A new WwTW will help preserve the bathing areas of Cornwall's tourist trap, the Lizard Peninsula. South West Water's Steve Cross and Hyder Consulting's Dominic Lovell report.
Britain's most southerly point, the peninsula is also home to many Sites of Special Scientific Interest (SSSI) and candidate Special Areas of Conservation (cSAC).
Placed on this idyllic canvas, the picture offshore has not been so rosy. Up until a few months ago, crude, macerated sewage from catchments centred on the villages of Mullion and Lizard was pumped into the coastal waters via dedicated outfalls. In the case of Mullion, the outfall terminated at the base of cliffs, only about 100m from a designated bathing water. Such a situation was unsustainable in an area that is very dependent on tourism for its long-term economic future. The solution has come in the form of a new £5.65M plant serving the two villages' combined population of just over 3,000. This came on stream in June 2005.
Less is more
The plant is a case of less is more. The original scope, under the National Environment Programme, had been to construct two WwTWs, one for each village. However, taking into account the diversity and sensitivity of the Lizard Peninsula, and the need to work within environmental, ecological, archaeological and financial constraints, the two WwTWs were combined.
Since the two communities are 7km apart, the plant is located remotely between them. A total of 14km of transfer pipelines bring flows from Lizard via a main pumping station and two ancillary pumping stations; and from Mullion via a main transfer and an ancillary pumping station.
The new activated sludge process plant has a design capacity of 4743PE. Flow to full treatment (6DWF) is 54l/s with the effluent treated to meet a consent of 40mg/l BOD; 60mg/l SS.
The process consists of an inlet screen, anoxic tank, compact plant (aeration lane and final settlement tank combined), odour control and a final effluent pumping station. Activated odour treatment is incorporated into the scheme to meet the strict planning conditions. Sludge is stored in a 105m3 tank and provides a total of five days' storage. The sludge produced at the works is transported to South West Water's existing sludge-treatment facility at Hayle WwTW.
As part of the process of ensuring that discharges from the new scheme comply with the Bathing Water Regulations, Hyder Consulting constructed Infoworks models for both the Mullion and Lizard catchments. The verified models were submitted to the Environment Agency for approval. To meet the three spills per bathing season driver at Mullion, 100m3 of storm attenuation was required. This was achieved by constructing a 74m long oversized sewer, which terminated at the wet well of the Mullion Transfer Pumping Station. A small package pumping station was also constructed to collect flows from a smaller sub-catchment.
The existing outfall at Mullion was retained as the combined sewer overflow/emergency overflow (CSO/EO) discharge location for both pumping stations and negated the need to discharge to a fluvial watercourse. All CSO/EOs were screened to 6mm in both directions.
Flows are transferred from Mullion to the WwTW at a rate of 25l/s (6DWF). High-lift dry-well pumps are used to pump flows 7.1km, with a static head of 45m and friction head of 51m. The pumps are variable speed to reduce surge pressures within the pipeline.
The transfer pumping station at Lizard is positioned in a particularly sensitive area adjacent to SSSI/cSAC and is located in a copse of local importance adjacent to a protected species of Babington Leek. The design of the pumping station specified prefabricated plastic for both the storm attenuation tanks (100m3) and also the pumping station. Using this material enabled the contractor to construct the pumping station much more quickly than using conventional methods while minimising disruption.
Two satellite pumping stations were also constructed to collect flows in low-lying areas. These were located in SSSI/cSAC in the beautiful Church Cove area of Lizard. The existing sewerage was located in the middle of the only access road to Church Cove. Constructing the proposed final effluent sewer in this road would have caused major disruption to local residents and visitors alike, so in order to avoid this, the team decided that a better option would be to lay the pipeline through the SSSI/cSAC. This proposal was accepted and agreed with English Nature.
The Lizard catchment also included an existing pumping station located at Ruan Minor, which collected flows from Ruan Minor and Cadgwith. The pumping station forms an integral part of the new scheme and continues to transfer flows to Church Cove. However, instead of discharging flows through the outfall, they are collected at the new Church Cove pumping station and passed forward to the WwTW. Due to the length of transfer from Ruan Minor PS to the works, calcium nitrate is dosed to prevent the occurrence of septicity and combat existing odourous sewage at Church Cove.
Making use of the existing assets was a high priority for the team members. The topography of the Lizard peninsula is very flat plateau and there are no rivers or streams with the capability of accommodating the treated effluent. The existing outfall at Church Cove (Lizard), constructed in the late 1960s, was utilised to discharge treated effluent and also the CSO/EO from the Church Cove Transfer
pumping station. The outfall is a 200mm aluminium pipe, which
terminates 460m offshore, 20m below mean high water springs and is an ideal location to discharge the final effluent.
Design and sustainability issues
To minimise the transfer of waste spoil from the pipelines, the design incorporated a bund around the WwTW, which has a capacity of 5,800m3. The bund also acts as a screen around the works and significantly reduced construction traffic movements and waste taken to landfill. Similarly at the main transfer pumping station in Mullion, a bund was incorporated for aesthetic and sustainability reasons.
Minimising the footprint of the plant was an important requirement and this influenced the specification of a compact plant. The plant consists of an outer annulus (24m diameter) aeration zone with a central final settlement tank (16m diameter). The structure was constructed using pre-cast, post-tensioned concrete panels. These panels were used for the construction of both the compact plant and sludge holding tank.
In order to speed up the construction of the transfer pipelines and to reduce friction head loss, 200 outside diameter, MoPVC, 12.5bar MoPVC pipework was used. These pipes are lightweight spigot/socket pipes and avoided the use of heavy machinery for installation.
The scheme has been operational since June 2005 and has been producing high-quality final effluent exceeding the consent standards.
The new plant will not feature in the tourist brochures. After all, effective sewage treatment does not tend to be a tourism draw. But, by bringing first-time sewage treatment to these coastal communities, it has delivered environmental benefits that will help secure the area's tourist-industry lifeblood.