Drive algorithm cuts sludge pumping demand by 90 percent
24 June 2011
ABB has introduced a new method of controlling sludge in sewage treatment plants that cuts pumping demand by up to 90 percent. Known as 'reactive pump control', the method uses an ABB low voltage ac drive, combined with a specially written software algorithm to measure viscosity and achieve the flow control demanded by the sludge pumps.
With reactive pump control, the drive starts pumping on an internally set time delay and stops pumping when sludge viscosity drops to a pre-determined value. The pump maintains a constant speed using the viscosity measurement to ensure only the correct percentage of solids is pumped. It can typically achieve savings of over £42,000 for a sewage treatment plant, through reduced energy use, reduced sludge transport vehicles and lower maintenance of the sludge pumps.
The method tackles much of the waste of resources involved in current sludge pumping, leading to reductions in operating costs, cuts in transport of sludge and savings in capital equipment for new projects.
Steve Ruddell, division manager, ABB Discrete Automation & Motion, says: “The new reactive pump control method is the brainchild of one of our ABB Drives Alliance members, IDS, showing the depth of skills and knowledge that such companies have, as well as the commitment to innovation throughout the ABB network.”
In current wastewater treatment practice, sludge is pumped from settling tanks to either holding tanks or decanting tanks using progressive cavity (PC) pumps, from where it is pumped into road tankers for transfer to digesters.
With this method before the sludge can be pumped from the settling tanks, it needs to be of a certain viscosity. Measuring this viscosity is achieved using PLCs and sensors inserted within the sludge to determine the percentage of dry solids.
Yet, whatever the level of solids present in the sludge, the pumps need to be run frequently to prevent clogging. As well as wasting energy, this also leads to a requirement for monthly maintenance of the pumps.
This excessive pumping also leads to holding tanks having a relatively low percentage of solids and storage tanks requiring regular manual decanting. Road tankers also take on higher water content than required, while further energy is used to pump the de-canted water back to the head of the works.
Payback for a typical reactive pump control project is about ten months. This pump control method also helps achieve a reduction in capital costs, removing the need for surge vessels, pressure relief valves, electromechanical timers, sensors, PLC and instrumentation.
Another benefit is that settling tanks do not need de-canting prior to tanker arrival, while the number of tankers needed is halved as the water content in the sludge in the holding tank is much reduced.
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