Chain and Flight Skimmer

Chain and flight skimmers remove floating oils, grease, and scum from clarifier surfaces using an endless chain system with attached flights that travel across the water surface. The chain-driven flights continuously scrape floating materials toward collection troughs, typically removing 90-95% of surface contaminants in primary and secondary clarifiers. These mechanical skimmers operate at slow speeds (2-6 feet per minute) to minimize turbulence while effectively collecting materials ranging from light oils to heavy scum blankets. The primary limitation is mechanical complexity, requiring regular chain tensioning, sprocket maintenance, and potential downtime during repairs, making them less suitable for facilities with limited maintenance resources.

• Primary Clarifiers (0.5-20 MGD): Chain and flight skimmers continuously remove floating scum and grease from clarifier surfaces. The chain-driven flights travel along the water surface, pushing accumulated materials to collection troughs. Selected for reliable continuous operation and ability to handle variable scum loads without operator intervention. Upstream from secondary treatment processes where grease removal is critical.
• Secondary Clarifiers (1-50 MGD): Used in activated sludge systems to remove biological foam and floating solids. The mechanical action prevents surface mat formation that can cause effluent quality issues. Chosen over other skimming methods for consistent performance in high-flow applications. Integrates with RAS/WAS pumping systems downstream.
• DAF Systems (2-25 MGD): Essential for removing float sludge in dissolved air flotation applications. Flights continuously sweep concentrated solids from surface while maintaining optimal contact time. Selected for ability to handle high solids loading rates typical in DAF operations.

Daily Operations: Operators visually inspect chain movement and flight alignment during routine rounds. Monitor drive motor amperage and unusual noise patterns. Adjust operating speed based on scum accumulation rates - typically 1-2 fpm for normal conditions, up to 3 fpm during high loading periods. Check collection trough discharge and clear any blockages.

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Maintenance: Monthly lubrication of drive components and chain tensioning adjustments. Quarterly inspection of flight condition and chain wear patterns. Annual drive motor service and bearing replacement. Requires standard mechanical PPE and lockout/tagout procedures. Maintenance staff need basic mechanical skills for chain adjustment and motor service. Critical to maintain proper chain tension to prevent derailment.

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Troubleshooting: Chain derailment typically indicates improper tension or worn sprockets - warning signs include irregular movement patterns and increased motor amperage. Flight damage from debris impact shows as uneven skimming patterns. Drive motor overload often results from excessive debris loading or mechanical binding. Typical service life: 15-20 years for mechanical components, 8-12 years for flights depending on abrasive conditions and maintenance quality.

• Drive Mechanism: Gear-reduced motors (0.5-2 HP typical) provide continuous chain movement at 1-3 fpm surface speed. Stainless steel or aluminum construction for corrosion resistance. Selection based on basin dimensions and required torque capacity.
• Chain System: Stainless steel roller chain with corrosion-resistant coatings spans basin length. Standard #60-#100 chain sizes depending on loading requirements. Must resist municipal wastewater chemistry and mechanical wear.
• Flight Assemblies: HDPE or fiberglass flights (6-24 inches wide) attach to chain at 8-20 foot intervals. Design includes adjustable height settings for varying water levels. Material selection balances durability with buoyancy requirements.
• Collection Trough: Stainless steel or concrete troughs (12-36 inches wide) receive skimmed materials. Include drain connections and access ports. Sizing based on expected scum production rates of 0.1-0.5 gal/1000 gal treated.
• Tensioning System: Adjustable sprockets maintain proper chain tension during operation. Include overload protection to prevent damage from debris loading.

• Surface Loading Rate: 800-1,200 GPD/ft² for primary clarifiers, 400-600 GPD/ft² for secondary clarifiers. Higher rates increase carryover risk and reduce settling efficiency.
• Chain Speed: 1-3 ft/min typical, with variable speed drives standard. Slower speeds (1-1.5 ft/min) for heavily loaded plants, faster (2-3 ft/min) for lightly loaded or when enhanced mixing is desired.
• Skimmer Beach Slope: Minimum 1:12 (8.3%) for gravity drainage, 1:8 (12.5%) preferred for reliable scum removal. Steeper slopes reduce scum buildup but increase structural costs.
• Flight Spacing: 8-12 ft centers for rectangular clarifiers up to 100 ft length, 6-8 ft for shorter basins. Closer spacing improves collection but increases maintenance complexity.
• Scum Trough Capacity: Size for 2-5% of influent flow as scum volume, with minimum 6-inch depth. Include 100% overflow capacity for peak conditions.
• Drive Power: 0.5-2 HP for basins up to 100 ft length, 3-5 HP for longer installations. Include 150% service factor for intermittent heavy loading conditions.
• Material Selection: 316 SS for all wetted components in municipal applications. Aluminum acceptable only for covers and above-water structural elements.

• Fixed vs. Variable Speed Operation? Variable speed (0.5-3 ft/min) costs 15-25% more but essential for plants >5 MGD or with significant load variations. Fixed speed adequate for smaller, consistent-load facilities. Wrong choice leads to either inadequate scum removal during peak loads or excessive power consumption during low flows.
• Chain-Only vs. Chain-and-Flight Configuration? Chain-and-flight required for basins >75 ft length or surface loading >1,000 GPD/ft². Chain-only systems fail to adequately move scum in longer basins, causing surface mat formation and odor issues. Need basin dimensions, loading rates, and scum characteristics.
• Manual vs. Automatic Scum Removal? Automatic systems justified for plants >2 MGD or facilities with limited operator coverage. Manual removal acceptable for smaller plants with dedicated operators. Automatic systems cost 40-60% more but reduce labor by 2-4 hours daily. Wrong choice impacts both O&M costs and treatment reliability.
• Drive Location: End-Mounted vs. Center-Mounted? Center-mounted drives required for basins >120 ft length to prevent excessive chain tension and wear. End-mounted adequate for shorter basins and costs 20-30% less. Need exact basin geometry and structural constraints for proper selection.

• Primary: Division 46 23 61 - Clarifier Skimmers and Collectors
• Secondary: Division 40 41 33 - Rectangular Clarifiers (when integrated with basin design)
• Note: Chain and flight skimmers typically specified within clarifier sections rather than standalone equipment specifications.

• Material/Equipment Verification: Verify 316SS construction for chains, flights, and drive components; Confirm AWWA D100 compliance for steel tank construction; Check drive unit torque ratings and safety factors
• Installation Requirements: Crane access for drive unit installation (typically 3-5 tons); Precise rail alignment critical - tolerance ±1/8"; Electrical coordination for VFD integration
• Field Challenges: Chain tensioning during startup requires experienced technician; Concrete tolerances affect rail installation; Weather protection needed during installation
• Coordination Issues: 16-20 week lead times typical; Early electrical rough-in coordination essential

• WesTech Engineering - Enviro-Clear rectangular clarifiers with integrated chain and flight systems, widely used in 1-50 MGD plants.
• Lakeside Equipment - Raptor chain and flight collectors, popular for retrofit applications.
• Evoqua Water Technologies - Envirex collectors, strong presence in larger municipal facilities.
• Walker Process Equipment - Chain and flight systems, established in Midwest municipal market.

• Traveling bridge collectors - Preferred for wider tanks (>40 ft), 15-25% higher cost but better access for maintenance.
• Suction header systems - Lower maintenance alternative for smaller plants (<5 MGD), roughly 30% less expensive but limited to lighter solids loads.
• Tube settlers with mechanical collection - Space-efficient option for upgrades, similar cost but requires more sophisticated controls and higher operator skill level.

Establish strong relationship with manufacturer's field service team early - their startup support is invaluable for proper chain tensioning and drive calibration. Budget 2-3% of equipment cost for annual service contracts. Consider purchasing spare chain links and flights during initial order to avoid future premium pricing. Many operators report better performance with slightly slower chain speeds than manufacturer recommendations, improving solids capture.