Circular Clarifiers

Circular clarifiers remove suspended solids from water and wastewater through gravity settling in a circular basin with rotating collection mechanisms. Raw water enters at the center and flows radially outward while solids settle to the bottom, where rotating scrapers continuously move sludge toward a central hopper for removal. Typical overflow rates range from 600-1,200 gallons per day per square foot (gpd/sf) for primary treatment and 400-800 gpd/sf for secondary treatment. The primary trade-off is achieving optimal hydraulic loading versus maintaining adequate detention time for effective solids separation in space-constrained sites.

• Primary Clarification (Water Treatment): Used after coagulation/flocculation to remove settled floc and floating debris. Typically 80-120 ft diameter for 5-25 MGD plants, with 2-4 hour detention time. Selected for consistent performance and lower footprint than rectangular basins. Feeds directly to filtration.

• Secondary Clarification (Wastewater): Separates activated sludge from treated effluent in 80-150 ft diameter units handling 1-50 MGD. Chosen for superior solids separation and RAS control compared to rectangular clarifiers. Connects between aeration basins and disinfection.

• Thickening Applications: Concentrates waste activated sludge from 0.5-1.5% to 3-6% solids in 40-80 ft diameter units. Selected for gravity thickening efficiency and minimal operator attention. Feeds downstream dewatering equipment.

• Lime Softening: Removes precipitated calcium carbonate and magnesium hydroxide in 60-120 ft diameter clarifiers. Preferred for handling high solids loading rates (up to 8 lb/sf/day) with excellent clarification performance.

Daily Operations: Operators monitor clarifier performance through visual inspection of surface conditions, effluent clarity, and sludge blanket depth using sludge judge. Key adjustments include RAS rates (25-100% of influent flow), waste sludge rates, and scum removal frequency. Surface loading rates maintained at 400-800 gpd/sf for secondary clarifiers, with SVI testing guiding operational decisions.

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Maintenance: Monthly lubrication of drive mechanisms and quarterly alignment checks of scraper arms. Annual replacement of effluent weir plates and drive chain inspection. Safety requirements include confined space entry procedures for clarifier access, fall protection for walkways, and lockout/tagout for rotating equipment. Maintenance requires millwright skills for mechanical components and basic welding for repairs.

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Troubleshooting: Common failures include drive mechanism overload from excessive sludge accumulation, effluent weir clogging causing uneven overflow, and scum accumulation from inadequate removal. Warning signs include unusual noise from drives, visible sludge car

• Center Feed Well: 8-12 ft diameter concrete structure with energy dissipation baffles. Distributes influent at 0.5-1.0 fps velocity to prevent short-circuiting. Sizing based on peak hydraulic loading.

• Rotating Sludge Scraper: Steel rake arms spanning clarifier radius, rotating 0.02-0.05 rpm via center drive mechanism. Moves settled solids to central hopper. Drive units typically 1-5 HP depending on diameter.

• Effluent Weir/Launder: Adjustable steel or fiberglass weir plates maintaining 10-15 ft weir loading rates. V-notch weirs provide uniform overflow distribution. Launders sized for 0.2-0.4 fps velocity.

• Scum Removal System: Surface skimmer blades or rotating scum pipe collect floating materials. Beach or scum box receives collected material. Essential for consistent surface loading control.

• Sludge Withdrawal: Central hopper with 6-12 inch sludge pipes and automated withdrawal valves. Hopper sizing accommodates 2-4 hours sludge storage at design solids loading.

• Surface Overflow Rate (SOR): 400-800 gpd/sf for primary clarifiers; 300-600 gpd/sf for secondary clarifiers. Higher rates acceptable with plate settlers (up to 1,200 gpd/sf).

• Solids Loading Rate: 15-40 lbs/day/sf for primary; 0.2-1.5 lbs/day/sf for secondary (biological). Thickening applications: 2-6 lbs/day/sf.

• Detention Time: 1.5-3.0 hours primary; 2-4 hours secondary. Minimum 90 minutes under peak flow conditions.

• Weir Loading Rate: 10,000-20,000 gpd/linear foot for effluent weirs. Scum weirs typically 500-2,000 gpd/lf.

• Diameter Range: 20-200 feet typical for municipal applications. Depths: 8-16 feet sidewater depth, with 1-3 feet additional for sludge storage.

• Peripheral Speed: 2-5 fpm for rake arms. Torque requirements: 10,000-100,000 ft-lbs depending on diameter and sludge characteristics.

• Sludge Withdrawal: 0.5-5% of influent flow rate. Pump cycling: 15-60 minutes on/off based on blanket monitoring.

• Effluent Quality Targets: <30 mg/L TSS and BOD for secondary; <150 mg/L TSS for primary treatment applications.

• Should we use center-feed or peripheral-feed configuration? Center-feed standard for <100 ft diameter; peripheral-feed for larger units or high solids loading (>25 lbs/day/sf). Wrong choice causes short-circuiting, reducing removal efficiency by 15-30%. Need influent flow pattern analysis and jar testing data.

• What sludge removal mechanism is appropriate? Chain-and-flight for diameters <60 ft and high grit loads; rotating bridge for 60-150 ft; traveling bridge for >150 ft. Undersized mechanisms cause sludge carryover and increased maintenance. Requires sludge production estimates and grit characterization.

• Do we need enhanced settling features? Plate settlers increase capacity 50-100% but add $200-400/sf. Flocculation wells improve performance 10-20% for biological systems. Decision based on expansion requirements and effluent quality margins.

• What level of automation is required? Basic SCADA monitoring versus advanced sludge blanket control and variable speed drives. Advanced systems reduce operator attention 60-80% but add $50,000-150,000 per unit. Consider staffing levels and operational complexity.

• Primary: 40 05 23 - Circular Clarifiers

• Secondary: 40 05 13 - Sedimentation Equipment (for package units)

• Related: 40 06 00 - Packaged Water Treatment Equipment (if integrated systems)

• Division 40 covers process water and wastewater treatment equipment, with 40 05 23

• Material/Equipment Verification: Verify concrete strength (4000 psi minimum) and steel reinforcement schedules. Confirm drive unit torque ratings match sludge characteristics. Check effluent launder levelness specifications (±1/8").

• Installation Requirements: 18-24 month lead times for custom concrete work. Requires 150-ton crane access for mechanism installation. Electrical coordination for VFD integration with plant SCADA.

• Field Challenges: Concrete curing delays in winter climates. Mechanism alignment critical during startup.

• WesTech Engineering - Superpulsator series with integrated sludge thickening

• Evoqua Water Technologies - Densadeg high-rate clarifiers for space-constrained sites

• Xylem/Leopold - Traditional center-feed clarifiers with Spiraflo launders

• Lakeside Equipment - Raptor series with integrated screening for smaller municipalities

• All maintain strong municipal references and field service networks.

• Rectangular Clarifiers: 15-20% lower capital cost, easier expansion. Preferred for linear plant layouts.

• Lamella/Tube Settlers: 60-70% footprint reduction, 25% higher cost. Ideal for plant expansions.

• Dissolved Air Flotation: Superior for low-density floc, algae removal. 40-50% higher O&M costs but handles variable influent better than conventional settling.

Manufacturer Relationships: Establish service agreements during procurement - drive unit gearbox failures typically occur 8-12 years post-installation. Cost Savings: Specify standard mechanism diameters (40', 60', 80', 100') to leverage manufacturer inventory. Consider concrete supplier partnerships for multi-phase projects. Pre-cast center wells can reduce construction time 4-6 weeks versus cast-in-place alternatives.