Chain-and-Flight Grit Collector

Chain-and-flight grit collectors continuously remove sand, gravel, and heavy organic matter from wastewater using an endless chain system with attached flights that scrape settled grit along the tank bottom. The chain travels over sprockets at each end of a rectangular tank, with flights spaced typically 8-12 feet apart to push accumulated grit up an inclined section for discharge. These systems typically achieve 95% removal efficiency for particles 150 microns and larger at design flow rates. The primary trade-off is higher maintenance requirements due to chain wear and potential jamming from debris compared to other grit removal technologies.

• Primary Clarifier Underflow: Chain-and-flight collectors remove settled grit from primary clarifier bottoms in plants 2-25 MGD. Selected for reliable continuous removal without hydraulic disruption. Feeds to grit classifiers upstream of digesters.

• Aerated Grit Chamber Bottoms: Most common application in 0.5-15 MGD plants using rectangular aerated grit chambers. Chain system provides consistent scraping velocity (0.5-1.0 fpm) regardless of flow variations. Connects upstream to headworks screening, downstream to grit washing/dewatering.

• Pre-Aeration Basin Grit Removal: Used in extended aeration plants (1-10 MGD) where grit settles in pre-aeration zones. Selected over pumped systems due to lower maintenance and power requirements in remote locations.

• Secondary Clarifier Grit Accumulation: Backup removal system in plants experiencing grit carryover from primary treatment, typically 5-50 MGD facilities with high inflow infiltration during wet weather events.

Daily Operations: Operators monitor chain speed and flight integrity during routine rounds, typically 2-3 times per shift. Key observations include unusual noise, chain alignment, and grit discharge consistency. Speed adjustments made based on grit accumulation rates, with typical settings of 0.5-1.0 fpm for normal conditions.

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Maintenance: Monthly lubrication of drive components and quarterly chain tension adjustment required. Annual replacement of wear strips and flight hardware typical. Confined space entry procedures necessary for basin access. Maintenance staff need basic mechanical skills and fall protection training for work around moving equipment.

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Troubleshooting: Chain derailment most common failure, usually from improper tension or worn sprockets. Warning signs include irregular scraping sounds, visible chain slack, or reduced grit removal efficiency. Typical service life: 10-15 years for mechanical components, 5-8 years for chains/flights. Drive motor failures often result from inadequate weatherproofing in outdoor installations.

• Drive Mechanism: Motorized head sprocket (typically 0.5-2 HP) with variable speed capability (0.3-1.5 fpm chain speed). Gear reducers sized for 50:1 to 100:1 reduction. Selection based on basin length and anticipated grit loading.

• Chain and Flight Assembly: Corrosion-resistant chain (304/316 SS or polymer-coated) with attached flights spaced 6-10 feet apart. Flight materials include HDPE, fiberglass, or stainless steel. Sizing based on basin width (typically 6-20 feet municipal applications).

• Collection Hopper: Stainless steel or concrete hopper at discharge end with 60-degree minimum slope. Capacity sized for 2-4 hours grit storage at design loading rates of 1-4 cubic feet per MG treated.

• Tensioning System: Adjustable tail sprocket with pneumatic or manual tensioning to maintain proper chain tension (typically 500-1500 lbs). Critical for preventing chain derailment and ensuring consistent flight contact with basin floor.

• Flow Rate Parameters: Peak flow capacity: 0.5-15 MGD per channel for standard units, Overflow rate: 500-800 gpm/ft² (typical municipal range), Detention time: 45-90 seconds at peak flow, Channel width: 3-12 feet (standard manufactured sizes), Channel depth: 6-12 feet water depth

• Hydraulic Loading: Horizontal velocity: 0.7-1.2 fps (critical for grit settling), Scour velocity: <1.4 fps to prevent organics carryover, Head loss: 2-6 inches through channel at design flow

• Mechanical Parameters: Chain speed: 3-8 fpm (varies by manufacturer - Headworks, Lakeside), Flight spacing: 6-10 feet on centers, Grit removal capacity: 1-4 cubic feet per MG treated, Chain tension: 15,000-25,000 lbs typical operating load

• Performance Criteria: Grit removal efficiency: 95% for particles >65 mesh (0.21mm), Organic content in removed grit: <3% by weight, Available head: minimum 12 inches for proper hydraulics

• Single vs. Multiple Channel Configuration? Above 5 MGD, specify minimum two channels for redundancy. Single channel acceptable only below 2 MGD with bypass capability. Wrong decision eliminates maintenance flexibility and creates single point of failure during peak flows.

• Aerated vs. Non-Aerated System? Aerated systems required when BOD loading exceeds 200 mg/L or septic conditions exist. Air requirement: 3-5 cfm per foot of channel length. Non-aerated sufficient for typical domestic wastewater. Wrong choice leads to septicity, odors, and poor grit washing.

• Grit Washing Integration Level? Integral washing (Headworks AquaGuard) vs. separate classifier depends on organics content and disposal requirements. Integral systems handle <15% organics effectively. Separate washing needed for higher organic content or strict disposal specifications.

• Drive Configuration Selection? Chain drive systems require 5-15 HP motors depending on channel length and loading. Backup power essential for facilities >10 MGD. Under-powered drives cause chain failures and operational shutdowns during peak flows.

• Primary: Division 46 23 13 - Grit Removal Equipment

• Secondary: Division 46 21 00 - Wastewater Collection Piping (inlet/outlet connections), Division 46 23 00 - Wastewater Treatment Equipment (general mechanical systems)

• Material/Equipment Verification: Chain material certification (stainless steel grade verification), Drive component specifications and warranty terms, Concrete anchor bolt templates and structural loads

• Installation Requirements: Precise channel dimensions and elevation control, Overhead crane access for maintenance, Electrical coordination for VFD integration

• Field Challenges: Chain tensioning during startup, Alignment issues with concrete tolerances

• Coordination Issues: 16-20 week lead times typical, Sequencing with concrete pour schedules

• Headworks International - PISTA Grit Systems (rectangular units 4-40 ft wide)

• Lakeside Equipment - Raptor Grit Removal Systems (flows 0.5-100 MGD)

• WesTech Engineering - ChainFlight Grit Collectors (standard widths 6-20 ft)

• Evoqua Water Technologies - Grit King systems (municipal installations nationwide)

• Vortex Grit Chambers - Better for smaller plants (<5 MGD), 20-30% lower capital cost but higher energy consumption

• Aerated Grit Chambers - Preferred when combined organics removal needed, similar capital cost but more complex operations

• Horizontal Flow Grit Chambers - Lowest cost option for smaller facilities, but requires more frequent cleaning and provides less consistent removal efficiency

Establish direct relationships with manufacturer field service teams early - they provide invaluable startup support and troubleshooting expertise. Consider oversizing drive motors by 25% for challenging grit conditions. Specify stainless steel chains even when carbon steel is acceptable; the 15-20% cost premium pays back through reduced maintenance. Always include spare chain links and flight paddles in initial procurement.