Biologically Activated Filters

Biologically Activated Filters (BAF) combine physical filtration with biological treatment in a single unit, using attached-growth biomass on granular media to remove both suspended solids and dissolved organics. Water flows through a packed bed of specialized media (typically expanded clay, plastic, or sand) where microorganisms form biofilms that metabolize contaminants while the media provides physical straining. Typical BOD removal efficiencies range from 85-95% in municipal applications with hydraulic loading rates of 1-4 gpm/ft². The primary trade-off is higher operational complexity compared to conventional filtration, requiring careful backwash management to maintain both biological activity and filtration capacity.

• Post-Secondary Clarification (5-25 MGD plants): BAF follows secondary clarifiers for tertiary treatment, removing residual BOD/TSS and providing nitrification. Selected for compact footprint versus conventional sand filters. Upstream: secondary effluent, chlorine contact downstream.
• Combined Sewer Overflow Treatment (2-15 MGD peak flows): BAF provides rapid biological treatment during wet weather events. High-rate design (10-15 gpm/sf) handles variable hydraulic loading. Selected for automated operation and resilience to shock loads.
• Small Plant Secondary Treatment (0.5-5 MGD): BAF replaces activated sludge in space-constrained facilities. Integrated nitrification/denitrification with recycle streams. Upstream: primary effluent, downstream: disinfection.
• Drinking Water Biofiltration (1-20 MGD): BAF removes ammonia, iron, manganese, and biodegradable organics post-coagulation/sedimentation. Selected for biological stability and reduced chlorine demand. Operates at 2-5 gpm/sf loading rates.

Daily Operations: Operators monitor head loss gauges (target 2-6 inches), effluent turbidity (<2 NTU), and dissolved oxygen levels (2-4 mg/L for nitrification). Flow distribution adjustments maintain even loading across filter cells. Visual inspection for media carryover or uneven backwash patterns.

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Maintenance: Weekly automated backwash cycles require minimal intervention. Monthly underdrain inspection and quarterly media sampling for biomass activity. Annual media addition (5-10% replacement typical). Requires confined space entry procedures for cell access. Basic mechanical skills sufficient for routine valve/sensor maintenance.

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Troubleshooting: Rapid head loss increase indicates biomass overgrowth or media fouling - increase backwash frequency. Poor effluent quality suggests insufficient biomass or oxygen limitation. Media loss during backwash indicates excessive air scour rates. Expected 15-20 year media life with proper operation. Warning signs include uneven backwash expansion or persistent odors.

• Filter Media: Typically expanded clay (Biolite) or plastic media (Biofor rings) in 6-12 foot depths. Porosity 40-50%, specific surface area 300-500 m²/m³. Selection based on required biomass retention and head loss characteristics.
• Underdrain System: Perforated laterals or nozzle plates distribute backwash air/water uniformly. Stainless steel or HDPE construction. Sized for 3-5 scfm/sf air scour, 15-20 gpm/sf water backwash rates.
• Backwash System: Combined air/water wash removes excess biomass weekly. Includes air blowers (15-25 HP), backwash pumps, and waste handling. Automated 15-20 minute cycles maintain 2-6 inch head loss.
• Process Control: PLC-based systems monitor head loss, effluent quality, dissolved oxygen. Automated backwash initiation, flow distribution between cells. Integration with plant SCADA for remote monitoring.

• Hydraulic Loading Rates: 2-8 gpm/ft² for municipal applications, with 4-6 gpm/ft² typical for secondary effluent polishing. Higher rates (6-8 gpm/ft²) acceptable for nitrification-only applications.
• Empty Bed Contact Time (EBCT): 10-30 minutes depending on treatment objectives. 15-20 minutes typical for combined BOD/TSS/ammonia removal. Shorter contact times (10-15 minutes) sufficient for nitrification of well-nitrified secondary effluent.
• Backwash Requirements: Air scour at 3-5 scfm/ft² for 3-5 minutes, followed by water backwash at 12-18 gpm/ft² for 8-12 minutes. Combined air/water backwash reduces media loss and improves cleaning efficiency.
• Media Specifications: GAC bed depth 4-8 feet, typically 6 feet. Effective size 0.8-1.2 mm, uniformity coefficient <2.0. Iodine number >900 mg/g for new carbon.
• Operating Parameters: Headloss buildup 6-10 feet typical between backwash cycles. Maximum recommended headloss 12 feet. Filtration rates maintain 24-48 hour backwash intervals for stable biological activity.
• Underdrain Design: False bottom with 0.5-1.0% open area, supporting gravel layers totaling 12-18 inches depth.

• What hydraulic loading rate balances treatment efficiency with capital cost? Loading rates below 4 gpm/ft² provide excellent effluent quality but increase basin size 25-40%. Above 6 gpm/ft² risks breakthrough during peak flows. Engineers need influent variability data, peak flow factors, and treatment targets to optimize sizing.
• Should the system prioritize BOD removal or nitrification? BOD-focused designs use shorter EBCT (10-15 minutes) and higher loading rates, while nitrification requires longer contact times (20-30 minutes) and lower organic loading. Wrong choice results in 20-30% performance shortfall. Requires detailed influent characterization and regulatory requirements analysis.
• What backwash strategy minimizes media replacement costs? Air-first backwash reduces GAC losses to <2% annually versus 5-8% with water-only systems. However, air scour systems add $150,000-300,000 capital cost for 5-20 MGD plants. Decision requires 20-year lifecycle cost analysis including media replacement at $800-1,200/ton.
• How much redundancy is required for reliable operation? Minimum two trains for <5 MGD, three trains for larger plants. Undersizing by one train increases loading rates 50-100% during maintenance, causing treatment failure.

• Primary: Division 46 13 43 - Biological Filtration Equipment
• Secondary: Division 46 13 13 - Granular Media Filtration Equipment (for media and underdrain components)

• Material/Equipment Verification: Media certification and gradation analysis, Underdrain system pressure testing, Air distribution uniformity verification
• Installation Requirements: Precise media placement and leveling, Underdrain integrity during backfill, Air piping leak testing before startup
• Field Challenges: Media segregation during placement, Achieving uniform air distribution, Backwash system commissioning complexity
• Coordination Issues: SCADA integration for automated backwash, Lead times: 16-20 weeks for custom underdrains

• Veolia - BioFor systems with Biolite media, widely used in North America with 200+ installations
• Evoqua - Leopold Clariflow BAF with Biomag media, strong municipal presence
• Xylem - Sanitaire BAF systems with Kaldnes media, established track record
• Headworks - BioMag systems with proprietary media, growing municipal market share

• Conventional activated sludge - Lower capital cost but higher energy/footprint; preferred for >20 MGD plants
• Moving bed biofilm reactors (MBBR) - Similar performance, easier operation, 10-15% higher cost
• Membrane bioreactors (MBR) - Superior effluent quality, 40-60% higher cost, suitable for tight discharge limits or reuse applications

Establish media supplier relationships early - replacement media availability varies significantly between manufacturers. Budget 15-20% contingency for underdrain modifications during startup. Negotiate performance guarantees carefully; effluent quality depends heavily on influent characteristics. Consider pilot testing for challenging applications. Standardize on one manufacturer's media type to simplify operations and spare parts inventory.