Tank Mixers

Tank mixers are mechanical devices that create fluid motion in storage tanks, basins, and reactors to prevent settling, maintain uniform chemical distribution, or suspend solids. A motor-driven impeller rotates at controlled speeds, generating flow patterns that circulate the entire tank volume. Performance is commonly expressed as turnover rate—typically 4 to 8 turnovers per hour for most municipal applications, meaning the mixer circulates the full tank volume that many times hourly. You'll find tank mixers in equalization basins, chemical feed tanks, anaerobic digesters, and flocculation chambers. The key trade-off: higher mixing intensity prevents settling but increases energy costs and can shear biological flocs or chemical aggregates. Choosing the right mixer type (top-entry, side-entry, portable) and impeller design depends on tank geometry, solids concentration, and whether you're blending liquids or keeping solids suspended—these are fundamentally different mixing objectives that require different approaches.

• 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

Misconception 1: Bigger motors always mix better, so specifying higher horsepower ensures adequate mixing.

​

Reality: Oversized mixers waste energy and can damage floc or create excessive turbulence. Proper mixing depends on impeller design, placement, and speed—not just motor size.

​

Action: Ask manufacturers to justify their recommended horsepower based on your specific tank geometry and mixing objective (blending versus solids suspension).

​

Misconception 2: Any mixer can handle any application if it fits the tank dimensions.

​

Reality: Mixers designed for chemical blending fail quickly in high-solids digesters. Impeller type, seal design, and shaft materials must match your fluid characteristics.

​

Action: Specify solids concentration, viscosity, and any abrasive materials to your equipment supplier before selecting a mixer type.

Impeller drives fluid movement by converting motor torque into radial or axial flow patterns within the tank. Common designs include pitched-blade turbines for solids suspension and hydrofoil impellers for efficient low-shear mixing in municipal basins. Impeller selection affects energy consumption and mixing effectiveness—undersized impellers leave dead zones while oversized units waste power and may damage floc.

​

Motor and gearbox provide rotational power to the impeller shaft, typically mounted on a bridge or platform above the tank. Motors range from fractional horsepower for small clarifiers to 25+ HP for large anaerobic digesters, paired with speed reducers for optimal impeller rpm. Proper sizing prevents overheating during high-viscosity conditions like cold-weather digester startup or thickened sludge mixing.

​

Shaft transmits torque from the gearbox to the submerged impeller, often spanning 10 to 40 feet in municipal tanks. Shafts are typically 304 or 316 stainless steel with diameter selected to resist bending and torsional loads. Shaft deflection causes vibration and premature seal failure—you'll notice unusual noise or leakage before catastrophic failure occurs.

​

Mechanical seal prevents liquid from migrating up the shaft into the gearbox, positioned where the shaft enters the tank. Seals use spring-loaded faces (carbon vs. ceramic or silicon carbide) with external flushing systems for grit-laden applications. Seal failure shows as dripping or pooling around the shaft—it's the most common maintenance item and requires shutdown to replace.

​

Mounting system secures the mixer to the tank structure, either as a fixed bridge installation or a portable rail-mounted unit. Fixed mounts use steel beams anchored to tank walls; portable systems slide on rails for maintenance access or multi-tank service. Mounting rigidity affects vibration transmission—loose bolts or corroded welds amplify shaft deflection and accelerate component wear you'll feel during routine inspections.

Daily Operations: You'll monitor mixer operation by listening for unusual vibration or grinding sounds during routine rounds. Normal operation is steady hum with no visible leakage around the shaft seal. Check for proper impeller submergence if tank level varies—low levels cause vortexing and air entrainment you'll see as surface turbulence. Notify maintenance immediately if you detect new vibration patterns or see fluid weeping from the seal area.

​

Maintenance: Seal replacement is the primary task, typically every 1-3 years depending on grit loading and runtime. You'll need confined space entry protocols if accessing submerged components, plus lockout/tagout for electrical isolation. Gearbox oil changes occur annually and can be done in-house with basic tools. Impeller inspection requires draining the tank—schedule this during planned outages and expect vendor assistance for shaft alignment checks on larger units.

​

Troubleshooting: Increased vibration signals bearing wear or shaft imbalance—check mounting bolts first before calling for service. Seal leakage starts as occasional drips and progresses to steady weeping; replace seals before gearbox contamination occurs. Motor overheating indicates excessive load from thickened sludge or impeller fouling—reduce feed solids or clean the impeller if accessible. Mixers typically run 15-20 years with proper maintenance; premature failure under five years suggests sizing issues requiring engineering review.

Tank mixer selection depends on interdependent variables including tank geometry, process requirements, and mixing intensity needed to maintain solids suspension or blend chemical additions. Understanding how these parameters interact helps you evaluate manufacturer proposals and ask informed questions during equipment selection.

​

Impeller Diameter (inches) determines the zone of influence and mixing intensity each unit can achieve. Municipal tank mixers commonly use impellers between 24 and 72 inches in diameter. Larger impellers move more water per revolution and create broader circulation patterns suitable for large equalization basins, while smaller impellers generate higher localized shear for chemical blending or preventing solids settlement in compact tanks. The diameter you need depends on whether you're creating gentle bulk motion or aggressive localized mixing.

​

Motor Horsepower (HP) provides the energy input required to maintain your target mixing intensity throughout the tank volume. Municipal installations commonly specify motors between 1 and 25 HP depending on tank size and application. Higher horsepower supports larger impellers, higher rotational speeds, or more viscous fluids, while lower horsepower suffices for gentle blending in smaller tanks where energy costs matter more than mixing intensity. Oversizing wastes energy; undersizing allows solids to settle or creates dead zones.

​

Rotational Speed (RPM) controls the tip speed and shear environment the impeller creates. Most municipal tank mixers operate between 35 and 80 RPM at the impeller. Higher speeds increase turbulence and shear, which helps break up floc or disperse chemicals quickly, while lower speeds create gentle rolling motion that prevents settling without damaging biological floc in activated sludge or anaerobic digestion applications. Speed selection balances mixing effectiveness against potential process disruption.

​

Mixing Intensity (velocity gradient, sec⁻¹) quantifies the turbulence level and affects both process performance and energy consumption. Municipal applications commonly target velocity gradients between 20 and 80 sec⁻¹ depending on process needs. Higher intensity promotes rapid chemical dispersion and prevents stratification but increases energy costs and can shear fragile floc, while lower intensity maintains gentle suspension suitable for biological processes where you want mixing without disrupting microbial aggregates. Your target intensity should match your process chemistry and settling characteristics.

​

Submergence Depth (feet) affects hydraulic loading on the mixer and influences circulation patterns within the tank. Municipal tank mixers typically operate with submergence between 3 and 30 feet below the liquid surface. Deeper submergence increases structural loads on the mounting system and may require more robust seals, while shallow submergence risks surface vortexing or air entrainment that can disrupt biological processes or create foaming. Proper submergence ensures the impeller operates in a stable hydraulic environment without cavitation or surface disturbance.

​

All values are typical ranges—actual selection requires manufacturer consultation and site-specific analysis.

What mixer configuration best suits your tank geometry and process requirements?

​

• Why it matters: Configuration affects mixing effectiveness, installation cost, and long-term maintenance accessibility.
• What you need to know: Tank diameter, depth, side water depth, internal obstructions, and required turnover rate.
• Typical considerations: Top-entry mixers suit deeper tanks with open access and simpler installation. Side-entry units work better in shallow or covered tanks where vertical clearance limits overhead access. Draft tubes enhance circulation in stratified tanks but add structural complexity.
• Ask manufacturer reps: How does your recommended mounting position achieve uniform velocity distribution in our tank geometry?
• Ask senior engineers: Which configuration has performed best in similar tank dimensions at our other facilities?
• Ask operations team: What mounting arrangement allows easiest impeller inspection without confined space entry or dewatering?

​

How much mixing intensity do you need to maintain solids suspension or blending?

​

• Why it matters: Under-mixing causes settling and dead zones; over-mixing wastes energy and increases mechanical wear.
• What you need to know: Solids concentration, particle settling velocity, required blend time, and acceptable velocity gradients.
• Typical considerations: Gentle blending for chemical storage requires lower intensity than solids suspension in equalization basins. Flocculation demands controlled shear to build floc without breakup. Anaerobic digesters need complete mixing without excessive turbulence that disrupts biological activity.
• Ask manufacturer reps: What impeller diameter and speed combination delivers our target velocity gradient at minimum power?
• Ask senior engineers: Have we experienced settling issues in similar applications that would justify higher mixing intensity?
• Ask operations team: Can you adjust mixing intensity seasonally, or does fixed-speed operation simplify your maintenance schedule?

​

What motor and drive arrangement balances performance with operational flexibility?

​

• Why it matters: Drive selection affects energy consumption, speed control capability, and equipment lifespan under variable loading.
• What you need to know: Required power range, process variability, duty cycle, and available electrical service characteristics.
• Typical considerations: Fixed-speed drives suit consistent processes with predictable loads and minimize control complexity. Variable frequency drives enable intensity adjustment for changing conditions but require additional electrical infrastructure. Direct-drive units reduce mechanical losses compared to gear-reduced systems but limit torque multiplication for high-solids applications.
• Ask manufacturer reps: How does your recommended drive system accommodate our peak torque requirements during startup with settled solids?
• Ask senior engineers: Does our plant's operational strategy justify VFD cost through demand response or process optimization?
• Ask operations team: Would speed adjustment help you manage foam, scum buildup, or seasonal temperature changes?

Lead Times: Standard mixers typically ship in 8-12 weeks; custom configurations or large horsepower units extend to 16-20 weeks. Important for project scheduling—confirm early.

​

Installation Requirements: Top-entry units need overhead clearance for removal (typically 1.5× impeller diameter) and structural support for mounting flanges. Submersible units require lifting equipment rated for wet weight and guide rail systems for removal. All installations need electrical coordination for motor starters and cable routing from MCC.

​

Coordination Needs: Structural engineer must verify tank wall or cover loading for mounting points. Electrical must provide properly sized starters, VFD compatibility if specified, and conduit routing. Mechanical contractor coordinates with basin construction for embedded anchors or guide rails.

Flygt (Xylem) – Submersible mixers and propeller-style units; known for compact submersible designs suitable for tight spaces and retrofit applications.

​

Landia – Top-entry and submersible mixers; specializes in chopper-style mixers for high-solids applications and digesters.

​

Philadelphia Mixing Solutions – Top-entry and side-entry industrial mixers; offers custom-engineered solutions for unusual tank geometries or challenging process conditions.

​

This is not an exhaustive list—consult regional representatives and project specifications.

• 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