Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | FEBRUARY 2015 | 23 Above: The temporary treatment plant in action downstream of the dam Below: Grouting work in progress on the dam's south side Project focus: Water resources • Perspectives Chris Olley, Bachy Soletanche: "The project has secured the future of the dam, and has been a huge suc- cess as far as the impact on the community and all stakeholders is con- cerned. It was also a very interesting project from a technical perspective, and the dam panel, who have signed the work off, were very pleased with how it went." Les Metcalfe, head of contracts and commercial at South West Water: "Strengthening the grout curtain in the bedrock below Wimbleball Dam was a challenging project for all concerned. A great deal of care was taken to protect the environment, with the Siltbuster 'mini water treatment works' deployed to prevent any pollution downstream. The results are very positive, with leakage through the dam reduced to historically low levels not seen since it was constructed." • Innovations ● The main grout work used descending stage drilling and grouting, to ensure maximum stability as the dam's foundations were strengthened ● The temporary water treatment process downstream consisted of two treatment plants and a temporary dam with a feeder channel ● A three stage treatment process was deployed using pH adjustment, coagulation and flocculation, and solid/ liquid separation fine cementitious product, with high- pressure water flush in fractured rock, seepage was unavoidable," continues Olley. "This gave rise to the require- ment to plan, monitor and treat the watercourse to prevent any possible pollution events. Given the sensitivity of the area, and a multi-million pound fish farm less than one mile down the road, this was paramount to the project." Turbidity and pH monitoring In recognition of the sensitivity of the receiving watercourse, five remote wa- ter quality monitoring stations were installed by RSHydro Ltd to monitor the quality of the water downstream of the dam, and to raise an alarm in the event that water quality parame- ters exceeded user defined levels. The monitoring system automatically re- corded and reported pH, temperature, dissolved oxygen, redox potential, specific conductivity, turbidity and water level. Water treatment specialists Silt- buster were then brought in to provide a temporary treatment solution for the length of the project. "To mitigate the risk of potential pollution associated with the grout- ing programme, Siltbuster installed two Water Treatment Plants," says Dave James, Engineering Manager at Siltbuster. "A 50 l/s Siltbuster Water Treatment Plant was connected to the dam drainage system and this unit was operated continuously. A tempo- rary dam was constructed across the feeder channel downstream of the dam. The quality of the water passing over the dam was monitored, and in the event that this water quality decreased, a second (150 l/s) Silt- buster Water Treatment Plant would automatically be activated to pump water from upstream of the temporary dam, treat the water to achieve the required standard and discharge the water downstream of the temporary dam." There were two main pollution concerns arising from the grouting work: one, that the pH of the seepage water would be raised to a highly alkaline level by the calcium oxide in the cement, and two, that the turbid- ity of the discharge water would be high because of suspended solids. Both of these elements had the ability to damage the aquatic ecosystem and are hence highly regulated by the Environment Agency. Typically, it is necessary to achieve a pH of between pH6 and pH9 and a total suspended solids content of less than 60 mg/l.

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