Water plant construction overcomes obstacles

Bat protection and old mineshafts were two of the difficulties faced by South West Water at a wastewater plant upgrade in Cornwall. Chris Rogers, principal engineer at Pell Frischmann, explains how a solution was reached

The construction of South West Water's latest membrane plant to protect the waters off the north Cornish coast is a boon for all who enjoy this dramatic stretch of coastline. Until recently, the town of St Agnes was served by a treatment works consisting of chemically assisted settlement (CAS) followed by a prototype ultraviolet disinfection plant.

The Environment Agency, which regulates discharges, placed a requirement that full secondary treatment followed by disinfection or equivalent be provided. This meant that South West Water (SWW) had to act.

In normal circumstances, it is not difficult to comply with these requirements. But much of Cornwall is in an Area of Outstanding Natural Beauty, has stretches of heritage coastline, and is full of historic mineworkings. The existing site containing the CAS plant is just behind St Agnes Football Club, and has shared access.

There was no land in the area to expand on to, and options to use the current site had to be explored. South West Water employed Pell Frischmann as one of its partners to commence investigations into how to proceed.

A visit to the site revealed just how many old mine workings there are in Cornwall - there were two capped shafts within the treatment works site itself.

A further planning restraint meant that any development would not only have to be within the walls of the existing site, but also could not extend above the height of the boundary walls.

High standard
With these parameters set, the team decided to use a membrane bioreactor (MBR) plant due to their compact nature and high standard of final effluent. The plant would be designed to treat the future consented flow of 36l/s delivered from the Peterville terminal pumping station.

Although the team acknowledged that the MBR process is one of the most compact available, it was not clear at the time if this process could be accommodated on the site while keeping the existing plant operating.

Outline design work indicated that it should just about be manageable if every scrap of the existing site was used. But this was not as simple as it seemed because, not only did the mineshafts have to be avoided, but also there were other mine passages and workings running beneath the site. Furthermore, one of the capped mineshafts had been modified to act as a roost for bats, and these could not be disturbed without further investigation.

More information on the site and mineworkings was required before the outline design could proceed any further, so the team enlisted the services of geotechnical engineers and environmental specialists in order to investigate the mine workings and determine if these were being used by bats. The analysis of background data identified the area as being one of the UK's strongholds for the Lesser and Greater Horseshoe Bats.

All bats are listed as protected species under the Wildlife and Countryside Act 1981 and work has to be carried out under strict EU guidance. Following consultations with abandoned mine specialists and permission from Natural England, a confined-space entry to one of the capped shafts was carried out.

This investigated the extent of the interconnecting passages and determined if the shaft was a live bat roost. The confined space entry survey confirmed the presence of mine passages off the shaft, but no bats or evidence of bat use. Permission was gained from the regulatory authorities to seal the bat entry for the duration of the construction project following which, on completion the shaft was re-opened for use by bats as a potential roost site.

Background research on the mineworkings was done and this, together with a review of archive ground investigations and construction records, provided the scope for a ground investigation to prove the location of mineworkings beneath the proposed new structures.

The investigation showed a large proportion of the site for the proposed new structures was underlain by mineworkings. The existing access road to the site had been piled and designated as the safe way out in the event of a collapse in the vicinity during a previous scheme. This again demonstrated the fragility of the ground conditions.

Detailed design
Once the outline design determined that it was feasible to carry out construction on the site, two of SWW's other partners joined the team to provide their expertise; Biwater as process contractor and Alfred McAlpine as civil and principal contractor. With the team fully assembled, detailed design was undertaken, commencing with proposals for the piling layout.

This was a complex operation as the only part of the site left where construction of the MBR Plant and ancillary equipment was feasible was the most heavily undermined area. The mine workings below this area included a shallow cavern with a longest roof span in excess of 15m with vertical slot workings, which had removed most of the bedrock down to over 20m depth.

To allow reuse of the site, the MBR plant was supported on a complex group of piled foundations of up to 20m length. These were installed through the zone of the mineworkings and into sound rock underneath. Raking piles were used to avoid the zone of the deepest workings and supplementary ground anchors were installed for stability and subsidence protection.

Innovative methods were used to provide support to the rock roof of the mine workings below the plant, including the use of rock dowels bonded to a redundant tank base. The redundant tank base was initially used as a piling platform to establish a safe method of working. In addition, piled king post walls and excavation braces were used to maintain safety during construction. Monitoring of the underground environment was carried out to provide early warning of any subterranean movements. The design and construction procedures employed allowed safe development of a very challenging area of ground that some would consider unsuitable for development. This satisfied one of the key project aims of establishing the new plant within SWW's existing asset boundary.

To allow the piling to proceed, the existing UV plant had to be removed from site. But this could not just be taken away, as that would have been in breach of SWW's discharge consent. To ensure compliance during construction, a temporary Wedeco UV plant was hired and craned into the far corner of the site where it remained during the construction phase.

The remainder of the works were far from simple. Although an area had been identified for the MBR plant, all the auxiliary equipment had to be installed in the existing works building.

This building was already full and the team had to fit in a replacement generator, new blowers, larger control panels, new 6mm screens, new fine screens, grit removal and a sludge thickener. A redesign of the inside of the building was done, allowing the installation of all this equipment together with a detailed maintenance plan.

Construction phase
St Agnes Football Club allowed a compound to be set up at its ground and site progress meetings were held in the clubhouse. This arrangement allowed the works to run smoothly throughout the construction phase, and the team celebrated when the piling was complete and the MBR tank began to emerge from below ground.

But, true to form on this complex site, this soon brought more difficulties due to the amount of pipework, instrumentation and equipment that had to be installed in and around the tanks. The team had to be aware that all plant had to be kept below the wall levels. And the existing works had to be kept operational and within consent.

With commissioning around the corner, a plan was prepared to cover all eventualities - as switching on the new plant would mean taking the old plant off line with no going back. Logistical difficulties were overcome such as finding enough potable water to rinse the new system through prior to commissioning and agreeing how this could be discharged.

Commissioning went to plan with the new works taking over the process stream. The final effluent is of such good quality that it could easily be mistaken for potable water, that is aside from a slight smell and a warning from the operator that it should not be imbibed.

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