Solids-Contact Clarifiers

Solids-contact clarifiers combine coagulation, flocculation, and sedimentation in a single unit, using recycled sludge to enhance particle removal efficiency. Raw water enters the central mixing zone where chemicals are added and mixed with recycled solids to form dense, settleable flocs. These flocs then settle in the outer clarification zone while clear water overflows at the rim. Typical surface loading rates range from 1.0-2.5 gpm/sf, significantly higher than conventional clarifiers. The key trade-off is operational complexity - maintaining proper sludge blanket levels and chemical feed rates requires skilled operators and continuous monitoring to prevent process upsets.

• Raw Water Treatment (5-50 MGD): Solids-contact clarifiers excel in high-turbidity surface water treatment, combining coagulation, flocculation, and sedimentation in one basin. Selected for footprint efficiency and superior turbidity removal (0.5-2.0 NTU effluent). Upstream: raw water intake and chemical feed systems. Downstream: filtration and disinfection.

• Lime Softening Plants (2-25 MGD): Handles heavy precipitate loads from lime/soda ash softening processes. The upflow design and sludge blanket effectively capture calcium carbonate and magnesium hydroxide solids. Selected for ability to process 15-25% solids concentrations while maintaining clarified water quality.

• High-Rate Treatment Upgrades (1-20 MGD): Retrofits existing conventional plants requiring capacity increases without expansion. Provides 2-3x higher surface loading rates (3-5 gpm/ft²) compared to conventional clarifiers. Selected when land constraints prevent horizontal expansion.

• Iron/Manganese Removal (0.5-15 MGD): Combines oxidation and clarification of precipitated iron/manganese hydroxides. The sludge blanket contact improves removal efficiency to <0.05 mg/L Fe, <0.02 mg/L Mn.

Daily Operations: Operators monitor sludge blanket depth hourly using interface detectors or manual sampling. Key parameters include effluent turbidity (<2 NTU), sludge blanket level (maintain 3-6 feet below surface), and recirculation flow rates. Chemical feed adjustments based on raw water quality changes. Surface loading maintained within design range through flow distribution control.

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Maintenance: Weekly cleaning of effluent weirs and scum removal systems. Monthly calibration of sludge blanket detectors and flow meters. Quarterly inspection of mixing impellers and drive systems. Annual drain-down for concrete inspection and mechanical overhaul. Requires confined space entry procedures, fall protection, and lockout/tagout. Maintenance staff need basic

• Mixing Zone/Reaction Well: Central mixing chamber where raw water contacts recirculated sludge. Typically 8-12 feet diameter with 2-4 minute detention time. Constructed of concrete with stainless steel impellers. Sized for 150-300 fpm upflow velocity to maintain proper sludge blanket contact.

• Sludge Blanket Zone: Suspended solids layer maintained 3-6 feet below clarified water surface. Functions as filtration medium, typically 15-25% solids concentration. Monitored via sludge blanket detectors and sampling ports at multiple depths.

• Clarification Zone: Upper portion providing 1.5-2.5 hours detention time for final settling. Surface loading rates of 2.5-5.0 gpm/ft² depending on application. Includes effluent weirs and scum removal systems.

• Sludge Recirculation System: Centrifugal pumps recirculate 5-15% of influent flow to maintain sludge blanket. Variable speed drives allow 50-150% flow adjustment. Typically 316 SS construction for chemical resistance.

• Sludge Withdrawal System: Continuous or intermittent waste sludge removal maintaining proper blanket level. Includes flow measurement and automatic control systems.

• Overflow Rate (Surface Loading): 0.5-1.5 gpm/sf for conventional applications, up to 2.0 gpm/sf with high-rate tube settlers. Critical for settling efficiency and footprint optimization.

• Solids Loading Rate: 8-15 lbs/day/sf for typical municipal applications. Higher rates (up to 20 lbs/day/sf) possible with enhanced mixing and proper sludge blanket management.

• Detention Time: 2-4 hours total, with 45-90 minutes in reaction zone and 1.5-3 hours in settling zone. Longer times needed for cold water or high turbidity applications.

• Upflow Velocity: 0.5-1.0 fpm in settling zone to maintain proper sludge blanket without carryover. Velocities above 1.2 fpm typically cause blanket disruption.

• Sludge Blanket Depth: 2-4 feet optimal depth, maintained at 25-40% of total clarifier depth. Deeper blankets improve contact but risk carryover.

• Recirculation Ratio: 5-15% of influent flow for internal recirculation systems. Higher ratios improve mixing but increase energy costs.

• Weir Loading: 10-15 gpm/linear foot maximum to prevent hydraulic disturbances. Lower rates (8-12 gpm/lf) preferred for consistent performance.

• Mixer Power: 0.5-2.0 HP/MG for flocculation zone, 2-5 HP/MG for reaction zone mixing intensity.

• What overflow rate can the plant achieve while meeting turbidity targets? Plants treating low-turbidity source water (2-5 NTU) can typically operate at 1.2-1.5 gpm/sf, while high-turbidity applications (>20 NTU) require 0.8-1.0 gpm/sf. Wrong decision leads to poor settling, increased chemical usage, and filter loading. Need: source water quality data, treatment targets, seasonal variations.

• Should the design include tube settlers or lamella plates? High-rate media allows 50-75% footprint reduction and overflow rates up to 2.0 gpm/sf, but adds $200-400/sf in capital costs and maintenance complexity. Wrong choice affects project economics and long-term O&M. Need: site constraints, budget, maintenance capabilities.

• What sludge handling capacity is required? Solids production ranges from 0.5-2.0% of flow depending on source water quality and chemical dosing. Undersizing leads to blanket carryover; oversizing wastes energy and chemicals. Need: jar testing data, chemical dosing estimates, seasonal solids loading variations.

• How should mixing zones be configured? Proper velocity gradients (20-50 s⁻¹ flocculation, 50-100 s⁻¹ reaction zone) are critical for floc formation and contact efficiency. Poor mixing reduces treatment effectiveness and increases chemical consumption. Need: pilot testing, CFD analysis, mixing equipment specifications.

• Primary: 40 05 00 - Water Treatment Equipment

• Material/Equipment Verification: Verify stainless steel grades for sludge contact zones, Confirm drive motor sizing for varying sludge densities, Check polymer feed system compatibility

• Installation Requirements: Crane access for center mechanism installation, Precise concrete tolerances for drive mounting, Utility coordination for multiple feed points

• Field Challenges: Weather protection during startup/commissioning, Temporary bypassing during tie-ins

• Lead Times: 16-20 weeks typical, 24+ weeks for custom configurations.

• WesTech Engineering - Superpulsator series, dominant in municipal market with 500+ installations

• Evoqua Water Technologies - Accelator units, strong in retrofit applications

• Veolia - Densadeg clarifiers, popular for high-rate applications

• Westech/Ovivo - ACTIFLO systems for compact footprint requirements

• All maintain extensive municipal reference lists and pilot testing capabilities

• Conventional Clarifiers: Lower capital cost, simpler O&M, preferred for <5 MGD plants or when land isn't constrained. Roughly 40-50% lower equipment cost.

• Dissolved Air Flotation: Better for low-temperature/low-turbidity water, 15-20% higher capital cost but superior cold weather performance.

• Membrane Filtration: Higher reliability, smaller footprint, 3-4x capital cost but eliminates clarification variability entirely.

Establish relationships with manufacturer field service early - their startup support is critical for achieving design performance. Negotiate extended warranty periods covering drive mechanisms and polymer systems. Consider bulk polymer purchasing agreements during design phase for 20-30% cost savings. Always specify redundant polymer feed systems; single-point failures cause immediate performance degradation requiring manual operation.