Wednesday, 24 February 2010

Thicker sludge saves time and money

Recent improvements in pump technology prompted Severn Trent Water (STW) (GB) to initiate 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 which, nine months after it began, has been heralded a success.

STW's Mark Bryan summarises 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 Ltd.

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 explains the role of instrumentation in achieving the objectives: “For many years thickened sludges have been processed and transported at low solid concentrations of around 2-3% due to concerns over the poor reliability of controlling instrumentation when higher sludge concentrations have been required. 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. 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 commented: “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% 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% for the trial).

The EMS system is now pumping sludge with solids of over twice the concentration of the traditional system (progressing cavity pump). Paul Hawthorne added: “There is a major drive in the water industry to save energy and to reduce the carbon footprint. We are delighted that this project has managed to demonstrate one of the ways in which this can be achieved.”

Summarising, Mark Bryan said he is delighted with the results of the trial and added that he anticipated this will lead to the new auto-desludging process being rolled out at other STW plants. He explained: “The trial has shown that the original objectives have been met; the new pump runs for approximately 3 hours each day (dependant on incoming solids loading), as opposed to 8-16 hours 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% 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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