Blending into the background
Review of the upgrade process at Slapton WwTW and Torcross pumping station, which involved significant time and environmental constraints
The area of south Devon surrounding the villages of Torcross and Slapton was used for rehearsals of the D-Day landings, which may explain why bomb disposal teams were twice called to deal with unexploded Second World War ordnance during the recent upgrade of Slapton WwTW.
South West Water put together a team that included Purac looking after process, mechanical and electrical input, MJ Gleeson Construction Services for the civil engineering input and Jacobs Babtie as the civil design consultant to undertake the upgrade. The upgrade involved design, construction and installation of equipment for a new pumping station at Torcross, to remove the raw sewage outfall, install 4.5km of twin pumping mains between Torcross and Slapton, and upgrade the existing WwTW at Slapton, in line with the latest consents required by the Environment Agency (EA). The total contract value was £4.9M.
If unexploded bombs weren't enough to cope with, the project also involved special attention to environmental issues. The work included activity around Slapton Ley National Nature Reserve - the largest freshwater lake in the south-west of England, a site of special scientific interest (SSSI) and a bird sanctuary. As a final concern, the new pumping station was to be sited under the car park of a major hotel in Torcross - work had to be completed by Easter so as not to impinge on the hotel's seasonal trade.
The public toilets in Torcross were sited on the hotel's car park, so this building was converted to house the motor control centre for the new pumping station and a new toilet block was built as part of the project by the seafront car park to compensate for the loss of the previous public toilets. For an area such as this, there was considerable interest and not a little concern at the nature of the work being carried out.
The team involved undertook a significant amount of community activity prior to work beginning to reassure local residents. During the work the team provided a weekly update on progress so local people felt involved and relaxed about the work going on. The new hotel car park, the new public toilets and some excellent new traditional earth-filled Devonian stone wall screening at Slapton WwTW, which is sited very close to a public footpath, were all important factors in gaining acceptance of the work from the locals.
Prior to the work being undertaken, sewage flows from the Torcross catchment were discharged via a sea outfall, which was clearly unacceptable under current environmental legislation. The existing Slapton WwTW was more than 50 years old and consisted of Copa Sac screens, a macerator, a storm overflow with a storm tank, two primary settlement tanks, one stone trickling filter and humus tank and a nitrifying BAF, before discharge of the final effluent into the sensitive waters of the Slapton Ley.
The new scheme saw the combining of the two village catchments and treating flows in the new plant before returning the effluent to Torcross 4.5km away for discharge via the existing sea outfall, which has also been upgraded to meet the new requirements. It was imperative the new scheme, while able to cope with a static population of around 1,000 people, could cope with a seasonal influx of visitors that would see the population treble between Easter and September. The new WwTW provides preliminary and secondary treatment for the sewage from the Slapton and Torcross catchments, together with thickening for the resultant sludges. The scheme involved the provision of a new inlet works where influent is screened to 6mm. Preliminary treatment provides grit removal and fine screening with the secondary treatment as an activated sludge process.
The final effluent is then pumped the 4.5kms back to Torcross prior to discharge via a pumped sea outfall that was replaced due to the poor state of the existing one. The treatment plant at Slapton is fed by a pumping station at Torcross and by the existing gravity main from the Slapton catchment. There is separate storm separation and storage at Torcross pumping station before transfer and at the treatment inlet works for the Slapton flows.
Storm flows that overflow the storage at Torcross are pumped to the sea together with return final effluent via the existing outfall. Storm flows that overflow the storage at Slapton are discharged to Slapton Ley via the existing storm overflow. The new final effluent consent is a standard of 40mg/l BOD and 60mg/l suspended solids (95%ile on spot samples). Surplus sludge is being exported from the site to Kingsbridge by road tankers as thickened sludge, at approximately 5% dry solids, for further treatment.
"It has been a challenging project, but a really interesting one," said Purac's George Handley. "The fantastic location has been a joy to work in and the end product - both from the point of the new facility which is excellent, but also from the warmth of the local people who have responded so well to the ongoing communication programme we have operated with them, has proved that making that little extra effort to keep everybody on board should be an intrinsic part of every project of this type."