Water & Wastewater Treatment

18 | november 2014 | WWT | www.wwtonline.co.uk better relative cost," Burns continues. "The chosen site was right next door to our existing works – we own the road that leads down to this work as well - so it allows us to run three assets in that location and operate them using the same teams." Scottish Water engaged Barhale Construction as their main contractors for the project, who brought in reed bed specialists ARM Ltd to design and construct the wetland. One of the main challenges was the poor ground conditions: the site contained landfill material including colliery spoil, domestic waste and sewage sludge, and some of this waste was found to extend down to depths of around 12 metres. In addition, there were disused mine shaˆs below the surface which needed to be filled and grouted before the main excavation work could start. In this context, the choice of a vertical, aerated wetland system was ideal, explains Patrick Hawes, Consultant at ARM Ltd. "The landfill site chosen for the project was a very old one, and there were mine workings underneath it, so bringing the ground back into use was going to cost a lot," says Hawes. "For this reason, the objective was that the reed bed had to be as small as possible, while being able to cope with the predicted load. So the use of aerated reed bed technology was ideal, because you maximise your treatment per metre squared and therefore reduce the total area of the reed bed." The FBA system enabled the bed to be designed much deeper – 2m – than a conventional reed bed, which typically goes down only 0.6m in depth. A deeper bed gives improved performance because the air bubbles can stay in the bed longer and provide a better oxygen transfer rate. The two-unit system is designed to ensure the reed bed can cope with the variable flows of CSO discharges, Hawes explains. "The nature of CSOs is that they are event-driven: you don't get a continuous flow, you get an intermittent flow and therefore the system has to be able to handle high peaks and then nothing, and you don't know when these flows are going to arrive as it depends on the weather. "The answer in this case was to have an upfront balancing system, a 20,000 cubic metre balancing tank, which collects the CSO water, which is then pumped through at a constant rate [4.6 litres per second] into the reed bed so we know and can control what the bed receives. There are limits to what the reed bed can handle, around 4000 cubic metres a day. If there's any excess beyond that, then it will be spilled into the final burn, although in that instance it's likely to be so diluted so as not to need treatment." Burns says that early designs for the flow balancing system had situated the tank underground, but Scottish Water thought there might be The 20,000 cubic metre flow balancing sha , seen here under construction, Project focus: Wastewater treatment "This was a site that nobody had cared for for a long period of time, and we've returned it to a state where it looks great, reeds are growing and we've enhanced the environment as well" Eddie Burns, project manager, Scottish Water

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