Spiral Screens

Spiral screens are rotating cylindrical screening devices that remove coarse solids from wastewater influent through continuous filtration and automated cleaning. Raw wastewater enters the rotating perforated drum, where solids are captured on the interior surface while liquid passes through. An internal spiral conveyor lifts trapped debris to the discharge point as the drum rotates at 2-4 RPM. These screens typically achieve 85-95% removal efficiency for solids larger than the screen opening (typically 6mm). The primary limitation is their susceptibility to plugging with stringy materials like rags and wipes, requiring frequent washwater for cleaning and occasional manual intervention during peak debris loading events.

• Primary Headworks Screening: Spiral screens serve as the first mechanical screening stage, typically handling 2-15 MGD flows with 6mm openings. They remove large debris, rags, and plastics before primary treatment, connecting downstream to grit removal or primary clarifiers

• Secondary Fine Screening: Installed after primary treatment for tertiary applications, processing 1-8 MGD with 3mm openings. They remove fine solids before membrane bioreactors or advanced treatment processes

• Bypass/Peak Flow Treatment: Used in storm overflow applications handling 5-25 MGD peak flows with 10mm openings. They provide reliable screening during high-flow events when conventional screens may blind

• Sidestream Applications: Process return flows from dewatering operations or digester supernatant, typically 0.5-2 MGD with variable opening sizes based on upstream processes

Daily Operations: Operators monitor drum rotation speed (typically 1-2 RPM), spray wash pressure, and screenings discharge consistency. Visual inspection for blinding or unusual debris accumulation occurs during routine rounds. Flow monitoring ensures proper hydraulic loading, with adjustments made for seasonal debris variations or upstream process changes.

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Maintenance: Weekly lubrication of drive components and monthly inspection of spray nozzles for plugging. Quarterly bearing inspection and semi-annual gear reducer oil changes. Confined space entry procedures required for internal drum inspection. Maintenance requires basic mechanical skills and standard PPE including hard hat, safety glasses, and non-slip footwear.

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Troubleshooting: Common failures include spray nozzle plugging (reduced wash effectiveness), bearing wear (unusual noise/vibration), and drive system overload (excessive debris loading). Warning signs include increased differential levels, reduced screenings discharge, or motor current spikes. Typical service life ranges 15-20 years with proper maintenance, with drum replacement being the most common major repair.

• Spiral Drum Assembly: Perforated stainless steel cylinder (304/316 SS) with helical flights, ranging 4-12 feet diameter for municipal applications. Opening sizes from 1-15mm based on application

• Drive System: Variable speed gear reducers (0.5-3 RPM) with 3-15 HP motors for municipal sizes. Includes torque monitoring and overload protection

• Spray Wash System: High-pressure wash headers (80-120 PSI) with stainless steel nozzles positioned along drum length. Flow rates typically 50-200 GPM depending on drum size

• Screenings Discharge Conveyor: Integral screw conveyor or separate belt system for screenings removal. Capacity ranges 5-50 cubic yards per day for typical municipal applications

• Support Structure: Carbon steel frame with corrosion-resistant coatings, designed for outdoor installation and equipment access requirements

• Flow Rate: 0.1 to 15 MGD per unit (typical municipal range 2-8 MGD)

• Screen Opening: 1mm to 6mm (3mm most common for municipal primary screening)

• Approach Velocity: 0.6-1.2 m/s (2-4 ft/s) at maximum flow

• Through-Screen Velocity: 0.8-1.5 m/s at design flow

• Headloss (Clean): 150-300mm (6-12 inches) at design flow

• Headloss (Fouled): Maximum 600mm (24 inches) before cleaning cycle

• Solids Loading: 5-15 m³/hr per unit for typical municipal waste

• Screenings Removal: 0.5-2.0 m³/1000 m³ treated (varies by season/location)

• Channel Width: 600-2000mm (2-6.5 ft) depending on capacity

• Installation Angle: 35-45° from horizontal (optimizes drainage and cleaning)

• Power Requirements: 1.5-7.5 kW per unit depending on size

• Materials: 316L stainless steel for wetted parts, carbon steel frame

• Cleaning Frequency: Continuous or intermittent based on differential level control

• Bypass Capability: Essential for maintenance - typically 150% of average daily flow

• What screen opening size balances removal efficiency with operational requirements? 1mm removes 85-95% of solids but requires frequent cleaning and higher power. 3mm removes 70-80% but reduces maintenance. 6mm only suitable for rough screening upstream of fine screens. Consider downstream processes and local discharge requirements.

• How many units are needed for redundancy and peak flow handling? Minimum two units for plants >2 MGD. Size for 100% capacity with largest unit out of service. Peak flow multiplier typically 2.5-3.0x average flow. Inadequate redundancy causes bypass during maintenance.

• What level differential triggers cleaning cycles? Typical range 150-300mm (6-12 inches). Lower settings increase cleaning frequency and power consumption. Higher settings risk screen blinding and overflow. Site-specific calibration needed based on waste characteristics.

• Is wash water supply adequate for cleaning system? Requires 2-5% of throughput flow at 40-60 psi. Inadequate pressure reduces cleaning efficiency. Consider potable water backup if plant water unavailable during startup or emergencies.

• Division 40 - Process Integration

• Section 40 05 23 - Screens and Screening Equipment

• Primary specification location for municipal water/wastewater screening equipment

• May reference Division 33 (Utilities) for civil/structural integration and Division 26 (Electrical) for motor controls and instrumentation

• Material/Equipment Verification: Verify 316SS construction for all wetted components, Confirm motor IP65 rating and explosion-proof certification if required

• Installation Requirements: Crane access for 8,000-15,000 lb units, Concrete channel tolerances ±1/4 inch, 480V/3-phase power with VFD compatibility

• Field Challenges: Channel modifications often required for retrofits, Bypass pumping duration 2-4 weeks typical

• Coordination Issues: Screenings conveyance integration with existing systems, Lead times 16-20 weeks for custom channel configurations

• Headworks International - RotaRake spiral screens, widely used in 1-50 MGD plants with 6mm-25mm openings

• Lakeside Equipment - Raptor series spiral screens, popular for smaller municipalities with flows under 10 MGD

• Huber Technology - RakeMax spiral screens, known for heavy-duty construction in larger facilities

• Parkson Corporation - AquaGuard spiral screens, competitive pricing for mid-size applications

• Static wedge wire screens cost 40-60% less but require frequent manual cleaning and work poorly with high organics

• Drum screens handle higher flows (20-100 MGD) but cost 50-80% more with complex wash systems

• Bar screens with manual raking cost 70% less for small plants under 2 MGD but require daily operator attention and produce inconsistent solids capture

Establish relationships with local manufacturer reps early - they provide valuable sizing verification and can expedite delivery for emergency replacements. Specify removable wear plates at screen contact points; replacement costs $2,000-5,000 versus $25,000+ for full screen replacement. Consider oversizing channels by 6 inches to accommodate future screen upgrades. Many operators prefer manual backup rakes as insurance against automation failures during peak flow events.