Automatic Backwash Cloth Filters

Automatic backwash cloth filters provide tertiary filtration in municipal wastewater treatment plants, removing suspended solids to achieve effluent TSS levels below 5 mg/L. The system operates by passing secondary effluent through synthetic cloth media panels that capture particles, while automated backwash sequences reverse flow direction to clean the fabric without manual intervention. Typical installations handle 1-20 MGD flows with filtration rates of 2-4 gpm/sf, achieving 85-95% TSS removal efficiency. The primary limitation is vulnerability to grease and oil fouling, which can blind the cloth media and require costly replacement, making upstream screening and degritting critical for reliable long-term operation.

• Tertiary Filtration for TSS Polishing: Primary application following secondary clarifiers in 2-25 MGD plants requiring <10 mg/L TSS effluent. Cloth filters achieve 5-8 mg/L TSS consistently with 2-4 gpm/sf loading rates. Selected over sand filters for lower footprint (60% smaller) and reduced backwash water consumption (1-3% vs 5-8%)
• Phosphorus Removal Enhancement: Used downstream of chemical precipitation systems where coagulant addition creates fine floc requiring polishing. Typical installations handle 0.5-15 MGD flows, achieving 0.1-0.3 mg/L total phosphorus when combined with ferric chloride or alum dosing upstream
• Peak Flow Management: Installed parallel to existing filters to handle wet weather flows up to 2x design capacity. Common in 5-50 MGD plants with combined sewer systems, providing consistent effluent quality during storm events when conventional filters become hydraulically limited
• Membrane Pre-treatment: Protects downstream RO/MF systems by removing particles >10 microns. Reduces membrane fouling rates by 40-60% in water reuse applications, extending cleaning cycles from 30 to 45-60 days

Daily Operations: Operators monitor headloss gauges (target 6-8 inches), flow rates, and turbidity meters during routine rounds. Backwash frequency automatically adjusts based on loading, typically every 20-45 minutes during peak flows. Visual inspection of cloth condition and spray nozzle alignment takes 5-10 minutes per unit. Solids removal requires daily attention with manual rake-out or automated conveyor operation.

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Maintenance: Weekly cloth inspection for tears or blinding, monthly spray nozzle cleaning, and quarterly bearing lubrication. Cloth replacement every 12-18 months costs $8,000-15,000 per unit depending

• Filter Cloth Panels: Woven polypropylene or polyester fabric with 10-20 micron openings, sized 6-8 feet wide by 8-12 feet long per panel. Municipal plants typically use 8-24 panels per unit. Cloth selection based on chemical compatibility - polypropylene for chlorinated effluent, polyester for non-chlorinated applications
• Backwash System: Includes spray nozzles (316 SS), backwash pumps (5-15 HP), and filtrate storage tank (15-30 minutes detention). Backwash pressure typically 40-60 psi with 0.5-1.0 gpm/sf cloth area. Automated based on headloss (6-12 inches) or time intervals (15-60 minutes)
• Drive Mechanism: Motorized drum rotation system (0.25-1.0 HP) with variable speed control (0.1-2.0 rpm). Stainless steel construction with sealed bearings rated for continuous outdoor operation. Chain and sprocket drives preferred over belt systems for reliability
• Solids Handling: Includes dewatering trough, solids pump (3-10 HP centrifugal), and waste storage hopper. Captures 200-800 lbs/day solids in typical 5 MGD applications. Solids concentration typically 2-4% without polymer addition

• Flow Rate Capacity: 0.1-25 MGD per unit, with multiple units for larger plants. Standard municipal sizes: 1, 2.5, 5, 10, 15, 20 MGD
• Hydraulic Loading Rate: 2-8 gpm/ft² of cloth area, typically 4-6 gpm/ft² for tertiary filtration. Higher rates (6-8 gpm/ft²) acceptable for secondary effluent polishing
• Filtration Media: 10-20 micron polyester or polypropylene cloth. Pore size selection based on target TSS removal (10 micron for <5 mg/L effluent TSS)
• Operating Head Loss: 2-8 feet clean, 8-15 feet dirty before backwash initiation. Design for maximum 20 feet total system head
• Backwash Requirements: 15-30 seconds duration, 15-25 gpm/ft² cloth area. Backwash water consumption 0.5-2% of forward flow
• Solids Loading: 10-50 mg/L TSS influent typical. Maximum design loading 100 mg/L TSS for secondary effluent applications
• Detention Time: 2-5 minutes in filter chamber at design flow
• Materials: 316SS wetted parts, carbon steel structure with protective coating. Cloth tensioning systems rated for 10+ year service life

• Single vs. Multiple Unit Configuration? Above 10 MGD, specify multiple units for redundancy. Single units acceptable below 5 MGD if bypass capability exists. Wrong decision eliminates maintenance flexibility and creates single point of failure. Need: peak flow analysis, redundancy requirements, site constraints
• Cloth Media Selection: 10 or 20 micron? 10 micron achieves <5 mg/L TSS but requires more frequent backwash and higher head loss. 20 micron suitable for <10 mg/L targets with lower O&M. Wrong choice impacts permit compliance and operating costs. Need: effluent TSS requirements, influent characteristics, energy costs
• Backwash Trigger: Head Loss or Time-Based? Head loss control (typically 8-12 feet) optimizes cloth utilization but requires reliable instrumentation. Fixed interval (2-6 hours) simpler but may waste backwash water or allow excessive fouling. Need: influent variability, instrumentation reliability preferences, operator staffing levels
• Cloth Tensioning: Manual or Automatic? Automatic tensioning adds $15,000-25,000 but reduces maintenance and improves performance consistency. Manual acceptable for smaller plants with dedicated staff. Need: maintenance capabilities, budget constraints, performance criticality

• Division 40 - Process Integration
• Section 40 23 13 - Cloth Disk Filters
• Primary specification location covering complete filter systems including drives, controls, and backwash equipment
• Cross-reference Section 40 05 00 for process piping connections and Section 26 00 00 for electrical controls integration

• Material/Equipment Verification: Verify cloth media specifications and NSF certification, Confirm stainless steel grade (316SS minimum for municipal), Review drive motor and gearbox specifications
• Installation Requirements: Level concrete pad critical for proper operation, Adequate overhead clearance for maintenance access, Electrical coordination for VFD integration
• Field Challenges: Cloth media damage during shipping/installation, Alignment issues affecting backwash efficiency
• Coordination Issues: 12-16 week lead times typical, Early electrical coordination required for controls integration

• Aqua-Aerobic Systems - AquaDisk cloth media filters, dominant in North American municipal market with installations at facilities like Aurora, CO and Broomfield, CO
• Nordic Water - Hydrotech disc filters, strong presence in Canada and northern US markets
• WesTech Engineering - TETRA cloth media filters, established in western US municipalities
• Parkson Corporation - DynaSand cloth-assisted filters, niche applications combining cloth and media filtration

• Traveling bridge sand filters - Better for higher TSS loads (>30 mg/L), roughly 20% higher capital cost but lower O&M
• Membrane bioreactors (MBR) - When space is extremely limited, 3-4x higher cost but superior effluent quality
• Conventional sand filters with polymer - Lower capital cost (40-50% less) but higher chemical costs and larger footprint requirements, suitable for plants with available land

Establish strong manufacturer relationships early - they provide critical startup support and troubleshooting expertise. Consider bulk cloth media purchases during initial procurement for 3-5 year inventory at reduced costs. Specify redundant units for critical applications; single-unit failures can compromise entire treatment trains. Request factory training for operators during equipment delivery rather than waiting for startup - reduces commissioning time significantly.