Water and Effluent Treatment Magazine
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20 WET NEWS JANUARY 2015 Upgrading sludge processing at Anglian Water's Pyewipe water recycling centre in Grimsby involved meticulous sludge management. The challenge: Preparing sludge for eco improvements The 1.2 megawatt CHP plants process methane from the sludge, and are used to generate power for the boilers T he Pyewipe water recy- cling centre processes 400m3 of sludge every day from the sewage works on site, along with an additional 600m3 of sludge and 100 tonnes of cake from other sites on the An- glian Water network. The thermal drying process that had been used for the three digesters on site for many years had been effective but it was not very energy efficient. John Jennings, treatment optimiser at Anglian Water, explains: "The site has its own CHP plants which are powered using methane from the sludge digestion during processing. However, despite energy effi- ciency measures like this, the thermal drying process was using huge amounts of energy which had both cost and sus- tainability implications." The decision was made to convert the sludge dewatering process to a heat, pasteurisa- tion and hydrolysis (HPH) advanced digestion process which would generate energy efficiencies while reducing operational requirements on site. Methane extraction In order to implement this change, Anglian Water needed to ensure that the sludge was at the right temperature and vis- cosity for the transition. The thermal drying process required that the sludge be maintained at 3% dry solids and a constant temperature of 37°C was needed to allow meth- ane extraction for use in the CHP, which helped power the thermal dryer. These levels were main- tained thanks to a fixed pump connected to each digester, which re-circulated the sludge through a heat exchanger. The level of dewatering car- ried out prior to storage of the sludge in the digesters was gradually increased over a three-month period to increase viscosity to 6% dry solids ready "It soon became clear that the pumps simply couldn't cope and instead of increasing the temperature of the sludge, we were actually seeing a drop in temperature. We needed to address that fall in temperature quickly ..." John Jennings, Anglian Water THE CONCEPT • Immediate stop gap solution found with diesel pumps to prevent any further loss of temperature • More long-term solution specified to minimise energy consumption and costs during the transition from thermal drying to HPH • Managing sludge solids content and tempera- ture meant that the HPH plant could be utilised as soon as commissioning was complete NEED TO KNOW 1 The HPH project at Pyewipe represented an investment of £28 million by Anglian Water 2 It was part of a strategy designed to improve efficiency, enhance capacity and reduce operational costs 3 Failure to circulate the sludge effectively could have been a major setback for the project with financial and operational implications THE VERDICT "The fact that Sykes Pumps was able to put an interim solution in place immediately and then get the specialist kit to site so quickly meant that we could continue to focus on the major challenges of modifying our process and keep the capex scheme on schedule" John Jennings, treatment optimiser, Anglian Water • INSIGHT PUMPS & VALVES for conversion to the HPH process. The temperature was also needed to be raised gradually over the same period from 37°C to 39°C to prepare it for heat treatment during pasteurisation. Jennings continues: "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 used in the process had to work signifi- cantly harder to circulate the sludge. "It soon became clear that the pumps simply couldn't cope and instead of increasing the temperature of the sludge, we were actually seeing a drop in temperature. We needed to address that fall in temperature quickly in order to keep the switch over to the new HPH plant on schedule." n