Grit Classifier

Grit classifiers separate inorganic grit particles from organic matter in wastewater treatment headworks, using hydraulic principles to achieve particle size separation typically in the 0.2-2.0 mm range. Raw grit slurry enters the classifier where upward water velocity creates a fluidized bed, allowing lighter organic materials to overflow while heavier inorganic particles settle and discharge through the underflow. Well-designed classifiers achieve 85-95% organic removal efficiency from captured grit. The primary trade-off is balancing wash water flow rates against grit loss - insufficient washing leaves organic content while excessive flow carries away valuable grit particles.

• Primary Grit Removal Systems (2-50 MGD): Grit classifiers are installed downstream of aerated grit chambers or vortex grit tanks to separate organic material from captured grit. The classifier receives grit slurry at 8-15% solids and produces clean grit at 65-85% solids for disposal. Selected because manual grit washing is labor-intensive and creates odor issues. Connects to grit pumps upstream and grit containers/conveyors downstream.
• Headworks Upgrade Projects (0.5-20 MGD): Retrofitted into existing plants where original grit systems lack washing capability. Particularly common when upgrading from manually-cleaned channels to mechanized systems. WHY selected: eliminates putrescible organics that cause odors in landfills and reduces grit disposal weight by 40-60%.
• Combined Sewer Plants (5-50 MGD): Essential in CSO-impacted facilities where high organic loading contaminates grit. Classifiers handle variable flows from 50-150% design capacity during wet weather events.

Daily Operations: Operators monitor grit discharge consistency and adjust screw speed based on visual inspection - properly washed grit appears gray/black without visible organics. Check wash water pressure (maintain 20-25 PSI) and flow rates. Document runtime hours and grit volumes. Typical adjustment: reduce speed if organics remain, increase if excessive grit carryover occurs.

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Maintenance: Weekly bearing lubrication and spray nozzle inspection. Monthly drive alignment checks and wear plate examination. Annual gearbox oil changes and screw flight replacement (typically 3-5 year intervals). Requires confined space entry training for hopper cleaning. Standard PPE includes hard hat, safety glasses, and slip-resistant footwear. Lockout/tagout procedures essential for screw maintenance.

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Troubleshooting: Excessive vibration indicates worn bearings or bent screw flights. Plugged spray nozzles cause poor washing - evident from brown, odorous grit discharge. Drive overloads suggest material bridging in hopper or excessive grit loading. Warning signs include unusual noise patterns and irregular discharge. Typical service life: 15-20 years for mechanical components, 8-12 years for wear

• Inclined Screw Conveyor: Stainless steel (304 or 316) screw at 15-25° angle lifts and dewaters grit. Diameter ranges 9-24 inches for municipal applications. Variable speed drives (5-25 RPM) optimize washing efficiency. Selection based on grit loading: 2-8 cubic yards per day typical.
• Wash Water System: Counter-current spray nozzles provide 3-8 GPM per cubic foot of grit. Stainless steel headers with removable nozzles for cleaning. Pressure typically 15-30 PSI from plant service water or dedicated pumps.
• Grit Discharge Hopper: Reinforced concrete or stainless steel collection point with 60° minimum slope. Sized for 4-8 hour grit storage at design capacity. Includes level sensors and cleanout access.
• Drive Assembly: Gear reducers with 20:1 to 40:1 ratios, typically 2-5 HP motors with VFD control. Sealed bearings rated for wash-down environments. NEMA 4X enclosures standard.

• Flow Parameters: Peak hourly flow rate: 0.5-75 MGD (typical municipal range), Grit loading rate: 0.5-8.0 cubic feet per hour per MGD, Hydraulic loading: 800-1,200 gpm/ft² of classifier surface area
• Physical Sizing: Screw conveyor diameter: 6"-24" (most common 9"-12"), Incline angle: 25-35 degrees (30° optimal for most applications), Tank depth: 8-20 feet depending on flow, Retention time: 2-4 minutes in classification zone
• Performance Criteria: Grit removal efficiency: 95% for particles >150 microns, Organics content in classified grit: <3-5% by weight, Wash water requirement: 10-25% of grit volume, Power consumption: 0.5-2.0 HP per MGD capacity
• Operational Ranges: Grit concentration: 1-15 cubic feet per MG influent, Specific gravity differential: 1.5-2.65 (grit vs. organics), Wash water pressure: 15-25 psi, Underflow solids concentration: 60-75%

• What is the expected grit loading and particle size distribution? Need detailed grit characterization study showing cubic feet per MG and size gradation. Undersizing leads to overflow and downstream equipment damage. Oversizing wastes 15-25% on capital costs. Critical threshold: >2 ft³/MG requires larger screw diameter and multiple units for redundancy.
• Is organic separation performance critical for disposal costs? Municipal landfills typically require <5% organics content. Without effective washing, disposal costs increase 200-400%. Need wash water system sizing based on 15-20 gpm per cubic foot of grit. Poor separation leads to odor complaints and regulatory issues.
• What redundancy level is required during peak wet weather events? Single units handle up to 10 MGD; larger plants need multiple parallel classifiers. N+1 redundancy essential for plants >25 MGD. Failure during storm events causes grit accumulation in downstream processes, requiring expensive emergency cleanouts costing $50,000-150,000.
• How will classified grit be dewatered and disposed? Determines discharge consistency requirements (60-75% solids) and conveyor design. Truck loading requires different discharge height than container systems. Wrong specification adds $25,000-75,000 in modification costs.

• Primary: Division 46 - Water and Wastewater Equipment, Section 46 21 00 - Grit Removal Equipment
• Secondary: Division 40 - Process Integration (for complete grit handling systems including conveyors and storage)

• Material/Equipment Verification: Verify 316SS construction for all wetted parts, Confirm bearing seals rated for grit service, Check drive motor IP65 rating minimum
• Installation Requirements: Concrete anchor bolt templates critical - verify field dimensions, Overhead crane access for maintenance removal, 480V/3-phase power with VFD integration
• Field Challenges: Grit discharge chute alignment with conveyors, Washwater system pressure/flow verification
• Coordination Issues: Coordinate with grit removal equipment lead times (12-16 weeks typical)

• Headworks International - GritSnail spiral classifiers (municipal standard, 0.5-50 MGD range)
• Lakeside Equipment - Grit-Pak and spiral grit classifiers (proven municipal track record)
• Smith & Loveless - PISTA grit systems with integrated classification (package plant focus)
• Huber Technology - RoSF4 spiral classifiers and ROTAMAT systems (European technology, growing US presence)

• Grit pumps with cyclone separators - Lower maintenance, higher power consumption, preferred for smaller plants (<5 MGD). Cost: 60-70% of screw classifiers.
• Aerated grit chambers with manual removal - Lowest capital cost but high labor requirements. Suitable for <1 MGD facilities.
• Vortex grit systems - Higher removal efficiency, 20-30% premium over spiral classifiers, preferred for stringent discharge requirements.

Specify stainless steel hardware throughout - carbon steel bolts fail within 2-3 years in grit service. Establish relationships with local manufacturer reps for rapid parts supply; bearing replacements are common maintenance items. Consider standardizing on single manufacturer across multiple units for parts inventory efficiency. Negotiate service training as part of initial purchase - proper lubrication schedules extend equipment life significantly.