Deep-bed Denitrification Filters

Deep-bed denitrification filters remove nitrate from treated wastewater effluent through biological reduction using carbon-based media like sand, anthracite, or specialized plastic carriers. Denitrifying bacteria in anoxic conditions convert nitrate to nitrogen gas using methanol or other carbon sources as electron donors. These filters typically achieve 85-95% nitrate removal efficiency in municipal applications ranging from 1-25 MGD. The primary trade-off involves ongoing chemical costs for carbon addition and periodic backwashing requirements that generate nitrogen-rich waste streams requiring additional treatment consideration.

• Tertiary Treatment for Effluent Compliance: Deep-bed denitrification filters are primarily used after secondary clarifiers and before disinfection in plants requiring total nitrogen limits below 8 mg/L. The filters receive nitrified effluent (15-25 mg/L NO3-N) and reduce it to 3-8 mg/L through biological denitrification using methanol or acetate dosing. Selected for consistent performance and minimal footprint compared to anoxic basins.
• BNR Process Polishing: Installed downstream of modified Bardenpho or A2O processes where seasonal nitrogen spikes exceed permit limits. Handles variable influent nitrogen loads (8-15 mg/L NO3-N) during high-flow periods or when upstream BNR performance declines. Provides operational flexibility without major process modifications.
• Retrofit Applications: Common in older plants (5-25 MGD) adding nitrogen removal capability. Fits between existing secondary treatment and chlorine contact chambers, utilizing existing piping infrastructure. Selected over complete BNR retrofits due to lower capital costs and shorter construction timelines.

Daily Operations: Operators monitor effluent nitrate levels using online analyzers or daily grab samples, adjusting carbon feed rates to maintain 2-5 mg/L residual NO3-N. Flow rates typically range 4-8 gpm/sf with headloss monitoring at 2-4 feet maximum before backwashing. Carbon dosing requires daily tank level checks and pump calibration verification.

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Maintenance: Backwash cycles every 1-3 days depending on upstream solids loading, requiring 20-30 minutes of automated operation. Monthly media sampling checks for biological activity and potential media loss. Carbon feed pumps need quarterly calibration and annual diaphragm replacement. Standard PPE includes safety glasses and chemical-resistant gloves for methanol handling.

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Troubleshooting: High effluent nitrates indicate insufficient carbon dosing or short-circuiting through cracked media beds. Rapid headloss buildup suggests upstream solids breakthrough or biological fouling requiring extended backwash cycles. Media replacement typically needed every 8-12 years when

• Filter Media: Typically 4-6 feet of sand (0.45-0.55 mm effective size) over 12-18 inches of gravel underdrain support. Some installations use anthracite/sand dual media for enhanced solids removal. Media depth selection based on required detention time (8-12 minutes) and nitrogen loading rates.
• Carbon Source Feed System: Methanol or sodium acetate storage tanks (500-2,000 gallons) with metering pumps delivering 2-4 mg/L carbon per mg/L NO3-N removed. Systems include day tanks, level controls, and flow-paced dosing based on influent nitrogen concentrations.
• Backwash System: Air scour blowers (10-25 HP) and backwash pumps sized for 15-20 gpm/sf air followed by 12-15 gpm/sf water wash. Includes wash water storage tanks, troughs, and automated valve sequences. Backwash frequency typically 24-72 hours based on headloss accumulation.
• Underdrain Collection: Leopold or Johnson screen laterals with 0.5-1.0 mm slot openings for uniform water distribution and media retention. Designed for 6-8 gpm/sf filtration rates with minimal pressure drop across the underdrain system.

• Hydraulic Loading Rate: 2-6 gpm/ft² (typical 4 gpm/ft² for municipal applications)
• Empty Bed Contact Time (EBCT): 8-15 minutes (minimum 10 minutes for reliable nitrate removal)
• Influent Nitrate-N: 5-25 mg/L (design basis for carbon dosing calculations)
• Target Effluent Nitrate-N: <3 mg/L (often <1 mg/L for advanced treatment)
• Operating Pressure: 15-40 psi (gravity systems preferred where feasible)
• Backwash Rate: 12-15 gpm/ft² for 8-12 minutes
• Media Depth: 4-6 feet (sand/anthracite combination typical)
• Carbon Source Requirements: 3-4 lbs COD per lb NO₃-N removed
• Temperature Range: 50-85°F (affects reaction kinetics significantly)
• pH Range: 6.5-8.5 (optimal 7.0-7.5)
• Dissolved Oxygen: <0.5 mg/L in denitrification zone
• Filter Bed Expansion: 15-25% during backwash
• Headloss Criteria: Clean bed 2-4 feet, terminal 8-10 feet
• Minimum Freeboard: 3-4 feet above expanded media level

• What carbon source and dosing strategy will achieve target nitrate removal while minimizing operational costs? Methanol typically requires 2.5-3.0 mg/L per mg/L NO₃-N removed versus 3.5-4.0 mg/L for ethanol. Wrong selection impacts annual chemical costs by $50,000-200,000 for 10-50 MGD plants. Requires influent nitrate characterization, carbon source availability, and safety considerations.
• Should the system operate in upflow or downflow configuration? Upflow provides better carbon utilization and prevents media stratification but requires more complex hydraulics. Downflow offers simpler operation but may need higher carbon doses. Decision affects capital costs by 15-25% and operational complexity significantly.
• What media configuration optimizes both denitrification and solids removal? Dual-media (sand/anthracite) provides filtration plus biological treatment versus single-media systems. Affects effluent turbidity (<0.1 NTU achievable), backwash frequency (24-72 hours), and total lifecycle costs.
• How will dissolved oxygen be controlled to maintain anoxic conditions? Pre-deoxygenation versus relying on biological oxygen consumption affects system reliability and carbon requirements. Impacts carbon dosing by 20-30% and requires influent DO monitoring strategy.

• Division 40 - Process Integration
• Section 40 05 23 - Denitrification Systems
• Primary application with biological treatment components. May also reference Section 40 20 00 (Water Treatment Equipment) for filtration components and Section 40 05 13 (Chemical

• Material/Equipment Verification: Media specifications (effective size, uniformity coefficient), Underdrain system capacity and materials, Carbon source feed system accuracy (±2%)
• Installation Requirements: Crane access for media placement, Electrical for blowers, pumps, instrumentation, Concrete curing time before media loading
• Field Challenges: Media segregation during placement, Underdrain blockage prevention, Backwash water disposal coordination
• Coordination Issues: SCADA integration with existing plant systems, Lead times: 16-24 weeks for custom underdrains, 8-12 weeks for standard media

• Evoqua Water Technologies - MEMCOR AnoxKaldnes MBBR systems with K1/K3 carriers for municipal retrofits up to 50 MGD
• Veolia - AnoxKaldnes biofilm reactors with specialized media configurations
• Headworks International - BioMag ballasted flocculation systems adapted for denitrification applications
• WesTech Engineering - Trident deep-bed filters with specialized underdrain systems for biological applications in 1-25 MGD plants

• Membrane Bioreactors (MBR) - Higher capital cost (2-3x) but smaller footprint, preferred for space-constrained sites
• Sequencing Batch Reactors (SBR) - Lower capital cost, flexible operation, suitable for smaller plants (<5 MGD) with variable loading
• Moving Bed Biofilm Reactors (MBBR) - Similar cost, easier retrofits into existing tankage, better for plants with existing biological treatment requiring nitrogen removal upgrades

Establish relationships with media suppliers early - delivery delays can extend commissioning by months. Consider split media deliveries to reduce storage requirements. Negotiate manufacturer startup assistance; biological seeding expertise is critical for achieving target nitrate removal within 60-90 days. Pre-purchase spare underdrain components - custom laterals have 12+ week lead times. Budget 15-20% contingency for unexpected biological performance optimization, including additional carbon source requirements during startup.