Automatic Straining Equipment

Automatic straining equipment removes suspended solids and debris from water streams through continuous mechanical screening without manual intervention. These systems typically employ rotating drums, traveling screens, or step screens with automated cleaning mechanisms that backwash or rake collected material into hopings or conveyors. Municipal installations commonly achieve 85-95% removal efficiency for particles larger than the screen opening size, typically ranging from 1-6mm mesh openings. The primary trade-off involves higher capital and maintenance costs compared to static screens, but this investment eliminates manual cleaning labor and provides consistent performance during peak flow events and storm conditions.

• Raw Water Intake Protection - Installed downstream of intake pumps, upstream of chemical feed systems. Removes debris, leaves, and fish that bypass intake screens. Selected for 24/7 operation without flow interruption. Typical 0.25-2.0mm openings protect downstream equipment from damage and reduce chemical consumption.
• Backwash Water Recovery - Positioned after filter backwash holding tanks, upstream of return pumps. Captures media particles and debris before water returns to plant headworks. Chosen for ability to handle high solids loading (500-2000 mg/L) while maintaining continuous flow during peak backwash periods.
• Effluent Polishing - Located after secondary clarifiers, before disinfection contact chambers. Removes residual suspended solids and biological flocs that could interfere with UV or chlorine disinfection. Selected for fine screening (0.5-1.0mm) to achieve consistent effluent quality below 10 mg/L TSS.
• Process Water Reuse - Installed upstream of plant service water systems protecting cooling water pumps, washdown systems, and laboratory equipment from process carryover.

Daily Operations - Monitor differential pressure across screen (typically 6-12 inches), verify drum rotation and spray wash cycles. Check solids discharge consistency and adjust rotation speed based on loading. Log power consumption and wash water usage. Typical attention required 2-3 times per shift during normal loading.

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Maintenance - Weekly inspection of spray nozzles for plugging, monthly bearing lubrication, quarterly drive belt tension checks. Screen media replacement every 2-5 years depending on abrasion. Requires confined space entry procedures for internal inspection. Basic mechanical skills sufficient for routine maintenance; manufacturer service recommended for major repairs.

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Troubleshooting - Excessive differential pressure indicates screen blinding or spray system failure. Irregular rotation suggests bearing wear or drive problems. Solids carryover indicates worn screen media or inadequate wash pressure. Warning signs include unusual noise, vibration, or power draw changes. Expected service life 15-20 years for mechanical components, 5-10 years for screen media in typical municipal applications.

• Rotating Drum Screen - Perforated stainless steel (316L) cylinder, 2-8 feet diameter, with 0.25-3.0mm openings. Rotates at 1-6 RPM driven by variable speed motor. Sizing based on hydraulic loading (5-15 gpm/ft²) and solids concentration.
• Spray Wash System - High-pressure nozzles (80-120 PSI) with automated sequencing. Stainless steel headers with self-cleaning capability. Flow rates 10-25% of process flow, typically using filtered effluent or potable water supply.
• Solids Removal Mechanism - Internal flights or external brush systems transport captured solids to discharge chute. Materials include UHMW polyethylene or stainless steel. Sized for anticipated solids loading (5-50 lbs/day typical).
• Level Control System - Upstream/downstream level sensors maintain consistent head differential (6-18 inches typical). Controls drum rotation speed and wash cycle frequency based on differential pressure or timer settings.
• Drive Assembly - Gear-reduced motors (0.5-5 HP) with variable frequency drives. Includes torque monitoring for jam detection and emergency stops.

• Flow Rate Requirements: Capacity range: 0.2-150 MGD (municipal applications typically 1-50 MGD), Peak flow multiplier: 2.0-3.0x average daily flow, Minimum turndown ratio: 3:1 for variable speed units
• Screen Opening Size: Coarse screens: 6mm-25mm (0.25"-1.0"), Fine screens: 1.5mm-6mm (0.06"-0.25"), Micro screens: 0.5mm-2mm (0.02"-0.08")
• Hydraulic Parameters: Approach velocity: 0.6-1.2 m/s (2-4 ft/s), Through-screen velocity: 0.3-0.9 m/s (1-3 ft/s), Maximum headloss (clean): 150-300mm (6-12 inches), Cleaning initiation headloss: 300-600mm (12-24 inches)
• Physical Constraints: Channel width: 0.6-6.0m (2-20 feet), Installation angle: 60-85 degrees from horizontal, Minimum upstream approach: 3x channel width, Screenings removal rate: 0.5-15 m³/ML treated (2-60 ft³/MG)
• Power Requirements: Screen drive: 1-15 kW depending on size, Wash water pressure: 550-690 kPa (80-100 psi), Wash water flow: 2-5% of through-screen flow

• What screen opening size is required based on downstream equipment protection needs? Fine screens (1.5-6mm) protect membranes and UV systems but cost 2-3x more than coarse screens (6-25mm). Micro screens (0.5-2mm) may eliminate primary clarification but require 40-60% more headroom and higher maintenance. Decision requires downstream equipment analysis and lifecycle cost evaluation including power, maintenance, and screenings handling.
• Should the installation be in-channel or in a separate structure? In-channel installations save 30-50% on civil costs but limit access for maintenance and future modifications. Separate channels allow redundancy and easier maintenance but increase footprint by 40-60%. Decision depends on available space, redundancy requirements, and maintenance philosophy. Flow rates above 25 MGD typically require separate channels.
• What level of automation and remote monitoring is justified? Basic systems with local control cost $150-250K while fully automated systems with SCADA integration cost $300-450K. Remote monitoring prevents overflow events (avg. fine $50-150K) but increases complexity. Decision requires evaluation of staffing levels, response times, and regulatory compliance requirements.
• How will screenings be handled and disposed of? Screenings volume ranges from 0.5-15 m³/ML. Washing/compaction reduces volume 60-80% but adds equipment cost ($75-150K). Direct disposal costs $200-400/tonne while processed screenings

• Material/Equipment Verification: Verify 316SS construction for potable water applications, Confirm ANSI flange ratings match system pressure class, Check backwash flow requirements against available capacity
• Installation Requirements: Minimum 10 pipe diameters upstream, 5 downstream for proper flow distribution, Adequate clearance for element removal (typically 1.5x housing length), Level concrete pad with anchor bolt template
• Field Challenges: Electrical connections often require local disconnect and NEMA 4X enclosure, Backwash piping routing frequently conflicts with other utilities
• Coordination Issues: Coordinate backwash disposal with plant hydraulics early in design, Lead times typically 12-16 weeks for municipal-grade units

• Lakos - Model ASF-Series automatic self-cleaning strainers, widely used in municipal applications up to 3,000 GPM per unit.
• Eaton Filtration - Model 9000 Series automatic backwash strainers, common in water treatment plants from 1-50 MGD capacity.
• Bollfilter - Model 6.18 automatic strainers, popular for municipal raw water intake protection.
• Amiad Water Systems - Model SAF automatic self-cleaning filters, frequently specified for municipal pretreatment applications.

• Static wedge wire screens - Lower cost ($15,000-25,000 vs $35,000-50,000), but require manual cleaning. Preferred for seasonal applications or where labor availability isn't constrained.
• Bag filters with automatic changeout - Similar cost, better fine particle removal, higher maintenance. Good for applications requiring <50 micron filtration.
• Traveling water screens - Much higher cost ($150,000+) but handle larger debris loads. Preferred for raw water intakes with heavy debris loading exceeding 100 lbs/day.

Size backwash pumps conservatively - municipal installations often see higher differential pressures than manufacturer curves suggest due to variable debris loading. Establish service agreements during commissioning; most manufacturers offer 2-year extended warranties that include annual PM visits. Consider duplex configurations for critical applications rather than oversizing single units. Lakos and Eaton provide excellent field support, while some manufacturers require factory technicians for startup, adding $3,000-5,000 to project costs.