Traveling Bridge Filters

Traveling Bridge Filters are continuous backwash granular media filters featuring a mobile bridge mechanism that travels across the filter surface, providing automated cleaning without taking the filter offline. The system uses a siphon-assisted backwash process as the bridge moves along rails above rectangular filter cells, typically at speeds of 2-6 feet per minute. Municipal installations commonly achieve 95-98% turbidity removal while maintaining continuous operation at filtration rates of 4-8 gpm/sf. The primary limitation is higher capital cost compared to conventional gravity filters, with complex mechanical components requiring specialized maintenance expertise.

• Secondary Clarifier Scum Removal: Traveling bridge filters serve as scum collection systems on 60-200 ft diameter clarifiers in 5-50 MGD plants. The bridge spans the full diameter, continuously collecting surface scum via rotating brushes or skimmers into collection troughs. Selected for large clarifiers where fixed equipment cannot reach the center effectively.

• Primary Clarifier Surface Cleaning: Used on 80-150 ft primary clarifiers in 10-100 MGD facilities. Bridge-mounted equipment removes floating debris, grease, and foam while integrated sludge collection arms scrape settled solids. Preferred over chain-and-flight systems for deeper tanks (12-16 ft) where structural loads favor bridge designs.

• Thickener Overflow Management: Applied to 40-120 ft gravity thickeners processing waste activated sludge. Bridge carries picket-fence mechanisms and overflow weirs, maintaining consistent surface conditions. Selected when thickener depth exceeds 12 ft or when integrated chemical feed systems require precise positioning above the sludge blanket.

Daily Operations: Operators monitor bridge position via SCADA displays, checking for proper travel speed and collection efficiency. Visual inspection confirms scum removal effectiveness and identifies any material buildup on collection mechanisms. Adjustments include travel speed optimization based on surface loading conditions and seasonal variations in scum production.

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Maintenance: Monthly lubrication of drive components and quarterly inspection of collection mechanisms for wear. Bridge rails require semi-annual cleaning and alignment checks. Safety protocols mandate lockout/tagout procedures and fall protection when accessing elevated components. Maintenance requires millwright-level mechanical skills for drive system repairs.

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Troubleshooting: Common failures include drive motor overload from excessive material buildup (indicated by high amperage), collection mechanism jamming from debris entanglement, and position sensor drift causing travel limit errors. Warning signs include unusual noise, erratic travel patterns, or incomplete scum removal. Typical service life ranges 15-20 years with proper maintenance.

• Drive System: Variable frequency drives (5-15 HP) power bridge movement via rack-and-pinion or cable systems. Motors sized for 0.5-2.0 ft/min travel speeds with emergency stop capability. Selection based on span length and collected material density.

• Collection Mechanisms: Rotating brushes (polyethylene bristles), skimmer blades (stainless steel), or suction headers depending on material type. Brush systems handle light scum; rigid skimmers for heavy debris. Typically 6-12 ft wide coverage per pass.

• Support Structure: Steel or aluminum bridge spans with center pier support for diameters >100 ft. Load ratings account for equipment weight plus 150% collected material capacity. Corrosion-resistant coatings essential in wastewater environments.

• Control Systems: PLC-based automation with position feedback, torque monitoring, and remote operation capability. Integration with plant SCADA for alarm reporting and operational data logging.

• Hydraulic Loading Rate: 2-6 gpm/sf (typical 3-4 gpm/sf for secondary effluent filtration)

• Filter Media Depth: 24-48 inches (30-36 inches standard for municipal applications)

• Backwash Flow Rate: 15-25 gpm/sf (typically 20 gpm/sf for granular media)

• Backwash Duration: 3-8 minutes per cycle (5-6 minutes typical)

• Air Scour Rate: 3-5 scfm/sf when air-assisted backwash is used

• Bridge Travel Speed: 1-3 ft/min during backwash cycle

• Effluent Turbidity: <2 NTU (typically <1 NTU for tertiary applications)

• Headloss Range: 2-8 feet through clean media (terminal headloss 8-12 feet)

• Filter Cell Dimensions: 12-30 feet wide, 20-100 feet long (optimized for bridge span)

• Freeboard: Minimum 3 feet above maximum water level

• Media Specifications: 0.45-0.55 mm effective size anthracite over 0.45-0.65 mm sand (dual media standard)

• Underdrain Spacing: 6-12 inch centers for lateral systems

• Wash Water Storage: 1-3% of daily plant flow capacity

• What hydraulic loading rate should be specified based on upstream treatment performance? Loading rates of 2-3 gpm/sf for primary effluent applications versus 4-6 gpm/sf for secondary effluent. Under-sizing increases capital costs unnecessarily; over-sizing risks poor backwash distribution and channeling through media.

• Should single or dual media configuration be selected for the required effluent quality? Single media (sand only) adequate for >5 NTU influent to <2 NTU effluent. Dual media (anthracite/sand) required for <1 NTU effluent or when treating variable quality influent. Wrong choice affects long-term performance and backwash frequency.

• What bridge drive system capacity is needed for site-specific conditions? Standard 5-10 HP drives adequate for typical installations. Higher capacity (15-20 HP) required for facilities >40 MGD or extreme weather conditions. Undersized drives cause premature failure and maintenance issues.

• How many filter cells optimize operational flexibility versus capital cost? Minimum 3 cells for continuous operation during maintenance. 4-6 cells typical for 5-25 MGD plants. Insufficient cells compromise plant reliability; excess cells increase unnecessary complexity and cost.

• Primary: Division 40-25 13 - Water Treatment Equipment (Filtration Systems)

• Secondary: Division 46-71 13 - Packaged Water Treatment Equipment

• Material/Equipment Verification: Verify 316SS construction for all wetted components; Confirm NEMA 4X electrical enclosures for outdoor installations; Check polymer feed system compatibility with plant chemicals

• Installation Requirements: Concrete basin tolerance ±1/4" for proper rail alignment; Overhead crane access required during installation; 480V/3-phase power with VFD integration

• Field Challenges: Rail alignment critical - survey before concrete pour; Bridge deflection issues on spans >40 feet; Winter startup complications in northern climates

• Coordination Issues: SCADA integration requires early electrical coordination; Polymer feed system ties to plant chemical systems; 16-20 week lead times typical

• Lakeside Equipment - Raptor Complete Plant systems, dominant in North American municipal market

• WesTech Engineering - Trident series, strong in western US municipalities

• Evoqua Water Technologies - Envirex traveling bridge systems, established in Great Lakes region

• Headworks International - BioMag systems for smaller municipal applications (0.5-10 MGD range)

• Dissolved Air Flotation (DAF) - Preferred for high algae/low turbidity sources; 20-30% higher capital cost but better organics removal

• Conventional Gravity Clarifiers - Lower cost (40-50% less) for high-turbidity applications but larger footprint requirement

• Ballasted Flocculation (Actiflo) - Faster treatment, smaller footprint, but 50-75% higher operating costs due to microsand consumption and complexity

Establish direct relationship with manufacturer's regional service manager early - they often provide valuable sizing verification and can expedite warranty issues. Cost-saving opportunity: Specify standardized electrical components (Allen-Bradley, Schneider) rather than proprietary controls to reduce long-term maintenance costs. Many plants report 15-20% savings on spare parts inventory using standard industrial components versus manufacturer-specific items.