A grit classifier is the dewatering stage that follows grit chamber separation in the wastewater treatment headworks. A grit classifier in wastewater treatment receives the grit-water slurry extracted from a grit chamber and separates the grit from the carrier water, producing a relatively dry grit cake suitable for disposal while returning the separated water to the treatment process.
Grit extracted directly from a circular grit chamber or other grit removal equipment still contains a significant volume of water — typically 90% or more by volume. Without a dewatering stage, this wet grit would be heavy, difficult to handle, and costly to transport for disposal. The grit classifier solves this problem mechanically, with no chemical addition required.
What Is a Grit Classifier?
A grit classifier is a combination decantation and conveying unit that separates grit from water using gravity settling followed by mechanical screw conveying. The unit consists of a decantation tank where the grit-water mixture settles, and an inclined shaftless screw that lifts the settled grit out of the tank to a discharge point above the waterline.
As a grit classifier manufacturer and worldwide supplier, Vortex Engineering provides a wide range of design options to match different grit chamber capacities and grit loading rates.
The grit classifier is a standard companion to any grit removal system. Wherever a circular grit chamber, vortex grit chamber, or other grit separation equipment is installed, a grit classifier is needed downstream to complete the dewatering process before the separated grit can be disposed of.
How Does a Grit Classifier Work?
The grit classifier operates as a continuous three-stage process.
Decantation Stage
The grit-water mixture is pumped from the grit chamber into the decantation tank of the classifier — typically using an airlift pump or recessed impeller pump from the grit chamber extraction system. Inside the decantation tank, the mixture is given time to settle: grit, with its higher density compared to water, sinks to the bottom of the tank, while the clearer water remains above.
The decantation tank is sized to provide sufficient residence time for the grit particles to settle out before the water overflows to the return outlet. Tank geometry and the inlet arrangement are designed to minimise turbulence that could keep fine grit particles in suspension.
Screw Conveying and Dewatering
An inclined shaftless screw is positioned within the tank, with its lower end submerged in the settled grit and its upper end above the waterline at the discharge point. The screw rotates slowly, picking up settled grit from the bottom of the tank and conveying it upward along the incline.
As the grit travels up the inclined screw, it progressively drains free water back into the tank below — this dewatering action occurs naturally as the screw lifts the grit above the water surface. The shaftless screw design, without a central shaft, avoids the clogging issues that a conventional shafted screw would experience when conveying grit and debris.
Discharge
At the top of the inclined screw, the dewatered grit is discharged into a skip, container, or conveyor for transport to disposal. The discharged material has a significantly reduced water content compared to the original grit-water mixture extracted from the grit chamber, reducing the weight and volume that must be handled and transported.
The decanted water that remains in the tank overflows through a return outlet, typically directed back to the head of the treatment plant or to a designated return point in the treatment process.
Grit Classifier vs Grit Washing Classifier
Vortex Engineering offers both a standard grit classifier and a grit washing classifier, which differ in one important respect. Unwashed grit may contain 50 percent or more of putrescible, organic material — giving it a distinctly disagreeable odour and attracting insects and rodents if left untreated. The objective of grit washing is to produce a clean grit with a volatile solids content of less than 20 percent while retaining at least 95 percent of the depositable grit.
| Grit Classifier | Grit Washing Classifier | |
|---|---|---|
| Primary function | Dewatering | Dewatering plus organic washing |
| Washing mechanism | None | Integrated washing zone |
| Organic content of discharged grit | Moderate — some organics remain | Low — organics actively removed |
| Best for | Standard municipal grit handling | Plants requiring inert grit for disposal classification |
The grit washing classifier incorporates an additional washing stage that actively separates organic material from the inorganic grit, producing a cleaner, more inert discharge. This is particularly valuable where local disposal regulations classify washed grit differently from unwashed grit, or where odour control at the disposal stage is a priority. The standard grit classifier, without the washing stage, is sufficient for most municipal applications where dewatering alone meets disposal requirements.
Where the Grit Classifier Fits in the Grit Handling Process
The grit classifier is the second stage in a complete grit handling system, positioned immediately downstream of the grit separation equipment.
The typical grit handling sequence is:
Circular grit chamber (separation) → Grit classifier (dewatering) → Disposal container → Transport to landfill or reuse
The grit chamber performs the initial separation of grit from the bulk wastewater flow, using rotational or channel flow to settle dense inorganic particles. The grit-water slurry extracted from the chamber is then pumped to the grit classifier, which completes the dewatering process and produces the final discharge product ready for handling and transport.
Without the grit classifier, the wet grit-water mixture extracted from the grit chamber would require alternative dewatering arrangements or would need to be disposed of in its wet, heavy state — significantly increasing disposal volume and cost.
Key Design Parameters
Grit loading rate. The volume and mass flow rate of grit entering the classifier from the grit chamber determines the required decantation tank size and screw conveying capacity. This is derived from the grit removal rate of the upstream grit chamber.
Decantation residence time. The tank must provide sufficient residence time for grit particles to settle before the water overflows. Insufficient residence time results in fine grit carrying over with the decanted water, reducing dewatering performance.
Screw inclination angle. The angle of the inclined screw affects both conveying efficiency and dewatering performance — steeper angles provide better dewatering as grit travels further above the waterline, but require more conveying torque.
Screw speed. Slower screw speeds generally provide better dewatering, as the grit has more time to drain as it travels up the incline. Speed is balanced against the required throughput capacity.
Discharge height. The height of the discharge point above the operating floor or ground level must be sized to suit the container or conveyor receiving the dewatered grit.
Materials and Construction
Vortex Engineering grit classifiers are manufactured from stainless steel throughout — selected for durability against the highly abrasive nature of grit.
Tank and structural components: Stainless steel AISI 304 standard; AISI 316 available for more aggressive environments, including coastal or marine applications.
Shaftless screw: Wear-resistant stainless steel, designed to handle the abrasive nature of grit without premature wear. The shaftless design eliminates the central shaft bearing that would otherwise be exposed to constant grit abrasion.
Drive system: Motorised gearmotor drive, selected for continuous duty in the wet, abrasive environment of grit handling.
Fasteners: Stainless steel A2 or A4 depending on the installation environment.
Frequently Asked Questions
Properly designed grit classifiers typically reduce the water content of discharged grit to a level suitable for direct disposal, though exact figures depend on grit characteristics and classifier sizing. The key benefit is the significant volume and weight reduction compared to the raw grit-water mixture extracted from the grit chamber.
Yes, in virtually all practical installations. Without a dewatering stage, the wet grit-water mixture extracted from the grit chamber would be too heavy and voluminous for cost-effective handling and disposal.
A standard grit classifier dewaters the grit-water mixture through decantation and screw conveying. A grit washing classifier adds an active washing stage that removes organic material from the grit, producing a cleaner, more inert discharge suitable for stricter disposal classifications.
The water that overflows from the decantation tank is typically returned to the head of the treatment plant for reprocessing, as it still contains fine organic material and some residual fine grit.
Regular inspection should check the condition of the shaftless screw for wear, gearmotor function, and the cleanliness of the decantation tank. Annual inspection is recommended for most municipal applications, with more frequent checks for plants with high grit loading.
Unwashed grit pumped directly from a grit chamber may contain 50 percent or more of putrescible, organic material by weight. This organic content gives unwashed grit a distinctly disagreeable odour and can attract insects and rodents at the disposal site. Where this is a concern — either for operational or regulatory reasons — a grit washing classifier should be specified instead of a standard grit classifier.
The most common method of grit disposal is transport to a landfill, including codisposal with municipal solid waste. In some larger plants, grit is incinerated together with sludge solids. Depending on local regulations, grit may need to pass a paint filter liquid test before landfill disposal, or may require lime stabilisation for pathogen control. All disposal must conform with applicable environmental regulations.
The objective of the grit drying and dewatering process is to produce a clean, dry grit with a total solids concentration greater than 60 percent while retaining at least 95 percent of the depositable grit. This ensures the material is suitable for standard landfill disposal and is manageable to handle and transport.
Vortex Engineering designs and manufactures Grit Classifiers as part of its complete Grit Removal Equipment range and the full Wastewater Treatment Equipment lineup.
