Trickling Filters

Trickling filters are fixed-film biological treatment systems that remove organic matter and BOD from municipal wastewater by distributing effluent over a bed of media (typically plastic or rock) where biofilms develop. Wastewater trickles down through the media while air flows upward, creating aerobic conditions for microbial growth that consumes organic pollutants. These systems typically achieve 80-90% BOD removal at loading rates of 5-40 lbs BOD/1000 ft³/day. The primary trade-off is their large footprint requirement compared to activated sludge systems, making them less suitable for space-constrained municipal facilities.

• Secondary Treatment (BOD/TSS Removal) - Primary application at 2-20 MGD plants following primary clarifiers. Trickling filters provide 85-95% BOD removal with lower energy costs than activated sludge

• Nitrification (Ammonia Removal) - Two-stage systems use high-rate filters for carbonaceous removal followed by low-rate nitrification filters. Common at 5-50 MGD plants requiring ammonia limits <2 mg/L

• Roughing Treatment - Pre-treatment before activated sludge at high-strength facilities or industrial discharge plants. Reduces organic loading by 50-70% before biological treatment

• Polishing/Tertiary Treatment - Final treatment step after secondary processes for enhanced effluent quality. Low-rate operation (2-4 gpm/sf) provides additional BOD/TSS removal

Daily Operations: Operators monitor recirculation flow rates, distributor arm rotation (0.5-2 rpm), and effluent quality. Visual inspection identifies ponding, media clogging, or uneven distribution patterns. Flow adjustments maintain 1-4 gpm/sf hydraulic loading. Temperature and dissolved oxygen monitoring ensures proper biological activity.

​

Maintenance: Monthly lubrication of distributor bearings and drive mechanisms. Semi-annual media inspection for clogging or biological growth accumulation. Annual underdrain flushing removes accumulated solids. High-pressure washing every 2-3 years controls excessive biomass. Standard PPE includes hard hat, safety glasses, and slip-resistant footwear. Confined space procedures required for underdrain access.

​

Troubleshooting: Ponding indicates media clogging

• Media - Plastic structured media (Jaeger Tri-Packs, Brentwood AccuPac) or rock media. Plastic provides 95-98% void space, 25-35 sf/cf specific surface area

• Distribution System - Rotating arms (2-20 MGD plants) or fixed spray nozzles (>20 MGD). Rotating distributors provide 0.5-2 rpm rotation with hydraulic drive

• Underdrain System - Collects treated effluent and provides ventilation. Vitrified clay blocks, concrete, or plastic modules with 6-12 inch collection channels

• Recirculation System - Pumps return settled effluent to filter inlet, maintaining minimum wetting rates and diluting high-strength influent

• Hydraulic Loading Rate: 0.5-4.0 MGD/acre (typical municipal range 1.0-2.5 MGD/acre)

• Organic Loading Rate: 5-40 lbs BOD₅/1000 ft³/day (standard practice 15-25 lbs BOD₅/1000 ft³/day)

• Media Depth: 6-40 feet (plastic media), 6-10 feet (rock media)

• Recirculation Ratio: 0.5:1 to 4:1 (effluent to influent flow)

• Distributor Arm Speed: 1-3 RPM for rotary distributors

• Underdrain Slope: Minimum 1% grade toward collection point

• Ventilation Requirements: Natural draft or forced air at 0.1-0.5 cfm/ft² media surface

• Temperature Range: 32-104°F operational (performance drops below 55°F)

• What organic loading rate should be applied based on effluent quality requirements? - <15 lbs BOD₅/1000 ft³/day for <20 mg/L BOD₅ effluent; 15-25 lbs BOD₅/1000 ft³/day for 20-30 mg/L BOD₅ effluent

• Should plastic or rock media be specified for this application? - Rock media: <2 MGD plants, low maintenance capability; Plastic media: >2 MGD plants, higher loading rates needed

• What recirculation ratio is required for process stability? - 1:1 minimum for consistent wetting and temperature control; 2:1+ for plants with variable influent strength

• How should the system handle peak flow conditions? - Design for 2-3x average daily flow without washout; Consider equalization vs. larger distributor systems

• Division 40 - Process Integration

• Section 40 22 13 - Trickling Filter Equipment

• Secondary: Section 40 20 00 - Water and Wastewater Treatment Equipment (for integrated package systems)

• Material/Equipment Verification - Media certification (plastic fill density, surface area specifications); Underdrain system hydraulic capacity verification; Rotary distributor mechanical specifications and warranties

• Installation Requirements - Concrete basin waterproofing and structural integrity; Precise leveling for distributor arm operation; Electrical coordination for variable speed drives

• Field Challenges - Media installation sequencing and support; Distributor arm balancing and calibration

• Coordination Issues - HVAC systems for odor control integration; Lead times: 16-20 weeks for custom configurations, 12-14 weeks for standard units

• Evoqua (Envirex brand) - E-Series trickling filters, dominates larger municipal installations (10+ MGD)

• Tonka Equipment Company - circular and rectangular configurations, serves smaller communities effectively with standardized package units

• WesTech Engineering - biological treatment systems including trickling filters

• Parkson Corporation - Lamella trickling filter systems

• Activated sludge (ASP) - preferred for plants >10 MGD due to better process control and smaller footprint, though 15-25% higher operating costs

• Moving bed biofilm reactors (MBBR) - gaining popularity for upgrades, similar biological performance with 30% less space requirement

• Membrane bioreactors (MBR) - for tight discharge limits but 3-4x capital cost

• Trickling filters - remain cost-effective for smaller plants (<5 MGD) with moderate BOD removal requirements and limited operator staffing

Operational Insights: Size distributor arms 20% larger than calculated to handle peak wet weather flows - learned from multiple Midwest installations experiencing bypass issues. Specify stainless steel hardware throughout; galvanized components fail within 5-7 years in wastewater environments. Manufacturer Relations: Establish service agreements upfront; replacement parts availability varies significantly between suppliers, with Evoqua offering best long-term support but at premium pricing.