Water & Wastewater Treatment

PUMP CENTRE CONFERENCE & EXHIBITION 2014 PREVIEW 15 Reactive pump control saves utility £100,000 Case study S evern Trent Water (STW) is saving a total of £100,000 per year on energy and maintenance costs across a number of its wastewater treatment plants (WwTP) following the installation of technology from ABB and its author- ised value provider, Inverter Drive Sys- tems (IDS). The system was initially tri- alled at STW's sewage treatment site at Pye Bridge in Nottinghamshire. It reduced pumping time by 90% by running the pump on average for ten minutes per five hours, saving £42,000 a year. Since the installation at Pye Bridge, a number of other installations have be- come operational at Severn Trent sites - Rainworth, Boughton, and Calverton - which have seen a 49% reduction in pumping overall. Savings in energy and maintenance hours across the sites have totalled £100,000 a year. ABB says maintenance costs have been cut owing to the variable-speed drives, and gearbox failures have been eliminated. Each stoppage for mainte- nance and repair used to mean four hours of staff time to manually drain the sludge tanks. Control method Developed by IDS and known as reactive pump control, the new method of con- trolling sludge in WwTPs uses an ABB low voltage AC drive, combined with a specially written soŽware algorithm to measure viscosity and achieve the flow control demanded by the sludge pumps. The drive starts pumping on an internally set time delay and stops pumping when sludge viscosity drops to a pre-deter- mined value. Managing director of IDS, Blaise Ford, said, "The pump maintains a constant speed using the viscosity measurement to ensure only the correct percentage of sol- ids is pumped. Costs are cut through reduced energy use, reduced sludge transport vehicles and lower mainte- nance of the sludge pumps. "The method tackles much of the waste of resources involved in current sludge pumping, leading to reductions in oper- ating costs, and savings in capital equip- ment for new projects." In traditional wastewater treatment practice, sludge is pumped from settling tanks to either holding tanks or decant- ing 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 sol- ids present in the sludge, the pumps need to be run frequently to prevent clogging. As well as wasting energy, this also leads Severn Trent's trial at Pye Bridge led to a rollout of reactive pump control to a requirement for monthly mainte- nance of the pumps. This excessive pumping also leads to holding tanks having a relatively low per- centage 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 decanted water back to the head of the works. Reliability Reactive pump control does not need sensors and is therefore inherently relia- ble. Using proven components such as diaphragm pumps and ABB drives means the application needs no other control products. Capital costs are reduced as there is no need for surge vessels, pressure release valves or electromechanical tim- ers and can be retrofitted to existing dia- phragm pumps. Andy Berry, operations & mainte- nance standards, service delivery partner for the East, within STW's wastewater services, says, "The system at Pye Bridge is still working well, giving us power sav- ings and improved de-sludging through better control of the pumps." The reactive pump control technology won two innovation awards at the Water Industry Achievement Awards 2013, hosted by WWT and WET News. Stand S6

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