Centrifugal vs. Positive Displacement Pumps in Water and Wastewater
Pump selection is fundamental to water and wastewater system design, and the choice between centrifugal and positive displacement (PD) pumps affects energy efficiency, maintenance requirements, and operational flexibility. Understanding where each technology excels ensures reliable performance and optimal lifecycle cost for every pumping application in the treatment plant.
Centrifugal Pump Fundamentals
Centrifugal pumps use a rotating impeller to impart kinetic energy to fluid, which is then converted to pressure energy in the pump volute or diffuser. They are the most common pump type in water and wastewater, covering the vast majority of clean water, raw sewage, and low-solids pumping applications.
How they work: Fluid enters the pump at the impeller eye (center) and is accelerated radially outward by the spinning impeller vanes. The expanding volute casing converts velocity to pressure. Flow rate is determined by the intersection of the pump curve and system curve.
Key characteristics:
- Flow varies with system pressure (head-dependent)
- Best efficiency point (BEP) defines optimal operating range
- Cannot run dry without damage
- Flow rate controlled by throttling, VFD, or impeller trimming
- Smooth, continuous, non-pulsating flow
- Lower capital cost per unit flow than PD pumps
Common Centrifugal Pump Types in Water/Wastewater
| Type | Application | Typical Flow |
|---|---|---|
| End suction | Clean water, chemical feed | 10–5,000 gpm |
| Split case | Raw water intake, high flow | 100–50,000 gpm |
| Vertical turbine | Well pumping, deep wet wells | 50–20,000 gpm |
| Submersible | Lift stations, wet well mounting | 50–10,000 gpm |
| Non-clog | Raw sewage, solids handling | 50–10,000 gpm |
| Self-priming | Portable, above-grade | 50–3,000 gpm |
| Chopper | Rag-heavy influent | 50–5,000 gpm |
Representative centrifugal manufacturers: Ebara — centrifugal water/wastewater pumps; plus Flygt (Xylem), Grundfos, Sulzer, and Cornell for non-clog and submersible service.
Positive Displacement Pump Fundamentals
Positive displacement pumps trap a fixed volume of fluid and mechanically force it through the discharge. Flow is proportional to speed and nearly independent of discharge pressure, making PD pumps ideal for applications requiring precise flow control or handling of viscous, abrasive, or shear-sensitive fluids.
Common PD Pump Types in Water/Wastewater
Progressive cavity (PC): A helical rotor turns inside an elastomer stator, creating progressing sealed cavities. The dominant PD pump for sludge and biosolids handling. Flow range: 1–1,500 gpm. Handles solids up to 25% concentration.
Rotary lobe: Two or three lobed rotors counter-rotate within a housing. Gentler than PC pumps with easier maintenance (no stator replacement). Common for thickened sludge and polymer transfer. Flow range: 10–3,000 gpm.
Diaphragm/metering: A reciprocating diaphragm displaces precise volumes per stroke. Used for chemical dosing (polymer, coagulant, chlorine, caustic). Flow range: 0.01–100 gph. Accuracy ±1–2%.
Peristaltic (hose): Rollers compress a flexible hose, pushing fluid forward. No seals contact the fluid—ideal for abrasive slurries and chemicals. Flow range: 0.1–500 gpm. Hose life 2,000–4,000 hours.
Piston/plunger: Reciprocating high-pressure pumps used for membrane system feed, sludge injection, and high-pressure wash. Flow range: 1–500 gpm at pressures to 10,000 psi.
Representative PD manufacturers: LobePro — rotary lobe pumps (sludge/slurry/scum); PCM — progressing cavity pumps; plus NETZSCH, SEEPEX, and Moyno for sludge and dosing service.
Head-to-Head Comparison
| Parameter | Centrifugal | Positive Displacement |
|---|---|---|
| Flow behavior | Varies with head | Constant at given speed |
| Viscosity handling | Poor above 500 cP | Excellent to 100,000+ cP |
| Solids handling | <5% (non-clog) | Up to 25%+ (PC, lobe) |
| Shear | High | Low (PC, lobe) |
| Efficiency at BEP | 70–90% | 50–85% |
| Efficiency off-BEP | Drops significantly | Remains relatively flat |
| Self-priming | Usually not | Most types yes |
| Pulsation | None | Varies by type |
| Pressure capability | Low–medium (to 500 psi) | Medium–high (to 10,000 psi) |
| Capital cost | Lower | Higher per unit flow |
| Maintenance | Impeller, seal, bearings | Stator/hose/diaphragm wear parts |
Application Guidelines
Use Centrifugal Pumps When:
- Pumping clean water, raw sewage (<3% solids), or low-viscosity fluids
- High flow rates are needed (>500 gpm)
- Variable flow is required (VFD-controlled centrifugal pumps handle this efficiently)
- The application is within the pump's BEP range (±20%)
- Capital budget is a primary driver and the application matches centrifugal capabilities
Use Positive Displacement Pumps When:
- Handling thickened sludge, biosolids, or viscous fluids (>500 cP)
- Precise, repeatable flow control is required (chemical dosing)
- Low shear is critical (polymer-conditioned sludge must maintain floc integrity)
- Self-priming is needed (suction lift applications)
- High pressure at low flow is required
- Solids concentration exceeds what centrifugal pumps can handle
Energy Considerations
Centrifugal pumps are most energy-efficient when operated near BEP. Operating at 50% of BEP flow (throttled) wastes 20–40% of input energy. VFD control significantly improves part-load efficiency by reducing speed rather than throttling.
PD pumps maintain relatively constant efficiency across their operating range because flow is proportional to speed. This makes them more efficient than centrifugal pumps in applications with highly variable flow or viscosity. However, the mechanical friction in PD pumps limits peak efficiency to 50–85% depending on type.
For a facility with multiple pumping applications, the optimal strategy is centrifugal pumps (with VFDs) for clean water and low-solids service, and PD pumps for sludge, chemical feed, and high-viscosity applications.
Selection Process
- Define the hydraulic requirements: flow rate, total dynamic head, NPSH available
- Characterize the fluid: viscosity, solids content, abrasiveness, shear sensitivity, chemical compatibility
- Determine operating pattern: constant duty, variable duty, intermittent
- Evaluate lifecycle cost: capital, energy, maintenance parts, labor over 20 years
- Consider redundancy: duty/standby requirements, spare parts availability, lead times
When in
Do you know where your next client is? We do.
Violations, consent orders, full historical DMR data, plant details and rankings for every regulated facility — searchable with a smart AI interface.
This guide is provided for general informational and educational purposes only and does not constitute engineering advice. Treatment technology selection, sizing, and regulatory compliance are project-specific; design ranges and manufacturer information are summarized from public sources and may change over time. Verify all data against current regulations, applicable standards, and manufacturer documentation, and consult a qualified professional engineer before making design or procurement decisions.