Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | ocTober 2014 | 29 The move from thermal drying to HPH was a major project with work over four months Project focus: Pumps & pumping systems temperature of 37 °C in the digesters, by way of a fixed pump connected to each digester, which recirculated the sludge through a heat exchanger to ensure the required temperature. Jennings explains: "The solids content and the temperature were both critical to the process because the level of solid content was required for the thermal dryer and the 37 °C temperature was needed to allow methane extraction for use in the CHP plant that helped power the thermal dryer. "The infrastructure we had in place was ideal for managing the required viscosity and temperature but as we started to move towards introducing the HPH process, we found that it wasn't able to manage the required changes to the dry solids content and temperature parameters." The thermal drying process contin- ued to operate as usual while the HPH infrastructure was being constructed but in order to prepare for the switch • Innovations ● Utilising diesel pumps to circulate the sludge throughout the four-month changeover to the HPH plant would have been effective but this approach would have impacted sig- nificantly on the budget for project delivery because of the cost of providing fuel 24/7 dur- ing the scheme. ● By providing electric submersible wastewa- ter pumps, Sykes was able to optimise tem- perature management while reducing the financial implications of the job. over to the new process, Anglian Water began to increase the level of dry solids in the sludge from 3% to 6% by increasing the level of dewatering prior to storage of the sludge in the di- gesters. This was done gradually over a three-month period and coincided with a move to increase the tempera- ture of the sludge in the digesters from 37 °C to 39 °C to prepare it for heat treatment during the pasteurisation process. "While a 3% increase in dry solids and a two degree increase in tempera- ture may seem like small changes they were actually very significant. The level of dry solids was increasing by 50% and because the digesters were so large, it meant the fixed pumps had to work significantly harder to circulate the sludge," says Jennings. "It soon became clear that the pumps simply couldn't cope and in- stead of increasing the temperature of the sludge, we were actually seeing a drop in temperature. We needed to ad-

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