Pressure Filters

Pressure filters remove suspended solids from water using granular media (sand, anthracite, or GAC) within a closed steel vessel operating under 15-125 psi. Raw water enters the top, flows downward through filter media beds, and exits through underdrain systems as filtered effluent. These systems typically achieve 95-99% turbidity removal at loading rates of 2-8 gpm/ft². The key trade-off is higher capital cost and energy consumption compared to gravity filters, offset by smaller footprint requirements and ability to pump directly to distribution without intermediate storage.

• Secondary Effluent Polishing (2-25 MGD plants): Pressure filters follow secondary clarifiers to remove residual TSS before discharge or advanced treatment. Typically sized for 4-8 gpm/sf loading with 18-24" anthracite/sand media. Selected over gravity filters when site constraints limit footprint or when 15+ feet of available head eliminates pumping costs.
• Tertiary Filtration for Reuse (5-50 MGD): Essential upstream of RO systems, removing particles >10 microns that could foul membranes. Dual-media configurations achieve <2 mg/L TSS consistently. Preferred over cloth disk filters when influent TSS exceeds 15 mg/L or when backwash water recovery is critical.
• Primary Effluent Pre-treatment: Smaller plants (0.5-5 MGD) use pressure filters before biological treatment when influent contains high industrial solids. Coarse media (1.2mm sand) removes settleable solids while maintaining biological treatability.
• Backwash Water Treatment: Clarifier sludge thickening applications where pressure filters polish thickener overflow before return to plant headworks, preventing solids accumulation.

Daily Operations: Operators monitor differential pressure across each filter (typical 2-8 psi clean, 15-20 psi terminal), effluent turbidity (<1 NTU target), and flow rates. Automated systems initiate backwash cycles based on head loss or time intervals (24-72 hours typical). Manual adjustments include flow balancing between parallel units and backwash optimization during seasonal temperature changes.

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Maintenance: Monthly media sampling and annual media replacement (5-10% loss per year normal). Quarterly underdrain inspection requires confined space entry with full PPE and gas monitoring. Valve actuator maintenance every 6 months. Control system calibration quarterly. Skilled millwright required for major valve repairs; operators handle routine adjustments and media additions.

• Filter Vessel: Carbon steel with epoxy lining or stainless steel construction. Typical municipal sizes: 8-12 feet diameter, 12-16 feet height for 2-10 MGD applications. Design pressure 75-125 psi with 2:1 safety factor. ASME Section VIII certification required.
• Media Bed: Dual-media standard - 18" anthracite (0.95-1.05 mm) over 12" sand (0.45-0.55 mm). Total bed depth 30-36" with 18" freeboard minimum. Media selection based on required filtration rate and effluent quality targets.
• Underdrain System: Leopold or Wheeler bottom designs with 0.25-0.5% open area. Stainless steel construction prevents corrosion. Uniform distribution critical for backwash effectiveness and media retention.
• Backwash System: Air scour (3-5 scfm/sf) followed by water wash (12-15 gpm/sf). Separate air and water headers with automated sequencing. Backwash pumps typically 25-50% of forward flow capacity.
• Control Valves: Pneumatic actuated butterfly or ball valves (6-24" diameter). Influent, effluent, backwash, and waste isolation. 316SS wetted parts standard for municipal service.

• Filtration Rate: 2-8 gpm/ft² (typical 4-6 gpm/ft² for municipal applications)
• Small plants (0.5-2 MGD): 3-5 gpm/ft²
• Medium plants (2-20 MGD): 4-6 gpm/ft²
• Large plants (20-50 MGD): 5-8 gpm/ft²
• Operating Pressure: 15-125 psi working pressure
• Inlet pressure: 25-80 psi typical
• Pressure drop across clean media: 2-8 psi
• Maximum allowable pressure drop: 15-25 psi before backwash
• Media Specifications:
• Anthracite: 0.95-1.05 mm effective size, 1.4-1.6 uniformity coefficient
• Sand: 0.45-0.55 mm effective size, <1.65 uniformity coefficient
• Media depth: 24-36 inches total (dual media typical)
• Vessel Sizing:
• Diameter: 6-12 feet standard (larger custom available)
• Length-to-diameter ratio: 2.5:1 to 4:1 for horizontal vessels
• Minimum freeboard: 18-24 inches above media
• Backwash Requirements:
• Rate: 12-20 gpm/ft² (15 gpm/ft² typical)
• Duration: 8-15 minutes
• Air scour (if provided): 3-5 scfm/ft²

• What filtration rate should be specified based on raw water quality and treatment objectives?
• Threshold: >6 gpm/ft² increases breakthrough risk for turbidity <0.1 NTU
• Consequence: Undersizing leads to frequent backwashing; oversizing wastes capital
• Need: Historical turbidity data, peak flow projections, regulatory requirements
• Should the system use single or dual media configuration?
• Threshold: Turbidity >2 NTU or color >15 units typically requires dual media
• Consequence: Single media may not achieve <0.1 NTU consistently; dual media costs 20-30% more
• Need: Raw water characteristics, effluent quality targets, coagulation performance data
• What vessel orientation (horizontal vs. vertical) best fits the site constraints?
• Threshold: Horizontal vessels require 40-60% more floor space but 50% less headroom
• Consequence: Wrong choice impacts building costs significantly
• Need: Site dimensions, building height restrictions, maintenance access requirements
• Is air scour necessary for the media cleaning regime?
• Threshold: Systems without air scour require 25-50% higher backwash water rates
• Consequence: Adds complexity but reduces backwash water consumption by 30-40%
• Need: Water availability, energy costs, maintenance capabilities

• Primary: Division 46 23 13 - Packaged Water Treatment Equipment
• Secondary: Division 40 20 00 - Process Water Treatment (for larger custom)

• Material/Equipment Verification:
• Vessel ASME certification and hydrostatic testing documentation
• Media specifications and NSF certification
• Instrumentation calibration certificates
• Installation Requirements:
• Crane access for vessel placement (typically 12-20 ft diameter)
• Concrete pad design for 150-200 psf loading
• Piping stress analysis for thermal expansion
• Field Challenges:
• Limited headroom in existing facilities
• Backwash waste handling coordination
• Coordination Issues:
• 16-24 week lead times for custom vessels
• Early electrical/controls coordination required

• WesTech Engineering - Model PressurePac filters, strong municipal references including Flagstaff, AZ (12 MGD)
• Evoqua Water Technologies - Leopold pressure filter systems, installed at numerous facilities like Riverside, CA
• Roberts Filter Group - Traveling bridge filters, popular in smaller municipalities
• Tonka Water - Pressure leaf filters, competitive in 1-10 MGD range with proven municipal installations

• Gravity filters cost 20-30% less but require more space and higher structures
• Cloth media filters (Aqua-Aerobic, Parkson) offer smaller footprint, 30-40% higher capital cost but lower O&M
• Membrane bioreactors provide superior effluent quality at 3-4x capital cost, justified for tight discharge limits or water reuse applications

Establish relationships with local media suppliers early - freight costs can exceed $50/ton for specialty media. Specify oversized access ports (minimum 24") for maintenance; standard 18" ports create operator headaches. Consider modular designs for future expansion. Request factory acceptance testing for complex control systems. Budget 15-20% contingency for unforeseen piping modifications during retrofits in existing facilities.