Gravity Filters

Gravity filters remove suspended solids and turbidity from water using granular media beds (typically sand, anthracite, or multimedia configurations) with gravity-driven downflow. Raw water enters the top of the filter bed and flows downward through progressively finer media layers, where physical straining and biological action capture particles. Municipal gravity filters typically operate at 2-6 gpm/ft² loading rates and achieve turbidity removal to less than 0.1 NTU when properly designed and operated. The primary limitation is the need for regular backwashing cycles that temporarily take filters offline and generate waste backwash water requiring disposal or treatment.

• Primary Treatment Effluent Filtration (0.5-5 MGD plants): Gravity filters follow primary clarifiers to remove remaining suspended solids before secondary treatment. Selected for their low energy requirements and ability to handle variable flows without pumping. Upstream: primary clarifiers. Downstream: aeration basins or trickling filters.
• Tertiary Polishing (2-25 MGD plants): Most common application following secondary clarifiers to achieve <10 mg/L TSS for discharge permits. Gravity filters provide consistent effluent quality with minimal operator intervention. Upstream: secondary clarifiers. Downstream: chlorine contact or UV disinfection.
• Water Treatment Pre-filtration (1-15 MGD plants): Used after sedimentation basins to reduce turbidity before rapid sand filters. Selected when raw water has high organic content or seasonal algae blooms that could blind rapid filters. Upstream: sedimentation basins. Downstream: rapid sand filters.
• Side-stream Solids Capture: Applied to clarifier overflow or plant bypass flows during peak wet weather events, providing additional treatment capacity without major infrastructure expansion.

Daily Operations: Operators monitor head loss across filter beds (typically 2-8 feet), effluent turbidity (<2 NTU), and filtration rates. Manual adjustments include initiating backwash cycles when head loss reaches 6-8 feet or turbidity breakthrough occurs. Flow distribution between multiple filter cells requires periodic rebalancing based on individual filter performance.

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Maintenance: Backwash cycles performed every 24-72 hours depending on influent quality, requiring 15-30 minutes per filter. Monthly media sampling and annual media replacement (10-15% annually). Safety requires confined space procedures for underdrain inspection. Basic mechanical skills needed for valve operation and minor repairs. Hard hat and safety harness required for work above filter beds.

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Troubleshooting: Mud ball formation indicates inadequate backwash, causing shortened filter runs and poor effluent quality. Media loss during backwash suggests excessive wash rates or damaged underdrains. Channeling appears as uneven head loss distribution across filter surface. Typical filter media life spans 5-10 years with proper maintenance, while underdrain systems last 15-20 years.

• Filter Media Bed: Typically 24-36 inches of anthracite coal (0.95-1.05 mm effective size) over 12-18 inches of sand (0.45-0.55 mm). Media selection based on required filtration rate (2-6 gpm/sf) and backwash requirements. Anthracite provides deeper penetration and longer filter runs.
• Underdrain System: Perforated pipe or porous plate design distributes backwash water and collects filtered effluent. Leopold or Wheeler bottom systems common in municipal applications. Sizing based on backwash rate (15-20 gpm/sf) and structural loading.
• Backwash System: Includes wash water pumps (typically 500-2000 gpm), surface wash equipment, and waste collection troughs. Air scour systems (3-5 cfm/sf) increasingly specified for improved cleaning efficiency and reduced wash water consumption.
• Flow Control: V-notch weirs or adjustable gates maintain constant filtration rates during declining head conditions. Rate-of-flow controllers maintain 3-5 gpm/sf regardless of head loss variations.

• Hydraulic Loading Rate: 2-8 gpm/sf (typical 4-6 gpm/sf for conventional media, up to 12 gpm/sf for dual media). Higher rates require backwash capability assessment.
• Filter Media Depth: Single media 24-30 inches; dual media 12-18 inches anthracite over 12-18 inches sand; multimedia 60-72 inches total depth.
• Effective Size (ES): Sand 0.45-0.55 mm; anthracite 0.95-1.05 mm. Uniformity coefficient <1.65 for sand, <1.85 for anthracite.
• Underdrain System: Minimum 6-inch clearance below media. Leopold Type S or equivalent with 0.2-0.4% open area. Backwash rates 12-20 gpm/sf.
• Freeboard: Minimum 24 inches above media surface during backwash expansion (typically 50% for sand, 65% for anthracite).
• Head Loss: Clean bed 2-4 feet; terminal 8-10 feet maximum. Effluent turbidity target <0.3 NTU, preferably <0.1 NTU.
• Filter Box Dimensions: Typical 20x20 to 25x25 feet for municipal applications. Length-to-width ratio should not exceed 1.3:1 for uniform flow distribution.
• Backwash Duration: 5-10 minutes at design rate, with 2-3 minute air scour pre-treatment where applicable.

• What filtration rate balances capacity with performance requirements? Rates above 6 gpm/sf may compromise turbidity removal and reduce run lengths below 24 hours. Consider peak hourly flows, redundancy during maintenance, and regulatory limits. Higher rates require more sophisticated media gradation and underdrain design.
• Should the design include air scour capability? Air scour reduces backwash water consumption by 25-40% and improves cleaning efficiency. Required for rates above 5 gpm/sf or when treating high-turbidity water (>10 NTU). Adds $50,000-100,000 per filter but reduces operating costs.
• What media configuration optimizes performance for the source water quality? Single sand media adequate for <5 NTU influent; dual media recommended for 5-25 NTU; multimedia systems for challenging raw water. Consider seasonal variations, coagulation effectiveness, and downstream disinfection requirements.
• How many filter cells provide adequate redundancy? Minimum 3 cells for plants <5 MGD; 4-6 cells for larger facilities. Size for N-1 operation at maximum day demand. Insufficient cells force operation during maintenance, compromising water quality and regulatory compliance.

• Division 40 - Process Integration
• Section 40 05 00 - Common Work Results for Process Integration
• Section 40 20 00 - Water Treatment Equipment
• Primary: 40 20 00 covers filtration systems including media, underdrains, and controls. May reference 33 00 00

• Material/Equipment Verification: Media gradation certificates and uniformity coefficients, Underdrain system hydraulic calculations and materials compliance, Control valve sizing verification for backwash rates
• Installation Requirements: Concrete curing time (28-day minimum before media placement), Crane access for filter media installation and future maintenance, Electrical coordination for automated backwash sequences
• Field Challenges: Media segregation during placement requires careful layering, Underdrain blockage from construction debris, Initial turbidity breakthrough requiring extended settling
• Coordination Issues: Chemical feed system integration timing, SCADA programming for automated operations, Lead Times: 16-24 weeks for custom concrete construction, 12-16 weeks for package systems

• WesTech Engineering: DYNASAND continuous backwash filters for 0.5-50 MGD applications.
• Evoqua/Siemens: VORTISAND and conventional dual-media systems, strong municipal presence.
• Parkson Corporation: DynaSand and conventional gravity filters with integrated controls.
• Tonka Water: Package gravity filter systems for smaller municipalities (0.5-5 MGD). All maintain extensive North American municipal references and standardized designs.

• Membrane Filtration (MF/UF): Preferred for high-quality requirements or challenging raw water. 40-60% higher capital cost but lower chemical usage.
• Dissolved Air Flotation (DAF): Better for high algae/organics, similar capital cost, 20% higher O&M.
• Cloth Media Filters: Tertiary polishing applications, 50% lower footprint, comparable costs for <10 MGD plants. Consider when space-constrained or retrofit applications require minimal civil work.

Media Specification: Always require dual-source media suppliers and 10% overage - media shortages cause major delays. Manufacturer Relations: Establish service agreements upfront; filter troubleshooting often requires factory expertise within 48 hours. Cost Savings: Consider package systems for <5 MGD plants - 20-30% lower installed cost than cast-in-place concrete. Standardize filter dimensions across multiple cells to reduce construction complexity and spare parts inventory.