Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | sepTember 2014 | 23 Project focus: Wastewater treatment Bardenpho ASP was always going to be difficult. As a result three dimen- sional modelling of the site was used to plan pipelines and to avoid clashes with existing below-ground services. "With two existing plants in place there was not a great deal of room and space was clearly going to be an issue. The irregularly-shaped foot- print available to use, overhead HV cables, a shallow groundwater table, poor ground strength, and existing buried services all had to be consid- ered," says Garrett. The physical geometry of the large Bardenpho ASP structure had to take all of these constraints into account while remaining in step with the governing process design. In an effort to reduce footprint, the process depth was set at seven metres - the upper limit for ASP plants. However this still meant a structure 92m long and 62m wide was required. A total of four lanes were selected to provide opera- tional flexibility and were arranged in a three-pass serpentine arrange- ment to ensure optimum plug-flow conditions. A close working relationship between Southern Water, 4Delivery and civil, structural and geotechnical engineers was instrumental in the smooth execution of the project. Garrett notes: "Collaboration was key in the design and construction phases and Southern Water devel- oped a very close relationship with 4Delivery during the project which contributed significantly towards its overall success." This close working relationship led to a series of design workshops which culminated in agreement on the fol- • Perspectives Jon Kenrick, project manager, southern Water: "From an engineering point of view, this has been an exciting scheme to work on. It will have huge environmental benefits – the end result will be a much greener treatment works that will treat wastewater to even higher standards before releasing it into the Solent." stewart Garrett, senior project manager, southern Water: "It was vital to keep the WwTW running during the upgrade work. A significant part of this project's uniqueness comes from the two- part construction process that was adopted to keep existing capacity up to sufficient levels." lowing design approach: 1) Treatment of the made ground layers using Vibro-stone columns which create less noise and are more cost-effective than pre-cast concrete piles. The structural de- sign had to allow for up to 50mm of potential settlement. 2) A propped cantilever system with tie-beams between opposing walls at coping level was utilised to avoid unnecessarily thick walls. This al- lowed the 7.5m high walls to taper in thickness from 500 to 350mm. 3) The base slab design was also more efficient as a result of adopting this solution, being only 300mm thick and 500 mm thick below the main walls. This approach brought about a number of benefits. The propped can- tilever design cut the amount of in-si- tu concrete that was required and the tie beams made installation quick and simple, with no requirement for con- nection into wall reinforcement. The beams were designed in reinforced concrete and included overhangs at each end for a simple hooked connec- tion which will help to ensure their longevity. In total, 68 16m long beams were made to hold the treatment tank, each weighing eight tonnes. The tank itself is the length of ten buses and home to millions of bacteria that will break down particles in the wastewater. The site also incorporates two final settlement tanks, measuring 33 metres in diameter. The construction phase Commenting on the scale of the con- struction phase Garrett says: "During the construction of the upgrade over 10,000 cubic metres of concrete were used, enough to fill four Olympic swimming pools. In addition 11,000 tonnes of reinforced steel, 1,500 me- tres of new pipeline and 3,000 metres of new ducting were used." To create space for the Bardenpho ASP, two existing primary settlement tanks and an ASP structure had to be demolished. However, as Garrett observes, the day-to day operations had to con- tinue: "Millbrook serves the South- ampton region which has a popula- tion of approximately 135,000 and the site treats a Flow to Full Treatment (FFT) of 850 l/ per second. Serving such a large region it was clear that the current permanent condition of the plant had to be maintained during the upgrade work." To ensure this treatment capac- 3D site modelling was used to plan pipelines and avoid clashes with exist- ing below-ground services ►

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