Technically Speaking: sludge dewatering at Bran Sands
Real-time control processes are allowing the optimisation of the sludge dewatering process at Northumbrian's Bran Sands WWTW, Teesside
by Stuart Ainsworth, Hach UK and Dr Andreas Schroers, Hach Germany
At Bran Sands on Teesside, Northumbrian Water’s site houses a regional sludge treatment centre and effluent 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 annually processes 40,000 tonnes of 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 significantly 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 first 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 flow 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 fluctuating 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.
The quality of the DS measurement results as a prerequisite for a safe and reliable control of the sludge dewatering is assured by Prognosys, a predictive diagnostic system running on each Hach real time controller. The system continuously analyses internal signals of the connected sensors, thereby detecting potential irregularities and alerting the user to upcoming instrument maintenance before measurement results and ultimately the dewatering process are affected. The RTC-SD calculates the optimum amount of polymer based on the measured dry solids content and the amount of the incoming sludge to be treated, and uses the calculated result to control the dosing pump directly.
Stabilised Processes – Reduced Chemical Consumption
With this solution, the primary goal, to feed the thermal hydrolysis process with sludge having a constant dry solid content of 18% could be achieved immediately after the start-up phase. Figure 4 clearly shows that after commissioning the control solution a very stable ratio of polymer to dry solids could be established. By eliminating polymer overdosing the polymer consumption could be reduced by 40 %. Subsequently, the dosing of anti-foaming agents previously required for process stability was reduced by 75 %.
Finally, the stabilisation and optimisation of the thermal hydrolysis process and the reduced effort for the manual control of the process.
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