The Cotton Valley Sludge Treatment Centre (STC) is one of 10 operated by Anglian Water to dewater and treat sludge is anaerobically to produce bioenergy. Anglian Water is the largest water company in England and Wales by geographic area, supplying water and recycling services to over six million domestic customers, processing over 142,000 tonnes of dry solid sewage sludge in 2020-21 to generate 115GWh of bioenergy.
In 2008, an advanced digestion thermal hydrolysis process (THP) was commissioned to speed up the anaerobic digestion process. As part of an upgrade, a sludge handling facility was designed to handle approx. 160 tonne/day of imported dewatered sludge from satellite works.
Progressive cavity (PC) pumps not supplied by Seepex were installed to back-mix the dewatered sludge and pump it into the THP. The two pumps were costing overall about £100,000 per annum due to changing rotors and stators every 3-6 months. Their inability to transfer no more than 3.5m3/h meant the site was not able to meet the design criteria, limiting the amount of imported raw cake being handled to only 120 tonne per day. Additionally, prior to maintenance, the silo had to be emptied to allow work on the pumps. In addition, cake imports from satellite sites had to be diverted while work was carried out, resulting in substantial logistical costs.
Seepex was invited to survey the site based on its experience in pumping and mixing dewatered sludge within Anglian Water’s water recycling treatment sites and sludge treatment centres. Leigh Thornley, engineer for Seepex, says: “We took pride in Anglian Water coming to Seepex with the opportunity to resolve a long-standing problematic application.”
Seepex was asked for a complete turnkey package, which included the removal of the existing pumps which were installed under a 7m-high silo. The new pumps required custom-made hoppers, which were 5m long, to fit under the existing silo. They had to be able to handle 5.5m3/hr of 16% DS (dry solids) sludge which had been back-mixed within the pump from 25% DS, and to transfer this approximately 60m.
Seepex supplied and installed two custom-made pumps from its BTES range with a dimensionally identical hopper to fit within the same footprint of the original pumps, while delivering over 55% increased throughput capacity from 3.5m3/hr to 5.5m3/hr. These pumps are designed to fit under silos, have custom made hoppers to suit the application and additionally incorporate an isolation devise to enable maintenance of the pumps to take place whilst sludge is left in the silo. Part of the Seepex solution was to offer standardisation of pump size with other sludge treatment centres within Anglian Water, resulting in commonality of spare parts.
Due to the complexities of replacing the existing pumps and the restricted space, the Seepex Service Team carried out 3D modelling of the working area to ensure the pumps would fit first-time. Over a two-week period, the old pumps were removed and the new pumps installed all within the agreed project timeline.
Prior to the installation of the new pumps, Anglian Water were processing approx. 120 tonne per day of imported sludge cake with both pumps running. With the new pumps installed, the customer is now able to exceed this to 180 tonne while maintaining the same footprint and motor size.
Since the pumps have been installed, no spare parts have been used and the service life has considerably exceeded that of the previous pumps.
“Due to the reliability and performance of the new Seepex pumps, there is an increase in uptime of the cake import facility at Cotton Valley,” explains Luke Fish, production manager at Anglian Water. “This has not only allowed us to increase the sludge throughput on site and therefore generate more biogas but has also given us significant savings in logistics and raw cake storage costs.”