WAS Pumps for
Waste Activated Sludge
Reliable Waste Activated Sludge Pumping for Wastewater Treatment Plants
Waste Activated Sludge (WAS) can present a challenging pumping duty. Although fresh WAS is generally a relatively dilute biological slurry, it can contain biological floc, fine suspended solids, fibres, hair, fats, grit, entrained air and other material that can make reliable pumping difficult.
Hydro Innovations supplies a range of WAS pumps for municipal and industrial wastewater treatment applications, including Gorman-Rupp Super T Series, Ultra V Series and 6400 Series solids-handling centrifugal pumps.
Whether the duty involves withdrawing WAS from a secondary clarifier, transferring it to a holding tank or feeding a sludge thickening process, our team can help select and configure a pump to suit the application.
What Is Waste Activated Sludge (WAS)?
Waste Activated Sludge, commonly referred to as WAS, is the excess biological sludge produced during the activated sludge wastewater treatment process.
Within the biological reactor or aeration basin, microorganisms consume biodegradable organic matter in the wastewater. These microorganisms reproduce and form biological floc, which is then separated from the treated water in the secondary clarifier.
The sludge collected at the bottom of the secondary clarifier is generally split into two streams:
RAS – Return Activated Sludge
Sludge returned to the biological treatment process to maintain the required concentration of microorganisms.
WAS – Waste Activated Sludge
Excess biological sludge removed from the process and transferred to further solids treatment.
A simplified process looks like this:
Biological Reactor → Secondary Clarifier → WAS Pump → Sludge Treatment
While WAS is often grouped under the broader term “sewage sludge”, it has different characteristics to primary sludge. WAS predominantly consists of biological solids and microorganisms generated during the activated sludge process.
Why Can WAS Be Difficult to Pump?
Fresh Waste Activated Sludge may contain approximately 0.5–1.5% dry solids, depending on the wastewater treatment plant and process. This means the majority of the material may still be water, but that does not necessarily make it easy to pump.
WAS can contain a mixture of:
- Biological floc
- Fine suspended solids
- Fibres and hair
- Fats and organic material
- Grit and inorganic particles
- Entrained air
- Rags and other contaminants
These characteristics can create problems for pumps that are not suited to solids-laden wastewater.
The characteristics of sludge can also change considerably as it moves through the treatment process. Fresh WAS is comparatively fluid, while thickening increases the solids concentration and can substantially increase viscosity.
For this reason, pump selection should be based on the actual WAS duty rather than treating every sludge application the same way.
WAS Pump Options
Hydro Innovations has several Gorman-Rupp pump options suitable for Waste Activated Sludge applications. The most appropriate pump will depend on factors including flow, head, solids concentration, suction conditions and the type of material present in the sludge.
Super T Series
Sewage and Industrial Wastewater Stringy materials with Eradicator Flows to 200 L/S
Ultra V Series
Sewage and Industrial Wastewater Pump Stringy materials with Eradicator Pressures to 95 metres
6400 Series: Solids-Handling End-Suction Centrifugal Pumps
Centrifugal Pumps for Wastewater Interchangeable parts with Super T Series Multiple impeller options
Gorman-Rupp Super T Series WAS Pumps
The Gorman-Rupp Super T Series is a heavy-duty self-priming centrifugal wastewater pump with proven solids-handling capability.
Super T Series pumps have been extensively used for sludge transfer and are regularly applied to Waste Activated Sludge (WAS) and Return Activated Sludge (RAS) duties in sewage treatment plants.
One of the major advantages of the Super T Series is its ability to be installed high and dry above the wet well, rather than submerged in the wastewater.
This provides operators with easy access to the pump for inspection, monitoring, servicing and blockage removal.
Super T Series advantages include:
- Self-priming operation
- Suction lifts up to 7.6 metres
- Solids-handling capability
- Surface-mounted installation
- Easy access to pump internals
- Large removable inspection cover
- Removable rotating assembly for easier maintenance
- Eradicator options for stringy materials
- Eradicator Plus options for difficult solids-handling applications
- A range of construction materials for abrasive or corrosive applications
For WAS containing rags, fibres, wet wipes or other stringy material, Super T Series pumps can be configured with Gorman-Rupp’s Eradicator Solids Management System.
Gorman-Rupp Ultra V Series WAS Pumps
For Waste Activated Sludge duties requiring higher pressures, the Gorman-Rupp Ultra V Series provides the solids-handling benefits of a self-priming wastewater pump with increased head capability.
Ultra V Series pumps can produce heads approaching and exceeding 95 metres in suitable configurations, making them an option for applications where a conventional solids-handling self-primer may not provide sufficient pressure.
Like the Super T Series, Ultra V pumps can be mounted high and dry above the wet well, keeping the pump accessible to operators.
Ultra V Series advantages include:
- High-head solids-handling performance
- Self-priming to 7.6 metres
- Surface-mounted installation
- Easy access for monitoring and servicing
- Removable rotating assembly
- Eradicator technology available for stringy material
- Flexible flow and head configurations
The Ultra V Series is particularly useful where WAS must be transferred over longer distances or against higher system pressure.
Gorman-Rupp 6400 Series WAS Pumps
The Gorman-Rupp 6400 Series provides an end-suction centrifugal pump option for solids-laden wastewater and sludge applications.
The range is based on proven Gorman-Rupp solids-handling technology and shares many components with the Super T Series.
This gives asset owners an alternative where a conventional end-suction pump arrangement is preferred rather than a self-priming installation.
6400 Series features include:
- Solids-handling end-suction centrifugal design
- Multiple impeller options
- Eradicator Solids Management options
- Eradicator Plus options
- Vortex impeller options
- Replaceable wear components
- Multiple material options
- Interchangeable components with selected Super T Series pumps
Where Are WAS Pumps Used?​
Waste Activated Sludge pumping can occur at several points within the wastewater treatment process.
Secondary Clarifier WAS Withdrawal
The most direct WAS pumping duty is removing excess activated sludge from the secondary clarifier.
Some of the settled activated sludge is returned to the biological reactor as RAS, while the excess WAS is removed and sent for further treatment.
WAS Holding and Transfer
WAS may be transferred to a holding or storage tank before being sent to the next stage of treatment.
The pump must handle biological sludge reliably while meeting required flow and head conditions.
WAS Thickening Feed
WAS is commonly sent to thickening processes to reduce volume before further treatment.
WAS pumps can feed equipment such as:
- Gravity thickeners
- DAF thickeners
- Rotary drum thickeners
- Gravity belt thickeners
- Centrifugal thickeners
As sludge becomes thicker, its pumping characteristics change significantly, and pump selection should be reassessed.
WAS Pumps That Are Easier to Access and Maintain
Pump accessibility can have a major impact on maintenance.
Traditional submersible installations require pump retrieval from the wet well for servicing.
Gorman-Rupp Super T and Ultra V self-priming pumps provide a high-and-dry alternative, allowing surface access.
Benefits include:
- Easier Monitoring
- Faster servicing
- No wet-well pump retrieval
- Reduced lifting equipment requirements
- Improved safety and cleanliness
- Easier access to pump internals
All pumps are selected for efficiency, robust construction, and long life, offering low cost of ownership.
Selecting the Right WAS Pump
There is no single pump configuration suitable for every WAS application. Selection depends on:
Required flow rate
How much WAS must be transferred?
Total dynamic head
System pressure requirements.
WAS solids concentration
Percentage of dry solids.
Sludge characteristics
Fibres, rags, grit, etc.
Suction conditions
Wet well, clarifier, or tank.
Pipework
Diameter, length, and layout.
Operating duty
Continuous or intermittent operation.
Installation type
Self-priming vs end-suction.
Hydro Innovations can assist with full pump selection.
Why Hydro Innovations for Waste Activated Sludge Pumping
Hydro Innovations specialises in pumps for demanding wastewater applications.
We supply Gorman-Rupp solids-handling pumps across Australia and help customers select equipment based on real application requirements.
Our range includes:
- Super T Series self-priming pumps
- Ultra V Series high-head pumps
- 6400 Series end-suction pumps
- Eradicator solids-handling technology
- Sludge pumping solutions for higher-solids duties
What is a WAS pump?
A pump used to transfer Waste Activated Sludge within a wastewater treatment plant.
What does WAS mean?
Waste Activated Sludge.
What is the difference between WAS and RAS?
RAS is returned to the biological process; WAS is removed for further treatment.
What type of pump is used for WAS?
Solids-handling centrifugal pumps such as Super T, Ultra V, and 6400 Series.
Can centrifugal pumps handle WAS?
Yes, when correctly selected for the duty.
Can self-priming pumps be used for WAS?
Yes, in suitable applications.
How much solids does WAS contain?
Typically around 0.5–1.5% in fresh form.
Can WAS pumps handle rags and fibres?
Yes, with Eradicator-equipped configurations.
What happens to WAS after removal?
It is typically thickened, stabilised, dewatered, and converted to biosolids.
Need a Reliable WAS Pump?
Hydro Innovations can help select the right Waste Activated Sludge pump based on your system requirements.
Contact Form
Our Team is available to answer your pump query within the hour.
21-23 Clyde Street,
Rydalmere NSW 2116