A partnering approach to project management helped Biwater Treatment and Sutton & East Surrey Water overcome the physical constraints of installing a new water-softening plant at Woodmansterne WTW.
Built in 1905 in Chipstead, Surrey, the Woodmansterne works treats water abstracted from eight boreholes in the London basin chalk aquifer, and currently puts an average of 25Ml/d into the Sutton water resource zone.
The works relied on a precipitator plant using hydrated lime to meet the legal requirement for water softening, but to meet increasing demand, the plant had been running in excess of capacity. Sutton & East Surrey decided to address the inability to soften efficiently at higher demands and the problems associated with an ageing treatment plant by carrying out modernisation which will double potential output to 50Ml/d.
Alongside extensions and overhauls, the work focused on a completely new softening process. Caustic soda and microfiltration were considered, but softening using lime pellet reactors was settled on because it offered closer process control and easier sludge disposal.
After a tender stage based on an IChemE Red Book specification, the contract was awarded to Biwater Treatment and converted to Green Book principles with a target cost of £6.12M. The decision to adopt a partnering approach to the project was influenced by a number of factors, including a 104-week deadline and the need to limit shutdowns to a maximum of 24 hours. Onsite space restrictions added to the challenge. The works is surrounded by residential development, so the new softening plant had to be built on the site of the two existing precipitator settlement tanks.
Keeping it live
During the first phase one of the tanks had to remain live until the pellet reactor plant was ready to go into service, while the other had to be demolished without breaching the shared dividing wall, or threatening the integrity of the adjacent pipeline delivering softened water to the filters.
Demolition began with sheet piling to protect the pipeline and diamond sawing through the roof, walls and floor of the first tank to isolate it from its neighbour. Low-noise construction techniques were used to minimise disturbance to the school backing onto the settlement tank area.
Once the softening plant building was erected, process equipment had to be craned into position. Lifting was only possible from one side of the building. Ten major items were positioned in this way, including the lime silo with its attendant mixing tanks and stock tanks, the sand silo, the CO2 stripper and the three pellet reactors. The 9m-high reactors had to be transported in sections which were assembled and welded onsite before lifting into the building, where they occupy all three floors from basement to eaves.
With three subcontractors involved, timing for each stage of the operation demanded detailed planning. The lack of space meant all equipment had to be installed as soon as it arrived on site. Work below ground was equally complex, calling for re-laying of the pipes carrying water from the eight boreholes to link with the new manifold supplying the pellet reactors. Each of the new pipes had its own independent wash-out facilities.
Successful commissioning of the new softening plant completed the first phase of the installation programme, allowing demolition of the precipitators and the second settlement tank to make way for the wastewater-recovery and waste-handling systems included in phase two. These were housed in the second part of the new building, which is also home to the motor control centre, offices and staff facilities. Access to the second building was relatively easy, although the 50t hopper which stores pellets removed from the reactors had to be temporarily erected and provided with vehicle access, with the permanent supporting structure added as construction of the building progressed.
The third and final phase of the work involved demolition of the old lime house, where the redundant precipitation settlement tanks were located, to make way for a wastewater lagoon. The lagoon is a simple, unlined basin which allows wash-water from the borehole pipelines, the only wastewater from the new softening process, to percolate back into the aquifer.
Biwater Treatment’s site manager, Ian Stirling, was in no doubt the project would not have stayed on schedule without the degree of cooperation between client and contractor made possible by the partnering approach: “The space and access constraints were challenging enough, but we also had to maintain supplies throughout the work.”
All operational decisions were taken by a core team consisting of the project managers for both parties, Sutton & East Surrey’s site representative and operations manager, and Biwater’s contract engineer. Plans for each stage were agreed at monthly meetings, sharing open-plan site offices helped engender a climate of co-operation.
“The free exchange of technical expertise contributed to the success of the project at all levels,” said Mr Stirling. “We had our own ideas, of course, but Sutton & East Surrey have considerable practical experience of treating hard water, and it was useful to know what had worked best for them at operational level.”
Biwater’s electrical, mechanical and civil experts provided specialist input as required, often via a video link to the company’s offices in Heywood. The link proved to be a considerable timesaver,especially for relatively short meetings such as drawing approval sessions, which normally last an hour or so, but eat up an entire day when travelling time is taken into account.
For Mr Stirling, partnership working was exemplified by the role of Gary Taylor, Sutton & East Surrey’s site representative: “Under the traditional ‘them-and-us’ approach, the site representative is there to keep an eye on what the contractor is doing, but at Woodmansterne, because of the team approach, a policing role was unnecessary.” Instead, Gary formed part of the site management team, getting involved in all aspects of construction management and quality assurance as well as liaising with the water company.
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