Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | january 2015 | 17 where wastewater treatment plants are being encroached by residential and commercial development," says Strange. "Engaging with our custom- ers is very important to us, with odour control in particular being an under- standable focus for local residents. As development has encroached on our sites we have been driven to cover more of our primary settlement tanks." Odour control A major source of odour is the off-gas- es emanating from the primary sludge removal process in the open settle- ment tanks. The accepted practice is to provide a covering over the tanks, extract the contaminated air and scrub it to remove odours. With traditional travelling bridge technology, surface scum and bottom sludge scrapers re- quiring mounting on the exterior wall of the rectangular tanks, considerable headroom clearance is required for the moving machinery so covers become a major construction challenge; as well as covering more materials that are vulnerable to the corrosive damp atmosphere, there is high electricity consumption from handling the large volumes of enclosed air. For these reasons, Thames were drawn to the bottom scraper and surface scum scrapers provided by Zickert: its surface scum scraper de- sign has a low profile enabling covers to be installed at a much lower level compared to conventional travelling bridges, leading to lower air volumes to be purified and therefore a consid- erable saving in energy consumption. Meanwhile, the bottom scraper is a framework of plough-shaped, profiled blades which move the sludge in a reciprocating action to the hopper end of the tank. "With the minimal headroom re- quirements of the Zickert top and bot- tom scraper technology, we were able to create an initial low volume cover design for some of the first tanks to be refurbished at Beckton," says Strange. "This cover design was improved to be even closer to the top of the water at Mogden and Crossness, for more energy-saving air extraction for odour treatment." Thames Water decided to invest in the technology based on the proven performance of some existing installa- tions at Beckton and Mogden. Starting in 2011, the most recent upgrade work has involved both new build of rectangular primary settlement tanks and refurbishment of existing tanks, completed this year.

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