Siltbuster Group
Small wastewater treatment works challenges with stormwater overflow
Small wastewater treatment works challenges with stormwater overflow
Background
United Utilities (UU) operates a small wastewater treatment works serving 352 residents in 254 homes in the village of Cargo, on the Solway Plain in Cumbria. The small size, scale and location of the site brought multiple challenges with stormwater overflow spills into the local water course.
Alongside the environmental impact, the unconsented spills also attracted high levels of scrutiny from villagers, local groups and the media.
With funding made available from Ofwat, UU engaged Siltbuster’s water treatment specialists to create an interim solution with a view to implementing a long-term fix.
The improvements at Cargo form part of UU’s ongoing commitment to creating better rivers, including a total £914m investment to reduce spills from 158 storm overflows in Cumbria.
The challenge
The works at Cargo only needed to process 5 litres per second of flow, but were still experiencing regular overflows even at times of modest use, with 343 spills recorded in a year. It was clear that more treatment capacity was needed to mitigate the risk, with the aim of eliminating spills altogether.
There were also logistical issues with access to the site because of its location and small size – in particular, a large tree branch made transporting large equipment to site difficult.
The solution
Following a full site survey and liaison with UU, Siltbuster specified a complete side stream – effectively creating a new small treatment works at Cargo.
Once flows reached a level that threatened to overwhelm the existing works, flow would be diverted through the side stream, which increased the total flow that could be handed to 17 l/s and added 75 m3 of capacity.
It was agreed that a temporary solution would be installed first, to address the immediate issues, test capability and act as a proof of concept for a more permanent solution.
Siltbuster specified an HBR20 lamella clarifier, which separates out solids from wastewater using a series of inclined metal plates to encourage contaminants to settle out. This feeds into an MBBR10 treatment unit, which provides aerobic biological treatment to remove organic matter, and finally a D25 DAF (Dissolved Air Flotation) unit which removed the smallest particles. This process handled the overflows and safely treated the wastewater so it could be discharged into the water course within consent.
These modular units, assembled to specification and tested at Siltbuster’s headquarters in Monmouth, were transported to site where they could easily be installed as a ‘plug and play’ solution.
To overcome the issue of the tree branch obstructing access to site, Siltbuster fitted a lid to the MBBR unit and delivered it on its side, avoiding causing any damage. From start to finish, the job was up and running, with a period of 14 weeks from the initial enquiry to full installation.
The result
Siltbuster’s solution had an impact immediately, reducing spills from 343 a year to just one. Using the temporary equipment gave UU reassurance that the system would work and enabled monitoring to take place, and adjustments to be made, before equipment was purchased as a permanent solution.
For the final installation, the HBR20 was upgraded to a higher capacity HB50 unit. The MBBR already on site as part of the temporary solution was left in place and purchased by UU to avoid any issues removing and replacing it on site.
The system is now in place and running flawlessly – protecting the local environment, providing peace of mind for local people and ensuring the site remains in consent.
