Inclined Screw Presses

Inclined Screw Presses serve as the primary dewatering equipment for biosolids in municipal wastewater treatment plants, removing water from thickened sludge through mechanical compression and drainage. The unit consists of a slowly rotating screw conveyor (typically 1-3 RPM) within a perforated cylindrical screen, where sludge enters at the lower end and travels upward against gravity while water drains through the screen openings. These systems typically achieve 18-25% dry solids content from feed sludge at 3-6% solids concentration. The primary trade-off involves higher capital costs compared to belt filter presses, but with significantly lower polymer consumption and reduced operator attention requirements.

• Primary Sludge Thickening: Installed downstream of primary clarifiers, inclined screw presses concentrate primary sludge from 2-4% to 8-12% solids. Selected for continuous operation and ability to handle fibrous materials without plugging. Thickened sludge feeds digesters or belt filter presses.

• WAS Thickening: Processes waste activated sludge from secondary clarifiers, increasing solids from 0.8-1.2% to 4-6%. Chosen over gravity thickeners for smaller footprint and consistent performance. Polymer addition upstream improves capture rates to 95%+.

• Scum Thickening: Handles primary scum and DAF float, reducing volume by 85-90% before disposal or digestion. Selected for ability to process variable consistency materials and self-cleaning action that prevents buildup.

• Digested Sludge Dewatering: Light-duty dewatering of digested sludge to 18-22% solids before composting or landfill disposal in plants under 5 MGD where belt presses aren't economical.

Daily Operations: Operators monitor inlet flow rates, polymer feed rates, and discharge solids content hourly. Torque readings indicate loading conditions - high torque suggests heavy feed or worn components. Filtrate turbidity provides immediate feedback on polymer dosing effectiveness. Speed adjustments optimize solids capture versus cake dryness.

​

Maintenance: Weekly bearing lubrication and daily visual inspections for unusual wear or vibration. Monthly polymer system cleaning prevents buildup. Screen replacement every 2-3 years depending on abrasive content. Requires confined space entry procedures and lockout/tagout. Maintenance staff need basic mechanical skills for bearing replacement and belt tensioning.

​

Troubleshooting: Excessive filtrate turbidity indicates insufficient polymer or worn screens. High torque with low throughput suggests plugged screens or worn flights. Unusual vibration warns of bearing failure or shaft misalignment. Typical service life 15-20 years with proper maintenance, though screens may require replacement every 24-36 months in high-grit applications.

• Perforated Cylindrical Screen: 304/316 stainless steel with 0.5-3.0mm slot openings. Screen diameter ranges 200-1200mm for municipal applications. Slot size selected based on sludge type - finer slots for WAS, coarser for primary sludge.

• Internal Screw Conveyor: Single or dual-shaft design with variable pitch flights. Constructed from stainless steel or mild steel with hardface welding. Pitch decreases toward discharge to increase compression. Speed typically 1-8 RPM.

• Drive Assembly: Gear reducer with 10-50 HP motor for municipal sizes. Torque monitoring prevents overload damage. Variable frequency drives provide speed control for optimizing performance across varying feed conditions.

• Polymer Feed System: Dilution and metering equipment for flocculant addition. Critical for achieving target solids concentrations and filtrate quality. Dosing rates typically 2-8 lbs/dry ton.

• Flow Capacity: 5-500 gpm per unit (typical municipal range 25-200 gpm)

• Solids Loading Rate: 8-25 lbs DS/hr/ft² screen area

• Inlet Solids Concentration: 0.5-6% by weight

• Cake Dryness: 15-35% solids (typical 20-28% for municipal sludge)

• Screw Speed: 1-8 rpm (variable frequency drive controlled)

• Screen Openings: 0.5-3.0mm (typically 1.0-2.0mm for municipal applications)

• Capture Rate: 85-95% solids removal

• Washwater Requirements: 2-8 gpm per unit

• Power Requirements: 1-15 HP per unit (typically 3-7.5 HP)

• Inlet Pressure: 5-25 psig maximum

• Materials: 304/316 stainless steel construction standard

• Footprint: 4-12 ft length, 2-4 ft width per unit

• Inclination Angle: 25-35 degrees from horizontal

• Polymer Dosing: 2-8 lbs active polymer per dry ton solids

• Throughput per Unit: 50-400 lbs DS/hr typical range

• Single vs. Multiple Unit Configuration? Threshold: >150 gpm or >300 lbs DS/hr typically requires multiple units. Single units limit redundancy and maintenance flexibility. Need peak flow data, redundancy requirements, and maintenance philosophy. Wrong decision creates capacity bottlenecks or excessive capital costs.

• Screen Opening Size Selection? Threshold: 1.0mm for primary sludge, 1.5-2.0mm for waste activated sludge, 0.75mm for mixed sludge. Affects capture rate vs. blinding frequency. Need sludge characterization data and acceptable solids loss limits. Wrong selection causes poor capture (<85%) or excessive maintenance.

• Polymer System Integration Requirements? Threshold: >0.5% inlet solids requires polymer conditioning. Affects mixing tank sizing, dosing pump capacity, and dilution water needs. Need polymer jar testing results and storage/handling constraints. Inadequate polymer system reduces cake dryness by 5-10 percentage points.

• Washwater Source and Pressure Requirements? Threshold: 40-60 psig optimal, minimum 25 psig. Plant water vs. effluent affects corrosion and biological growth. Need available pressure data and water quality analysis. Insufficient pressure reduces screen life and increases maintenance frequency.

• Primary: Division 46 23 61 - Sludge Thickening and Dewatering Equipment

• Secondary: Division 40 32 00 - Mechanical Dewatering Equipment (older format)

• Standard specification covers mechanical dewatering equipment for municipal wastewater treatment applications including performance criteria and testing requirements

• Material/Equipment Verification: Verify 316SS construction for all wetted parts, Confirm polymer feed system compatibility and calibration certificates, Review electrical specifications for VFD integration

• Installation Requirements: Concrete pad with 6-8 inch thick reinforcement for vibration control, Overhead crane access (minimum 2-ton capacity for maintenance), Dedicated polymer feed lines with backflow prevention

• Field Challenges: Alignment criticality requires precision millwright work, Polymer dilution water quality affects performance significantly

• Coordination Issues: 16-20 week lead times typical; coordinate with building construction early, Electrical/controls integration requires early vendor coordination meetings

• HUBER Technology: ROTAMAT® Screw Press RoS3Q series (municipal installations at 50+ US plants)

• Kuhn North America: Disruptor® D-Series models D6-D12 (proven at facilities like Aurora, CO)

• ANDRITZ-SEPARATION: SEPARATION Screw Press S-Series (installations across Canada/US)

• Jacopa: JWI Screw Press systems (growing municipal market presence, competitive pricing)

• Belt Filter Presses: Better for variable sludge types, higher capital cost (~30% more), require more operator attention. Preferred when space allows and labor costs are low.

• Centrifuges: Higher dewatering efficiency (22-28% solids), but 2-3x operating costs due to energy and polymer consumption. Choose for space-constrained sites or when cake disposal costs dominate.

• Gravity Belt Thickeners: Lower capital cost (~40% less) but only achieve 4-6% solids concentration, requiring downstream dewatering.

Polymer Optimization: Start commissioning at 50% of design polymer dose - many plants over-dose initially, creating sticky cake and higher operating costs. Work directly with manufacturer's field service during startup; their experience prevents months of optimization struggles.

​

Maintenance Access: Specify removable floor grating around discharge chute rather than fixed concrete - saves hours during routine cleaning and allows better cake consistency monitoring.