Water & Wastewater Treatment

24 | FEBRUARY 2015 | WWT | www.wwtonline.co.uk An overview of the project, showing the location of the grouting work and the two treatment stations downstream Project focus: Water resources Three-stage process The treatment solution therefore con- sisted of three elements: pH adjust- ment, coagulation and flocculation, and solid/liquid separation. While the 2003 grouting work had used citric acid as the neutralising agent for pH adjustment, the Siltbuster system was configured to operate using carbon di- oxide, which was judged to be a safer, more controllable solution for such a scale of work. "The results are very positive, with leakage through the dam reduced to historically low levels not seen since it was constructed." Les Metcalfe, South West Water At the coagulation and flocculation stage, the system was set up to ensure that only small amounts of dosing chemicals were required, with the dosing pump varying its speed in re- sponse to the flow rate being treated, and a turbidity meter ensuring that chemicals were not added to water that was already clear. The third and final stage, solid/ liquid separation, was achieved with the help of Lamella Clarifiers, which were preferred to settlement lagoons or hydrocyclones. The grouting work at Wimbleball Dam has been hailed as a success, with seepage levels reduced to their lowest ever levels. Moreover, the monitoring system provided by RSHydro confirmed that the treatment solution was a success. While at times the untreated dam seepage water was highly alkaline, between pH12 and pH14, the treated water released into the River Haddeo was comfortably between pH6 and pH8. During the grouting works the total volume of water treated by the Siltbuster Treat- ment System was an impressive 1.4M cubic metres - equivalent to 6.5% of the volume of Wimbleball Lake. Les Metcalfe, head of contracts and commercial at South West Water, says: "Strengthening the grout curtain in the bedrock below Wimbleball Dam was a challenging project for all concerned. The reservoir is a key resource for South West Water and our neighbours Wessex Water, and the dam is an iconic structure and a popular place for outdoor recreational activities. A'er careful selection of the designer and contractor, we feel both CH2M Hill and Bachy Soletanche rose to the challenge impressively to work as a single team towards successful outcome within a joint target cost. "There were a number of innova- tive approaches and best practice in evidence, and we received plaudits from residents for the way we com- municated and maintained access. A great deal of care was taken to protect the environment, with the Siltbuster 'mini water treatment works' deployed to prevent any pollu- tion downstream. The results are very positive, with leakage through the dam reduced to historically low levels not seen since it was constructed."

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