With phosphorous reduction required at more than 800 wastewater treatment works over the next five years, Duncan Wildgoose, BDM head of wastewater & water treatment, Xylem UK, discusses ballasted clarification, one of the solutions achieving very low levels.
The 2025-2030 Water Industry National Environment programme (WINEP) is the largest yet, with a total expenditure commitment of £22.1billion. The actions included for phosphorous removal are estimated to result in a 33% reduction in loadings to English rivers.
A range of options to meet very low phosphorus levels emerged in AMP7, among them ballasted clarification which we expect to see even wider deployment of in AMP8. The process uses a dense material to intensify biological or conventional chemical flocs and improve the settling of suspended solids in water.
Xylem’s ballasted clarification systems are:
BioMag® – for secondary processes and intensification
Uses magnetite to deliver fast-settling clarification technology for biological floc, allowing capacity expansion and performance improvement with minimal plant modifications at the lowest cost possible.
CoMag® – for tertiary processes and storm treatment
Uses magnetite as a weighting agent into traditional chemical floc to efficiently reduce total phosphorous. The system settles floc up to 30 times faster than conventional treatment.
In AMP7, we worked with four utilities to install 14 ballasted clarification systems. This case study highlights one of the projects:
Yorkshire Water, Knostrop Wastewater Water Treatment Works.
Knostrop WwTW is Yorkshire’s largest treatment site, serving a population equivalent of 990,000 in and around the city of Leeds, with a full flow to treatment of 5,600l/s.
Discharges from the site were required to undergo advanced biological treatment to reduce levels of phosphorous returned to the River Aire.
The Aire flows 148km from the Yorkshire Dales to the River Ouse, with a catchment that encompasses 22 Sites of Special Scientific Interest (SSSI), four Special Areas of Conservation (SACs) and two Special Protection Areas (SPAs).
Knostrop’s new compliance level was set at 0.4mg/l total phosphorus. To achieve this, in 2023 Yorkshire Water began a £60m upgrade of the site, implementing a range of advanced treatment technology from solutions providers including Xylem.
The solution
A CoMag® ballasted clarification system was installed to provide a new tertiary solids removal process, as part of the wider upgrade at Knostrop WwTW. Unique to Xylem, the CoMag process reduces phosphorus through the addition of magnetite, which easily embeds into the floc and results in the accelerated solid settlement.
As part of the CoMag process, sludge is recycled and passed through a magnetic drum, which continuously recovers most of the magnetite, resulting in more sustainable treatment and lower operational expenditure. Due to the efficiency of the process, just one 32m high-rate CoMag clarifier was required at Knostrop WwTW.
Results
Following commissioning in September 2024, Knostrop’s CoMag system provides tertiary solids removal to 75% of the site’s full flow to treatment, helping achieve average total phosphorous of 0.25mg/l – exceeding the target of 0.4mg/l. The results were immediate and the ultra-low level of 0.1mg/l was recorded during the performance test period.
The required surface area to treat the same volumes – 75% of the site’s FFT – in a traditional final settlement tank is 14,136m2, compared to the 804m2 for CoMag. Thanks to significantly faster settlement, the CoMag clarifier can be much smaller than conventional final settlement tanks.
Importantly, the innovative technology continuously recovers most of the magnetite, which will support the plant’s sustainability requirements and budget.
Sam Akeroyd, capital delivery programme manager, Yorkshire Water, said: “The CoMag system from Xylem has provided Yorkshire Water with a stand-out solution for reducing phosphorus levels to meet new environmental consents.
“We can now confidently fulfil and surpass our commitments to reduce phosphorus at our Knostrop STW, while meeting our cost and ongoing operational objectives.”
Following the success at Knostrop, Yorkshire Water has selected CoMag technology for another large site requiring phosphorous removal.
Xylem’s range of phosphorous removal technologies can be found at:
xylem.com/en-uk/applications/phosphorus-removal-and-recovery/
