Helical Skimmer

Helical skimmers remove floating debris and scum from clarifier surfaces using a continuous spiral blade that rotates around the tank perimeter. The helical mechanism sweeps surface contaminants toward a central collection point while maintaining constant surface contact through adjustable depth settings. Typical installations achieve 95-98% surface debris removal efficiency in primary and secondary clarifiers ranging from 30 to 150 feet in diameter. The primary trade-off involves higher maintenance requirements compared to chain-and-flight systems, as the continuous rotation and surface contact create more wear points requiring regular bearing and drive component servicing.

• Primary Clarifiers (0.5-10 MGD plants): Helical skimmers continuously collect floating scum and grease in circular primary clarifiers, typically 30-100 feet diameter. Connected to scum pumps downstream and positioned ahead of secondary treatment. Selected for reliable operation in high-grease municipal wastewater where blade skimmers may clog.
• Secondary Clarifiers: Used in activated sludge plants for biofilm and foam removal, particularly effective during filamentous bulking episodes. Handles variable foam depths better than fixed blade systems.
• Dissolved Air Flotation (DAF) units: Removes concentrated float solids in 1-20 MGD plants using DAF for primary treatment. Helical design prevents float re-entrainment during collection, feeding directly to float thickening or dewatering.
• Oxidation Ditches: Removes surface foam and floating debris in package plant applications (0.5-5 MGD), where simple maintenance requirements suit smaller operation staffs.

Daily Operations: Operators monitor scum discharge consistency and adjust rotation speed via VFD controls (typically 0.5-3 RPM range). Check discharge flow rates and grease trap levels. Visual inspection for proper flight submersion and even scum removal pattern across basin surface.

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Maintenance: Monthly lubrication of gear reducers and bearings using EP-2 lithium grease. Quarterly inspection of flight integrity and attachment bolts. Annual gear oil changes and drive alignment checks. Requires confined space entry procedures and fall protection PPE for center column access. Basic mechanical skills sufficient for routine maintenance.

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Troubleshooting: Uneven scum removal indicates worn flights or improper speed settings. Excessive torque loads suggest flight damage or debris accumulation. Typical service life 15-20 years for flights, 10-15 years for drives. Early warning signs include increased amperage draw, unusual noise, or visible flight deflection during operation.

• Helical Flight Assembly: Continuous spiral blade, typically 304 SS construction, 6-24 inch pitch depending on basin diameter. Sized for 1-3 RPM rotation speed in 30-150 foot diameter basins. Selection based on expected scum volume and consistency.
• Drive Unit: Gear reducer with 0.5-2 HP motor, typically 20:1 to 60:1 reduction ratio. TEFC motors standard for outdoor installations. Sized for continuous duty with 150% torque capacity for startup loads.
• Center Column: Structural steel support, 8-16 inch diameter, houses rotating shaft and bearings. Hot-dip galvanized or stainless steel in corrosive environments.
• Scum Discharge System: Beach or trough collection with 4-8 inch discharge piping. Includes adjustable weirs for variable scum depths and connection to scum pumps rated 10-100 GPM.
• Support Structure: Radial arms and walkway system, aluminum or galvanized steel, designed for 50 PSF live load per ASCE standards.

• Flow Rate Parameters: Surface overflow rates: 300-800 GPD/ft² (typical municipal range), Peak hydraulic loading: 1,200-1,500 GPD/ft² maximum, Skimmer peripheral speed: 3-8 ft/min (optimum 4-6 ft/min), Scum removal rate: 0.1-0.5 GPM per skimmer arm
• Physical Dimensions: Tank diameter: 20-200 feet (municipal clarifiers), Skimmer arm length: 8-100 feet radius, Blade depth: 6-18 inches below surface, Drive torque requirements: 1,000-15,000 ft-lbs
• Performance Metrics: TSS removal efficiency: 85-95% (primary), 90-98% (secondary), Scum collection efficiency: >95% surface material removal, Solids loading: 20-50 lbs/day/ft² (primary clarifiers), Detention time: 1.5-3.0 hours (primary), 3-6 hours (secondary)
• Operational Parameters: Drive motor: 0.5-10 HP (variable frequency drive recommended), Gear reducer ratio: 1,000:1 to 3,000:1 typical, Operating temperature range: 32-120°F, Corrosion resistance: 316 SS minimum for municipal applications

• What surface loading rate can the clarifier handle during peak flow conditions? Engineers must determine if existing or planned clarifiers can maintain <800 GPD/ft² during 2-4x average flow events. Exceeding 1,200 GPD/ft² causes solids carryover and process upset. Requires accurate flow projections and peaking factors for the specific service area.
• Does the application require dual-zone skimming capability? Facilities with high grease/scum loads (>200 mg/L influent FOG) need skimmers with both surface and subsurface collection. Single-zone skimmers fail when scum accumulates below surface level. Decision requires influent characterization and seasonal variation analysis.
• What drive system redundancy level is acceptable for this facility? Critical plants (>10 MGD or no bypass capability) require dual-drive systems or emergency manual operation. Single-drive failures cause immediate process problems and potential permit violations. Decision depends on plant criticality, staffing levels, and bypass capacity.
• Can existing clarifier structures support helical skimmer loads? Retrofit installations must verify structural capacity for 15,000+ lb equipment loads plus wind/seismic forces. Inadequate support causes equipment failure and safety hazards. Requires structural analysis of existing concrete and steel components.

• Primary: 46 13 16 - Clarifier Skimmers
• Secondary: 40 30 00 - Site Clarifiers (for new installations with integrated skimming systems)
• Note: Helical skimmers are specialized equipment within clarifier systems, requiring coordination between mechanical (Division 46) and process equipment

• Material/Equipment Verification: Verify 316 SS construction for all wetted components, Confirm torque ratings match calculated scum loads, Review bearing specifications for continuous duty
• Installation Requirements: Crane access for center pier installation, Precise alignment tolerances (±1/8" typical), Electrical coordination for variable speed drives
• Field Challenges: Weather delays during mechanical installation, Concrete tolerance issues affecting fit-up
• Coordination Issues: SCADA integration timing with controls contractor, Lead times: 16-20 weeks for standard units, 24+ weeks for custom sizes

• Lakeside Equipment - RAPTOR helical screw skimmers (2-200 ft diameter range)
• WesTech Engineering - Helical Skimmer systems with integrated controls
• Hydro-Dyne Engineering - Custom helical flight skimmers for municipal clarifiers
• Schlumberger Water Services - PISTA helical skimmer assemblies
• All maintain strong municipal references with installations at 1-50 MGD facilities across North America.

• Chain and Flight Skimmers - Lower capital cost ($15-25K vs $35-50K), preferred for rectangular clarifiers, higher maintenance requirements.
• Rotating Tube Skimmers - Best for heavy scum loads, 20-30% higher cost, excellent for primary clarifiers.
• Surface Wash Systems - Lowest cost option ($8-15K), suitable for facilities with minimal scum production, requires separate collection system.

Establish relationships with regional manufacturer reps early - they provide valuable sizing verification and can expedite delivery for urgent projects. Consider standardizing on one manufacturer across multiple clarifiers to reduce spare parts inventory. Negotiate package deals for multiple units; savings of 8-12% are common. Always specify redundant drive systems for critical applications - motor failures are the most common maintenance issue.