Progressive Cavity RAS Pumps
Controlled return activated sludge pumping for reliable recycle flow between secondary clarification and the biological treatment process.
What is return activated sludge?
In an activated-sludge process, mixed liquor flows from the aeration basin to a secondary clarifier where biological solids settle. A portion of those solids is returned to the biological process as RAS, while excess biomass is removed as waste activated sludge. Maintaining the required recycle rate helps sustain the desired microorganism concentration in treatment.
Displacement is approximately proportional to speed, allowing recycle flow adjustment with a VFD.
Smooth positive-displacement delivery supports stable process control.
PCPs can move activated sludge when the pump and suction system are correctly selected.
Relatively low-shear pumping is useful for biological sludge service.
RAS pump selection factors
A RAS pump should not be selected from flow alone. Pump speed, suction conditions, sludge properties and operating hours all affect reliable service.
Flow control and VFD operation
Because progressive cavity pump capacity is closely related to rotational speed, a VFD can provide straightforward adjustment of RAS flow as plant conditions change. The allowable speed range must still respect pump size, inlet conditions, sludge properties and motor requirements.
RAS vs. WAS
RAS is recycled to maintain biological solids in treatment, while WAS removes excess biomass. WAS—and particularly thickened WAS—can have different viscosity, solids concentration and downstream pressure requirements.
Explore WAS & TWAS Pumps →Rotor, stator and maintenance considerations
Rotor and stator life depends on operating speed, pressure, sludge abrasiveness, elastomer compatibility and lubrication by the pumped fluid. Pump size should allow the required recycle flow without unnecessary speed.
Protect the pump
Prolonged dry running should be prevented, and positive-displacement discharge pressure should be protected with appropriate controls or relief provisions.
- Pressure protection
- Dry-run protection
- Motor overload protection
- VFD speed limits
Need help selecting a RAS pump?
Send the required recycle flow range, solids concentration, discharge pressure, suction conditions, temperature and existing pump information.