Rotary Drum Thickeners

Overview

Rotary Drum Thickeners concentrate dilute sludge streams by mechanically dewatering biosolids on a slowly rotating perforated drum. The unit operates by feeding liquid sludge onto the interior of a horizontal cylindrical screen that rotates at 1-3 RPM, allowing water to drain through while conveying thickened solids to discharge. Typical performance achieves 4-8% solids concentration from 0.5-2% feed sludge at hydraulic loading rates of 2-5 gpm/ft² of drum area. The primary trade-off is limited thickening capability compared to gravity thickeners, making it suitable mainly for waste activated sludge applications in smaller municipal plants.

Common Applications
  • Primary Sludge Thickening (2-15 MGD plants): Rotary drum 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 for consistent performance with fibrous materials and minimal polymer usage compared to belt thickeners.
  • Waste Activated Sludge (WAS) Thickening (5-50 MGD plants): Thickens secondary sludge from 0.8-1.2% to 3-5% solids upstream of dewatering. Handles variable biological loads better than gravity thickeners. Essential for plants with limited space or high WAS production rates exceeding 15,000 gpd.
  • Combined Sludge Applications (10-50 MGD plants): Processes blended primary and WAS streams, achieving 4-6% final solids concentration. Preferred when separate thickening isn't cost-effective and downstream belt presses require consistent feed solids.
Operator Experience

Daily Operations: Operators monitor polymer feed rates (2-8 lb/dry ton), drum rotation speed, and effluent clarity hourly. Key parameters include feed flow rate, solids capture (target >90%), and cake solids concentration. Adjustments focus on polymer dosage based on visual effluent quality and upstream process changes.

Maintenance: Weekly bearing lubrication and spray nozzle inspection. Monthly drive alignment checks and polymer system calibration. Quarterly drum screen cleaning requires confined space entry with proper PPE. Annual bearing replacement and drive service typically require millwright skills and 2-person crews for safety.

Troubleshooting: Screen blinding indicates inadequate wash water pressure or polymer overdosing. Excessive carryover suggests insufficient polymer or high organic loading. Poor cake formation often results from polymer underdosing or aged polymer solutions. Typical service life ranges 15-20 years with proper maintenance, with screen replacement every 7-10 years.

Major Components
  • Rotating Drum Assembly: Perforated stainless steel cylinder (6-12 ft diameter, 8-20 ft length) with 0.5-1.5mm openings. Rotates at 1-3 rpm, creating gentle mixing action. Sizing based on 2-4 gpm/sq ft hydraulic loading for municipal applications.
  • Polymer Feed System: Dilutes concentrated polymer (0.1-0.5%) to working strength. Includes static mixers and flow-proportional controls. Critical for achieving target capture rates of 90-95% in municipal sludge applications.
  • Drive Mechanism: Variable frequency drive with 5-15 HP motor provides precise speed control. Includes torque monitoring for process optimization and overload protection during upset conditions.
  • Wash Water System: High-pressure spray nozzles (60-100 psi) prevent screen blinding. Typically requires 5-15% of feed flow as wash water, recycled to plant headworks.
Design Criteria
  • Hydraulic Loading Rate: 2-8 gpm/ft² of drum surface area for typical municipal WAS applications. Higher rates (6-8 gpm/ft²) suitable for well-conditioned sludge; lower rates (2-4 gpm/ft²) for difficult-to-thicken biosolids.
  • Solids Loading Rate: 8-25 lbs/hr/ft² dry solids basis. Primary sludge handles higher loadings; activated sludge requires conservative approach at 8-15 lbs/hr/ft².
  • Drum Dimensions: Standard diameters 6-12 feet, lengths 8-40 feet. Submergence typically 40-60% of drum diameter. Screen openings 0.25-0.75mm for municipal applications.
  • Rotational Speed: 1-6 rpm variable speed drive standard. Slower speeds increase capture efficiency but reduce capacity.
  • Polymer Dosage: 2-8 lbs active polymer per dry ton solids. Anionic polymers typical for WAS applications.
  • Thickened Solids Concentration: 4-8% for WAS, 8-12% for primary sludge. Performance depends heavily on polymer conditioning and feed characteristics.
  • Capture Rate: 95-98% TSS removal expected with proper polymer dosing and operational control.
  • Wash Water Requirements: 15-25 gpm per foot of drum width for screen cleaning.
Key Design Decisions
  • What feed solids concentration and variability can the unit handle? Feed TSS typically 0.3-1.2% for WAS applications. Units struggle below 0.2% TSS or with highly variable feeds exceeding ±50% concentration swings. Poor feed consistency leads to polymer overdosing, increased operating costs, and inconsistent thickened solids quality. Requires detailed characterization of plant's sludge production patterns and seasonal variations.
  • How will polymer mixing and feed distribution be configured? Polymer contact time of 30-60 seconds critical before drum entry. Insufficient mixing causes poor flocculation and reduced capture rates below 90%. Over-mixing shears floc and degrades performance. Decision affects equipment layout, pumping requirements, and polymer consumption rates of 4-12 lbs/dry ton.
  • What backup and redundancy level is required? Single units create process bottlenecks during maintenance. Dual units allow 50-75% capacity operation during outages but increase capital costs 60-80%. Consider plant's storage capacity, peak flow periods, and downstream process flexibility when determining redundancy requirements.
  • How will screenings and wash water be handled? Screenings typically 1-3% of feed volume but require separate handling system. Wash water recycle affects plant hydraulic loading and may require dedicated treatment. Inadequate planning creates operational headaches and regulatory compliance issues.
Specification Section
  • Primary: Division 46 - Water and Wastewater Equipment, Section 46 55 63 - Sludge Thickening Equipment
  • Secondary: Division 40 - Process Integration (for control systems integration with existing plant SCADA and process equipment coordination)
Submittal + Construction Considerations
  • Material/Equipment Verification: Verify 316SS construction for all wetted parts, Confirm polymer dosing system compatibility, Check drive motor sizing for actual solids loading
  • Installation Requirements: Reinforced concrete pad with anchor bolt template, Overhead crane access for drum removal, Electrical classification per plant standards
  • Field Challenges: Precise leveling critical for proper operation, Polymer feed line routing requires careful planning
  • Coordination Issues: 16-20 week lead times typical, Coordinate with building steel for support structure
Popular Manufacturers and Models
  • HUBER Technology - RoDisc series (municipal installations at 15+ MGD plants)
  • Andritz - DRUM-THICKENER models (proven at 50+ North American facilities)
  • Parkson Corporation - RotaDrum units (popular for 1-20 MGD applications)
  • Alfa Laval - ALDRUM thickeners (growing municipal presence, strong industrial background)

All maintain active North American service networks with municipal references available.

Alternative Equipment
  • Gravity Belt Thickeners - Lower capital cost, simpler operation, preferred for smaller plants (<5 MGD). Typically 20-30% less expensive than rotary drums.
  • Dissolved Air Flotation (DAF) - Better for difficult-to-thicken sludges, higher chemical costs but superior performance on waste activated sludge. 40-50% higher capital cost.
  • Centrifuges - Higher throughput, smaller footprint, but significantly higher power consumption and maintenance requirements.
Real-World Tips

Establish direct manufacturer service relationships early - local reps provide faster troubleshooting than corporate support. Negotiate spare parts packages during initial purchase for 15-20% savings versus individual orders. Consider standardizing on single manufacturer across multiple units to reduce training requirements and parts inventory. Most successful installations include operator training as part of startup - budget 2-3 days minimum for comprehensive training.

Connect with Local Representative
If you need help with sizing, system compatibility,  maintenance planning, or sourcing, connect with your local manufacturer's representative. They can assist you in selecting the right equipment for your specific application and site conditions.

Connect with a Local Distributor

If you need help with sizing, system compatibility,  maintenance planning, or sourcing, connect with your local manufacturer's representative. They can assist you in selecting the right equipment for your specific application and site conditions.