Anglian Water trials using variable speed drives (VSDs) to control the speed of Archimedes screw pumps, resulted in a three-month energy savings of £26,000 across three wastewater treatment sites. It also helped to reduce wear on the associated belts, drive chains and bearings.
Each site uses three Archimedes screw pumps each controlled by a 90 kW motor. They move activated sludge from a sump to the aeration dyke. Smart ‘multi-pump’ software built into the VSDs lets them communicate with each other. The VSDs adjust the speed of the screws to keep sludge balanced between them and ensure throughput remains constant. Moreover, this ensures the transfer of sludge to the aeration dyke, with none remains on the actual screws to trickle back down to the sump.
In the original configuration, each screw motor operated alone in a duty/assist/assist cycle. The duty screw would run at full speed whilst an ultrasonic detector measured the sludge level in the sump. As this approached the selected depth, they brought a second screw and then a third as needed. When the level of sludge in the sump dropped, they would stop one of the assist screws. Yet, this would cause the sludge in the screw to fall back into the sump, raising concerns within Anglian Water of how much energy they were wasting re-pumping this liquid.
Archimedes screw pumps
Anglian Water studied the use of VSDs on Archimedes screw pumps at the three locations. They installed 90 kW ABB VSDs on the screws at Newton Marsh in Tetney, Lincolnshire. Rather than switch the screws on and off, they ran them non-stop, with signals from the level sensor in the sump determining the correct speed for the motor and screw. This varied between 25-60 Hz to maintain a fixed level in the sump.
Previous attempts at applying VSDs to Archimedes screw pumps showed little benefit as the power consumption per cubic metre of sludge remained constant over a wide range of speeds and flow rates. The main difference in this case was by ensuring the drives communicate with each other to balance the motor speeds they found it worked well.
Energy measurements showed that the new VSDs at Newton Marsh reduce energy use by 13.7 percent. At Flag Fen, the control system saved 19.4 percent and final savings achieved at Hibaldstow were greater at 32 percent.
The VSDs also helped to reduce starting currents when compared with the variable voltage soft starters used before. By using the VSDs, starting currents fell from 600 A to 180 A. Moreover, using VSDs also helped to reduce wear on belts, drive chains and bearings.