The 5km tunnel, which will be 4.5km in diameter, will be driven at depths of between 10m to 30m and will run through an area of old mine workings raising potential technical problems.

Two designs are currently under consideration, a shallow tunnel that would cross major infrastructure road and rail routes or a deeper tunnel within stable bedrock that would need a lift pump station. The winning firm will need to connect the tunnel at both ends to the existing sewer network.

In addition to the main tunnel works further ancillary works within the wider sewage catchment area comprising network augmentation and alterations, watercourse engineering, storage structures shafts and tanks, flow management and CSOs are proposed.

The primary purpose of the tunnel is to convey combined sewer flows and provide storm storage in both the tunnel and shafts.