The Hidrostal Prerostal system.
The original site at the Troqueer Wastewater Works was constructed in the 1950s and extended again in the 1970s. Hidrostal installed three of their shaft driven dry well Prerostal pumps in 1974 when Dumfries & Galloway Regional Council required inlet pumps to handle raw sewage across a range of flows.
The pumps were originally chosen as they presented a solution that could achieve flow matching from the same size of pumps through hydraulically induced turndowns. This natural and sustainable hydraulic solution uses Hidrostal Screw Centrifugal pumps installed alongside Hidrostal Prerostal Basins.
The pumps were installed in a vertical dry well configuration above the shallow wet well basins with suction bellmouths installed into the wet well area below. Shaft driven by Brook Crompton 37 kW, eight pole motors, each pump is capable of delivering 350 litres per second at 6m head. Reduced flows can also be achieved through pre-rotation at lower sump levels.
The benefit of this system to operator Scottish Water was that fewer models of fixed speed pumps were required to achieve a wide range of flow turndowns. The system was also efficient at self-cleaning. In addition, a Prerostal system requires a much smaller installation footprint, particularly when compared to a conventional Archimedes screw pump, which significantly reduces construction costs.
The three original 37kw Brook Compton motors driving the 40-year-old Preorstal system.
Archimedes screws
During their first 14 years of operation the pumps ran daily as the main duty pumps. In 1988, two Archimedes screws were added to the installation in order to handle increased dry weather flows of up to 600 l/s.
The Prerostal system then assisted with the higher flows under storm conditions and for the past 27 years has handled the raw sewage on storm flows in addition to the Archimedean screws, without fault. Scottish Water engineers claim the performance of the pumps over the last four decades has been exemplary. Tam Kearney, senior operator at Troqueer, said: “The inlet pumps have handled everything thrown at them. I have never seen a blockage in my 30 years of working on this site. I would say that they have been the most reliable piece of mechanical equipment at Troqueer.”
“In my 20 years operating this site I have only seen the bearings on shaft drives 1 and 2 replaced. Apart from that no other mechanical replacements have been made,” added David Gillespie, M & E technical at Troqueer.
Now Hidrostal has supplied two new dry well submersible low shear RAS/SAS pumps and submersible final effluent pumps as part of the upgrade The original pumps were used for forwarding raw sewage to the treatment works. They were chosen for their ability to handle raw sewage without blocking and for the ability to forward flows uniformly.
The enhanced operation is now running more efficiently than ever with the promise of many more years of trouble-free service.
