications of the early obligation AMP7 requirements, with a small footprint and the lowest whole life cost solution.

An Innovative Solution

Through close collaboration with water treatment specialist Evoqua, ballasted clarification technology, the CoMag® system, was identified as the solution. In one of the first applications of the technology in the UK, the CoMag system was installed as a tertiary solids removal process to lower the phosphorus to these new challenging levels. With the CoMag system, the addition of magnetite (a high density naturally occurring mineral) increases the weight of a conventional precipitated floc. The magnetite easily embeds into the floc and results in the accelerated settlement of the solids – up to 10 x faster than conventional methods. This enables the resulting clarifier to be up to 90% smaller than a conventional clarifier, which in turn results in a small footprint area to process such a significant treatment flow.

Importantly, as well as the CoMag system doing the job of improving effluent quality and meeting new regulatory Environment Agency (EA) quality requirements for water entering the local watercourse, it delivers sustainability and operational benefits for Severn Trent. The innovative technology continuously recovers most of the magnetite, which results in a more sustainable process and lower operational expenditure. It is also fully automated, requiring minimal ongoing operational input.

" Following extensive research and testing, Evoqua delivered our tertiary treatment requirements, as our project specifications were met. With installation now complete (June 2021), sample results indicate that the project has been successful. "

Ian Johnson, Lead Project Manager, Severn Trent


The results of the installation of the Evoqua CoMag system at Severn Trent’s Cannock site were immediate. The total phosphorus level entering the local water course is now frequently below 0.1 mg/l. The CoMag system is therefore significantly reducing the impact on the ecosystem and environment. In addition, the iron level is consistently below 1 mg/l which is comfortably below the EA consent for Fe of 3 mg/l.

The CoMag system has successfully future-proofed the plant and will treat the expected average flow of just over 19,400 m3 per day, with a peak design flow of 48,200 m3 daily.


" The Evoqua CoMag system has met all of its objectives – it’s increased our treatment capacity and improved the quality of tertiary filtration, all without increasing the site’s footprint while meeting our whole life cost requirements. With restricted space available, Evoqua’s solution has provided us with real innovation to meet our goals – even down to the unique ‘L’ shaped arrangement of the reaction tanks to fit into what space we had. "

Ian Johnson, Lead Project Manager, Severn Trent

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