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Raising the game

Water & drainage - Culmore wastewater treatment works is being rebuilt to comply with environmental standards, as Bernadette Redfern discovers.

The Culmore wastewater treatment works in Derry is undergoing a substantial £15M transformation.

It is one of the nine sites in Northern Ireland that fell foul of the European Commission after it was discovered the plant's treatment levels were not adequate to protect the quality of the River Foyle, into which it discharges.

The Commission gave the UK government a final warning in January to clean up or face a trial in the European Court of Justice.

But at the time of the warning, consultant Black & Veatch, in joint venture with Irish contractor Dawson Wam, had already been employed by the Water Service to renovate the site. Bringing the plant up to scratch meant refurbishing the inlet works and six existing primary settlement tanks, and adding a secondary activated sludge plant with six final settlement tanks. The new works will treat a maximum daily fl ow of 225,000m 3. Secondary treatment in the activated sludge plant will aerate the sewage. Oxygen stimulates naturally occurring microorganisms to digest organic carbon compounds, which will change the remaining waste solids to a form that can be settled and removed as sludge by sedimentation. This will take place in the six additional settlement tanks. The remaining sludge will then be pumped out from the base of the tanks and dewatered, with the final cake taken to an incinerator in Belfast.

Constructing the infrastructure was made easier thanks to a change in the design. 'The ground conditions really were awful, very silty and alluvial, ' says Black & Veatch site manager Alan Cole. Groundwater levels at the site were also subject to tidal influence.

'One of the main reasons we won the job was because Hyder managed to work up a solution that didn't require any piling, ' Cole notes.

Hyder design manager Rob Dustan explains: 'The original preliminary design had the structures sitting about 1.5m lower than they are now, which was potentially in the zone of tidal influence. We were very concerned that sea water ingress would flood the works every tide cycle, requiring a cofferdam to be in place during construction.

Discussions on buildability at the partnering meetings established that there was a need to avoid this, which we did by setting the formation level above the influence of the tidal zone.

'We wanted to lift the base level of the structures but we still had to make the hydraulics work and we couldn't simply increase to footprint area of everything to keep the volume the same, ' he continues.

This meant some clever process design solutions had to be employed to ensure that structures such as the final settlement tanks were still efficient. The team ended up with a non-conventional design for the tanks that has reduced their volume by 15%, without affecting the hydraulic performance.

Black & Veatch is guarded about giving away design details as the firm believes it has added a powerful commercial weapon to its engineering arsenal.

Construction has otherwise been straightforward. The process has been speeded by negotiating access to land adjoining the site, aiding the storage of materials and spoil.

Civils work is complete and the project is now in mechanical and electrical fit-out phase.

Construction is due to finish approximately three months earlier than scheduled, with commissioning set for December this year.

'It's a challenge. Some of the people on site aren't happy with it but we will do our best to help them understand the new systems, ' says Black & Veatch business development director John Aldridge.

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