Knostrop Wastewater Treatment Works (WwTW) represents Yorkshire Water’s most extensive treatment facility, catering to a population equivalent of 990,000 residents in and around Leeds, with a maximum treatment capacity of 5,600 liters per second.
In response to anticipated population growth, the site was mandated to implement advanced biological treatment processes to lower the phosphorous levels discharged into the River Aire. This requirement was stipulated by the UK Environment Agency’s Water Industry National Environment Programme (WINEP).
The River Aire stretches 148 kilometers from the Yorkshire Dales to the River Ouse and has a watershed that includes 22 Sites of Special Scientific Interest (SSSI), four Special Areas of Conservation (SACs), and two Special Protection Areas (SPAs).
Elevated phosphorus levels in waterways can lead to algal blooms, which deplete oxygen and harm aquatic ecosystems. To mitigate these blooms, WINEP is enforcing stricter phosphorus limits on numerous wastewater treatment facilities across England.
Knostrop’s new compliance standard was established at 0.4 mg/l total phosphorus. To meet this target, Yorkshire Water initiated a £60 million site upgrade in 2023, incorporating various advanced treatment technologies from providers like Xylem. A CoMag® ballasted clarification system was installed to enhance the tertiary solids removal process as part of the overall Knostrop WwTW enhancement. The CoMag process, unique to Xylem, effectively reduces phosphorus by incorporating magnetite a naturally occurring, high-density mineral that enhances the weight of standard precipitated floc. This magnetite integrates seamlessly into the floc, facilitating accelerated solid settlement up to ten times quicker than traditional techniques.
In the CoMag process, sludge is recycled and passed through a magnetic drum that continuously recovers most of the magnetite, promoting more sustainable treatment and reducing operational costs. The entire system is fully automated, requiring minimal ongoing management.
Thanks to the rapid settlement capabilities, the CoMag clarifier can be significantly smaller than conventional final settlement tanks. This not only optimizes space but also improves overall process efficiency, allowing for high-performance treatment within a more compact area.
Due to the process’s efficiency, only one 32-meter high-rate CoMag clarifier was necessary at Knostrop WwTW. After commissioning in September 2024, the CoMag system provides tertiary solids removal for 75% of the site’s full treatment flow, achieving an average total phosphorus level of 0.25 mg/l surpassing the 0.4 mg/l target. The results were immediate, with an ultra-low level of 0.1 mg/l recorded during the performance testing phase.
The surface area needed to treat the same volume 75% of the site’s full treatment flow in a traditional final settlement tank is 14,136 m², compared to just 804 m² for CoMag.
Duncan Wildgoose, Xylem’s Business Development Manager – Head of Treatment Wastewater & Water, stated: “Xylem is honored that its CoMag system was chosen for Yorkshire Water’s significant upgrade of Knostrop wastewater treatment works. This technology processes a substantial treatment flow, increases treatment capacity, and enhances tertiary filtration quality, all while occupying a smaller footprint than conventional methods. The benefits to the River Aire ecosystem will be realized quickly due to the CoMag process’s efficiency and speed. Crucially for Yorkshire Water, the innovative technology continuously recovers most of the magnetite, supporting the plant’s sustainability goals and budgetary needs.”
Sam Akeroyd, Capital Delivery Programme Manager at Yorkshire Water, remarked: “The CoMag system from Xylem has delivered an exceptional solution for lowering phosphorus levels in line with new environmental regulations. We can now confidently meet and exceed our commitments to reduce phosphorus at our Knostrop STW, while achieving our financial and ongoing operational objectives.”
Source: Xylem
