Rotating Biological Contactors

Rotating Biological Contactors (RBCs) are secondary treatment systems that remove organic matter and ammonia through attached-growth biological processes. Large plastic media discs rotate slowly through wastewater, allowing biofilms to alternately contact organics in the liquid phase and oxygen in the air phase. Typical installations achieve 85-95% BOD removal and 80-90% ammonia oxidation at loading rates of 2-4 gpd/sf. The key trade-off is mechanical complexity and higher maintenance requirements compared to activated sludge systems, particularly for drive mechanisms and media replacement.

• Secondary Treatment for Small-Medium Plants (0.5-10 MGD): RBCs serve as the primary biological treatment following primary clarification. The rotating media provides consistent oxygen transfer and biomass contact, making them ideal for facilities with limited operational staff. Effluent typically flows to secondary clarifiers before disinfection.
• Nitrification Applications (2-25 MGD): RBCs excel in nitrification due to their consistent oxygen supply and long solids retention time. Often configured as tertiary treatment after conventional activated sludge, or as dedicated nitrification trains. The slow rotation (1-2 RPM) prevents biomass shearing while maintaining aerobic conditions throughout the biofilm.
• Small Community Treatment (0.5-5 MGD): RBCs are selected for their operational simplicity and consistent performance with minimal supervision. They handle flow variations well and require less skilled maintenance than activated sludge systems. Common in rural municipalities where operational expertise is limited.
• Industrial Pretreatment Polishing: Used downstream of industrial pretreatment to remove residual organics before discharge to municipal collection systems, particularly effective for food processing and brewery waste streams.

Daily Operations: Operators monitor rotation speed, biomass thickness on discs, and effluent quality through visual inspection and routine sampling. Key parameters include dissolved oxygen (maintain 2-4 mg/L), pH (6.5-8.5), and biomass color (brown indicates healthy aerobic conditions). Flow distribution adjustments ensure even loading across multiple shafts.

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Maintenance: Monthly lubrication of drive systems and bearing inspection. Semi-annual gear oil changes and belt tension adjustments. Annual shaft alignment checks and media cleaning if necessary. Requires basic mechanical skills and standard PPE (hard hat, safety glasses, non-slip footwear). Confined space entry procedures for tank maintenance.

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Troubleshooting: Excessive biomass growth indicates organic overloading - reduce influent flow or increase rotation speed. White/gray biomass suggests oxygen limitation or toxic loading. Bearing failure warning signs include unusual noise, vibration, or temperature rise. Drive system problems typically manifest as speed variations or motor overheating. Expected service life: 15-20 years for media, 10-15 years for drives, 25+ years for concrete structures.

• Rotating Media Discs: High-density polyethylene or polystyrene discs mounted on central shaft, typically 12-foot diameter for municipal applications. Disc spacing ranges 1-2 inches, with total media surface area of 100,000-150,000 sq ft per shaft. Selection based on organic loading (5-15 lbs BOD/1000 sq ft/day).
• Drive System: Low-speed gear reducers (1-2 RPM) with 5-25 HP motors depending on shaft length and media loading. Variable frequency drives increasingly common for energy optimization. Backup drives essential for continuous operation.
• Shaft and Bearings: Stainless steel shafts (6-12 inch diameter) with heavy-duty pillow block bearings rated for continuous submersed operation. Shaft lengths typically 25-100 feet for municipal installations.
• Tank Structure: Concrete basins with 8-12 feet depth, designed for 40% media submergence. Includes influent distribution boxes, effluent weirs, and access walkways. Insulation and covers required in northern climates.

• Hydraulic Loading Rate: 1.0-4.0 gpd/sf of media surface area (typical municipal range 2.0-3.0 gpd/sf)
• Organic Loading Rate: 5-15 lbs BOD₅/1000 sf-day for carbonaceous removal; 2-5 lbs BOD₅/1000 sf-day for nitrification
• Media Surface Area: 100,000-150,000 sf/acre-ft of media volume (standard density discs)
• Rotational Speed: 1.5-2.0 rpm (peripheral speed 50-100 fpm at disc edge)
• Submergence: 35-45% of disc diameter submerged in wastewater
• Disc Diameter: 10-12 feet standard; 8 feet for smaller plants (<2 MGD)
• Shaft Length: Up to 25 feet per shaft assembly
• Drive Power: 0.5-2.0 hp per shaft (includes 50% safety factor)
• Temperature Range: 35-95°F operational (performance drops significantly below 50°F)
• Minimum Flow Velocity: 0.5 fps through RBC tank to prevent solids deposition
• BOD₅ Removal: 85-95% with proper loading rates
• Effluent Quality: 15-25 mg/L BOD₅, 15-30 mg/L TSS achievable

• Single-stage vs. multi-stage configuration? Threshold: >20 mg/L effluent BOD₅ target requires 2-4 stages in series. Single-stage limited to 25-30 mg/L BOD₅ removal. Wrong decision results in permit violations or oversized/costly systems. Need influent BOD₅, effluent targets, and nitrification requirements.
• Standard vs. high-density media selection? Threshold: Organic loading >10 lbs BOD₅/1000 sf-day requires high-density media (200,000+ sf/acre-ft). Standard density adequate for typical municipal loads. Wrong choice causes biomass washout or insufficient treatment. Need peak organic loading calculations and space constraints.
• Enclosed vs. open tank design? Threshold: Cold climates (<40°F winter temps) require enclosures with heating/ventilation. Open systems acceptable in moderate climates. Wrong decision causes winter treatment failure or unnecessary capital costs. Need local climate data and year-round temperature requirements.
• Drive system redundancy requirements? Threshold: Plants >5 MGD typically require backup drives or dual-drive systems. Smaller plants may accept single drives with rapid repair capability. Wrong decision risks extended downtime. Need bypass capacity and maintenance resources assessment.

• Primary: Division 46 - Water and Wastewater Equipment, Section 46 55 00 - Biological Wastewater Treatment Equipment
• Secondary: Division 40 - Process Integration (for control systems integration)

• Material/Equipment Verification: Verify media material (HDPE vs. polystyrene) and density specifications; Confirm drive system sizing and backup power requirements; Check structural steel coating specifications for corrosive environments
• Installation Requirements: Level concrete foundations critical for proper shaft alignment; Adequate crane access for media installation and future maintenance; Electrical coordination for variable frequency drives
• Field Challenges: Media shipping damage common - inspect before installation; Cold weather installation affects bearing lubrication; 16-20 week lead times typical
• Coordination Issues: Interface with upstream screening and grit removal; Effluent weir coordination with downstream clarifiers

• Evoqua/Envirex: Bio-Surf RBC systems, standard municipal packages 0.1-10 MGD capacity
• Walker Process: Biocyl rotating contactors with enclosed designs for cold climates
• Pollution Control Systems: Rotating biological contactor packages focused on smaller municipal plants
• Lakeside Equipment: Raptor RBC systems with integrated screening, popular for package plant applications under 2 MGD

• Activated Sludge: Preferred for larger plants (>5 MGD) with lower footprint requirements, 20-30% higher operating costs
• Moving Bed Biofilm Reactors (MBBR): Better process control and higher loading rates, 40-50% higher capital cost but smaller footprint
• Lagoon Systems: Lower cost option for smaller communities with available land, significantly longer detention times required but minimal operational complexity

Establish strong relationships with manufacturer service teams - RBC troubleshooting often requires specialized knowledge of bearing systems and media replacement techniques. Negotiate spare parts packages upfront, particularly for drive components and media segments. Consider enclosed designs in northern climates to prevent freezing issues. Budget 15-20% contingency for site-specific modifications during installation, as RBC foundations require precise tolerances.