Submersible Mixers

Submersible mixers provide mechanical mixing and solids suspension in municipal treatment basins through direct-drive propellers mounted on waterproof motor assemblies. These units operate fully submerged, creating directional flow patterns that maintain uniform conditions in basins ranging from 50,000 to 5 million gallons. Typical installations achieve mixing velocities of 0.5-1.5 ft/sec with power densities of 5-20 HP per million gallons, depending on application requirements. The primary trade-off involves balancing adequate mixing energy against excessive turbulence that can disrupt biological processes or cause foam generation in activated sludge systems.

• Anoxic Zones in BNR Processes: Submersible mixers maintain 0.3-0.5 fps velocity in anoxic basins for denitrification, positioned between aeration zones and clarifiers. Selected for gentle mixing without oxygen transfer, typically 0.5-2.0 HP units in 5-25 MGD plants

• Equalization Basins: Prevent solids settling during flow variations, maintaining uniform influent characteristics. Mixers sized at 10-20 HP per million gallons, positioned to create bulk flow patterns. Critical upstream of primary treatment to optimize downstream processes

• Sludge Holding Tanks: Keep biosolids in suspension before dewatering or digestion. Typically 5-15 HP mixers in 50,000-200,000 gallon tanks, preventing septicity and maintaining pumpability. Positioned to eliminate dead zones while minimizing foam generation

• Contact Tanks for Chemical Addition: Provide rapid mixing for coagulant, polymer, or disinfectant contact. Usually 1-5 HP units creating localized turbulence near chemical feed points, ensuring proper dispersion before flocculation or distribution

Daily Operations: Operators monitor amperage draw (should remain within 10% of nameplate), unusual vibrations, and mixing effectiveness through visual observation of surface patterns. Typical adjustments include mixer positioning via guide rails and rotation timing in intermittent applications. Most installations run continuously with minimal operator intervention.

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Maintenance: Quarterly inspections of guide rails and cables, annual motor oil changes, and impeller inspection during plant shutdowns. Requires confined space entry procedures and lockout/tagout. Crane or hoist needed for mixer removal. Bearing replacement and seal service typically every 3-5 years, requiring millwright-level mechanical skills.

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Troubleshooting: High amperage indicates impeller damage or fouling, while low amperage suggests worn impeller or motor issues. Excessive vibration signals bearing failure or impeller imbalance. Typical service life 15-20 years with proper maintenance. Oil leaks and moisture alarms provide early warning of seal failure, allowing planned maintenance before catastrophic failure.

• Impeller Assembly: Cast iron or stainless steel propellers, typically 24-72 inches diameter for municipal applications. Three-blade designs common for general mixing, four-blade for high-solids applications. Selection based on required thrust and tank geometry

• Motor Housing: Sealed TEFC motors from 0.5-50 HP, oil-filled for cooling and protection. 460V three-phase standard, with thermal protection and moisture sensors. Sizing based on required mixing intensity and duty cycle

• Guide Rail System: Stainless steel rails allow mixer positioning and retrieval without dewatering. Includes chain/cable assemblies rated 150% of mixer weight. Critical for maintenance access in deep tanks

• Discharge Volute: Directs flow pattern and maximizes hydraulic efficiency. Cast iron construction with replaceable wear rings. Designed for specific tank geometry and mixing requirements

• Support Structure: Includes mounting brackets, thrust bearings, and alignment systems. Must handle dynamic loads and prevent vibration transmission to tank structures

• Thrust Output: 50-2,000 lbf for typical municipal applications. Anoxic zones require 100-400 lbf, while aerobic zones need 200-800 lbf based on solids concentration

• Power Rating: 0.5-25 HP motors standard. Small basins (0.1-1.0 MG) typically use 1-5 HP units, while larger tanks (2-10 MG) require 10-25 HP mixers

• Propeller Diameter: 24"-72" range. Larger diameters provide better mixing efficiency at lower speeds, reducing maintenance

• Mixing Zone Radius: 15-50 feet effective radius depending on thrust and basin geometry. Rule of thumb: 1 lbf thrust per 1,000-2,000 gallons mixed volume

• Solids Concentration: Design for 2,000-8,000 mg/L MLSS in activated sludge systems. Higher concentrations require increased thrust density

• Turnover Rate: Target 2-4 turnovers per hour for biological processes. Anoxic zones can operate at lower rates (1-2 turnovers/hour)

• Submergence Depth: Minimum 3 feet above propeller, maximum 40 feet for standard units. Deeper installations require special motor cooling considerations

• Velocity Gradient: Maintain 20-80 sec⁻¹ G-value for biological mixing applications

• Question 1: What mixing intensity is required for the specific process application? Anoxic denitrification: 10-30 sec⁻¹ G-value, 50-150 lbf thrust; Aerobic biological: 30-80 sec⁻¹ G-value, 200-500 lbf thrust. Wrong decision consequences: Inadequate mixing causes dead zones and poor treatment; excessive mixing wastes energy and can damage floc structure. Need: Process type, basin geometry, expected solids concentration

• Question 2: Should mixers be fixed-mount or portable/retrievable systems? Fixed-mount: <15 HP, permanent installations, lower cost; Retrievable: >10 HP, maintenance access critical, rail/chain systems. Wrong decision consequences: Fixed units require basin dewatering for service; retrievable systems cost 20-30% more initially. Need: Maintenance philosophy, basin configuration, budget constraints

• Question 3: What redundancy level is appropriate for the application? Critical processes: N+1 or N+2 redundancy (100-200% backup); Non-critical: N+0 acceptable with spare inventory. Wrong decision consequences: Insufficient backup causes process failure; excessive redundancy wastes capital and energy. Need: Process criticality, permit requirements, maintenance capabilities

• Primary: Division 40 - Process Integration - Section 40 30 00 - Process Mechanical Equipment

• Secondary: Division 43 - Process Gas and Liquid Handling, Purification, and Storage Equipment - Section 43 21

• Material/Equipment Verification: Verify 316SS wetted parts minimum; Confirm IP68 sealing ratings; Check motor efficiency ratings (IE3/NEMA Premium)

• Installation Requirements: Crane access for removal (mixers 15+ HP); Electrical conduit routing to junction boxes; Guide rail systems require precise alignment

• Field Challenges: Concrete anchor bolt tolerance ±1/4"; Cable entry sealing critical; Mixer orientation affects performance

• Coordination Issues: Coordinate with structural for mounting loads; 12-16 week lead times typical

• Xylem/Flygt: 4640 series submersible mixers (municipal standard), 2-75 HP range

• Grundfos: Amarex KRT mixers, popular in smaller plants (0.5-10 MGD)

• KSB: Amaprop series, strong European presence expanding in North America

• Sulzer: ABS XRW mixers, gaining market share in retrofit applications. All offer municipal-specific impeller designs and corrosion-resistant materials

• Surface Aerators: Preferred for shallow basins (<12 ft), 20-30% lower capital cost but higher energy consumption

• Jet Mixing: Better for deep tanks (>20 ft), using existing pumps. 15-25% higher operating costs

• Mechanical Draft Tube Mixers: Vertical shaft mixers for large basins, lower maintenance but require structural platforms. Submersible mixers typically most cost-effective for 12-20 ft depth municipal applications

Installation: Always order spare lifting chains/cables - they're the first failure point. Install guide rails 1/8" wider than spec to account for concrete tolerances. Manufacturer Relations: Xylem/Flygt offers excellent field support but premium pricing. Grundfos provides competitive pricing with solid municipal references. Cost Savings: Bundling multiple units reduces per-unit cost 10-15%. Consider factory-mounted VFDs to reduce electrical installation costs.