Floating Mechanical Mixers

Floating Mechanical Mixers provide surface-mounted mixing for lagoons, basins, and large tanks where fixed installations are impractical or costly. These units float on the water surface while their submerged propellers create circulation patterns extending 8-12 times the propeller diameter. Typical power densities range from 5-15 HP per million gallons for effective mixing in municipal applications. The primary trade-off is reduced mixing efficiency compared to fixed systems due to surface positioning, requiring careful sizing and placement to achieve uniform velocity gradients throughout the basin depth.

• Equalization Basins (2-15 MGD plants): Floating mixers prevent stratification and maintain uniform BOD/TSS concentrations in flow equalization tanks. Selected over fixed mixers because they accommodate 8-12 foot water level fluctuations without performance loss. Upstream from primary clarifiers, downstream from screening/grit removal.

• Anaerobic Digesters (5-50 MGD plants): Maintain uniform temperature and solids distribution in 20-40 foot diameter digesters. Chosen for their ability to handle varying sludge levels (15-35 feet typical) while providing 0.02-0.05 HP/1000 ft³ mixing intensity. Connected between thickeners and biogas collection systems.

• Lagoon Systems (0.5-5 MGD plants): Provide mixing in facultative lagoons ranging 6-12 feet deep. Selected because they eliminate dead zones in irregular basin geometries and handle seasonal water level variations of 3-6 feet. Positioned upstream of polishing lagoons or discharge structures.

• Emergency Storage Basins: Maintain water quality during bypass events or plant maintenance. Float-mounted design allows operation across 10-20 foot level variations without mechanical modifications.

Daily Operations: Operators monitor amperage draw (should remain within 90-110% of nameplate), observe mixing patterns for dead zones, and check float stability. Water level documentation ensures proper impeller submergence (3-5 feet typical). Remote monitoring systems track runtime hours and motor temperatures, with daily visual inspections for debris accumulation or unusual vibration.

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Maintenance: Quarterly inspections include float integrity checks, mooring system tension, and gear oil analysis. Annual maintenance requires crane access for impeller removal, typically 4-6 hour procedures requiring confined space entry permits and fall protection. Impeller rebalancing every 3-5 years prevents premature bearing failure. Two-person teams minimum with electrical lockout procedures.

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Troubleshooting: Excessive amperage indicates impeller damage or debris fouling, while decreased current suggests drive coupling failure. Unusual vibration patterns warn of bearing wear (typical 8-12 year service life). Float listing indicates pontoon damage requiring immediate shutdown. Ice formation in northern

• Impeller Assembly: Typically 4-8 foot diameter axial flow propellers generating 0.3-0.8 fps tip speeds. Bronze or stainless steel construction for corrosion resistance. Selection based on basin geometry and required turnover rates (4-6 turnovers/day typical).

• Drive Motor: 5-75 HP TEFC motors with 1750 RPM operation through gear reducers. Sized for continuous duty with 1.15 service factors. Selection considers mixing intensity requirements and power availability.

• Float Structure: HDPE or fiberglass pontoons providing 150-200% buoyancy safety factor. Dimensions typically 8x12 to 12x20 feet for stability. Must accommodate motor weight plus dynamic loads from impeller operation.

• Mooring System: Adjustable cable or chain assemblies allowing 360-degree rotation while maintaining position. Includes tensioning hardware and anchor points rated for wind and wave loads.

• Gear Reducer: Right-angle reducers providing 10:1 to 40:1 speed reduction with sealed lubrication systems. Sized for continuous operation with 2.0+ service factors for municipal duty cycles.

• Tank Geometry: Basin depth 8-30 feet, diameter 20-150 feet. Minimum 3:1 depth-to-impeller diameter ratio required for effective mixing.

• Mixing Power: 0.5-2.0 HP per million gallons tank volume for biological processes, 2-5 HP/MG for chemical mixing applications. Actual power ranges 1-75 HP for municipal installations.

• Impeller Selection: Diameter typically 4-12 feet for municipal basins. Tip speed 3-8 ft/sec for biological mixing, up to 12 ft/sec for chemical applications to prevent foam generation.

• Flow Patterns: Axial flow impellers generate 3-8 times impeller diameter circulation patterns. Target velocity gradient (G-value) 20-80 sec⁻¹ for biological processes, 100-300 sec⁻¹ for rapid mix.

• Positioning: Multiple units spaced 1.5-2.0 impeller diameters apart. Submergence depth 0.5-3.0 times impeller diameter below surface to prevent vortexing and air entrainment.

• Environmental Limits: Operating temperature range -10°F to 120°F, wind loads up to 90 mph design with guy wire systems for basins >100 feet diameter.

• What mixing intensity is required for the process application? Biological processes need G-values 20-80 sec⁻¹, while chemical mixing requires 100-300 sec⁻¹. Under-mixing leads to dead zones and poor treatment efficiency. Over-mixing wastes energy and can damage biological floc. Need process kinetics, basin geometry, and target detention time.

• How many mixers versus what individual unit size? Single large mixers (20-75 HP) provide lower installed cost but create mixing dead zones in basins >80 feet diameter. Multiple smaller units (5-25 HP each) improve redundancy and mixing uniformity but increase maintenance complexity. Need basin dimensions, required turnover rate, and reliability requirements.

• What impeller type matches the application viscosity and solids content? High-efficiency axial flow impellers work for clean water and low-solids biological processes (<3,000 mg/L MLSS). Radial flow or pitched-blade turbines needed for high-solids digestion (>4% solids) or viscous polymer mixing. Wrong selection causes poor mixing or excessive power consumption. Need fluid properties and solids concentrations.

• What anchoring and positioning system handles site conditions? Guy wire systems work for basins >100 feet diameter with wind exposure. Bridge-mounted units provide better access but require structural analysis. Floating systems need wave action assessment and anchor point design for 50-year storm loads.

• Primary: Division 46 23 61 - Packaged Water Treatment Equipment Mixers

• Secondary: Division 40 32 17 - Wastewater Treatment Mixers (for biological treatment applications)

• Notes: Selection depends on process application - potable water

• Material/Equipment Verification: Verify 316SS propeller/shaft materials for municipal wastewater, Confirm IP68 motor sealing ratings, Check cable specifications (SOOW jacket minimum)

• Installation Requirements: Concrete anchor blocks typically 2-3x mixer weight, Shore power connections require GFCI protection, Minimum 3-point mooring system for stability

• Field Challenges: Cable management during seasonal water level changes, Propeller fouling from debris/rags, Anchor dragging in soft basin bottoms

• Coordination Issues: Electrical rough-in timing with basin construction, Crane access for installation/maintenance, Lead times: 12-16 weeks standard, 20+ weeks for custom configurations

• Xylem Flygt - 4680 series floating mixers (0.5-15 HP), dominant in North American municipal market

• Sulzer ABS - XRW floating mixers (1-20 HP), strong lagoon applications

• KSB - Amaprop floating units (2-25 HP), growing municipal presence

• Grundfos - Biobooster floating mixers, newer to municipal market but gaining traction in smaller plants

• Fixed-mount submersible mixers - 20-30% less expensive, better for constant water levels, permanent basins

• Coarse bubble diffusion - Lower power consumption (0.5-1.0 HP/1000 ft³), better oxygen transfer if needed, higher maintenance

• Jet mixing systems - Effective for deep basins (>15 feet), typically 40-60% higher capital cost but lower maintenance requirements

Specify removable propellers for maintenance - saves significant downtime versus full unit removal. Establish service agreements during procurement; manufacturer field service costs $1,500-2,500/day plus travel. Consider dual smaller units over single large mixer for redundancy in critical applications. Cable loops should include 25% extra length for water level fluctuations and future repositioning needs.