Disc Thickeners

Disc thickeners concentrate municipal wastewater sludge by removing water through gravity settling and mechanical dewatering within rotating disc assemblies. Raw sludge enters the unit where slowly rotating discs create laminar flow patterns that promote flocculation and allow solids to settle while clarified water overflows. These units typically achieve 3-6% total solids concentration from feed sludge containing 0.5-2% solids, with hydraulic loading rates of 0.5-2.0 gpm/ft². The primary limitation is their sensitivity to shock loads and requirement for consistent polymer dosing to maintain optimal performance.

• Primary Sludge Thickening (2-20 MGD plants): Disc thickeners concentrate primary sludge from 2-4% to 6-8% solids before anaerobic digestion. Located between primary clarifiers and digesters, they reduce digester volume requirements by 40-50%. Selected over gravity thickeners when footprint is limited and consistent thickening performance is critical.

• WAS Thickening (5-50 MGD plants): Handles waste activated sludge from secondary clarifiers, thickening from 0.8-1.2% to 3-5% solids. Feeds thickened sludge to digesters or dewatering equipment. Chosen over dissolved air flotation when polymer consumption must be minimized and gentle handling preserves sludge characteristics.

• Combined Sludge Applications (10+ MGD plants): Processes blended primary and secondary sludges simultaneously, achieving 5-7% solids concentration. Installed upstream of centrifuge dewatering systems to optimize overall solids handling efficiency and reduce polymer costs by 20-30%.

Daily Operations: Operators monitor influent flow rates, effluent clarity, and sludge blanket levels hourly. Key adjustments include disc speed optimization (typically 1-2 RPM), polymer dosing rates, and sludge withdrawal timing. Performance tracking focuses on solids capture efficiency (>95%) and thickened sludge consistency.

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Maintenance: Weekly bearing lubrication and polymer system cleaning prevent most issues. Monthly disc inspection requires confined space entry with proper ventilation and gas monitoring. Annual gearbox oil changes and drive alignment checks require millwright skills. Disc replacement typically occurs every 8-12 years with specialized contractor assistance.

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Troubleshooting: Poor thickening indicates polymer overdosing or disc fouling from grease/debris. Excessive torque signals bearing wear or sludge buildup requiring immediate shutdown. High effluent solids suggest disc damage or hydraulic overloading. Warning signs include unusual vibration, temperature increases, or polymer consumption spikes. Expected service life ranges 15-20 years with proper maintenance.

• Rotating Disc Assembly: Stainless steel discs (6-12 feet diameter) with 2-3mm spacing rotate at 1-3 RPM. Disc quantity ranges from 50-200 units depending on capacity (50-2000 GPM). Material selection includes 304SS standard or 316SS for aggressive conditions.

• Drive System: Variable frequency drives control disc rotation with 5-15 HP motors for municipal sizes. Includes torque monitoring for process optimization and overload protection during upset conditions.

• Polymer Feed System: Dilution and metering pumps deliver 0.5-3.0 lbs polymer per dry ton solids. Features include flow-paced control and multiple injection points for uniform mixing.

• Effluent Collection: Perforated plates and collection troughs capture separated liquid. Designed for 15-25 GPM/ft² surface loading with adjustable weirs for level control.

• Sludge Discharge: Scraper mechanisms and pneumatic pumps remove thickened sludge. Includes variable speed control matching downstream equipment requirements.

• Solids Loading Rate: 8-25 lb/hr/ft² disc area (municipal WAS typically 12-18 lb/hr/ft²)

• Hydraulic Loading Rate: 0.5-2.0 gpm/ft² disc area (design at 1.0-1.5 gpm/ft² for reliable operation)

• Feed Solids Concentration: 0.3-1.2% TS (optimum 0.6-0.8% for municipal activated sludge)

• Thickened Solids Concentration: 3-6% TS achievable (municipal WAS typically 4-5% TS)

• Disc Diameter: 9.8-11.5 ft (3-3.5m) standard municipal sizes

• Disc Spacing: 0.75-1.0 inch between discs

• Drive Power: 3-15 HP for municipal applications (0.5-50 MGD plants)

• Rotational Speed: 0.5-2.0 RPM (typically 1.0 RPM)

• Polymer Dosage: 4-12 lb active polymer/dry ton solids

• Capture Efficiency: 95-98% TSS removal expected

• Torque Requirements: Size for 150-200% of normal operating torque

• Minimum Basin Depth: 8-10 ft below disc assembly for proper hydraulics

• What is the peak hourly solids loading and can the unit handle 150% of average loading? Need 24-hour flow data and solids production rates. Undersizing results in poor capture (<90%) and thin cake (<3% TS). Requires polymer usage data from similar plants and pilot testing for loading >15 lb/hr/ft².

• Will polymer mixing and feed systems provide adequate flocculation within the 2-3 minute retention time? Requires polymer type selection (emulsion vs. dry), mixing energy calculations, and feed point locations. Poor mixing results in 20-30% higher polymer usage and reduced capture efficiency.

• Can the site accommodate the required 12-15 ft headroom and structural loading of 150-200 psf? Includes disc assembly, lifting equipment, and maintenance access. Inadequate clearance prevents disc removal for maintenance. Foundation must handle dynamic loads from unbalanced rotation.

• What backup thickening capacity exists during maintenance shutdowns lasting 8-24 hours? Consider redundant units, bypass storage, or temporary equipment rental. Single units create operational vulnerability requiring emergency sludge storage or hauling.

• Primary: Division 46 23 61 - Sludge Thickening Equipment

• Secondary: Division 40 32 00 - Mechanical Water and Wastewater Equipment (for integrated systems including polymer feed and controls)

• Material/Equipment Verification: Verify 316SS construction for all wetted parts, Confirm polymer feed system compatibility and flow ranges, Check vibration monitoring and emergency shutdown systems

• Installation Requirements: Substantial concrete pad (typically 12-18 inches thick), Overhead crane access for bowl removal (8-10 ton capacity minimum), Dedicated electrical room with climate control

• Field Challenges: Precise leveling critical (±0.5mm tolerance), Polymer feed line routing and calibration, Noise/vibration isolation from adjacent structures

• Coordination Issues: HVAC coordination for heat removal (15-25 kW typical), Process piping tie-ins during limited shutdown windows, Lead times: 16-24 weeks typical, up to 36 weeks for custom configurations

• Andritz SEPARATION: D-Series disc thickeners (D2L-3000 to D6L-3000 models) - strong municipal presence, especially larger plants.

• HUBER Technology: RoDisc series (RoDisk 3, RoDisk 6) - popular for 1-20 MGD plants, good service network.

• Alfa Laval: ALDEC G3 series - premium option with advanced automation.

• Flottweg: C-Series centrifuges - German engineering, growing North American municipal market share.

• Belt Filter Presses: Lower capital cost ($400-600K vs $800K-1.2M), higher labor requirements. Preferred for plants <5 MGD with limited staffing.

• Screw Presses: Emerging technology, $300-500K range, good for plants wanting to minimize polymer use. Still proving long-term reliability.

• Gravity Belt Thickeners: Much lower cost ($100-200K) but only achieve 4-6% solids vs 18-22% for disc thickeners. Suitable when downstream processes can handle thinner sludge.

Manufacturer Relationships: Establish service agreements early - disc stack replacement requires factory-trained technicians and costs $150-300K. Negotiate training packages and spare parts inventory.

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Cost-Saving Opportunities: Consider refurbished units for smaller plants (30-40% savings). Standardize on one manufacturer across multiple facilities to leverage parts inventory and operator training. Bundle polymer systems with centrifuge purchase for better pricing and single-source warranty.