Traveling Bridge Grit Collector

Traveling bridge grit collectors remove sand, gravel, and other heavy inorganic particles from wastewater using a mechanized bridge that travels across rectangular grit chambers. The system employs a scraper mechanism mounted on rails to continuously collect settled grit from the chamber floor and transport it to a discharge hopper. Typical removal efficiency ranges from 95-98% for particles larger than 0.2mm at surface loading rates of 500-1,200 gal/min/ft². The primary limitation is higher capital cost compared to aerated grit systems, making them most cost-effective for plants above 5 MGD capacity where consistent grit removal is critical.

• Primary Treatment - Grit Removal: Traveling bridge grit collectors are primarily installed in rectangular grit chambers (10-50 feet wide, 80-200 feet long) following bar screens and ahead of primary clarifiers. They continuously traverse the chamber length, collecting settled grit while maintaining constant velocity flow. Selected for their ability to handle variable flows (2-25 MGD) without velocity control structures.

• Aerated Grit Chambers: Used in spiral-flow aerated grit systems where air diffusers create controlled circulation patterns. The traveling bridge removes grit from collection hoppers while organic material remains suspended. WHY selected: eliminates need for multiple chain-and-flight collectors, reducing maintenance complexity in 15-40 foot wide chambers.

• Combined Sewer Systems: Essential in plants receiving combined sewer overflows where grit loads vary dramatically during storm events. The traveling mechanism handles intermittent high-grit periods without clogging, unlike fixed mechanical systems.

Daily Operations: Operators monitor bridge travel cycles (typically 10-20 minutes per pass), checking for smooth operation and proper grit discharge. Flow velocity verification ensures 0.7-1.2 fps maintained in chambers. Visual inspection of collected material quality - excessive organics indicates aeration adjustment needs. Bridge position indicators and cycle counters track performance.

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Maintenance: Weekly lubrication of drive chains, monthly inspection of scraper blade wear and guide wheel alignment. Quarterly gear reducer oil changes and annual motor servicing. Confined space entry required for underwater inspections - full PPE including gas monitors, harnesses, and attendants. Mechanical aptitude needed for drive component replacement and blade adjustments.

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Troubleshooting: Drive motor overloads indicate debris jams or excessive grit loading - typically resolved by reversing bridge direction. Uneven grit removal patterns suggest blade wear or misalignment. Excessive organics in discharge indicates insufficient aeration velocity. Equipment service life averages 15-20 years with proper maintenance, with drive components requiring replacement every 8-12 years.

• Bridge Structure: Steel frame spanning chamber width (15-50 feet typical) with reinforced concrete guide rails. Constructed from galvanized or stainless steel, sized for 150-200% design flow loading conditions plus equipment weight.

• Collection Mechanism: Traveling scraper blade or bucket system moves along chamber bottom at 2-8 feet/minute. Blade heights range 6-18 inches with rubber seals. Stainless steel construction standard for corrosion resistance.

• Drive System: Variable-speed motor (3-10 HP typical) with gear reducers providing precise speed control. Includes limit switches, overload protection, and reversing capability for debris clearing.

• Grit Discharge: Bucket elevator or conveyor system lifts collected grit to 8-15 feet above chamber. Capacity ranges 2-20 cubic yards/hour depending on plant size. Includes dewatering features and washwater systems.

• Controls: PLC-based systems with timer controls, manual override, and alarm functions for equipment protection.

• Flow Parameters: Design flow rate: 0.5-50 MGD (0.35-35 cfs), Peak hourly flow: 2.5-3.0 × average daily flow, Minimum flow velocity: 0.7-1.2 fps (maintains grit suspension), Maximum flow velocity: 1.5 fps (prevents organics carryover)

• Basin Dimensions: Length-to-width ratio: 3:1 to 5:1, Detention time: 2-5 minutes at design flow, Basin depth: 8-16 feet (typical 10-12 feet), Bottom slope: 1-2% toward collection point

• Performance Criteria: Grit removal efficiency: 95% for particles >65 mesh (0.21mm), Organics content in removed grit: <3-5% by weight, Bridge travel speed: 3-8 fpm (adjustable), Collection cycle frequency: 15-60 minutes (flow dependent)

• Mechanical Specifications: Drive motor: 1-5 HP (basin size dependent), Bucket capacity: 1-8 cubic feet per linear foot, Maximum lifting capacity: 50-150 lbs/linear foot, Bridge rail span: 10-50 feet typical

• Electrical: Control voltage: 120V/480V, 3-phase, Variable frequency drive standard for speed control

• Continuous vs. Intermittent Operation? Continuous operation for flows >5 MGD or high grit loads >50 cubic feet/MG. Intermittent operation acceptable for smaller plants with timer or flow-proportional control. Wrong choice leads to either excessive wear/energy costs or inadequate grit removal during peak flows. Need 24-hour flow profiles and historical grit accumulation data.

• Single vs. Multiple Bridge Configuration? Single bridge adequate for basins <30 feet wide. Multiple bridges required for wider basins or redundancy needs at plants >10 MGD. Incorrect sizing causes incomplete cleaning, dead zones, or excessive capital cost. Requires basin geometry, redundancy requirements, and maintenance access evaluation.

• Bucket Design: Perforated vs. Solid Bottom? Perforated buckets (3/8" holes) for high organics content, reducing disposal volume by 20-30%. Solid buckets for high inorganics or when dewatering isn't critical. Wrong selection affects disposal costs ($50-150/ton) and downstream grit processing. Need influent characterization and disposal cost analysis.

• Drive Location: Center vs. End-Mounted? Center-mounted drives for spans >25 feet, providing better load distribution. End-mounted acceptable for smaller basins with lower maintenance requirements. Incorrect choice leads to premature wear, alignment issues, or accessibility problems during maintenance.

• Primary: Division 46 - Water and Wastewater Equipment, Section 46 21 13 - Grit Removal Equipment

• Secondary: Division 40 - Process Integration (for controls integration with plant SCADA systems)

• Material/Equipment Verification: Verify 316SS construction for all wetted components, Confirm drive motor IP65 rating minimum, Check bridge rail material specifications (aluminum vs. stainless)

• Installation Requirements: Crane access for 15,000-25,000 lb bridge assembly, Precise channel dimensional tolerances (±1/4"), Electrical service 480V/3-phase typical

• Field Challenges: Channel concrete finishing critical for rail alignment, Grit pump coordination with bridge controls

• Coordination Issues: 16-20 week lead times standard, Early electrical rough-in coordination essential

• Headworks International: BioGrit systems with integrated washing capabilities for 1-50 MGD plants

• Lakeside Equipment Corporation: Raptor traveling bridge collectors, widely specified in Midwest municipalities

• WesTech Engineering: rectangular grit removal systems with proven track record in 10-100 MGD facilities

• Huber Technology: RakeMax systems popular in Northeast installations

• Vortex Grit Chambers: Lower capital cost ($150K vs. $300K), better for <5 MGD plants, minimal moving parts

• Aerated Grit Chambers: Proven technology, 20-30% lower equipment costs, but higher energy consumption

• Horizontal Paddle Grit Classifiers: Suitable for retrofit applications, compact footprint, typically 40% less expensive than traveling bridge systems but lower removal efficiency

Manufacturer Relations: Establish direct contact with regional service managers during design phase - they provide invaluable sizing verification and installation support. Cost Savings: Specify standard bridge lengths (20', 30', 40') rather than custom dimensions to reduce costs 15-20%. Consider dual-bridge installations for redundancy rather than single oversized units in facilities above 20 MGD.