Bio-towers

Bio-towers are fixed-film biological treatment systems that remove organic pollutants and nitrogen from wastewater using structured plastic media in tall, enclosed reactors. Wastewater is distributed over high-surface-area media where biofilms develop, consuming BOD and converting ammonia to nitrate through nitrification. These systems typically achieve 85-95% BOD removal and can nitrify completely at loading rates of 5-15 lbs BOD/1000 ft³/day. The primary trade-off is their substantial height requirement (20-40 feet) and higher energy costs for pumping compared to conventional activated sludge processes.

• Secondary Treatment Following Primary Clarification: Bio-towers serve as the main biological treatment step in 2-15 MGD plants, receiving primary effluent at 150-300 mg/L BOD. They're selected for consistent performance in cold climates and minimal operator attention compared to activated sludge. Effluent flows to secondary clarifiers for biomass separation.
• Nitrification Applications: In 5-25 MGD facilities requiring ammonia removal, bio-towers provide reliable nitrification with minimal temperature sensitivity. Two-stage configurations achieve 90%+ ammonia removal, with first stage for BOD removal and second stage for nitrification before final clarification.
• Small Plant Upgrades: Existing 0.5-5 MGD trickling filter plants retrofit with high-rate bio-tower media to increase capacity 2-3x without expanding footprint. The upgrade maintains existing clarifiers and pumping while meeting stricter discharge limits.
• Industrial Pretreatment: Municipal plants receiving high-strength industrial waste (food processing, breweries) use bio-towers for initial BOD reduction before conventional secondary treatment, handling 500-1500 mg/L influent BOD.

Daily Operations: Operators monitor distributor rotation speed and arm levelness, checking for plugged orifices during morning rounds. Flow measurement verification includes influent, recirculation, and effluent rates. Visual inspection of media for ponding, excessive sloughing, or odors. Typical adjustments include recirculation ratio changes based on loading and seasonal temperature variations.

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Maintenance: Monthly distributor bearing lubrication and quarterly orifice cleaning using high-pressure water. Semi-annual media inspection requires confined space entry with gas monitoring and ventilation. Annual distributor arm balancing and drive system service. Maintenance requires basic mechanical skills, fall protection training, and confined space certification. Underdrain flushing performed annually or as needed for flow distribution issues.

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Troubleshooting: Ponding indicates excessive organic loading or plugged underdrains, requiring

• Structured Media: Cross-fluted PVC or polypropylene sheets providing 25-60 ft²/ft³ surface area. Standard modules are 2'×2'×1' or 2'×2'×2' blocks. Media selection based on loading rates: 25-35 ft²/ft³ for high-rate applications, 45-60 ft²/ft³ for nitrification. Typical depth ranges 16-25 feet in municipal applications.
• Rotary Distributor: Center-pivot arm with 2-6 distribution arms rotating 1-3 RPM. Arms feature adjustable orifices (¼" to ½") spaced 12-18" on center. Flow rates typically 0.5-4.0 MGD per distributor. Speed control via hydraulic drive or variable frequency drive maintains even distribution.
• Underdrain System: Vitrified clay blocks or precast concrete with 6-12" collection channels sloped 1-2% to central collection point. Includes ventilation ports every 10-15 feet for natural draft aeration. Designed for 1.5-3.0 gpm/ft² peak hydraulic loading.
• Recirculation System: Centrifugal pumps (50-500 HP) providing 1:1 to 4:1 recirculation ratios. Includes flow measurement and VFD control for load management.

• Hydraulic Loading Rate: 0.5-4.0 gpm/ft² (typical municipal range 1.0-2.5 gpm/ft²)
• Organic Loading Rate: 5-40 lbs BOD₅/1000 ft³/day (standard design 15-25 lbs BOD₅/1000 ft³/day)
• Media Depth: 12-30 feet (most common 20-25 feet for municipal applications)
• Recirculation Ratio: 0.5:1 to 4:1 (typical 1:1 to 2:1 for consistent loading)
• Influent BOD₅: 100-400 mg/L (post-primary clarification)
• Expected BOD₅ Removal: 80-95% (design for 85-90% in municipal service)
• Underdrain Air Flow: 0.5-2.0 cfm/ft² of media (natural draft systems)
• Media Specific Surface: 25-60 ft²/ft³ (plastic media typical 27-35 ft²/ft³)
• Distributor Arm Speed: 1-3 rpm (optimize for uniform distribution)
• Head Loss Through Media: 0.1-0.5 feet (clean media, increases with biofilm growth)
• Temperature Range: 40-95°F operational (performance drops below 50°F)
• Minimum Wetting Rate: 0.1 gpm/ft² (prevent media drying during low flows)

• What organic loading rate should be used for sizing? Municipal applications typically use 15-25 lbs BOD₅/1000 ft³/day. Exceeding 30 lbs/1000 ft³/day risks incomplete treatment and odor issues. Under-loading below 10 lbs/1000 ft³/day wastes media volume. Decision requires accurate influent BOD₅ characterization and peak loading analysis.
• Is forced ventilation required or adequate natural draft? Towers over 20 feet with loading >20 lbs BOD₅/1000 ft³/day typically need forced air (0.5-1.5 cfm/ft²). Natural draft adequate for lower loadings but risks anaerobic conditions during summer peaks. Wrong decision leads to odor complaints or unnecessary operating costs.
• What recirculation ratio optimizes performance? Higher ratios (2:1-3:1) improve low-flow performance but increase pumping costs 15-30%. Lower ratios (<1:1) risk uneven wetting and reduced efficiency during diurnal flow variations. Requires detailed flow profile analysis and lifecycle cost evaluation.
• Single-stage or multi-stage configuration? Two-stage systems achieve 90-95% BOD₅ removal versus 80-85% single-stage, but require 40-60% more footprint and complexity. Decision depends on effluent requirements and available land area.

• Division 40 - Process Integration
• Section 40 30 00 - Biological Wastewater Treatment
• Primary specification location for bio-towers

• Material/Equipment Verification: Media specifications and void ratios critical for performance; Verify underdrain system materials and flow distribution; Confirm blower capacity and turndown ratios
• Installation Requirements: Concrete curing time extends schedule 4-6 weeks; Crane access for media placement and equipment installation; Electrical coordination for variable frequency drives
• Field Challenges: Media segregation during placement requires careful handling; Underdrain blockage during construction common issue; Weather protection needed during startup
• Coordination Issues: 16-20 week equipment lead times typical; Media delivery timing critical to prevent double-handling

• Evoqua Water Technologies - Biofor systems for 1-50 MGD applications with extensive municipal references including Riverside, CA and Aurora, CO
• RDP Technologies - Biostyr and Biocarbone systems, strong in Canada with installations at Calgary and Edmonton
• Veolia - Biostyr fixed-film reactors, notable projects include Phoenix, AZ expansions
• Westech Engineering - Custom bio-tower designs for smaller municipalities under 10 MGD capacity

• Membrane Bioreactors (MBRs) - Higher capital cost (30-40% premium) but smaller footprint, preferred for space-constrained sites
• Moving Bed Biofilm Reactors (MBBR) - Lower capital cost, easier retrofit applications, better for smaller plants under 5 MGD
• Conventional Activated Sludge - Lowest capital cost but requires larger footprint and higher operational complexity, suitable when land is available and skilled operators present

Establish direct relationships with manufacturer technical support early - they provide invaluable troubleshooting during startup and optimization. Consider phased media loading to reduce initial capital costs while maintaining expansion capability. Specify spare blower capacity at 110% rather than standard 100% - bio-towers often require higher air flows than initially calculated during peak loading conditions.