Oxidation Ditch Equipment

Oxidation ditches provide biological nutrient removal and secondary treatment by circulating mixed liquor through oval or circular channels using horizontal brush aerators or submersible mixers. The extended aeration process maintains dissolved oxygen levels of 1-3 mg/L while achieving complete nitrification and partial denitrification through alternating aerobic and anoxic zones. Typical systems handle 0.5-20 MGD flows with SRTs of 15-30 days, achieving BOD removal efficiencies exceeding 95% and total nitrogen removal of 70-85%. The primary trade-off is higher energy consumption (60-90 kWh per million gallons) compared to conventional activated sludge due to extended aeration requirements and mechanical mixing.

• Extended Aeration Secondary Treatment (0.5-15 MGD): Oxidation ditches serve as complete biological treatment systems, receiving screened and degritted primary effluent. The oval racetrack configuration provides 18-36 hour detention times for complete carbonaceous and nitrification treatment. Selected for small communities requiring simple operation, minimal sludge production, and consistent effluent quality without primary clarification.
• Nutrient Removal Upgrade (2-25 MGD): Existing plants retrofit oxidation ditches with anoxic zones and internal recycle pumps for BNR. The long SRT (15-30 days) and plug-flow hydraulics create ideal conditions for simultaneous nitrification-denitrification. Selected when discharge permits require <10 mg/L TN and <1 mg/L TP.
• Industrial Pretreatment Integration (5-50 MGD): Municipal plants treating significant industrial loads use oxidation ditches for shock loading equalization and high-strength waste treatment. The large mixed liquor volume buffers toxic events and maintains stable biomass. Connected downstream to secondary clarifiers and tertiary polishing.

Daily Operations: Operators monitor DO levels (maintain 1-3 mg/L), MLSS concentrations, and brush aerator performance through visual inspection of spray patterns and foam characteristics. SVI testing guides sludge wasting decisions. Flow pacing controls automatically adjust aerator speeds, but operators fine-tune based on effluent ammonia and nitrate levels.

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Maintenance: Brush aerator bearings require monthly greasing, with annual gearbox oil changes and brush replacement every 2-3 years. Confined space entry procedures required for channel inspection and cleaning. Operators need basic mechanical skills for belt adjustments and minor electrical troubleshooting. RAS pump maintenance follows standard centrifugal pump protocols.

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Troubleshooting: Brush aerator bearing failure creates excessive noise and vibration - typically 5-8 year service life. Filamentous bulking appears as poor settling and white foam accumulation. Channel short-circuiting shows as uneven DO distribution and poor mixing patterns. Poor nitrification indicates

• Horizontal Brush Aerators: Galvanized steel or aluminum rotors with synthetic bristles provide oxygen transfer (2.0-3.5 lbs O2/hp-hr) and channel velocity (0.8-1.2 fps). Sizes range from 3-12 feet diameter with 5-150 HP motors. Selection based on oxygen demand calculations and channel geometry.
• Concrete Channel System: Reinforced concrete oval configuration, typically 8-15 feet deep with 3:1 side slopes. Channel widths range 15-40 feet for municipal applications. Design factors include soil conditions, groundwater, and construction access for equipment maintenance.
• Mixed Liquor Recirculation Pumps: Submersible or dry-pit pumps (100-2000 GPM) create internal recycle for BNR processes. Typically 3-10 times average daily flow. Materials include cast iron volutes with 316 SS impellers for mixed liquor service.
• Clarifier Integration: Dedicated secondary clarifiers (typically 800-1200 gpd/sf) with RAS pumping systems. Return rates of 50-100% of influent flow maintain 3000-6000 mg/L MLSS concentrations in the oxidation ditch.

• Flow and Hydraulic Parameters: Design flow: 0.5-50 MGD (typical municipal range), Peak flow factor: 2.5-4.0 times average daily flow, Hydraulic retention time: 8-24 hours (typically 12-18 hours), Mixed liquor velocity: 0.8-1.2 ft/s minimum for solids suspension, Channel depth: 8-16 feet (typically 12-14 feet for efficient mixing)
• Process Loading Criteria: BOD loading: 15-40 lbs BOD/1000 ft³/day (food-to-microorganism ratio), MLSS concentration: 2,000-4,500 mg/L (typically 3,000-4,000 mg/L), Sludge age: 15-30 days for nitrification, 20+ days for nutrient removal, Dissolved oxygen: 1.5-3.0 mg/L (2.0 mg/L minimum for nitrification)
• Equipment Sizing: Rotor diameter: 36-120 inches (Lakeside, Andritz standard sizes), Power density: 8-20 HP/MG (varies with configuration and DO requirements), Wire-to-water efficiency: 2.5-4.0 lbs O₂/HP-hr for brush rotors, Channel width-to-depth ratio: 1.5:1 to 3:1 for optimal mixing patterns

• What aeration configuration meets oxygen demand while maintaining velocity? Brush rotors vs. fine bubble diffusers vs. hybrid systems. Brush rotors provide 2.5-4.0 lbs O₂/HP-hr with inherent mixing. Fine bubble systems achieve 4-6 lbs O₂/HP-hr but require separate mixing. Wrong choice impacts both capital costs ($200-400/HP difference) and long-term O&M. Need influent BOD, peak loading, and nitrification requirements.
• How many channels provide optimal redundancy without excessive costs? Single channel (small plants <2 MGD), dual channel (2-15 MGD), or multi-channel (>15 MGD). Single channel risks total shutdown during maintenance. Oversizing for redundancy increases construction costs 30-50%. Need reliability requirements, maintenance philosophy, and bypass capabilities.
• What channel geometry optimizes mixing while minimizing short-circuiting? Length-to-width ratios of 10:1 to 40:1, with turning radius 1.5-3 times channel width. Poor geometry creates dead zones, reduces treatment efficiency, and causes septicity. Need tracer studies for existing plants, computational modeling for new designs.
• Does biological nutrient removal require zone configuration modifications? Anaerobic/anoxic zones require 20-30% of total volume, modified mixing patterns, and internal recycle pumping (3-5 times influent flow). Retrofit costs $1-3 million per MGD. Need effluent limits, influent characteristics, and future regulatory

• Material/Equipment Verification: Verify diffuser membrane material (EPDM vs silicone) for site conditions, Confirm blower capacity at actual site elevation and temperature, Check motor efficiency ratings meet utility rebate requirements
• Installation Requirements: Crane access for rotor/aerator installation in existing facilities, Electrical coordination for VFD integration with SCADA systems, Concrete anchor bolt templates critical for surface aerators
• Field Challenges: Diffuser grid installation in active tanks requires careful staging, Blower building ventilation often undersized in original design
• Coordination Issues: 12-16 week lead times for custom rotors; 6-8 weeks for standard diffusers

• Xylem (SANITAIRE) - SANITAIRE Silver Series II fine bubble diffusers, dominant in municipal market
• Evoqua - Envirex rotors and brush aerators, strong in package plant sector
• Ovivo - Lemna oxidation ditch systems with integrated clarifiers
• Environmental Dynamics Inc. - AIRE-O2 Triton surface aerators, popular for smaller facilities under 5 MGD

• Membrane Bioreactors (MBR) - 30-50% smaller footprint, 2-3x capital cost, preferred for tight sites or advanced treatment
• Sequential Batch Reactors (SBR) - Similar footprint, 10-15% lower energy, better for variable flows under 10 MGD
• Conventional activated sludge with separate clarifiers - 20-25% lower capital cost, preferred for flows over 50 MGD with available land

Budget 15-20% contingency for electrical upgrades when retrofitting aerators - older plants rarely have adequate power infrastructure. Establish service agreements during procurement; manufacturers prioritize customers with active contracts for emergency repairs. Consider standardizing on one manufacturer across multiple basins to reduce spare parts inventory and training requirements.