Rotary Fan Presses

Rotary Fan Presses are specialized dewatering equipment used primarily for municipal biosolids processing, combining pressure filtration with rotary drum technology to achieve continuous sludge dewatering. The system operates by feeding conditioned sludge onto a rotating perforated drum where vacuum and mechanical pressure extract water through filter media, typically achieving 18-25% dry solids content from activated sludge. Performance varies significantly with sludge characteristics, with typical throughput rates of 50-200 gallons per minute per meter of drum width depending on solids loading and polymer conditioning effectiveness. The primary trade-off is higher capital and maintenance costs compared to belt filter presses, while offering potentially better cake dryness and reduced polymer consumption.

• Primary Sludge Dewatering (2-20 MGD plants): Rotary fan presses handle raw primary sludge at 3-6% solids, producing 18-25% cake solids. Selected for gentle handling that preserves floc structure and reduces polymer demand by 20-30% versus belt presses. Upstream: gravity thickeners or DAF units. Downstream: cake conveyors to trucks or storage.
• Secondary Sludge Processing (5-50 MGD plants): Processes waste activated sludge (WAS) at 0.5-1.5% solids after polymer conditioning. Achieves 16-20% cake solids with lower shear than centrifuges. WHY: Produces drier cake than belt presses while handling variable solids loading from seasonal plant operations.
• Combined Sludge Applications (10-50 MGD plants): Dewaters blended primary/secondary sludge streams. Selected where space constraints limit multiple dewatering units. Upstream: blend tanks with polymer feed systems. Downstream: typically lime stabilization or anaerobic digestion.

Daily Operations: Operators monitor cake consistency, filtrate clarity, and amperage draw hourly. Key adjustments include drum speed (1-3 RPM), differential speed between drum/screw, and polymer dosing. Visual inspection through observation ports reveals cake formation quality. Typical staffing: 0.5-1.0 FTE for units processing 50-200 GPM.

​

Maintenance: Weekly bearing lubrication and spray wash cleaning. Monthly inspection of drum perforations and screw flights for wear. Annual bearing replacement and drive alignment. Safety requires lockout/tagout for internal access. PPE: respiratory protection for bioaerosols, slip-resistant boots. Requires millwright skills for mechanical components, basic electrical knowledge for VFD troubleshooting.

​

Troubleshooting: Poor cake formation indicates polymer underdosing or aged sludge. High amperage suggests bearing wear or solids buildup. Excessive filtrate solids indicate drum perforation plugging or polymer overdosing. Warning signs: unusual vibration, temperature rise in bearings. Expected service life: 15-20 years with proper maintenance, drum replacement

• Rotating Drum Assembly: Perforated stainless steel cylinder (6-12 ft diameter) with 0.5-2mm openings. Rotates at 1-3 RPM. Sizing based on hydraulic loading (2-8 gpm/ft²). Selection factors: solids type, required cake dryness, and available footprint.
• Internal Screw Conveyor: Helical flights transport dewatered cake to discharge. 316 SS construction handles abrasive solids. Pitch varies along length - tighter near discharge for final compression. Speed differential with drum creates conveying action.
• Filtrate Collection System: Internal piping collects separated liquid through drum perforations. Includes backwash capability for cleaning. Sized for peak hydraulic loads plus 25% safety factor.
• Drive System: Variable frequency drives control drum and screw speeds independently. Typical power: 15-75 HP total. Torque monitoring prevents overload during upset conditions.
• Polymer Feed Integration: Inline mixers or flash-mix chambers condition sludge before dewatering. Critical for optimizing polymer dose (2-8 lbs/dry ton typically).

• Hydraulic Loading Rate: 2-8 gpm/ft² (typical 4-6 gpm/ft²)
• Solids Loading Rate: 15-40 lbs DS/hr/ft² (typical 20-30 lbs DS/hr/ft²)
• Feed Solids Concentration: 0.5-3.0% (typical 1.0-2.0%)
• Cake Solids: 12-20% (typical 15-18%)
• Filtrate Turbidity: <10 NTU (with proper polymer dosing)
• Polymer Dose: 4-12 lbs/ton DS (typical 6-8 lbs/ton DS)
• Drum Diameter: 6-12 feet (municipal standard 8-10 feet)
• Drum Length: 8-20 feet (varies by capacity)
• Filter Media: 150-300 mesh stainless steel or synthetic
• Vacuum Level: 15-25 inches Hg
• Drum Speed: 0.1-2.0 rpm (typical 0.5-1.0 rpm)
• Wash Water: 1-3 gpm/ft² of drum surface
• Sludge Flow: 50-2,000 gpm
• Solids Processing: 500-15,000 lbs DS/day
• Filtrate Production: 95-98% of feed volume

• What is the required cake dryness versus energy consumption trade-off? Target cake solids of 15-18% achievable with standard vacuum (20" Hg) and 0.5-1.0 rpm drum speed. Higher dryness (>18%) requires increased vacuum (25" Hg) and slower speeds (0.3 rpm), increasing power consumption by 20-30%. Insufficient dryness (<15%) creates handling and disposal cost penalties exceeding $50/dry ton.
• How will seasonal sludge characteristics affect sizing margins? Design for worst-case winter conditions when sludge dewaterability drops 15-25%. Summer performance at 6 gpm/ft² may require winter deration to 4 gpm/ft². Undersized units cannot compensate with polymer addition alone. Size for 125% of average annual loading.
• What level of automation is justified for operational reliability? Basic manual operation suitable for <1 MGD plants with dedicated operators. Automated polymer feed control and drum speed adjustment essential for >5 MGD facilities or limited staffing. Full PLC integration adds $75,000-150,000 but reduces operator requirements by 50%.
• How does influent variability affect media selection and cleaning systems? High grease/oil content (>500 mg/L) requires synthetic media and heated wash systems. Standard 200-mesh stainless adequate for typical municipal sludge. Inadequate washing reduces capacity 10-15% annually.

Primary

• Material/Equipment Verification: Verify 316SS construction for all wetted parts, Confirm polymer injection system compatibility, Check belt material specifications (polyester vs. polypropylene)
• Installation Requirements: 12-18 month lead times typical, Concrete pads require 28-day cure before equipment setting, Overhead crane access essential for belt replacement
• Field Challenges: Belt tracking alignment critical during startup, Polymer feed line routing affects performance, Washwater pressure requirements often underestimated
• Coordination Issues: HVAC sizing impacts odor control effectiveness, Electrical load calculations frequently revised upward

• Andritz - SEPARATION Belt Filter Press BFP series, strong municipal references including 25 MGD Denver Metro plants
• Huber Technology - RoS3 Rotary Drum Thickener, installed at numerous 5-50 MGD facilities
• Parkson Corporation - AquaKlear belt systems for smaller municipals
• Alfa Laval - ALDRUM rotary units, though more industrial-focused than municipal

• Centrifuges - Higher capital cost ($800K vs $400K) but better for variable flows, preferred at plants >20 MGD
• Gravity Belt Thickeners - Lower cost ($200K) for thickening only, 4-6% solids typical
• Dissolved Air Flotation - Similar capital cost but requires more operator attention, better for facilities with existing DAF experience
• Rotary presses offer middle-ground performance and maintenance requirements

Establish direct relationships with manufacturer field service teams early - they provide invaluable startup support and troubleshooting expertise. Budget 15-20% additional polymer costs during first year while operators optimize dosing. Consider purchasing spare belts during initial procurement to avoid premium pricing later. Many plants achieve 10-15% better solids capture by upgrading polymer mixing systems beyond standard manufacturer packages.