The Ashbridges Bay Main Treatment Plant in Toronto, Canada, is using a novel pump design to solve some of its toughest sludge pumping applications.
The plant operates 24 hours a day, 365 days a year, so pump reliability in sludge handling is paramount. The plant is the largest of four sewage treatment facilities located within the borders of metropolitan Toronto. It has a design capacity of 150,000 gpm, providing wastewater treatment services to an estimated population of 1,250,000 people.
The first area of concern at the Ashbridges plant was the digested sludge disposal area. The sludge contains approximately 3 percent solids and has to pass through a grinder prior to being pumped to the centrifuges. The plant operates continuously at a flow of 250 gpm to each centrifuge. Unplanned downtime in this service could lead to the unavailability of the centrifuges, reducing the plants solids dewatering capacity.
The second pump problem area at the main treatment plant was transferring thickened waste activated sludge from the dissolved air flotation tanks to the digestion tanks. Here, the sludge is thickened from less than 1 percent to approximately 5 percent solids in a process that separates solids from liquid using dissolved air in water and a polymer coagulant. The resulting sludge usually contains some entrained air.
In 1995 staff began installing Discflo disc pumps in the sludge disposal facility. Since then, another nine pumps have been installed in the sludge disposal area. The latest batch of four pumps was started up at the end of 1998.
Disc Pump Design
The disc pump operates on the principles of boundary layer and viscous drag. It looks like a centrifugal unit but does the work of a progressive cavity or lobe pump. The pumping mechanism is called the Discpac - a series of parallel discs with either a smooth or a ribbed surface.
Disc Pump Design
As fluid enters the disc pump, it moves parallel to the discs, with the boundary layer creating a molecular buffer between the disc surfaces and the fluid. Energy is transferred to successive layers of molecules in the fluid between the discs, generating velocity and pressure gradients across the width of the Discpac. This combination of boundary layer and viscous drag creates a force that "pulls" the product through the pump in a smooth, pulsation-free flow.
Disc Pump Design
The key point here is that the fluid does not "impinge" on the moving parts of the pump. This non-impingement design is where the disc systems differs from other pump systems on the market, all of which use some kind of impingement device - such as a vane, impeller, paddle, lobe or screw - to "push" product through the pump. The non-impingement action prevents excessive wear from abrasion. In addition, the disc pump has no close tolerances, so it will not clog when pumping large and stringy solids.
Disc Pump Design
Since the startup, the Discflo pumps in both sludge thickening and sludge disposal have operated trouble free. No repairs have been required to date on any of the pumps, and downtime in the critical area of sludge disposal has been negligible, with only routine maintenance being required.
