Catenary Bar Screens

Catenary Bar Screens remove coarse debris from raw wastewater influent through a curved screening surface that follows a catenary (chain-like) curve profile. The curved design creates a self-cleaning effect as debris naturally slides down the screen face toward collection points, while allowing wastewater to pass through bar openings typically ranging from 6mm to 25mm. These screens can handle peak flows up to 15-20 MGD per unit with removal efficiencies of 85-95% for debris larger than bar spacing. The primary trade-off is higher capital cost compared to straight bar screens, though operational benefits include reduced maintenance requirements and improved hydraulic efficiency in applications with significant flow variations.

• Primary Headworks Screening (0.5-50 MGD): Catenary bar screens serve as the first mechanical screening stage after raw sewage enters the treatment plant. They're selected for their ability to handle high debris loads with minimal head loss (typically 0.5-1.5 feet). Upstream connections include influent channels with bypass gates; downstream flows to grit removal or primary clarifiers. The flexible screen surface reduces plugging compared to fixed bar screens.

• Combined Sewer Overflow (CSO) Facilities: These screens excel in CSO treatment due to their self-cleaning action during high flow events. The catenary design allows debris to ride over the curved screen surface rather than accumulating. Typical installations handle 5-25 MGD peak flows with 1/4-inch to 1/2-inch bar spacing.

• Pump Station Protection: Installed upstream of raw sewage pumps to prevent clogging and damage. The passive cleaning action reduces maintenance requirements in unmanned lift stations serving 1-10 MGD flows.

Daily Operations: Operators visually inspect screen condition during routine rounds, checking for excessive debris accumulation or uneven loading across the screen width. Flow measurements help verify hydraulic performance - head loss exceeding 2 feet indicates cleaning needs. No mechanical adjustments required during normal operation, making these screens ideal for smaller facilities with limited operator coverage.

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Maintenance: Monthly cleaning involves isolating the screen section and manually removing accumulated debris using long-handled tools. Operators require confined space training and fall protection when accessing screen channels. Annual inspection includes checking support anchor points and screen bar integrity. Typical screen life exceeds 15-20 years with proper maintenance. Replacement involves crane work and temporary bypass pumping.

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Troubleshooting: Excessive head loss indicates screen blinding from grease or fine debris - resolved through pressure washing or chemical cleaning. Uneven flow distribution suggests structural settlement or anchor point failure requiring immediate attention. Screen bar breakage creates bypass flow paths, reducing treatment effectiveness. Warning signs include unusual debris patterns, increased downstream pump cycling, or visible screen deformation during high flows.

• Curved Screen Assembly: Stainless steel 316L bars with 1/4" to 3/4" spacing, spanning 4-12 feet wide. The catenary curve (typically 15-30 degree arc) creates self-cleaning action as water velocity increases. Bar thickness ranges from 1/8" to 1/4" based on expected debris loading.

• Support Structure: Concrete or steel frame anchoring the screen ends while allowing center deflection. Critical for maintaining proper catenary shape under varying flow conditions. Includes access platforms and safety railings per OSHA requirements.

• Flow Control Gates: Upstream and downstream gates for isolation during maintenance. Typically manual knife gates for smaller installations (under 5 MGD) or motorized gates for larger facilities.

• Screenings Handling System: Debris collection hopper with manual or mechanical removal. Larger installations include conveyor systems and compaction equipment. Sizing based on 0.5-2.0 cubic feet of screenings per million gallons treated.

• Flow Parameters: Peak flow capacity: 0.5-50 MGD (0.35-34.7 cfs), Approach velocity: 1.5-3.0 ft/s (optimal 2.0-2.5 ft/s), Through-screen velocity: 2.0-4.0 ft/s, Channel width: 2-20 feet (multiple units for larger flows)

• Physical Specifications: Bar spacing: 0.25-2.0 inches clear opening (0.75" typical for municipal), Screen angle: 70-85 degrees from horizontal (75-80° typical), Head loss (clean): 0.1-0.3 feet, Maximum operating head loss: 1.5-2.0 feet before cleaning cycle, Screen depth: 6-25 feet (varies with plant hydraulics)

• Performance Criteria: Removal efficiency: 85-95% of solids >6mm, Screenings production: 0.5-3.0 ft³/MG (varies seasonally), Cleaning cycle frequency: 2-15 minutes typical operation, Power requirements: 3-15 HP per screen (includes rake and conveyance)

• Loading Rates: Hydraulic loading: 5-15 gpm/ft² screen area, Solids loading: 50-200 lbs/day per screen (highly variable)

• Bar Spacing Selection: 0.75" vs 1.0" vs custom spacing? Smaller spacing (0.75") captures more debris but increases head loss and screenings volume by 30-40%. Larger spacing (1.0"+) reduces maintenance but may pass material that damages downstream equipment. Need upstream debris characterization and downstream equipment vulnerability assessment.

• Single vs Multiple Screen Configuration? Flows >10 MGD typically require multiple units. Single large screens (>12 ft wide) have higher maintenance complexity and single-point failure risk. Multiple smaller units provide redundancy but increase civil costs 15-25%. Requires evaluation of redundancy requirements and maintenance capabilities.

• Integral Screenings Handling vs Separate Conveyance? Integral washing/compacting systems reduce civil work but limit flexibility and increase electrical complexity. Separate conveyance allows standardized components but requires additional space and interfaces. Decision impacts O&M costs by $15,000-30,000 annually.

• Standard vs Stainless Steel Construction? Carbon steel with coatings costs 40-50% less initially but may require replacement in 15-20 years in aggressive environments. Stainless 316L provides 30+ year life but doubles capital cost. Requires site-specific corrosion assessment and lifecycle analysis.

• Primary: Division 46 - Water and Wastewater Equipment - Section 46 21 00 - Water and Wastewater Preliminary Treatment Equipment

• Secondary: Division 40 - Process Interconnections (for integrated conveyance systems)

• Material/Equipment Verification: Verify 316SS construction for all wetted components, Confirm rake material and wear strip specifications, Check motor and gearbox ratings match site conditions

• Installation Requirements: Channel dimensions must accommodate screen frame and rake travel, Electrical rough-in coordination for variable frequency drives, Bypass provisions during installation critical for operational plants

• Field Challenges: Concrete tolerances affect screen fit and alignment, Access requirements for maintenance often underestimated

• Coordination Issues: Lead times typically 12-16 weeks for standard units, Custom channel configurations add 4-6 weeks

• Headworks International - CatRake series, strong in 1-20 MGD plants with proven municipal track record

• Kuhn North America - RakeMax systems, popular for larger installations 10-50 MGD

• JWC Environmental - BarScreen catenary units, competitive in smaller municipal market under 10 MGD

• Lakeside Equipment - Raptor catenary screens, established presence in Midwest municipal projects

• Mechanical bar screens cost 20-30% less but require more frequent maintenance and have higher power consumption

• Rotating drum screens work well for high-flow applications over 25 MGD but need larger footprints

• Step screens excel in retrofit situations with limited channel depth, typically 15-25% more expensive than catenary but easier installation in existing structures

Establish relationships with manufacturer service representatives early - they're invaluable for troubleshooting and parts availability. Many municipalities negotiate service contracts during initial purchase for better pricing. Consider standardizing on one manufacturer across multiple facilities to reduce spare parts inventory and training requirements. Retrofit applications often require custom modifications that significantly impact costs and schedules.