Water & Wastewater Treatment

18 | january 2015 | WWT | www.wwtonline.co.uk Project focus: Tanks Zickert offers significant advantages over conventional sludge & scum scrap- ing technology with lower operating and infrastructure costs. The gentle action of the equipment produces a higher yield of sludge than convention- al scrapers, while its higher throughput reduces the amount of secondary treatment required and produces more primary sludge with a higher calorific value. At Crossness STW, eight new primary settlement tanks have been built, contributing towards increas- ing the sewage treatment capacity by 44% at the site, while supporting the need to provide energy-efficient odour control and deliver sludge to provide 20% of the plant's energy needs. The largest sludge treatment project is at Beckton STW, where a total of 24 existing sedimentation tanks have been retrofitted with the twinned sludge scrapers, adding to eight existing primary tanks already converted to the system. At Mogden, eight primary set- tlement tanks had previously been retrofitted with the technology, while a further eight existing primary settlement tanks and five new tanks built were also retrofitted in a 2nd phase which is part of Thames Water's £140M upgrade to increase sewage treatment capacity at the site by 50%. Energy saving "Considerable energy savings accrue with the implementation of close fitted tank covers," Strange points out. "An important further objective is to increase our use of sludge for digestion and gas production. For example we have made significant investment in new high yield thermal hydrolysis plants, especially at the bigger sites, and have a couple more sites to expand to. "To improve gas volumes, we have been looking at the performance of our Primary Settlement Tanks to remove sludge before thick blankets form. The use of Zickert technology can help achieve this as the technology means that the sludge has to come out fresher so we should be getting a better gas yield than with travelling bridges. We will be able to make comparisons over 2014/2015, particularly between Beckton and Long Reach, where travel- ling bridges have been retained, as the plant is not being encroached to the same degree." Operating efficiency Maintenance and tank cleaning is another long term operating cost issue that will be assessed and compared. "My operations colleagues will be looking at this, as well," Strange adds, "as it is part of the lifetime cost equa- tion. At the moment, it seems that it is easier to maintain the tanks fitted with Zickert, as most wear parts are outside the tank whereas the travelling bridge has to be completely shut down during maintenance." "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 Strange. "Over the three years of installa- tion, we were able to make further refinements to our tank installation design that maximised the immedi- ate advantages of the Zickert scraper technology." • Innovations ● Eight new primary settlement tanks have been built at Crossness, contributing towards increasing the sewage treatment capacity by 44% at the site ● At Beckton, 24 existing tanks had to be retrofitted with the technology ● All the tanks involved in the Thames Water project were rectangular in shape. However, the recently developed Zickert Rotating Sludge Scraper enables the same technology to be used for circular tanks. Top: Installation underway at Mogden Bottom: a close up of the surface scum scraper

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