Membrane Biological Reactors

Membrane Biological Reactors (MBRs) combine conventional activated sludge treatment with membrane filtration to produce high-quality effluent suitable for reuse or stringent discharge standards. The process uses submerged ultrafiltration or microfiltration membranes (0.04-0.4 micron pore size) to separate biomass from treated water, eliminating the need for secondary clarifiers. MBRs typically achieve >99% removal of suspended solids and turbidity <0.1 NTU, enabling direct discharge to sensitive receiving waters or advanced treatment trains. The primary trade-off is significantly higher energy consumption (2-4 times conventional treatment) due to membrane aeration requirements and frequent cleaning cycles.

• Municipal Wastewater Treatment (Primary Application): MBRs replace conventional activated sludge systems with secondary clarifiers in 1-25 MGD plants. Raw wastewater flows through preliminary treatment (screening, grit removal) directly to the MBR bioreactor. The integrated membrane filtration produces high-quality effluent suitable for direct discharge or advanced treatment. Selected for footprint reduction (60-80% smaller than conventional), consistent effluent quality regardless of influent variations, and elimination of secondary clarifier issues.

• Water Reclamation Facilities: MBRs serve as the biological treatment step in 0.5-10 MGD reclamation plants, upstream of RO or UV disinfection. The 0.04-0.4 micron membrane pore size removes pathogens and suspended solids, reducing downstream treatment burden. Critical for meeting Title 22 standards.

• Plant Upgrades/Expansions: Retrofit applications where existing clarifiers are converted to MBR systems, increasing capacity 2-3x within existing footprint while improving effluent quality from 30/30 to <5/<2 mg/L BOD/TSS.

Daily Operations: Operators monitor transmembrane pressure (target <6 psi), permeate flow rates, and mixed liquor suspended solids (8,000-12,000 mg/L optimal). Key adjustments include blower output based on dissolved oxygen levels (2-4 mg/L), permeate pump speeds to maintain design flux, and waste activated sludge rates. Daily membrane backwash cycles (30-60 seconds every 10-30 minutes) require monitoring for proper sequencing.

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Maintenance: Weekly membrane integrity testing using pressure decay or bubble point methods. Monthly chemical cleaning with sodium hypochlorite (200-500 mg/L, 30-60 minutes contact). Quarterly intensive CIP using caustic followed by acid cleaning. Blower maintenance every 2,000-4,000 hours including oil changes and belt inspection.

• Membrane Modules: Hollow fiber (Zenon/Suez ZeeWeed) or flat sheet (Kubota) configurations with 0.04-0.4 micron pore size. Modules sized 200-500 ft² surface area each, with 15-25 GFD typical flux rates. Selection based on fouling resistance, cleaning requirements, and replacement costs ($200-400/module).

• Blower Systems: Provide coarse bubble aeration for biological treatment (1-3 SCFM/ft² membrane area) and membrane scouring air (3-6 SCFM/ft² membrane area). Rotary lobe or centrifugal blowers sized 50-500 HP for municipal applications.

• Permeate Pumps: Variable speed pumps maintaining transmembrane pressure 2-8 psi. Centrifugal pumps sized 10-500 HP based on design flow and TMP requirements.

• Chemical Feed Systems: Sodium hypochlorite (50-200 mg/L) for membrane cleaning, caustic/acid for CIP cycles. Metering pumps with 0.1-10 GPH capacity.

• Membrane Tanks: Reinforced concrete or steel tanks housing membrane cassettes, designed for 8-12 hour HRT with mixed liquor suspended solids 8,000-15,000 mg/L.

• Flux Rate: 8-20 GFD (gallons per square foot per day) for municipal applications, with 12-15 GFD typical for new installations. Higher flux rates reduce membrane area but increase fouling potential and energy consumption.

• Transmembrane Pressure (TMP): Operating range 2-8 psi, with cleaning triggered at 10-12 psi. Design systems for maximum 15 psi to accommodate membrane aging and seasonal variations.

• Mixed Liquor Suspended Solids (MLSS): 8,000-15,000 mg/L typical, up to 20,000 mg/L for space-constrained sites. Higher MLSS concentrations reduce tank volume but increase blower energy and membrane fouling rates.

• Food-to-Microorganism Ratio (F/M): 0.05-0.15 lb BOD/lb MLVSS/day for stable operation and good effluent quality.

• Hydraulic Retention Time (HRT): 4-8 hours based on influent strength and treatment objectives. Shorter HRT reduces tank volume but may compromise nitrification.

• Solids Retention Time (SRT): 15-30 days for BOD removal, 20-40 days when nitrification required. Longer SRT improves treatment but increases oxygen demand.

• Air Scour Rate: 4-6 SCFM per square foot of membrane area for fouling control. Represents 60-80% of total plant energy consumption.

• What flux rate should be specified: conservative 10-12 GFD or aggressive 15-18 GFD? Conservative flux extends membrane life 7-10 years vs. 5-7 years but requires 30-50% more membrane area and higher capital cost. Need detailed life-cycle cost analysis including membrane replacement, energy, and chemical costs.

• Should the system be designed for peak month average day (PMAD) or maximum month maximum day (MMMD) flows? PMAD sizing (typical) allows temporary flux increases to 25+ GFD during peak events but risks membrane damage. MMMD sizing prevents overloading but increases capital cost 20-40%. Requires accurate flow projections and peak duration analysis.

• What MLSS concentration: standard 10,000-12,000 mg/L or high-rate 15,000-18,000 mg/L? Higher MLSS reduces bioreactor volume 25-35% but increases membrane fouling, cleaning frequency, and blower energy 40-60%. Critical decision for space-constrained sites or energy-sensitive applications.

• Immersed flat sheet, immersed hollow fiber, or external crossflow configuration? Immersed systems have lower energy (0.3-0.5 kWh/1000 gal) but higher membrane replacement costs. External systems offer better cleaning but consume 2-4 kWh/1000 gal. Configuration affects long-term O&M significantly.

• Division 40 - Process Integration

• Section 40 30 00 - Biological Wastewater Treatment Equipment

• Material/Equipment Verification: Membrane material certification (PVDF, PES), Blower capacity at design flux rates, Chemical compatibility for cleaning systems

• Installation Requirements: Crane access for membrane cassette handling, Precise tank tolerances for membrane mounting, Dedicated electrical for variable frequency drives

• Field Challenges: Membrane damage during installation, Achieving proper air distribution uniformity

• Coordination Issues: SCADA integration complexity, Chemical feed system tie-ins

• Lead Times: 16-24 weeks for membrane modules

• Suez (ZEEWEED): ZW-500c cassettes for 0.5-20 MGD plants

• Evoqua: MBR-SE submerged systems, popular in 1-10 MGD range

• Kubota: Flat sheet modules (510/515 series) for smaller plants 0.1-5 MGD

• Koch Membrane Systems: PURON hollow fiber modules for mid-size applications. All maintain strong municipal references with established service networks across North America.

• Conventional Activated Sludge + Tertiary Filtration: 30-40% lower capital cost, higher footprint. Preferred for plants >20 MGD with available land.

• Moving Bed Biofilm Reactors (MBBR): Similar footprint, 25% lower O&M costs, less stringent effluent quality.

• Sequencing Batch Reactors (SBR): 20-30% lower capital cost, good for smaller plants <2 MGD with variable loading. MBRs preferred when footprint is critical or reuse standards required.

Manufacturer Relationships: Establish service agreements early - membrane cleaning protocols vary significantly between suppliers. Kubota requires different backwash sequences than Suez systems. Cost-Saving: Size blowers for average flux (8-12 GFD) rather than peak design flux (15-20 GFD) - saves 20-30% on energy costs. Consider split-train designs for smaller plants to allow partial operation during maintenance periods.