How to meet a deadline

Yorkshire Water has finished refurbishment of one of its water treatment works as part of the AMP4 Large Scheme Programme. Ian Farmery and Mark Hayton outline the project.

Thornton Steward Water Treatment Works (WTW) in Yorkshire has been refurbished.

The project is a successful part of the Yorkshire Water Services (YWS) AMP4 Large Scheme Programme, and has been achieved on programme and within the authorised scheme budget.

The WTW is in North Yorkshire about four miles to the west of Bedale. Prior to the most recent scheme, it had undergone two major refurbishments.

The first, in the mid-1990s, replaced the majority of the existing M&E equipment and included reconstruction of the rapid gravity filter floors. The second, between 2002 and 2003, included construction of new manganese contactors, construction of the clean-water contact tank and upgrades to the sludge handling system.

Thornton Steward WTW is a YWS asset. It supplies a number of production management zones in North Yorkshire comprising:

  • Upper and Central Wensleydale
  • Lower Wensleydale
  • Catterick/Richmond
  • Northallerton/Thirsk

Although there were no statutory drivers, quality or otherwise, directly associated with Thornton Steward WTW, both Langthwaite WTW and Osmotherly WTW had water quality DWI compliance dates. This required their closure by the August 1, 2006 and December 31, 2006, respectively.

In order to facilitate this asset management programme (AMP4) rationalisation scheme, YWS would be compelled to place increased reliance on Thornton Steward. And it was recognised that the works output would need to increase to 33MLD to meet the demand at the design horizon of 2029.

Algal blooms

In addition to the above rationalisation driver, Thornton Steward WTW was considered unreliable and was unable to meet the design output of 22.7MLD. Typically, the secure output from the works was around 17MLD and this could further reduce to 13MLD during periods when raw-water conditions were unfavourable.

The existing process deficiencies at Thornton Steward resulted for the most part from the inability of the existing upflow clarifiers to treat the design flow.

Predominantly, this was caused by the periodic occurrence of algal blooms in the raw water reservoir; the development of filamentous algae in the clarifiers; and the adverse impacts of wind turbulence and light on the clarifiers.

The project proceeded on the basis of upgrading the works to a 33MLD plant and implementing measures to stabilise the treatment process.

The main elements of the existing process comprised:

  • Raw-water reservoir
  • Inlet works with flash mixing and the facility to dose alum, sulphuric acid and lime
  • Upflow clarifiers
  • Rapid gravity filters (RGFs)
  • Interstage pumping station
  • Manganese contactors
  • Contact tank
  • Clean water tank

The existing sludge-handling system comprises:

  • Washwater-holding tanks (which settle and decant on time basis)
  • Sludge-blending tank (blending washwater sludge with clarifier sludge)
  • WRc-type sludge thickeners
  • Thickened sludge tank
  • Filter press house

During a detailed feasibility study by Arup, a number of options were considered for improving the performance of the existing plant while also providing increased throughput.

Initially, the proposed scheme for the clarification stage considered the extension of the existing upflow clarifiers. But, due to the poor performance of the existing works and the potential problems with algae in the raw water, the preferred design incorporated the construction of a new dissolved-air floatation (DAF) plant.

The other key elements of the Arup notional scheme included:

  • Modifications to existing RGFs, including construction of a building to cover them
  • Provision of an additional two units of RGF capacity
  • Increase to existing interstage pumping capacity to meet the 33MLD design output
  • Provision of a single additional manganese contactor
  • Upgrade of chemical dosing to accommodate increased works output
  • Upgrade of the sludge-handling system including the provision of a new plate filter press

Under Yorkshire Water’s AMP4 Large Scheme Programme the ethos of partnering under the NEC Option C was demonstrated via early contractor involvement meetings.

Early meetings allowed Arup to present the notional scheme to contractors prior to the issue of the tender and gain valuable feedback on potential construction issues and risks that could then be incorporated into the tender documents.

The Arup notional scheme was issued for tender on a design and build basis in November 2005. Mid-tender reviews were arranged to allow the contractors bidding for the works to meet in confidence with YWS and Arup.

Earth Tech Morrison (ETM) was awarded the contract in May 2006.

The ETM design was based on the notional design produced during feasibility and incorporated the DAF clarification stage, two additional RGFs and an additional manganese contactor. But ETM also contributed proposals for further innovations and capital efficiencies in a number of areas.

These efficiencies were agreed and implemented successfully through the early involvement of the wider team and the effective management of the project timetable that was used to identify dates for key meetings during the initial detailed design phase.

As a result, during this period there were a number of well attended design development meetings. Particularly useful was the design walk through meeting.

At this meeting the project engineer presented designs to the project stakeholders to ensure a common understanding of the design concepts.

Lean approach

A key consideration during detailed design was to minimise the risk of the impact of construction on the existing treatment process and the number of shutdowns required to interface the new extensions to the existing process. This resulted in off-line construction arrangement.

Construction began on site in July 2006 with excavation for the new DAF plant the first major activity.

ETM adopted a LEAN approach in the programming design and delivery of the works while using a buffered programme, whereby activities for all parties were programmed on the basis of minimum agreed durations.

The aim was to offer the best chance to meet the completion date – or an early completion with a significant element of the buffer intact (typically 0 to 20%). By adopting this approach, the original contract stage one completion date, for increased throughput capability by June 2007 (stage one) and final completion by September 2007, were met. But, due to unforeseen operational constraints imposed by YWS, the refurbishment works to the existing RGFs had to be delayed. This resulted in a revised completion date of November 2007, which was met.

The key project participants were Yorkshire Water, client; ETM, principal contractor / designer; Arup, technical consultant; and Turner and Townsend, commercial consultant.

Buy-in from YWS operational staff has also been positive and has significantly aided the interface between the new scheme and the existing works.

Ian Farmery is Arup project manager from feasibility through to completion, and Mark Hayton is ETM project manager.

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