Retrofit saves Wessex Water recycle site from population overload
5 Nov 2019
When a fast growing West Country town put a strain on the water recycling plant, it was an opportunity to address system quality as well as capacity...
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.
So Andrew Gulliford, process design manager at Wessex Water, sought an alternative and identified WPL’s enhanced biological treatment.
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 12metre-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. Its Hybrid-SAF 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).
It 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, over a 20-year horizon, one that was significantly cheaper.
A key advantage is that it can be retrofitted into any vessel, regardless of shape or size, to deliver more efficient wastewater processing
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.
It also reduced health and safety risks, while fewer vehicle journeys meant less disruption for the local community and reduced carbon emissions.
The cells were manufactured within six to eight weeks, installed in two days and the process optimised within three weeks.
WPL’s technical director Andrew Baird commented:
“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.”
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.
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.
Sustainable gains
The 30% smaller site footprint at Sherborne is 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.
