MBR Membrane System

Overview

A membrane bioreactor (MBR) integrates biological treatment and physical separation into a single reactor, eliminating the need for a separate secondary clarifier. Fine ultrafiltration membranes (pore size 0.04–0.2 μm) submerged in the aeration basin continuously filter mixed liquor, producing permeate of exceptional clarity (<1 NTU) while concentrating sludge in the reactor.

The key advantage is process compactness and effluent quality suitable for direct reuse (non-potable irrigation, industrial cooling) or stringent discharge permits. MBRs handle variable inlet quality and support higher MLSS concentrations (8000–12000 mg/L vs. 3000–4000 mg/L in conventional activated sludge) in a smaller footprint.

Membrane Technology

Two dominant membrane types are employed:

Flat-Sheet (Plate) Modules: PVDF or polyethersulfone membranes mounted on plastic frames, submerged horizontally or vertically. Typical module size: 0.5–3.0 m² per frame. Advantages: easier cleaning, modular scaling. Disadvantages: higher fouling rate due to lower shear.

Hollow-Fiber Modules: PVDF fibers (0.7–0.8 mm inner diameter) bundled into cartridges, providing 100–200 m² per m³ tank volume. Advantages: high surface area, moderate fouling. Disadvantages: fiber breakage if grit penetrates, more complex cleaning.

Pore size selection:

• 0.04–0.06 μm: Removes viruses; highest fouling rate; used in potable reuse
• 0.1–0.2 μm: Standard municipal; balances fouling and pathogen removal

Permeation Mechanism

Membrane filtration is driven by transmembrane pressure (TMP), the pressure difference between the inside of the membrane fiber and the sludge tank:

TMP = P_suction + P_hydrostatic - P_backpressure

For a typical MBR with Permeate Pump applying 0.3 bar vacuum:

• P_suction = 0.3 bar (gauge)
• P_hydrostatic = ρgh / 100,000 Pa → ~0.05 bar for 0.5 m submersion
• P_backpressure = atmospheric (0 gauge)
• TMP ≈ 0.35 bar

Permeate flux (flow per unit area) is expressed as L/m²/h: Flux = Q_permeate / A_membrane

For 100 m³/day (4.2 m³/h) and 50 m² membrane area: Flux = (4200 L/h) / (50 m²) = 84 L/m²/h

Sustainable flux ranges from 10–30 L/m²/h depending on sludge settleability and fouling propensity. Higher flux accelerates TMP rise (fouling).

Fouling and TMP Rise

Over time, solutes, colloids, and microbial cells accumulate inside membrane pores (internal fouling) or form a biofilm on the surface (external fouling). This manifests as rising TMP while flow remains constant.

Fouling rate = dTMP/dt (bar/day)

Typical values:

• Low fouling: 0.01–0.05 bar/day (excellent sludge quality, optimized DO)
• Moderate fouling: 0.05–0.1 bar/day (standard operation)
• High fouling: >0.1 bar/day (indicates SRT too short, MLSS too high, or DO deficiency)

When TMP reaches 0.4–0.5 bar, in-situ chemical cleaning is triggered to restore initial flux.

Clean-in-Place (CIP) Protocol

Periodic chemical back-flush removes reversible (cake) and irreversible (internal/external) foulants:

Protocol:

1. Caustic Soak (NaOH, pH 11–13): 30–60 min circulation at 5–20 m³/h; removes proteins and humic matter
2. Acid Soak (HCl or citric acid, pH 2–3): 30–60 min circulation; dissolves mineral deposits (calcium, iron oxides)
3. Rinse: Fresh water flush until pH neutral
4. Sodium Hypochlorite (NaOCl, 200–500 mg/L available chlorine): 30–60 min hold or circulation; oxidizes biofilm

CIP frequency depends on fouling rate; typically every 2–4 weeks for municipal sludge, or more frequently for high-strength industrial waste.

Aeration and Mixing

Two air streams serve the MBR:

1. Aeration Blower (Fine Bubble Aeration System): Supplies DO for biological treatment (typically 3–5 mg/L setpoint) via fine-bubble diffusers in the aeration zone.

2. Air Scour Blower (Air Scour Blower): Produces coarse bubbles at 0.2–0.5 m³/min directly below the membrane module to create shear flow, preventing cake buildup. This scour stream is often intermittent (5 min on, 5 min off) to balance fouling control with energy efficiency.

Air scour intensity is critical: insufficient aeration causes rapid fouling; excessive aeration increases power consumption and biosolids oxidation. Optimal scour is 0.5–2 m³ air per m³ permeate produced.

Bioreactor Control

The Control Panel monitors three key parameters:

• Dissolved Oxygen (DO): Set point 2–4 mg/L; blower modulation via VFD or on/off control maintains target
• Mixed Liquor Suspended Solids (MLSS): Manual assessment or online sensor; typical range 8000–12000 mg/L; controlled via waste sludge withdrawal rate
• Transmembrane Pressure (TMP): Continuous monitoring; rising TMP signals fouling; triggers air scour intensification or CIP scheduling

Sludge age (SRT) is managed by waste sludge withdrawal. Typical SRT for nitrification: 15–25 days. For carbon removal alone: 5–10 days.

SRT = (MLSS × V_tank) / (waste_sludge_mass_per_day)

Permeate Quality

Post-membrane filtration:

• Turbidity: <1 NTU (vs. 5–15 NTU from gravity clarifier)
• Suspended Solids: <1 mg/L
• BOD: 2–5 mg/L (biological activity still occurs; not total removal)
• Bacteria: 10⁵–10⁶ CFU/mL (some cells bypass; disinfection required for potable reuse)

Viruses and colloids are significantly reduced (3–4 log removal) but not completely eliminated unless combined with UV or ozonation.

Advantages

• Compact: 0.5–1.0 m² footprint per m³/day (vs. 1.5–3.0 m² for clarifier + filtration train)
• Superior Effluent: Suitable for indirect potable reuse or agricultural irrigation
• Pathogen Control: 3–4 log removal without chemical disinfection
• Easy Capacity Expansion: Add membrane modules in parallel
• Consistent Sludge Quality: MLSS stabilized by membrane retention

Limitations

• High Capital Cost: Membrane modules: €100–300/m²
• Operating Complexity: CIP cycles, TMP monitoring, seasonal fouling variation
• Energy Intensive: 1–3 kWh/m³ (air scour and permeate pump combined)
• Membrane Lifespan: 5–10 years; replacement cost significant
• Fouling Sensitivity: Grit, oils, or high-strength surges accelerate degradation

Sludge Handling

MBR sludge is highly concentrated (8000–12000 mg/L) and gelatinous due to high-SRT biomass with substantial EPS (extracellular polymeric substances). Thickening via DAF System or centrifugation is typically necessary before dewatering via Belt Filter Press or Sludge Screw Press.

Solids yield from MBRs is often lower than conventional plants (0.4–0.6 kg TSS/kg BOD removed) due to longer SRT and endogenous respiration.

Standards and Guidelines

• USEPA Design Manual: Membrane Filtration
• ASABE EP496: Membrane Bioreactors for Municipal Applications
• WPCF Manual of Practice: Biological Processes
• EN 12255-10: Membrane bioreactors