Population growth in the UK town of Sherborne meant that the rural water recycling facility owned and operated by Wessex Water needed upgrading to manage overload. The company chose a sustainable enhanced biological treatment process, which achieved savings of 75%.
Initially Wessex Water was planning to construct two to three additional 30 m diameter trickling filters for the upgrade, but this posed an issue around footprint on this land-constrained site. Instead, it began looking for an alternative solution and was searching for high-rate processes and ways of using the existing abandoned structures onsite.
Andrew Gulliford, process design manager at Wessex Water, identified WPL’s enhanced biological treatment process, the Hybrid-SAF, via an article in an industry publication and invited the company to visit the Sherborne site. Repurposing potential
Sustainable and cost-effective WPL’s Hybrid-SAF technology comprises a submerged moving-bed, fixed-film reactor which can treat wastewater in a more sustainable and cost-effective way than traditional submerged aerated filters (SAFs). The precision-engineered treatment system 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 12 m diameter onsite sludge tank was identified. Together, Wessex Water and WPL 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.
Doubling capacity Retrofitting the circular vessel with modular WPL Hybrid-SAF cells allows the use of the entire 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 ahead 20 years, was an alternative that would significantly cut costs.
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 work required for new trickling filters.
WPL’s technical director Andrew Baird says: “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.
Close collaboration By meeting Wessex Water on site at the early stages of the project, WPL was able to fully understand its objectives. 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 when designing the optimal solution.
Wessex Water also shared the design vision with WPL, ensuring a future-proofed solution. The facility now has more flexibility in terms of how it can increase throughput at Sherborne for a population which has grown from 12,600 to 15,700 in the last few years.
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 risks were reduced and quality control easier to maintain.
Height and width restrictions for vehicles and plant both onsite and 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 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 more potential for power optimisation.
The new system requires lower levels of operator maintenance than 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 take out the existing process treatment vessel and dispose of it in landfill. A minimal amount of concrete was required to line the repurposed 12 m diameter tank. In addition, the WPL Hybrid-SAF’s neutrally buoyant media is manufactured from recycled materials.
Andrew Gulliford, process design manager at Wessex Water said, “Working closely with WPL at the earliest stages of planning for the Sherborne water recycling facility meant that together we could completely rethink 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.”
