Flexible Rake Bar Screens

Flexible Rake Bar Screens remove large debris and solids from raw wastewater influent using continuous flexible chains with attached cleaning rakes that travel around sprockets at the top and bottom of the screening channel. These screens operate automatically, with rakes moving upward along the bar rack to lift captured material above the water surface for discharge into a collection trough. Typical installations handle flows from 0.5 to 50 MGD with bar spacings of 0.5 to 2 inches, achieving 85-95% removal efficiency for debris larger than the bar spacing. The primary trade-off is higher maintenance requirements compared to static screens due to the continuous movement of chains, sprockets, and drive mechanisms in a harsh wastewater environment.

• Primary Headworks Screening: Installed after grit removal, flexible rake screens handle 2-50 MGD flows with 3/8" to 3/4" bar spacing. Selected for high solids loading (>100 ft³/MG) where rigid rakes would jam. Upstream from primary clarifiers, downstream to screenings handling.
• Peak Flow Bypass Channels: Handle 2-5x average flows during wet weather events. Flexible design prevents damage from debris surges and variable hydraulic conditions. Typically sized for 15-25 MGD peak capacity in 5-15 MGD average plants.
• Secondary Screening Applications: Post-primary treatment for enhanced solids removal before biological processes. 1/4" to 1/2" spacing captures fine materials. Critical where downstream membrane bioreactors or tertiary filtration require consistent influent quality.
• Retrofit Installations: Replace failed rigid rake systems in existing channels without major civil modifications. Flexible mechanism accommodates irregular channel geometry and allows continued operation during maintenance.

Daily Operations: Operators monitor rake cycle frequency (typically 15-60 minute intervals) and screenings discharge volume. Visual inspection for unusual debris accumulation or differential head loss across screen. Adjust cycle timing based on influent conditions and downstream process requirements.

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Maintenance: Weekly lubrication of drive components and chain tensioning checks. Monthly cleaning of bar rack and inspection of rake teeth wear. Semi-annual gearbox oil changes and drive alignment verification. Requires confined space entry procedures and lockout/tagout protocols. Maintenance staff need basic mechanical skills.

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Troubleshooting: Chain derailment indicates worn guides or improper tensioning - service life 3-5 years for chains. Excessive power draw signals debris jamming or worn drive components. Reduced capture efficiency suggests bar spacing changes or structural damage. Complete system replacement typically required after 15-20 years operation.

• Flexible Chain-Link Rake Assembly: Stainless steel 316L construction with polyurethane wear strips. Chain pitch typically 6-12 inches, rake width spans channel (4-20 feet). Selection based on debris loading and required capture efficiency.
• Drive Unit and Gearbox: Variable speed drives (0.5-3 HP) with 20:1 to 100:1 reduction ratios. Torque overload protection prevents damage during heavy loading. Sized for continuous duty with 150-300% overload capacity.
• Bar Rack Structure: Hot-dip galvanized or stainless steel frame with removable sections. Bar spacing 3/8" to 1" based on downstream equipment protection requirements. Approach velocity maintained at 2-4 fps.
• Screenings Discharge System: Integral conveyor or separate screw conveyor removes captured material. Discharge rates 5-50 ft³/day typical for municipal applications. Includes wash water system for cleaning.

• Channel Width: 2-20 feet typical for municipal applications
• Bar Spacing: 6mm (1/4") to 25mm (1") clear spacing, with 6-10mm most common for headworks screening
• Approach Velocity: 0.6-1.2 m/s (2-4 ft/s) through bars at average flow
• Peak Flow Capacity: Size for 2.5-3.0x average daily flow, up to 50 MGD maximum
• Head Loss: 150-600mm (6-24") clean water loss at peak flow
• Screen Angle: 75-85° from horizontal for optimal debris transport
• Rake Speed: 3-15 m/min (10-50 ft/min) variable based on loading
• Screenings Production: 0.5-3.0 ft³/MG typical for municipal wastewater
• Channel Depth: 3-12 feet below operating floor
• Redundancy: Minimum 50% firm capacity with largest unit out of service
• Power Requirements: 2-15 HP per screen depending on size and loading
• Materials: 316L stainless steel standard, Hastelloy C-276 for severe corrosion
• Cleaning Frequency: Continuous to intermittent based on loading conditions

• What bar spacing is required for downstream equipment protection? 6mm spacing protects membrane bioreactors and fine screens; 10-15mm adequate for conventional activated sludge. Wrong choice: 6mm causes excessive head loss and maintenance; 15mm allows damaging debris through. Need: downstream process requirements and debris characterization.
• How many units provide adequate redundancy and flow distribution? Minimum two units for plants <5 MGD; three units for 5-25 MGD; four+ units >25 MGD. Each unit should handle 50% peak flow minimum. Wrong choice: inadequate backup causes bypass events; oversizing increases capital costs 25-40%. Need: peak flow projections and bypass regulations.
• What cleaning frequency matches site debris loading? Continuous operation for heavy debris loads >2 ft³/MG; timer-based cleaning for light loads <1 ft³/MG. Wrong choice: under-cleaning causes head loss >24" and potential overflow; over-cleaning wastes energy and increases wear. Need: debris characterization study and seasonal loading variations.
• Is wash water system required for screenings cleaning? Required for screenings >15% moisture content or sticky debris. Systems need 50-100 GPM at 60-80 PSI. Wrong choice: inadequate washing increases disposal costs 40-60%; oversized systems waste 2-5% plant flow. Need: disposal method and moisture content requirements.

• Primary: Division 40 - Process Integration
• Section 40 31 33 - Screening Equipment
• Secondary: Division 46 - Water and Wastewater Equipment (for packaged screening systems with integrated controls and wash water systems)

• Material/Equipment Verification: Verify 316SS construction vs. carbon steel options; Confirm rake tine spacing matches design requirements; Check motor ratings and control panel specifications
• Installation Requirements: Requires 8-12 week lead times for custom channel widths; Channel modifications often needed for retrofit applications; Crane access essential for installation and maintenance
• Field Challenges: Channel wall plumbness critical - 1/4" tolerance typical; Electrical coordination with existing SCADA systems; Screenings handling system integration
• Coordination Issues: Early coordination with structural for anchor bolt placement; Bypass pumping requirements during installation

• Huber Technology - RakeMax series, widely specified for 1-50 MGD plants with proven municipal track record
• Headworks International - Hydro-Rake models popular in Western US, strong aftermarket support
• JWC Environmental - Monster series, cost-competitive for smaller municipalities
• Lakeside Equipment - Raptor series, established presence in Midwest/Northeast markets with extensive municipal references

• Step screens - Better for high-debris applications, 15-20% higher capital cost but lower O&M
• Drum screens - Preferred for combined sewer systems with high peak flows, similar capital costs
• Static screens - No moving parts, 40-50% lower cost but limited to smaller flows under 5 MGD and lower debris loads

Establish relationships with manufacturer service technicians early - they provide invaluable troubleshooting support and spare parts guidance. Many municipalities negotiate service contracts bundling multiple Huber or Headworks units for cost savings. Consider oversizing channels slightly during design phase; retrofit installations frequently encounter dimensional conflicts requiring expensive field modifications that could be avoided with conservative initial sizing.