MABR - Membrane Aerated Biofilm Reactor

Advanced biofilm reactor using gas-permeable membranes for oxygen transfer directly into the biofilm, enabling counter-diffusion kinetics and exceptional energy efficiency.

Overview

The Membrane Aerated Biofilm Reactor (MABR) represents a paradigm shift in biological wastewater treatment. Oxygen is supplied through gas-permeable hollow fiber membranes, diffusing directly into the biofilm from the opposite direction of substrate diffusion. This counter-diffusion mechanism creates unique stratified biofilm conditions enabling simultaneous nitrification-denitrification (SND) in a single biofilm.

When to Use

• Energy-efficient nitrification upgrades
• Simultaneous nitrification-denitrification (SND)
• Space-constrained retrofit applications
• Decentralized treatment systems
• Low-energy treatment requirements
• Nutrient removal with limited aeration capacity

Key Characteristics

• 75-90% reduction in aeration energy vs conventional
• Counter-diffusion oxygen transfer
• Bubble-free aeration (no off-gas)
• Self-stratifying biofilm for SND
• No air entrainment or foaming
• Modular, scalable design

Counter-Diffusion Theory

Biofilm Structure

In MABR, the biofilm develops on the outer surface of gas-permeable membranes:

Membrane | Aerobic Zone | Anoxic Zone | Bulk Liquid
(O2 source)  (nitrifiers)   (denitrifiers)  (substrate)
   ←O2        →←S             →←S              S→

Oxygen Diffusion Through Membrane

$$J_{O2} = \frac{P_{O2} \times (p_{O2,gas} - p_{O2,biofilm})}{d_m}$$

Where:

• J_O2 = oxygen flux (g O2/m2/d)
• P_O2 = membrane oxygen permeability (m2/d/atm)
• p_O2,gas = partial pressure in lumen (atm)
• p_O2,biofilm = partial pressure at membrane-biofilm interface (atm)
• d_m = membrane wall thickness (m)

Counter-Diffusion Kinetics

Oxygen flux into biofilm (from membrane): $$J_{O2} = D_{O2,f} \times \frac{dC_{O2}}{dz}\bigg|_{z=0}$$

Substrate flux into biofilm (from bulk): $$J_S = D_{S,f} \times \frac{dC_S}{dz}\bigg|_{z=\delta}$$

Where:

• D_O2,f = oxygen diffusion coefficient in biofilm (m2/d)
• D_S,f = substrate diffusion coefficient in biofilm (m2/d)
• z = depth into biofilm from membrane surface (m)
• delta = biofilm thickness (m)

Biofilm Stratification

The counter-diffusion creates distinct zones:

Zone	Location	DO Level	Process
Aerobic	Near membrane	2-8 mg/L	Nitrification
Transition	Mid-biofilm	0.5-2 mg/L	Partial nitrification
Anoxic	Near bulk	<0.5 mg/L	Denitrification
Anaerobic	Outer surface	0 mg/L	Fermentation (thick biofilm)

SND Efficiency

Simultaneous nitrification-denitrification efficiency:

$$\eta_{SND} = \frac{TN_{removed} - NO_x^{effluent}}{TN_{removed}} \times 100%$$

Typical SND efficiency: 50-90%

Design Equations

Oxygen Transfer Rate (OTR)

$$OTR = K_m \times A_m \times (p_{O2,gas} - p_{O2,biofilm})$$

Where:

• OTR = oxygen transfer rate (kg O2/d)
• K_m = overall mass transfer coefficient (kg/m2/d/atm)
• A_m = membrane surface area (m2)

Oxygen Transfer Efficiency (OTE)

$$OTE = \frac{OTR}{Q_{air} \times \rho_{O2}} \times 100%$$

MABR achieves OTE of 40-90% vs 15-35% for fine bubble diffusers.

Specific Oxygen Flux

$$J_{O2,specific} = \frac{OTR}{A_m}$$

Typical range: 2-8 g O2/m2/h

Membrane Area Requirement

For nitrification:

$$A_m = \frac{Q \times \Delta NH_4 \times 4.57}{J_{O2,specific} \times 24}$$

Where:

• 4.57 = O2 required per g NH4-N oxidized
• Delta NH4 = ammonia removal required (mg/L)

Surface Area Loading Rate

$$SALR_{NH4} = \frac{Q \times \Delta NH_4}{A_m}$$

Units: g NH4-N/m2/d Typical: 1-3 g NH4-N/m2/d

Energy Consumption

$$E_{MABR} = \frac{P_{blower}}{Q \times \Delta COD}$$

Typical: 0.1-0.3 kWh/kg COD removed (vs 0.8-1.5 for conventional)

Design Criteria

Oxygen Transfer

Parameter	Value	Units
Membrane OTE	40-90	%
Oxygen flux	2-8	g O2/m2/h
Air pressure	3-15	kPa (gauge)
Air flow per module	0.5-2	m3/h

Loading Rates

Application	SALR	Expected Performance
BOD removal	5-15 g BOD/m2/d	70-90% removal
Nitrification	1-3 g NH4-N/m2/d	>90% conversion
SND	0.5-2 g TN/m2/d	50-80% TN removal
Tertiary	0.5-1 g NH4-N/m2/d	<1 mg/L effluent

Process Conditions

Parameter	Optimal Range	Notes
Temperature	15-30 deg C	T correction needed below 15
Bulk DO	0-2 mg/L	Low bulk DO promotes SND
pH	7.0-8.5	Affects nitrification rate
Biofilm thickness	100-500 um	Self-regulating

Temperature Correction

$$k(T) = k_{20} \times 1.072^{(T-20)}$$

At 10 deg C: reduce design SALR by 50%

Parameters

Membrane Parameters

Parameter	Unit	Default	Range	Description
Membrane material	-	PDMS	PDMS/PP/PE	Polymer type
Fiber OD	mm	0.5	0.2-2.0	Outer diameter
Fiber ID	mm	0.3	0.1-1.5	Inner diameter
Wall thickness	um	100	50-300	Membrane wall
Packing density	m2/m3	200	100-500	Area per module volume
O2 permeability	Barrer	600	200-1000	Material property

Module Parameters

Parameter	Unit	Default	Range	Description
Module type	-	spiralWound	hollow/spiral/flat	Configuration
Module area	m2	40	10-100	Active membrane area
Module volume	m3	0.2	0.1-0.5	Module displacement
Fill fraction	%	30	20-50	Tank volume fraction

Process Parameters

Parameter	Unit	Default	Range	Description
Lumen pressure	kPa	8	3-15	Air supply pressure
Air flow rate	m3/h/module	1	0.5-2	Per module
Bulk DO	mg/L	0.5	0-2	In tank
HRT	hours	4	2-12	Hydraulic retention
MLSS	mg/L	3000	2000-5000	If hybrid system

Biofilm Parameters

Parameter	Unit	Default	Range	Description
Biofilm thickness	um	300	100-500	Active biofilm
Biofilm density	g VS/L	30	20-50	Volatile solids
k_0.5 NH4	g^0.5/m2/d	2.0	1.5-3.0	Nitrification rate
Aerobic fraction	%	40	30-60	Of biofilm depth

Membrane Types

Hollow Fiber

Property	Dense	Microporous
Material	PDMS, silicone	PP, PE, PVDF
Transfer mechanism	Solution-diffusion	Pore diffusion
Bubble formation	None	Possible at high P
Fouling tendency	Low	Moderate
OTE	50-90%	30-60%

Module Configurations

Configuration	Specific Area	Application
Spiral wound	200-300 m2/m3	Large scale
Curtain/cassette	100-200 m2/m3	Retrofit
Mat/panel	50-150 m2/m3	Modular
Tubular	20-50 m2/m3	High-fouling

Configuration

Example Configuration

{
  "type": "mabr",
  "id": "mabr-nitrification",
  "parameters": {
    "tankVolume": 500,
    "membraneModules": {
      "count": 20,
      "type": "spiralWound",
      "areaPerModule": 40,
      "packingDensity": 200
    },
    "totalMembraneArea": 800,
    "fillFraction": 0.30,
    "airSupply": {
      "pressure": 8,
      "flowPerModule": 1.0,
      "oxygenPurity": 21
    },
    "processConditions": {
      "bulkDO": 0.5,
      "temperature": 18,
      "pH": 7.5,
      "hrt": 4
    },
    "biofilm": {
      "targetThickness": 300,
      "salrNH4": 2.0,
      "expectedSND": 70
    },
    "hybridMLSS": 3000,
    "mixing": {
      "type": "mechanical",
      "power": 5
    }
  }
}

Worked Example

Given

• Flow: 5,000 m3/d
• Influent NH4-N: 35 mg/L
• Target: ❤️ mg/L effluent NH4-N
• Target: TN <10 mg/L (with SND)
• Temperature: 15 deg C
• Space constraint: max 300 m2 footprint

Design

Step 1: Required Ammonia Removal $$\Delta NH_4 = 35 - 3 = 32 \text{ mg/L}$$

Ammonia load = 5,000 x 0.032 = 160 kg NH4-N/d

Step 2: Temperature-Corrected SALR

At 15 deg C: $$SALR_{15} = SALR_{20} \times 1.072^{(15-20)}$$ $$SALR_{15} = 2.0 \times 0.70 = 1.4 \text{ g NH}_4\text{-N/m}^2\text{/d}$$

Step 3: Required Membrane Area $$A_m = \frac{160,000 \text{ g/d}}{1.4 \text{ g/m}^2\text{/d}} = 114,000 \text{ m}^2$$

Wait - this seems high. Let's verify with oxygen approach:

Step 3b: Oxygen Requirement O2 for nitrification = 160 kg NH4-N/d x 4.57 = 731 kg O2/d

At J_O2 = 4 g O2/m2/h: $$A_m = \frac{731,000}{4 \times 24} = 7,615 \text{ m}^2$$

This is more realistic. Using oxygen-limited design.

Step 4: Module Selection Using 40 m2/module spiral wound: $$N_{modules} = 7,615 / 40 = 191 \text{ modules}$$

Round to 200 modules

Step 5: Tank Volume Module volume = 0.2 m3 each Module displacement = 200 x 0.2 = 40 m3

At 30% fill fraction: $$V_{tank} = 40 / 0.30 = 133 \text{ m}^3$$

Minimum for HRT check: $$HRT = 133 / (5000/24) = 0.64 \text{ h}$$ - too short

Increase to V = 800 m3 for HRT = 3.8 h (adequate for SND)

Step 6: Dimensions At depth 4m: $$A_{floor} = 800/4 = 200 \text{ m}^2$$ < 300 m2 constraint

Tank: 14m x 14m x 4m

Step 7: Air Supply Air per module = 1.0 m3/h Total air = 200 x 1.0 = 200 m3/h

At 8 kPa gauge pressure, blower power approx 5 kW

Step 8: Energy Comparison

MABR energy: 5 kW x 24 h = 120 kWh/d

Conventional aeration for same O2:

• Fine bubble at 2 kg O2/kWh: 731/2 = 366 kWh/d

Energy savings: 67%

Step 9: SND Performance With bulk DO = 0.5 mg/L and MABR biofilm: Expected SND efficiency: 60-70%

Effluent NO3-N produced: 32 x (1-0.65) = 11 mg/L Available for denitrification in anoxic zone: 32 x 0.65 = 21 mg/L Effluent TN = 3 + 11 = 14 mg/L

To achieve <10 mg/L, need supplemental anoxic zone or increase SND.

Hybrid MABR Systems

MABR + IFAS

Combine MABR membranes with suspended growth:

• MABR handles nitrification
• Suspended growth handles BOD
• Excellent for upgrades

MABR + MBR

Post-MABR MBR for:

• Solid-liquid separation
• Polishing
• Water reuse applications

Design Considerations

Hybrid System	MABR Role	Benefit
MABR + AS	Nitrification	50% energy reduction
MABR + IFAS	SND	Compact, high N removal
MABR + MBR	Pre-treatment	Reduced MBR aeration
MABR + Anoxic	N removal	Complete BNR

Operation and Maintenance

Biofilm Management

• Self-regulating thickness (typically 200-400 um)
• Excessive biofilm controlled by:
  • Air scouring (periodic high flow)
  • Reducing substrate loading
  • Mechanical cleaning (rarely needed)

Membrane Integrity

Check	Frequency	Method
Pressure drop	Daily	Monitor lumen pressure
OTE monitoring	Weekly	O2 mass balance
Visual inspection	Monthly	Endoscope or removal
Integrity test	Quarterly	Pressure decay test

Common Issues

Issue	Indicator	Solution
Membrane fouling	Rising pressure drop	Air scour, clean
Biofilm excess	Reduced OTE	Increase air scour
Membrane damage	Bubbles visible	Replace module
Poor SND	High effluent NO3	Reduce bulk DO

Troubleshooting

Issue	Cause	Solution
Low nitrification	Insufficient O2	Increase air pressure
	Cold temperature	Reduce loading, heat
	High bulk DO	Reduce, promote SND
Poor SND	Bulk DO too high	Target <0.5 mg/L
	Thin biofilm	Increase loading
High effluent NH4	Overloading	Add membrane area
	pH inhibition	Adjust alkalinity
Rising head loss	Biofilm accumulation	Air scour cleaning
	Membrane fouling	Chemical cleaning
Membrane failure	Age/damage	Replace modules
	Over-pressure	Check air supply
Uneven treatment	Poor mixing	Increase mixer power

Advantages vs Conventional

Aspect	MABR	Fine Bubble	Benefit
OTE	40-90%	15-35%	2-4x higher
Energy	0.1-0.3 kWh/kg COD	0.8-1.5	60-80% savings
Footprint	Compact	Large	30-50% smaller
SND capability	Inherent	Requires zones	Simpler BNR
Off-gas	None	Yes	Indoor applications
Noise	Low	Moderate	Better environment
Foaming	None	Possible	Easier operation

Commercial Technologies

Technology	Manufacturer	Configuration
ZeeLung	Suez	Spiral wound cassettes
OxyMem	Suez/OxyMem	Hollow fiber curtains
Fluence MABR	Fluence	Spiral wound modules
SUBRE	Oxymem	Submerged modules

Related Topics

• MBBR - Moving Bed Biofilm Reactor
• IFAS - Integrated Fixed-film AS
• Nitrification-Fundamentals
• Simultaneous-Nitrification-Denitrification
• Aeration-Systems

References

1. Syron, E. & Casey, E. (2008). "Membrane-aerated biofilms for high rate biotreatment". Critical Reviews in Environmental Science and Technology.
2. Heffernan, B. et al. (2017). "MABR - An emerging wastewater treatment technology". Water Science & Technology.
3. Houweling, D. et al. (2017). "MABR demonstration results and full-scale design considerations". WEFTEC Proceedings.
4. Suez (2021). "ZeeLung MABR Design and Operations Manual".
5. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Edition.
6. Nerenberg, R. (2016). "The membrane-biofilm reactor (MBfR) as a counter-diffusional biofilm process". Current Opinion in Biotechnology.