Water & Wastewater Treatment

In the know Technically speaking: sludge dewatering Fig 2: The SC1000 controller with cover removed 38 | DECEMBER 2015 | WWT | www.wwtonline.co.uk 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 a ected. 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 (see Fig. 3) 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 aƒ er the start- up phase. Figure 4 clearly shows that aƒ er 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 e ort for the manual control of the process 1 Decanter Centrifuge Polymer To Hydrolysis Q constant + RTC-SD Module Control DS Š What is thermal hydrolysis? ● Thermal hydrolysis is a two-fold process combining high-pressure, high temperature treatment of sludge, followed by a rapid decompression. This cracking process makes the sludge more biodegradable and thereby improves digestion performance and biogas production. Prior to the hydrolysis process the sludge has to be dewatered. Precise control of the upstream sludge dewatering process, delivering sludge with constant dry solids content, is necessary in order to run the hydrolysis process under optimal conditions. The DS content is kept constant by measuring the dry solids content and the volume of the incoming sludge and dosing the polymer used for dewatering accordingly using a real time control system. Fig 3: Schematic representation of the RTC SD module. Control is based on a fl ow and dry solids measurement. An algorithm is used to calculate the optimal polymer dose. were di‹ cult to monetise, but the achieved reduction in raw material consumption led to annual savings of more than £150k leading to a payback period of less than two months for the complete investment. Fig 4: Above diagram, before optimisation: Very large variations in polymer dose rates leading to unsatisfactory cake quality (under-dosing) and antifoam requirement due to overdosing. Below diagram, a er optimisation: Very stable polymer dosing rates – average 5.2 g polymer / kg DS.

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