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Severn Trent Water (STW) has initiated a de-sludging trial to reduce costs and energy consumption by producing thicker sludge in the Primary Settlement Tank (PST).

Advanced monitoring instrumentation from Hach Lange played a key role in the trial.

STW’s Mark Bryan summarised the objective of the trial: ‘We believed that if we could monitor and control the settlement process through a real-time control mechanism, we would be able to produce thicker sludge, reduce operational intervention and save the energy and costs involved with further sludge thickening prior to the sludge-digestion process.’ The trial took place at STW’s Stoke Bardolph Sewage Treatment Works.

Monitoring and control instrumentation was provided by Hach Lange and a hydraulically operated ram pump with an adaptive control system was provided by EMS Industries.

A Sonatax SC probe was deployed for the measurement of sludge blanket level and a Solitax SC probe provided suspended solids and turbidity values.

All data was handled by an SC 1000 plug-and-play controller, which interfaced with the EMS adaptive-control system to manage the pump operation.

Hach Lange’s Jon Shepherd explained the role of instrumentation in achieving the objectives: ‘It was the aim of this trial to establish if thicker sludge could be created in the PST and pumped.

‘The benefit of pumping thicker sludge is that more solid material can be transported per litre of water pumped, which provides energy savings and improved process efficiency.

He added: ‘Our instrumentation reliably monitored the sludge levels under a number of different de-sludging scenarios, including pumping with various sludge densities and using different PST settlement times.’ Paul Hawthorne, technical director at EMS said: ‘The Hach Lange instrumentation enabled us to refine our adaptive pumping system, allowing the full capability of our pump to reliably pump thick sludge to be achieved.’ The trial demonstrated that sludge in excess of 5.5 per cent solids has been automatically controlled and regularly drawn from the PST.

Sludge blanket-level stability has been maintained by the EMS pump’s adaptive-control system, which varied the speed of the pump stroking action to suit changing sludge conditions.

Hach Lange’s SC1000 control unit enabled the pump to be run only when a minimum sludge blanket level had been achieved and for pumping to cease when sludge density reached the preset low level (5.5 per cent for the trial).

The EMS system is now pumping sludge with solids of over twice the concentration of the traditional system (progressing cavity pump).

Mark Bryan said: ‘The trial has shown that the original objectives have been met; the new pump runs for approximately 3hrs each day, as opposed to 8-16hrs with the previous system, resulting in considerable energy savings and increased process efficiency.

‘This project has proved that the EMS ram pump can operate comfortably at 5.5 per cent solids and that the accuracy and reliability of the Hach Lange instrumentation has been key to the success of the system.’

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