Tank Mixers

Tank mixers are mechanical mixing devices that create fluid circulation patterns within treatment tanks to maintain solids suspension, promote chemical reactions, and prevent stratification. These units typically consist of a submersible motor-driven impeller or surface-mounted mixer that generates flow velocities of 0.5-2.0 ft/s throughout the tank volume. Common applications include equalization basins, chemical contact tanks, and biological reactors in municipal plants from 0.5-50 MGD capacity. The primary trade-off involves balancing adequate mixing intensity against energy consumption, as oversized mixers can waste significant power while undersized units create dead zones and poor process performance.

• Equalization Basins (0.5-10 MGD): Tank mixers prevent solids settling and maintain uniform influent characteristics. Typically 1-3 mixers per basin depending on geometry, positioned to create circulation patterns that eliminate dead zones. Connected upstream of primary treatment, downstream of screening/grit removal

• Chemical Contact Tanks: Used in coagulation/flocculation basins and chlorine contact chambers to ensure proper chemical distribution and reaction time. Mixers maintain gentle rolling action (20-50 fpm tip speed) to promote floc formation without breakage. Critical for achieving target CT values in disinfection

• Sludge Holding Tanks: Prevents septicity and solids stratification in primary/secondary sludge storage. Essential for maintaining pumpable consistency before dewatering operations. Typically 0.5-2 HP units for tanks up to 500,000 gallons

• Anoxic Zones: Creates mixing without aeration in BNR processes, maintaining suspended solids while preserving anoxic conditions for denitrification

Daily Operations: Monitor amperage draw (should remain consistent ±10% of baseline), check for unusual vibration or noise. Verify mixing patterns through visual observation - no dead zones or surface foam accumulation. Adjust speed via VFD if equipped, typically operating 60-80% of design speed during normal conditions.

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Maintenance: Quarterly shaft alignment checks and seal inspection. Annual gearbox oil changes (ISO 220 gear oil). Impeller inspection every 2-3 years requires confined space entry with gas monitoring, fall protection, and lockout procedures. Mechanical seal replacement typically 3-5 year intervals depending on duty cycle.

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Troubleshooting: Increasing amperage indicates impeller fouling or bearing wear. Excessive vibration suggests shaft imbalance or loose mounting hardware. Reduced mixing effectiveness with normal power draw indicates impeller damage. Service life 15-20 years for drive units, 10-15 years for underwater components with proper maintenance.

• Impeller Assembly: Propeller-style (3-blade) or turbine designs in 316 SS or bronze. Diameters range 24"-72" for municipal applications. Pitch and blade angle determine pumping capacity - typically 0.75-1.5 blade-to-tank diameter ratio for optimal circulation

• Drive Unit: Gear reducers (20:1 to 100:1 ratios) with 1-25 HP motors. TEFC motors standard with VFD compatibility. Torque arm prevents rotation, sized for maximum impeller loading plus safety factor

• Shaft: 316 SS construction, 2"-6" diameter depending on torque requirements. Length varies 6'-30' for typical municipal tank depths. Mechanical seals or stuffing boxes at tank penetration

• Support Structure: Adjustable mounting pedestals or bridge-mounted systems. Must accommodate thermal expansion and vibration isolation. Designed for 150% rated torque loading

• Mixing Power Requirements: 0.5-3.0 HP per 1,000 gallons for complete mixing in rectangular clarifiers; 0.3-1.5 HP per 1,000 gallons for storage tank mixing. Typical municipal installations range 5-75 HP motors

• Thrust Ratings: Submersible mixers generate 50-500 lbf thrust depending on propeller diameter (24"-72") and motor size. Standard municipal sizing: 200-300 lbf thrust per 100,000 gallons tank volume

• Flow Circulation: Propeller-driven mixers achieve 10-20 times motor flow rate in circulation. 25 HP mixer typically circulates 8,000-12,000 GPM in activated sludge applications

• Solids Suspension: Minimum 0.75 HP per 1,000 gallons for 2% MLSS; 1.5 HP per 1,000 gallons for 4% MLSS in aeration basins. Sludge holding tanks require 2-4 HP per 1,000 gallons for 3-6% solids

• Operating Depth: Standard submersible units operate 3-30 feet depth. Shaft-mounted mixers extend to 40+ feet with guide rail systems

• Velocity Gradients: Flocculation basins require G-values 20-75 sec⁻¹; activated sludge mixing needs 50-200 sec⁻¹ velocity gradients

• What mixing intensity is required for the process application? Gentle flocculation (G=20-40 sec⁻¹) versus aggressive solids suspension (G=100+ sec⁻¹) determines motor sizing and propeller selection. Under-mixing causes settling/dead zones; over-mixing breaks floc or wastes energy. Need: process type, solids concentration, tank geometry

• Should mixers be submersible or shaft-mounted? Submersible units cost 20-30% less initially but require crane removal for maintenance. Shaft-mounted systems allow in-place servicing but need structural support and weather protection. Decision threshold: tanks >15 feet deep or frequent maintenance access favor shaft-mounted. Need: tank depth, maintenance philosophy, structural capacity

• What redundancy level is acceptable? Single mixer failure in critical processes (activated sludge, chlorine contact) can cause permit violations within hours. N+1 redundancy adds 50-100% capital cost but prevents process upset. Municipal plants typically require backup mixing for biological treatment, optional for storage tanks. Need: process criticality, permit requirements, budget constraints

• How will mixing patterns avoid dead zones? Tank aspect ratios >3:1 or complex geometries require multiple mixers or strategic positioning. Poor mixing distribution causes 20-40% effective volume loss and uneven treatment. Need: tank dimensions, baffle locations, inlet/outlet positions

• Division 40 - Process Integration: Section 40 82 00 - Mixing Equipment (Primary)

• Section 46 73 13 - Packaged Water Treatment Equipment Mixers (Alternative for pack

• Material/Equipment Verification: Verify 316SS construction for all wetted parts, confirm NEMA 4X motor enclosures for harsh environments, check impeller material specifications (cast iron vs. stainless)

• Installation Requirements: Crane access for maintenance - plan 15-20 ft clearance minimum, electrical disconnects within sight of equipment, guide rail systems require precise alignment during construction

• Field Challenges: Concrete anchor bolt placement critical - template accuracy essential, cable management in wet wells often problematic, access platforms frequently undersized for maintenance

• Coordination Issues: 16-20 week lead times typical for custom configurations, early electrical coordination prevents change orders

• Xylem/Flygt: Model 4640 submersible mixers (5-150 HP range) dominate municipal installations

• Sulzer: ABS Salomix series popular for anoxic zones

• Landia: GasMix units for digester applications with biogas handling

• Philadelphia Mixing Solutions: Top-entry mixers for rectangular basins

• Flygt holds roughly 40% market share in North American municipal applications

• Coarse bubble aeration systems: 30-40% lower capital cost but higher operating costs due to blower energy. Preferred for smaller basins (<500,000 gal)

• Jet mixing systems: Better for deep tanks (>20 ft) with 15-20% lower energy consumption

• Static mixers: Passive mixing for pipeline applications, 60-70% cost reduction but limited to specific geometries and flow conditions

Establish direct relationships with manufacturer field service reps - they provide invaluable troubleshooting support and spare parts availability. Consider standardizing on one manufacturer across facilities to reduce inventory costs and training requirements. Flygt's service network is particularly strong. Budget 3-5% of equipment cost annually for preventive maintenance contracts - typically pays for itself through reduced emergency repairs.