Water & Wastewater Treatment Magazine
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24 | MARCH 2018 | WWT | www.wwtonline.co.uk • CASE STUDY Sludge thickening at a small sewage treatment works Severn Trent Water recently conducted a successful trial of a Static Sludge Thickener provided by Plantwork Systems (PWS) at its Blockley Sewage Treatment Works in Gloucestershire. Blockley is a small rural site in the Cotswolds. It is challenging because it is remote, accessed by a narrow track and predominantly unmanned. Prior to the trial, sewage sludge from the site was being removed by tankers visiting 4-6 times per week, transporting sludge with a dry solids content of 0.6-1.2%. PWS's Static Sludge Thickener is a technology that offers an efficient means of gravity settlement, making it possible to achieve considerably thicker sludge and reducing handling and transportation costs. The Blockley site was selected for a trial of the technology in order to prove the business case for installing the Static Sludge Thickener on multiple sites, saving money through a reduction in tanker movements. The target for the Static Sludge Thickener at Blockley was to increase sludge thickness to 5.5%-6% dry solids, reducing tanker movements accordingly. Over the 12-week trial period, there were a number of site operational issues with the main works which inhibited the full benefit of the technology throughout the trial. Nevertheless, almost 200 cubic metres of sludge was removed with a dry solids weight of 8.65 tonnes: the average percentage dry solids achieved over the period was therefore 4.4%. Whilst this was below the original target, the trial was considered a success by Severn Trent Water, given the operational restrictions of the site. "The trial Static Sludge Thickener provided by PWS has been excellent," said Craig Pearson, Sludge & Logistics Optimisation Manager at Severn Trent Water. "It fulfilled all our expectations in terms of dry solids and saved us over 60% in tankering costs." Based on the sludge density achieved in the trial, it has been concluded that a payback period of 3-5 years is possible on a number of Severn Trent Water sites. This represents significant savings and provides a strong business case for future installations. A site is currently being selected for the first permanent installation. The Works: sludge quality The trial of the SST at Blockley