Back mixing technology increases throughput

ANGLIAN WATER supplies water and recycling services to over six million domestic customers, processing over 142,000t of dry solid sewage sludge in 2020-21 to generate 115GWh of bioenergy. The Cotton Valley Sludge Treatment Centre (STC) is one of Anglian Water’s ten main STCs where sludge is dewatered and treated anaerobically to produce 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. 160t a day of imported dewatered sludge from satellite works.

Problematic application

Progressive cavity (PC) pumps from another manufacturer were installed to back mix the dewatered sludge and pump it into the THP. The two pumps were costing overall circa £100k per annum due to rotors and stators change every 3-6 months. Their inability to transfer no more than 3.5m³/h (bottleneck and limiting gas production) meant the site was not able to meet the design criteria limiting the amount of imported raw cake being handled to only 120t per day. Additionally, prior to maintenance, the silo had to be emptied to allow the pumps to be worked on, but also cake imports from satellite sites had to be diverted whilst work was carried out resulting in substantial logistical costs.

SEEPEX was invited to survey the site based on their vast experience in pumping and mixing dewatered sludge within Anglian Water’s water recycling treatment sites and sludge treatment centres.

Custom-made

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, and being able to handle 5.5m3/hr of 16% DS sludge which had been back mixed within the pump from 25% DS and transfer this approx. 60m. 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.

SEEPEX supplied and installed two custom-made pumps from their BTES range with a dimensionally identical hopper to fit within the same footprint of the original pumps whilst delivering over 55% increased throughput capacity from 3.5m³/hr to 5.5m³/hr. SEEPEX BTES range 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.

First-time fit

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 by 50% (from 120 tonne up to 180 tonne) whilst maintaining the same footprint and motor size.

The new SEEPEX pumps offer ample flexibility to meet the site’s production demands at lower speeds, which has a direct impact on reducing spare parts consumption. Since the pumps have been installed, no spare parts have been used and the service life has considerably exceeded that of the previous pumps.

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