Rotary Drum Screens

Rotary drum screens are rotating cylindrical screening devices that remove coarse solids from raw wastewater through continuous mechanical filtration. The perforated or wedge-wire drum rotates slowly (2-6 RPM) while partially submerged in the flow, capturing debris on the exterior surface as water passes through the screen openings. These units typically achieve 15-25% BOD removal and 20-35% TSS removal at headworks applications. While effective for solids capture with minimal operator attention, rotary drum screens require significant headroom (12-16 feet typical) and have higher capital costs than static bar screens, making them most suitable for larger plants with space constraints and consistent debris loading.

• Headworks Screening (0.5-10 MGD): Rotary drum screens serve as primary screening at smaller plants, removing 6mm+ debris before grit removal. They're selected for their compact footprint and automated operation compared to bar screens. Typical installation follows influent pumping with downstream grit chambers or vortex separators.

• Tertiary Filtration (2-50 MGD): Post-secondary clarifier applications for enhanced TSS removal before disinfection. Units achieve 20-40% additional TSS reduction, critical for plants approaching NPDES limits. Flow ranges from 0.5-5.0 MGD per unit with multiple parallel installations.

• Sludge Thickening (1-25 MGD): WAS thickening applications where drum screens concentrate solids from 0.5% to 2-4% before anaerobic digestion. Selected for continuous operation and lower polymer consumption versus dissolved air flotation.

• Effluent Polishing: Final solids removal before UV disinfection or membrane systems, protecting downstream equipment from debris that could cause fouling or UV shadowing.

Daily Operations: Operators monitor drum rotation speed, spray wash pressure, and differential levels across the screen. Visual inspection for unusual debris accumulation or spray nozzle plugging occurs during routine rounds. Flow distribution adjustments maintain even loading across multiple units. Screenings removal rates typically range 0.5-2.0 cubic yards per MG treated.

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Maintenance: Weekly bearing lubrication and spray nozzle inspection prevent most failures. Monthly drive chain/belt tension checks and quarterly bearing replacement (depending on manufacturer - SKF bearings typically last 8,000-12,000 hours). Confined space entry required for internal drum inspection. Standard PPE includes hard hat, safety glasses, and non-slip footwear. Mechanical aptitude sufficient for most tasks.

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Troubleshooting: Excessive differential head indicates screen blinding from grease or fine debris - increase spray frequency or pressure. Unusual vibration signals bearing wear or debris lodged in drum. Drive motor overload trips suggest excessive solids loading or mechanical binding. Screen media replacement typically required every 5-8 years

• Rotating Drum Assembly: Stainless steel 304/316 construction with wedge wire screens, typically 0.5-3.0mm openings. Drum diameters range 4-12 feet for municipal applications, with rotational speeds 1-6 RPM. Selection based on hydraulic loading (50-150 gpm/ft²) and solids concentration.

• Drive System: Gear motor assemblies (0.5-5 HP) with variable frequency drives for speed control. Includes torque monitoring for automated operation and overload protection during high solids loading events.

• Spray Wash System: High-pressure wash (60-100 PSI) with filtrate or plant water, consuming 2-5% of throughput flow. Nozzle configuration varies by manufacturer - Huber, Parkson, and Headworks offer different spray patterns for optimized cleaning.

• Collection Trough: Captures screenings with integrated screw conveyors (6-12 inch diameter) for transport to disposal. Constructed from stainless steel with drainage provisions for dewatering.

• Level Control: Upstream/downstream level sensors maintain consistent submergence (typically 65-75% drum diameter) for optimal hydraulic performance and solids capture efficiency.

• Flow Parameters:

• Design flow rate: 0.1-25 MGD per unit (typical municipal range)
• Peak flow capacity: 2.5-3.0x design flow
• Approach velocity: 2-4 ft/sec maximum to prevent blinding
• Through-screen velocity: 0.5-1.5 ft/sec optimal

• Physical Specifications:

• Drum diameter: 6-12 feet (larger diameters for higher flows)
• Screen length: 8-20 feet (L/D ratio typically 1.5-2.5:1)
• Opening size: 1-6mm (municipal primary: 3-6mm, tertiary: 1-3mm)
• Submergence depth: 40-60% of drum diameter
• Rotation speed: 1-3 RPM (variable frequency drive controlled)

• Loading Criteria:

• Hydraulic loading: 5-15 GPM/ft² of submerged screen area
• Solids loading: 50-200 lbs/day per foot of screen width
• Headloss: 6-12 inches through clean screen at design flow
• Backwash pressure: 15-30 PSI, flow rate 2-5 GPM/ft² of screen area

• Performance Metrics:

• Solids removal: 10-30% TSS (primary), 50-85% TSS (tertiary applications)
• Screenings production: 0.5-2.0 ft³/MG treated
• Power consumption: 0.5-2.0 HP per MGD capacity

• What opening size matches your treatment objectives and downstream equipment protection requirements? Primary screening (3-6mm) removes rags and large debris; tertiary applications (1-3mm) provide enhanced solids removal. Smaller openings increase removal efficiency but require more frequent cleaning and higher differential pressure. Decision impacts: Wrong size causes either inadequate protection or excessive maintenance. Need: Influent characterization study and downstream equipment specifications.

• Should you specify internal or external drum configuration based on site hydraulics and maintenance access? Internal feed (flow enters drum center) suits gravity applications with 6-8 feet available head; external feed handles higher flows with less headloss but requires larger structures. Decision impacts: Wrong choice causes hydraulic problems or construction cost overruns. Need: Site hydraulic profile and available space constraints.

• What level of automation and redundancy fits your staffing and reliability requirements? Basic units with manual spray wash versus fully automated systems with backwash optimization, differential pressure monitoring, and emergency bypass. Decision impacts: Under-automation causes operational problems; over-automation wastes capital on small plants. Need: Staffing levels, criticality assessment, and O&M budget analysis.

• How do local solids disposal regulations affect screenings handling and dewatering requirements? Screenings compaction (50-60% solids) versus washing systems (35-40% solids) significantly impacts disposal costs and truck traffic. Decision impacts: Wrong choice affects long-term operating costs and permit compliance. Need: Local disposal facility requirements and transportation logistics.

• Material/Equipment Verification

• Verify 316SS construction for all wetted parts
• Confirm screen opening size matches design specifications
• Check drive motor sizing and VFD compatibility

• Installation Requirements

• Requires 8-12 week lead times for custom configurations
• Needs precise channel dimensions and elevation control
• Requires 480V/3-phase power and compressed air supply

• Field Challenges

• Channel modifications often required for retrofits
• Debris handling system integration critical
• Access for maintenance frequently overlooked

• Coordination Issues

• Screen manufacturer, channel contractor, and debris handling vendor coordination essential

• Huber Technology - ROTAMAT RoK 4 series, widely used in 1-50 MGD plants

• Xylem/Sanitaire - RotaDrum screens, popular for retrofit applications

• Lakeside Equipment - Raptor FalconRake systems, common in packaged plants

• WAMGROUP - WAM ROTOTEC series, growing presence in smaller municipal facilities

• Static wedge wire screens - Lower capital cost, no power required, suitable for smaller flows <5 MGD, but higher maintenance and potential for blinding.

• Step screens - Better debris capture, handle higher solids loading, preferred for combined sewer systems, typically 20-30% higher cost.

• Perforated plate screens - Lowest cost option, manual cleaning acceptable for <1 MGD plants, minimal automation requirements.

Establish relationships with local manufacturer reps early - they provide valuable application engineering support and can expedite warranty issues. Consider standardizing on one manufacturer across multiple projects for parts commonality and operator training efficiency. Negotiate spare parts packages during initial purchase when pricing leverage is highest. Request factory startup and training as part of equipment purchase rather than separate service contracts.