A recent upgrade at Wessex Water’s Sherborne water recycling facility using WPL’s Hybrid-SAF™ precision-engineered treatment system to retrofit existing infrastructure has doubled process capacity and achieved a potential cost-saving of 75% in capital expenditure.

Population growth in the town of Sherborne in Dorset meant that the rural water recycling facility owned and operated by Wessex Water required an upgrade to manage overload.

Initially the utility was planning to construct two to three additional 30m-diameter trickling filters, but such a development posed an issue around footprint on this land-constrained site.

 Learn how WPL implemented the solution - watch the video below

Instead, the utility was looking for an alternative solution and was scouting for high-rate processes and ways of utilising existing abandoned structures onsite. Andrew Gulliford, process design manager at Wessex Water, identified WPL’s enhanced biological treatment process via an article in an industry publication and invited the company to the Sherborne site.

A key advantage of WPL’s Hybrid-SAF™ precision-engineered treatment system is that it can be retrofitted into any vessel, regardless of shape or size, to deliver more efficient wastewater processing. During the initial collaborative planning stages of the project, the repurposing potential of an abandoned 12m-diameter onsite sludge tank was identified.

Together the partners calculated that a potential cost-saving of 75% in capital expenditure could be achieved by retrofitting this existing infrastructure with WPL’s technology as an alternative to the planned project. WPL’s Hybrid-SAF technology comprises a submerged moving-bed, fixed-film reactor and can treat wastewater in a more sustainable and cost-effective way than traditional submerged aerated filters (SAFs).

Retrofit of repurposed circular sludge tank doubles process capacity

Retrofitting the circular vessel with modular WPL Hybrid-SAF cells could utilise the entirety of the vessel, whilst providing secondary biological treatment for 50% of the works’ flow-to-full-treatment. By doubling the process capacity, a permanent alternative to the planned trickling filters was identified and, looking at a 20-year horizon, one that was significantly cheaper.

Off-site manufacture of the modular process technology cells meant that the onsite project delivery time would be a couple of days, rather than a possible 12 months for the civils work required for new trickling filters.

Computational fluid dynamics modelling – watch the video below

WPL’s technical director Andrew Baird commented:

“WPL’s Hybrid-SAF is a significant step forward for submerged biological treatment. Our research and development team has conceived the hydrodynamic profile underpinning the technology in a new way, which has been made possible by the use of a high specific surface area media.

“The result is that significant process efficiency advantages have been achieved, including reductions in cost, physical footprint and electricity consumption, all whilst increasing the overall process capacity of the site and improving environmental compliance. Being involved in the project at the start and working collaboratively with Wessex Water meant the best solution for the site could be identified very early on.”

The first flows entered the system on 1 October 2018 and the first data was recorded on 30 October. Results showed ammonia (NH3) levels at <0.4mg/l, well within the 10mg/l consent demanded by the Environment Agency.

Photo: Sherborne wastewater treatment works ammonia results 2018-2019 

Close collaboration early on delivers multiple delivery and operational benefits

Getting-in-early on the project and meeting the end-user face-to-face and onsite meant WPL was able to fully understand the objectives of the client. The full scope of the project, including everything from the type of media installed in the biozone to the logistics of restricted access to this rural site, could be considered in designing the optimum solution.

Wessex Water also shared the 2040 design horizon with WPL, which ensured a solution that was futureproofed. The utility now has more flexibility in how throughput can be increased at Sherborne for population growth from 12,600 to 15,700 in the catchment.

The modular WPL Hybrid-SAF cells were manufactured within six to eight weeks, installed in two days and the process optimised within three weeks. Offsite manufacture also meant onsite health and safety risk was reduced and quality control easier to maintain.

Height and width restrictions for vehicles and plant both onsite and in accessing the site were considered at the design stage. Off-site manufacture and shorter project time also meant fewer vehicle journeys, causing less disruption for the local community and reduced carbon emissions.

Sustainability benefits include reduced energy consumption 

The 30% smaller site footprint at Sherborne is also reflected in the energy consumption of the plant itself, which is reduced by a similar measure. Variable speed blowers are delivering 50-100% of design requirements, allowing greater headroom for power optimisation.

Lower levels of operator maintenance are required than with traditional treatment systems and individual cells can be replaced in a few hours without impacting on service or taking treatment vessels offline.

Other sustainability benefits include the repurposing of the tank, which meant there was no need to rip out the existing process treatment vessel and dispose to landfill. A minimal amount of concrete was required in lining the repurposed 12m-diameter tank. In addition, the WPL Hybrid-SAF’s neutrally buoyant media is manufactured from recycled materials.

The potential opportunities offered by WPL’s solution for carbon reduction are significant, including:

• Fabrication of construction - offsite build (manufactured in quality controlled environment in 6 weeks and installed in 2 days)
• repurposing the existing sludge tank (to biological treatment process) means the embedded carbon can be amortised over a longer period of time
• materials used - stainless steel frameworks have a lower carbon footprint than concrete with an equivalent life expectancy
• transportation of modules - minimal vehicle usage during offsite build stage
• operational - no desludging requirements

Andrew Gulliford, process design manager at Wessex Water said:

“Working closely with WPL at the earliest stages of planning for Sherborne water recycling facility meant that together we could completely reimagine the possibilities at this constrained site. Wessex Water customers will benefit from the 75% cost saving on this project and the energy efficiencies achieved will help deliver our sustainability targets.”

Contact Andrew Baird for further information at  This e-mail address is being protected from spambots. You need JavaScript enabled to view it  or direct dial on 02392 242 607