Grit Removal Systems: Types and Applications

Introduction

Grit — sand, gravel, cinder ash, eggshells, bone chips, coffee grounds, seeds, and other hard inorganic solids — enters the wastewater collection system from stormwater, food waste disposals, industrial discharges, and deteriorating sewer infrastructure. Left in the treatment process, grit causes:

• Abrasion wear on pumps, mixers, and blowers
• Settling in tanks — grit accumulates in aeration basins, digesters, and clarifiers, reducing active volume and requiring costly cleanouts
• Clogging of pipelines and headworks equipment
• Process interference — grit in biological systems affects mixing and sludge handling

Grit removal is the second headworks treatment step, installed after screening. Its job is to remove dense inorganic solids while retaining the organic solids (settleable BOD) that need to stay in the flow for biological treatment downstream.

Grit Characteristics

Why grit is difficult to remove selectively:

• Grit: specific gravity 2.6-2.7; particles 0.1-3mm
• Organic solids: specific gravity 1.02-1.06; particles 0.1-5mm

The overlapping size ranges mean you can't simply use a settling tank — organic solids would settle along with grit. Grit removal systems use physics (velocity, centrifugal force, or aeration) to create conditions where grit settles while organics remain suspended.

Typical design removal target: 0.21mm (65 mesh) grit at 1.5 ft/sec settling velocity (standard Wastewater Engineering criterion). Some specifications require removal of 0.15mm (100 mesh) grit.

Types of Grit Removal Systems

1. Vortex (Cyclonic) Grit Systems

How they work: Wastewater enters a circular or conical tank tangentially, creating a controlled vortex. The cyclonic motion imparts centrifugal force on the denser grit particles, driving them to the periphery and downward to a central grit hopper. A slow-speed rotating paddle or propeller maintains the vortex and keeps organics suspended.

Types:

• PISTA Grit (Smith & Loveless): Patented paddle-wheel mechanism; most widely installed in North America
• Hydro-Grit (Fluidyne): Tangential inlet, bottom-mounted auger
• SlurryCup (Hydro International, formerly Eutek): Combined vortex + hydrocyclone

Performance:

• 95%+ removal of 0.21mm grit
• Velocity control over wide flow range (important for plants with high I/I)
• Self-adjusting to flow rate changes
• Compact footprint

Best for: Medium to large municipal plants (> 1 MGD); plants with high peak-to-average flow ratios

2. Aerated Grit Chambers

How they work: A rectangular basin with diffused air injected along one side creates a helical (spiral) roll pattern in the cross-section. The cross-roll velocity is controlled to achieve the desired grit/organic separation. Grit settles toward a longitudinal hopper at the bottom while organics are kept in suspension by the spiral motion.

Performance:

• Good grit removal efficiency (85-95% of 0.21mm)
• Flow-velocity dependent — performance drops at flows below design
• Larger footprint than vortex systems
• Requires aeration (additional energy, blowers)

Advantages:

• Simple, proven technology
• Can be designed with air distribution control for variable flow
• Preaeration effect freshens septic sewage and strips H2S

Best for: Older/larger plants upgrading existing grit chambers; plants where preaeration is also desired

3. Horizontal-Flow Velocity Control Channels

How they work: A rectangular channel designed to maintain a specific velocity (typically 1.0-1.3 fps) at all flows using proportional flow-control weirs or automated gates. At the design velocity, grit settles while organics remain in suspension.

Performance:

• Low efficiency compared to vortex and aerated systems
• Heavily flow-dependent (works well only near design flow)
• Large footprint

Applications: Small plants with consistent flows; rarely specified today for new construction

4. Detritors (Stirred Tank)

How they work: A slow-speed propeller agitator in a circular tank keeps organics in suspension while grit settles to the floor. Floor scrapers move settled grit to a central sump for pumping to a grit classifier.

Applications: Common in UK; some US installations; good for large plants with fine grit

Grit Handling Downstream

Settled grit must be extracted from the collection hopper and processed before disposal:

Grit Pump

A submersible or centrifugal pump moves the grit slurry (10-15% solids) from the grit hopper to the classifier. Grit pumps are heavy-duty with replaceable wear liners to handle the abrasive slurry.

Grit Classifier (Grit Washer)

The grit slurry is processed to:

1. Classify — separate grit from fine organic particles by hydraulic upflow
2. Wash — rinse organics off grit surface to reduce odor and landfill concerns
3. Dewater — inclined screw conveyor dewaters the washed grit to 70-80% solids

Clean, dewatered grit is discharged to a dumpster or conveyor for landfill disposal. Grit washers reduce landfill tipping fees (grit is classified as non-hazardous clean fill at many facilities) and reduce odor complaints.

Manufacturers: Headworks International, Smith & Loveless, WesTech Engineering, Hydro International (formerly Eutek)

Grit Volume

Typical grit generation: 1-20 ft³ per million gallons treated, depending on sewer system age, type, and stormwater contributions. Systems with significant I/I produce far more grit during wet weather events.

Grit Removal Sizing Considerations

Design Flow: Size for peak instantaneous flow (PIF) — typically 2-3× ADF for sanitary sewers; much higher for combined sewers

Number of Channels: N+1 minimum; N+2 for plants where grit removal failure has historically caused serious downstream damage

Grit Particle Size Target:

• Standard specification: 95% removal of 0.21mm (65 mesh) or larger
• Enhanced specification: 95% removal of 0.15mm (100 mesh) for protection of fine equipment
• Some equipment (centrifuges, heat exchangers) benefit from 0.10mm (150 mesh) grit removal

Key Grit Removal Manufacturers

• Fluidyne — Hydro-Grit vortex grit removal (95% removal at 74 micron, no submerged moving parts)
• WTP Equipment — vortex grit removal and grit classifiers
• Smith & Loveless — PISTA Grit vortex systems, most widely installed in US
• Hydro International (formerly Eutek) — HeadCell, TeaCup, SlurryCup, Grit Snail
• Parkson Corporation — Grit removal systems
• WesTech Engineering — Grit handling and clarification
• JWC Environmental — Combined grit/screening systems
• Huber Technology — COANDA Grit systems (European approach, US market)