Disc Screens

Disc screens provide primary screening for municipal wastewater treatment by removing large solids and debris through rotating perforated discs. Multiple discs mounted on horizontal shafts rotate partially submerged in the flow channel, with solids trapped on the disc surfaces and continuously scraped off by fixed brushes or spray systems. Typical installations achieve 85-95% removal efficiency for solids larger than the disc perforations (typically 6-20mm). The primary trade-off is higher capital and maintenance costs compared to static bar screens, requiring regular brush replacement and more complex mechanical systems for similar screening performance.

• Primary Influent Screening (0.5-10 MGD): Disc screens serve as the first mechanical treatment stage, removing debris 6mm and larger before primary clarifiers. Located after bar screens but before grit removal, they protect downstream pumps and reduce organic loading. Selected for lower headloss (6-12 inches) compared to step screens and ability to handle variable flows without plugging.

• Secondary Effluent Polishing (2-25 MGD): Positioned after secondary clarifiers to capture residual solids and biofilm fragments before disinfection. Critical for plants with strict TSS discharge limits (<10 mg/L). Chosen over cloth filters when maintenance simplicity outweighs slightly lower removal efficiency.

• Sidestream Applications (1-50 MGD): Used on digester supernatant, belt press filtrate, and centrifuge centrate streams containing high solids concentrations. Protects return activated sludge systems from debris that could damage RAS pumps or clog diffusers.

Daily Operations: Operators monitor amperage draw on drive motors, typically 40-60% of nameplate during normal operation. Visual inspection confirms proper disc rotation, spray wash activation, and screenings discharge. Flow-paced operation adjusts automatically, but manual override allows increased wash frequency during high debris periods or storm events.

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Maintenance: Weekly lubrication of drive gearboxes and bearing assemblies using food-grade lubricants. Monthly inspection of spray nozzles for plugging, requiring removal and cleaning. Quarterly belt tension checks and annual gearbox oil changes. Standard PPE includes safety glasses, gloves, and slip-resistant footwear. Lockout/tagout procedures required for any mechanical work.

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Troubleshooting: High amperage indicates disc plugging or mechanical binding, resolved through increased wash frequency or manual cleaning. Reduced screening efficiency suggests worn wedge wire (5-7 year replacement cycle) or inadequate wash pressure. Bearing failure typically announces itself through increased vibration and noise, requiring immediate shutdown to prevent shaft damage.

• Rotating Disc Assembly: Stainless steel 316 discs (4-12 feet diameter) with wedge wire or perforated openings (1-6mm). Rotation speed 1-3 RPM creates differential screening action. Disc quantity (6-20 discs) determines screening capacity and redundancy.

• Drive System: Gear motor assemblies (1-5 HP) with variable frequency drives for flow-proportional operation. Includes torque monitoring for debris detection and automatic reversal capabilities during high loading events.

• Spray Wash System: High-pressure nozzles (80-120 PSI) with automated sequencing remove captured solids. Wash water consumption 2-5% of throughput flow, with backwash pumps sized for 50-100 GPM depending on screen size.

• Collection Trough: Stainless steel housing captures screenings for conveyance to containers or compactors. Includes level sensors and wash-down connections for routine cleaning.

• Flow Rate Capacity: 0.5-15 MGD per unit for municipal applications, with approach velocities of 2.5-4.0 ft/s through the screen opening to prevent blinding.

• Screen Opening Size: 2-6mm typical for primary screening applications. Smaller openings (2-3mm) for enhanced solids removal; larger openings (4-6mm) for high-flow, debris-heavy applications.

• Hydraulic Loading Rate: 15-25 gpm/ft² of effective screen area for continuous operation. Peak loading up to 35 gpm/ft² for short duration storm events.

• Solids Loading: Design for 2-8 ft³ screenings per MG treated, varying by collection system characteristics and upstream pretreatment.

• Head Loss: Clean screen head loss typically 6-12 inches. Maximum allowable head loss before backwash activation: 18-24 inches to maintain hydraulic capacity.

• Backwash Requirements: 50-150 gpm per unit at 40-60 psi, consuming 0.5-2% of plant flow depending on solids loading and cleaning frequency.

• Power Requirements: 3-15 HP per unit including drive motor and backwash pump. Higher power for larger units and frequent cleaning cycles.

• Removal Efficiency: 85-95% removal of particles >3mm, 70-85% removal of particles >2mm under typical municipal loading conditions.

• What solids loading and debris characteristics define cleaning frequency requirements? Plants with combined systems or high I&I may require cleaning every 15-30 minutes versus 1-4 hours for separated systems. Underestimating cleaning frequency leads to excessive head loss, bypass events, and premature equipment wear. Need detailed influent characterization including seasonal variations.

• Should the system operate as duty/standby or parallel flow configuration? Single units handling >8 MGD create significant bypass risk during maintenance. Parallel configuration provides redundancy but requires flow splitting structures. Wrong decision impacts capital costs by 25-40% and operational reliability. Consider peak flow capacity, maintenance windows, and bypass capabilities.

• What screen opening size optimizes removal efficiency versus maintenance burden? 2mm openings achieve 90%+ removal but increase cleaning frequency 3-5x versus 4mm openings. Smaller openings generate 40-60% more screenings volume requiring additional handling capacity. Balance removal targets against operational complexity and disposal costs.

• How does backwash water quality affect cleaning effectiveness and system reliability? Using plant effluent reduces cleaning effectiveness 15-25% versus potable water but eliminates external water supply requirements. Poor backwash quality accelerates disc wear and increases cleaning frequency. Consider water source reliability, quality, and long-term maintenance implications.

• Primary: Division 46 - Water and Wastewater Equipment, Section 46 21 13 - Screening Equipment

• Secondary: Division 40 - Process Integration (for control systems integration), Division 43 - Process Gas and Liquid Handling (for backwash systems)

• Material/Equipment Verification: Verify 316SS construction for all wetted parts, Confirm drive motor IP65 rating minimum, Check screen opening tolerance (±0.5mm typical)

• Installation Requirements: Concrete channel tolerances ±6mm critical, Overhead crane access for maintenance, 480V/3-phase power with VFD capability

• Field Challenges: Channel modifications often required for retrofits, Alignment issues with precast channels, Access limitations in existing headworks

• Coordination Issues: 16-20 week lead times typical, Early electrical rough-in coordination essential

• Headworks International - BioDisc BD series, widely used in 1-20 MGD plants with 6mm standard spacing

• Huber Technology - ROTAMAT RoDisc series, popular for larger installations up to 50+ MGD

• Lakeside Equipment - Raptor screens with disc configurations, common in Midwest municipal plants

• JWC Environmental - Monster disc screens, gaining market share in smaller municipal applications

• Static wedge wire screens - Lower capital cost (30-40% less), no moving parts, but higher headloss and manual cleaning requirements. Preferred for small plants <2 MGD.

• Rotating drum screens - Similar performance, 15-20% higher cost, better for high-grease applications. Common in food processing communities.

• Step screens - Higher capacity per footprint, 25% higher cost, preferred for major retrofits with space constraints.

Establish relationships with local manufacturer reps early - they often provide free preliminary sizing and can expedite warranty issues. Consider oversizing disc diameter by one size increment; the marginal cost increase (10-15%) provides significant operational flexibility for peak flows. Most successful installations include redundant units rather than single large screens, improving reliability and maintenance scheduling in municipal environments.