Bag Filters

Bag filters remove suspended solids from water and wastewater through physical straining using fabric filter bags housed in pressure vessels. Water flows from outside to inside the cylindrical bags, with particles captured on the exterior surface while clean filtrate passes through. These systems typically achieve 90-99% removal of particles larger than their rated micron size, with common municipal applications ranging from 1-100 micron filtration. The key trade-off is frequent bag replacement requirements - typically every 2-8 weeks depending on loading - which creates ongoing operational costs and labor demands that can exceed $50,000 annually for larger installations.

• Raw Water Intake Screening - Bag filters (50-200 micron) remove debris, algae, and suspended solids before coagulation/flocculation. Installed downstream of intake screens but upstream of chemical addition, they protect downstream equipment from fouling. Selected for their ability to handle variable debris loads and quick changeout capability during algae blooms
• Secondary Effluent Polishing - Fine bag filters (5-25 micron) provide tertiary treatment for TSS removal before disinfection. Positioned after secondary clarifiers and before UV or chlorination systems, they consistently achieve <5 mg/L TSS in 2-20 MGD plants. Selected for lower capital cost compared to sand filters and ability to handle variable biological solids loading
• Backwash Water Recovery - Coarse bag filters (100-500 micron) capture filter media and debris from backwash return flows. Installed in dedicated backwash recovery systems, they allow water reuse while protecting plant hydraulics. Selected for high dirt-holding capacity and resistance to flow surges during backwash cycles

Daily Operations - Operators monitor differential pressure gauges during routine rounds, logging readings to track fouling rates. Flow adjustments may be needed when pressure drop exceeds 15 PSI to maintain system capacity. Visual inspection of housing exteriors checks for leaks or unusual vibration indicating bag failure.

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Maintenance - Bag replacement typically occurs weekly to monthly depending on loading, requiring 15-30 minutes per housing. Operators need basic PPE (gloves, safety glasses) and lifting assistance for larger bags. Skill requirements are minimal - proper bag seating and housing reassembly are critical. Housing cleaning and gasket inspection extend service intervals.

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Troubleshooting - Rapid pressure rise indicates heavy loading or bag plugging requiring immediate changeout. Sudden pressure drop suggests bag failure or bypass, requiring flow shutdown and inspection. Typical bag service life ranges 1-8 weeks depending on micron rating and solids loading. Premature failure often results from improper installation or excessive flow rates.

• Filter Housing - Stainless steel (316SS) or coated carbon steel vessels sized for 2-8 fps approach velocity. Municipal units typically handle 50-2,000 GPM per housing. Selection factors include pressure rating (150-300 PSI), corrosion resistance, and access requirements for bag changes
• Filter Bags - Polypropylene, polyester, or PTFE construction in 1-500 micron ratings. Standard #2 size (7"x32") fits most municipal applications. Absolute vs. nominal ratings affect performance - absolute ratings provide consistent particle removal but shorter service life
• Support Basket - Perforated stainless steel or plastic basket maintains bag shape under differential pressure. Basket perforation size must be smaller than bag micron rating to prevent bypass. Proper fit prevents bag collapse at high flow rates
• Differential Pressure Monitoring - Gauges or transmitters track pressure drop across bags (typically 2-15 PSI clean, replace at 20-25 PSI). Critical for determining changeout intervals and preventing bag failure from excessive pressure

• Flow Rate Parameters: Design flow: 0.1-15 MGD per vessel for municipal applications; Peak flow capacity: 1.5-2.0 × average daily flow; Minimum turndown ratio: 3:1 without performance degradation
• Pressure Requirements: Operating pressure: 15-150 psig (typical municipal: 30-80 psig); Maximum allowable pressure drop: 15-25 psi across clean bags; Differential pressure alarm setpoint: 20-30 psi (bag replacement trigger)
• Loading Specifications: Surface loading rate: 1.0-8.0 gpm/ft² (typical: 2-4 gpm/ft²); Filtration rating: 1-200 microns (municipal secondary: 10-50 microns); Solids loading capacity: 5-15 lbs/bag depending on application
• Performance Metrics: Turbidity removal: 85-95% for secondary effluent polishing; TSS removal: 90-99% depending on influent characteristics; Bag service life: 30-90 days typical municipal operation; Vessel sizing: 6-48 bags per housing (standard configurations: 6, 12, 18, 24, 36, 48)
• Physical Constraints: Vessel diameter: 12"-60" for municipal applications; Installation space: Allow 3× vessel diameter for bag changeout access

• What influent solids concentration and particle size distribution will the system handle? Critical threshold: >50 mg/L TSS typically requires pre-filtration or frequent bag changes. Engineers need full particle size analysis and seasonal variation data. Wrong sizing leads to premature bag blinding, excessive maintenance costs ($2,000-5,000 annually per vessel), and potential bypass events
• What differential pressure operating range and alarm setpoints are appropriate? Key decision point: Clean bag ΔP <5 psi, replacement at 20-25 psi. Higher setpoints risk bag failure and solids breakthrough. Lower setpoints increase bag replacement frequency 2-3×. Requires accurate pressure instrumentation (±0.5% accuracy) and historical fouling rate data from similar applications
• What bag material and micron rating optimize performance versus cost? Critical selection: Polypropylene (standard), polyester (chemical resistance), or PTFE (high-temp). Rating selection: 10-25 microns for secondary effluent, 50-100 microns for primary applications. Wrong material selection can reduce bag life 50-75% or cause premature failure under chlorine exposure
• How many parallel vessels are required for redundancy and maintenance access? Minimum: 50% redundancy for continuous operation. Consider 3×50% or 4×33% configurations for >5 MGD plants. Single vessel systems acceptable only with adequate downstream storage and maintenance windows

• Primary: Division 40-05-23 - Bag and Cartridge Water Filters
• Secondary applications: Division 46-07-13 (Water Treatment Equipment) for integrated treatment systems

• Material/Equipment Verification: Verify media micron rating matches specifications; Confirm backwash system capacity and controls; Check structural loading for concrete pads
• Installation Requirements: Level concrete pads within ±1/4"; Adequate crane access for media changeouts; Electrical coordination for VFDs and controls
• Field Challenges: Media damage during shipping/installation; Backwash piping air entrainment issues; Control system integration complexities
• Coordination Issues: SCADA tie-ins with existing plant systems; Backwash waste handling integration
• Lead times: 16-24 weeks for standard units

• Pentair AES - AquaDisk series, strong municipal presence with 1-100 MGD installations
• Evoqua - Memcor CMF-S systems, proven in secondary effluent polishing
• Pall Corporation - Aria packaged systems for smaller plants (0.5-10 MGD)
• Veolia - Hydrotech Discfilter, popular for tertiary treatment upgrades

• Traveling bridge filters - Better for high solids loading, 15-20% higher capital cost but lower O&M
• Membrane bioreactors (MBR) - Higher treatment quality but 2-3x capital cost, preferred for tight discharge limits
• Conventional sand filters - Lower capital cost for larger plants (>25 MGD), higher footprint requirements. Bag filters excel in retrofit applications with space constraints

Establish relationships with local manufacturer reps early - they provide valuable troubleshooting support and spare parts inventory. Specify redundant media cleaning systems; manual cleaning during backwash failures saves costly downtime. Consider oversizing units 10-15% for future capacity or challenging influent conditions. Negotiate spare media packages during initial procurement for better pricing than future individual orders.