Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | DECEMBER 2015 | 37 In the know Technically speaking: sludge dewatering Optimal control Real-time control processes are allowing the optimisation of the sludge dewatering process at Northumbrian's Bran Sands WWTW, Teesside dry solids of indigenous and imported sewage sludge and has a power generation capacity of 4.7 MW. Besides a reduction of carbon emissions, the sludge treatment process leads to huge reductions in consumption of biogas and imported electricity (90 % and 50 % respectively) and thus signi• cantly saves on operating costs. The sludge is processed using the CAMBI process in which the sludge is subjected to a thermal hydrolysis process prior to anaerobic digestion. During the two-stage process, the sludge is • rst subjected to a high- pressure steam treatment and then expanded rapidly, resulting in a complete cell disruption of all microorganisms. This results in improved biodegradability and † ow properties of the treated sludge. Based on those improvements, the feed rates of the digesters can be doubled leading to maximised energy production and minimised energy and sludge transportation costs. The entry point into the process is the sludge dewatering. Here, the dry solids content (DS) of the sludge must be increased from ~2% to 17-18% DS regardless of its initial concentration and composition. Sludge dewatering requires mixing the incoming sludge with a polymer solution prior to the actual dewatering step which is taking place in a decanter centrifuge. In contrast to the commonly performed sludge thickening prior to the anaerobic sludge treatment where the DS content is increased to 6-7%, a much higher dry solids content must be reached here. In the past, adjusting the polymer dose had been done manually, resulting in a strongly † uctuating dry matter content leading to a non-optimal hydrolysis process. Furthermore, this approach resulted in a high consumption of polymer and subsequently also to an increased consumption of anti- foaming agents necessary to reduce the foam formation caused by excess polymer. Hence the primary objective of optimising sludge dewatering was therefore to keep the dry solids content constant at the desired 18% and also to reduce the consumption of chemicals. Real Time Control Solution Optimising sludge dewatering required the installation of a sensor continuously measuring the DS content of the incoming sludge. The installed Solitax inline sc probe connected to a SC1000 controller provides the measurement value for the sludge dewatering real time controller (RTC-SD) which in turn controls the polymer pump. Figure 2 shows the SC1000 controller (cover removed), the RTC-SD and the connector panel with connections to the polymer dosing pump and the sludge feed. The quality of the DS measurement results as a prerequisite for a safe and reliable control of the sludge Fig. 1: Sewage treatment plant and sludge treatment centre, Bran Sands, Teesside A t Bran Sands on Teesside, Northumbrian Water's site houses a regional sludge treatment centre and e˜ uent treatment works. It is one of Northumbrian Water's largest sites, treating the majority of sludge in the North East − with drying and digestion capabilities. The plant, situated at the estuary of the River Tees (Fig. 1) annually processes 40,000 tonnes of STUART AINSWORTH HACH UK DR. ANDREAS SCHROERS HACH GERMANY

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