Water and Effluent Treatment Magazine
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14 WET NEWS SEPTEMBER 2015 A three year project to increase capacity, improve odour control and deliver high-quality sludge for energy generation at Thames Water's major Mogden, Beckton and Crossness sewage treatment plants has come to fruition. Sludge handling technology has been upgraded as part of a multimillion pound investment in sewage treatment at Thames' major London sewage treatment plants. Following record-breaking contracts with Hydro International totalling more than £20M million, Thames' investment in Zickert bottom and surface scraper technology is proving its worth across a total of 53 new or refurbished primary settlement tanks. "What has emerged, to date, is that these projects have certainly met their initial aims, and further benefits may well accrue as part of longer term operation," says Garry Strange, Sludge Strategy manager for Thames Water. "Over the three years of installation, we were able to make further refinements to our tank installation design that maximised the immediate advantages of the Zickert scraper technology." These contracts were part of the £675M upgrade of the five treatment works at Beckton, Crossness, Mogden, Long Reach and Riverside as part of Thames Water's London Tideway Improvement programme, improving discharge quality, operating efficiency and biogas generation to supplement treatment plant energy requirements. "One of the key factors in Thames Water's strategy was consideration for nearby communities where wastewater treatment plants are being encroached by residential and commercial development," continues Strange. "Engaging with our customers is very important to us, with odour control in particular being an understandable focus for local residents." Damp atmosphere A major source of odour is from the off-gases emanating from the primary sludge removal process in the open settlement 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 requiring 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. The Zickert bottom scraper is a framework of plough-shaped, profiled blades that move the sludge in a reciprocating action • Thames Water has been upgrading its sludge handling technology where treatments plants have been encroached by residential development. Why keeping the neighbours happy is important ONSITE SludgE handling to the hopper end of the tank. The Zickert surface scum scraper design 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 considerable saving in energy consumption. "As development has encroached on our sites we have been driven to cover more of our primary settlement tanks. With the minimal headroom requirements of the Zickert top and bottom scraper technology, we were able to create an initial low volume cover design for some of the first tanks to be refurbished at Beckton. 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," adds Strange. Thames Water made the investment in Zickert sludge scrapers based on the performance of some existing installations 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, now completed in 2014. At Crossness STW, eight new primary settlement tanks have been built, contributing towards increasing the sewage treatment capacity by 44% at the site, while supporting the need to provide energy-efficient odour ProjEcT SPEcS • upgrade sludge handling technology at london STWs • Supply and install Zickert bottom and surface scraper technology • increase capacity, improve odour control and deliver high-quality sludge for energy generation TEchkNoW • The Zickert surface scum scraper design has a low profile enabling covers to be installed at a much lower level • The Zickert bottom scraper's profiled blades move the sludge in a reciprocating action to the hopper end of the tank • Zickert technology means the sludge is fresher "We will be able to make comparisons over 2014/2015, particularly between Beckton and long Reach, where travelling bridges have been retained, as the plant is not being encroached to the same degree. With the minimal headroom requirements of the Zickert top and bottom scraper technology, we were able to create an initial low volume cover design" Garry Strange, Thames Water NEEd To kNoW 1 Considerable energy savings accrue with the implementation of close fitted tank covers 2 Sludge handling technology was upgraded as part of a multimillion pound investment 3 Maintenance and tank cleaning is another long term operating cost issue that will be assessed and compared 4 a major source of odour is from the off-gases emanating from the primary sludge removal process Eight new primary settlement tanks have been built at Crossness STW, contributing towards increasing the sewage treatment capacity by 44% at the site