MBR - Membrane Bioreactor

Advanced biological treatment system combining activated sludge process with membrane filtration for solid-liquid separation, eliminating secondary clarifiers and producing high-quality effluent suitable for reuse.

Overview

The Membrane Bioreactor (MBR) integrates biological treatment with ultrafiltration (UF) or microfiltration (MF) membranes, replacing the conventional secondary clarifier. This configuration allows operation at high MLSS concentrations (8,000-15,000 mg/L) due to complete biomass retention, enabling compact reactor design and exceptional effluent quality.

When to Use

• Water reuse and reclamation applications
• Space-constrained sites requiring compact footprint
• Stringent effluent requirements (TSS <1 mg/L, turbidity <0.2 NTU)
• Retrofitting existing plants for capacity upgrades
• Industrial wastewater with high-strength or variable loads
• Applications requiring pathogen removal (log reduction credits)

Key Characteristics

• Complete solids separation via membrane barrier
• Decoupled SRT from settling performance
• High MLSS operation (8,000-15,000 mg/L)
• 50-70% footprint reduction vs conventional AS
• Excellent nitrification due to long SRT
• Superior pathogen removal (4-6 log)
• Higher energy consumption (0.4-0.8 kWh/m3)

Membrane Types

Configuration Comparison

Membrane Type	Configuration	Pore Size	Flux Range	Applications
Hollow Fiber	Submerged/External	0.03-0.4 um	15-35 LMH	Most common
Flat Sheet	Submerged	0.1-0.4 um	10-25 LMH	High solids
Tubular	External	0.01-0.1 um	50-150 LMH	Industrial
Ceramic	External	0.01-0.2 um	100-300 LMH	Harsh conditions

Hollow Fiber Membranes

Characteristics:

• High packing density (>300 m2/m3)
• Outside-in or inside-out flow
• Backwashable (outside-in)
• Lower capital cost per m2
• Fiber breakage risk

Major Suppliers:

Supplier	Product	Material	Pore Size
GE/Suez	ZeeWeed	PVDF	0.04 um
Koch	PURON	PES	0.04 um
Mitsubishi	SUR	PE	0.4 um
Toray	HSU	PVDF	0.08 um

Flat Sheet Membranes

Characteristics:

• Lower packing density (50-150 m2/m3)
• More robust to fouling
• Easier cleaning access
• Higher capital cost
• Better for high solids applications

Major Suppliers:

Supplier	Product	Material	Pore Size
Kubota	MF	PE/Chlorinated PE	0.4 um
Huber	VRM	PES	0.04 um
Alfa Laval	MBR	PVDF	0.2 um

Biokinetic Model Integration

ASM1 with SRT Decoupling

MBR allows complete biomass retention, decoupling SRT from hydraulic considerations:

Heterotrophic Growth: $$\mu_H = \mu_{H,max} \times \frac{S_S}{K_S + S_S} \times \frac{S_O}{K_O + S_O}$$

Net Biomass Production: $$P_{X,bio} = \frac{Y_H \times Q \times (S_0 - S)}{1 + b_H \times SRT}$$

MLSS Steady State: $$X_{MLSS} = \frac{SRT \times (Y_H \times r_S + f_d \times b_H \times Y_H \times SRT \times r_S)}{HRT}$$

Where:

• f_d = debris fraction from decay (0.1-0.15)
• Complete retention: X_effluent = 0

Nitrification: $$\mu_A = \mu_{A,max} \times \frac{S_{NH}}{K_{NH} + S_{NH}} \times \frac{S_O}{K_{O,A} + S_O}$$

At high SRT (>15 days), complete nitrification achieved even at low temperatures.

Oxygen Mass Balance

$$\frac{dS_O}{dt} = \frac{Q}{V}(S_{O,in} - S_O) + KLa \times (S_{O,sat} - S_O) - OUR$$

Oxygen Uptake Rate (OUR): $$OUR = \frac{1-Y_H}{Y_H} \times \mu_H \times X_H + \frac{4.57-Y_A}{Y_A} \times \mu_A \times X_A + b_H \times X_H + b_A \times X_A$$

Temperature Effects

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

Parameter	theta Value	Effect at 10C
mu_H,max	1.07	-49%
mu_A,max	1.10	-61%
b_H	1.04	-32%
b_A	1.04	-32%
Viscosity	1.024	+27% (reduces flux)

Membrane Flux and Fouling

Flux Equations

Darcy's Law: $$J = \frac{TMP}{\mu \times R_t}$$

Where:

• J = permeate flux (L/m2/h or LMH)
• TMP = transmembrane pressure (kPa)
• mu = dynamic viscosity (Pa.s)
• R_t = total resistance (1/m)

Total Resistance: $$R_t = R_m + R_c + R_p + R_f$$

Where:

• R_m = membrane intrinsic resistance
• R_c = cake layer resistance (reversible)
• R_p = pore blocking resistance
• R_f = irreversible fouling resistance

Critical Flux Concept

Operating below critical flux minimizes fouling:

$$J_{crit} = f(MLSS, aeration, SRT, EPS)$$

Empirical Correlation: $$J_{crit} = \frac{a \times v_{air}^b}{MLSS^c}$$

Typical values:

• a = 45-60
• b = 0.3-0.5
• c = 0.4-0.6

Fouling Mechanisms

Type	Cause	Recovery	Prevention
Cake	Biomass deposition	Air scouring	High aeration
Pore blocking	Colloids, EPS	Backwash	Pretreatment
Gel layer	SMP accumulation	CIP	Long SRT
Biofouling	Biofilm growth	CIP + chlorine	Maintenance clean
Scaling	Inorganics (Ca, Fe)	Acid clean	pH control

Membrane Aeration

Coarse Bubble Scouring: $$SADm = \frac{Q_{air}}{A_m}$$

Where:

• SADm = Specific Aeration Demand per membrane area (Nm3/m2/h)
• Typical range: 0.3-1.0 Nm3/m2/h

Process Aeration Requirement: $$SADp = \frac{Q_{air}}{Q_{permeate}}$$

Typical SADp: 8-25 Nm3/m3 permeate

Cleaning Cycles

Routine Cleaning Protocol

Cleaning Type	Frequency	Duration	Agent
Relaxation	Every 8-12 min	30-60 sec	None
Backwash	Every 10-15 min	15-30 sec	Permeate
Maintenance clean	Weekly	30-60 min	200-500 ppm NaOCl
Recovery clean	3-6 months	4-8 hours	NaOCl + citric acid

Chemical Cleaning (CIP)

Organic Fouling:

• NaOCl: 500-2000 ppm, 2-6 hours
• NaOH: 0.1-0.5%, 2-4 hours

Inorganic Fouling:

• Citric acid: 2-3%, 2-4 hours
• HCl: 0.1-0.5%, 1-2 hours (metallic)
• Oxalic acid: 1-2%, 2-4 hours (iron)

Recovery Targets:

• Permeability recovery: >90% of initial
• Frequency optimization: Cost vs performance

Design Criteria

Flux Rates

Application	Net Flux (LMH)	Peak Flux (LMH)	Notes
Municipal (HF)	18-25	30-40	Typical design
Municipal (FS)	12-18	20-30	Conservative
Industrial	10-20	20-35	Application dependent
High solids	8-15	15-25	MLSS >12,000
Reuse quality	15-20	25-35	Higher pretreatment

Peak Factor Allowance:

• Average day: Design flux
• Peak hour: 1.5-2.0 x design flux
• Consider 15% membrane contingency

MLSS Concentrations

Configuration	MLSS Range	Typical	F/M Ratio
Submerged HF	8,000-15,000	10,000	0.05-0.15
Submerged FS	10,000-18,000	12,000	0.04-0.12
External loop	5,000-10,000	8,000	0.08-0.20

Aeration Requirements

Membrane Scouring: $$P_{scour} = \frac{SADm \times A_m \times \rho_{air} \times g \times h}{\eta_{blower}}$$

Process Aeration: $$P_{process} = \frac{OD}{\alpha \times SOTE \times \rho_{O2}}$$

Total Specific Energy:

Component	Energy (kWh/m3)
Membrane aeration	0.2-0.4
Process aeration	0.1-0.3
Permeate pumping	0.05-0.10
Recirculation	0.05-0.15
Total	0.4-0.8

Parameters

Membrane Parameters

Parameter	Symbol	Unit	Default	Range	Description
Membrane type	-	-	Hollow Fiber	HF/FS	Configuration
Membrane area	A_m	m2	calculated	-	Total installed
Pore size	-	um	0.04	0.03-0.4	Nominal
Net flux	J_net	LMH	20	10-30	Design flux
Peak flux	J_peak	LMH	30	20-45	Maximum flux
TMP operating	TMP	kPa	15	5-40	Normal operation
TMP alarm	TMP_max	kPa	40	30-60	High limit

Biological Parameters

Parameter	Symbol	Unit	Default	Range	Description
MLSS	X	mg/L	10000	8000-15000	Mixed liquor
SRT	-	days	20	10-40	Solids retention
HRT	-	hours	6	4-10	Hydraulic retention
F/M	-	kg/kg/d	0.08	0.04-0.15	Food to mass
DO setpoint	-	mg/L	2.0	1.5-4.0	Dissolved oxygen
Temperature	T	C	15	10-30	Operating temp

Aeration Parameters

Parameter	Symbol	Unit	Default	Range	Description
SADm	-	Nm3/m2/h	0.5	0.3-1.0	Membrane air
SADp	-	Nm3/m3	15	8-25	Per permeate
Alpha factor	alpha	-	0.5	0.4-0.7	Oxygen transfer
Beta factor	beta	-	0.95	0.90-0.99	Salinity
Theta	theta	-	1.024	-	Temp correction

Cleaning Parameters

Parameter	Unit	Default	Range	Description
Relaxation interval	min	10	8-15	Between permeation
Relaxation duration	sec	45	30-60	Per cycle
Backwash interval	min	12	10-20	Between backwash
Backwash duration	sec	20	15-30	Per cycle
Backwash flux	LMH	35	25-50	Reverse flow
Maintenance clean freq	days	7	3-14	NaOCl clean

Configuration

Example Configuration

{
  "type": "mbr",
  "id": "mbr-train-1",
  "parameters": {
    "bioreactor": {
      "volume": 2500,
      "mlss": 10000,
      "srt": 20,
      "doSetpoint": 2.0,
      "temperature": 15,
      "mode": "aerobic",
      "biokinetics": {
        "model": "ASM1",
        "muHMax": 6.0,
        "muAMax": 0.8,
        "yH": 0.67,
        "yA": 0.24,
        "bH": 0.4,
        "bA": 0.15,
        "ks": 20,
        "kNH": 1.0,
        "kO": 0.2
      }
    },
    "membrane": {
      "type": "hollowFiber",
      "manufacturer": "ZeeWeed",
      "model": "ZW500D",
      "totalArea": 25000,
      "poreSize": 0.04,
      "netFlux": 22,
      "peakFlux": 35,
      "tmpOperating": 15,
      "tmpAlarm": 40
    },
    "aeration": {
      "membrane": {
        "sadm": 0.5,
        "blowerCapacity": 3500,
        "diffuserType": "coarseBubble"
      },
      "process": {
        "kla": 12,
        "alpha": 0.5,
        "beta": 0.95,
        "diffuserType": "fineBubble"
      }
    },
    "cleaning": {
      "relaxation": {
        "interval": 10,
        "duration": 45
      },
      "backwash": {
        "interval": 12,
        "duration": 20,
        "flux": 35
      },
      "maintenanceClean": {
        "frequency": 7,
        "naoclConcentration": 300
      },
      "recoveryClean": {
        "frequency": 180,
        "naoclConcentration": 1500,
        "acidType": "citric",
        "acidConcentration": 2.0
      }
    }
  }
}

Worked Example

Given

• Design flow: 10,000 m3/d average, 15,000 m3/d peak
• Influent: BOD 220 mg/L, TSS 200 mg/L, TKN 45 mg/L, TP 8 mg/L
• Effluent targets: BOD <10 mg/L, TSS <5 mg/L, NH4-N <2 mg/L
• Temperature: 12C minimum
• Water reuse application

Design Calculations

Step 1: Select Membrane System

• Application: Water reuse (stringent quality)
• Select: Hollow fiber, ZeeWeed 500D
• Design flux: 20 LMH (conservative for reuse)
• Peak flux: 32 LMH

Step 2: Calculate Membrane Area $$A_{m,avg} = \frac{Q_{avg}}{J_{net}} = \frac{10,000 \times 1000/24}{20} = 20,833 \text{ m}^2$$

$$A_{m,peak} = \frac{Q_{peak}}{J_{peak}} = \frac{15,000 \times 1000/24}{32} = 19,531 \text{ m}^2$$

Governing: Average flow case With 15% contingency: A_m = 20,833 x 1.15 = 24,000 m2

Step 3: Determine SRT for Nitrification

At 12C: $$\mu_{A,max,12} = 0.8 \times 1.10^{(12-20)} = 0.37 \text{ d}^{-1}$$

$$SRT_{min} = \frac{1}{\mu_{A,max} - b_A} = \frac{1}{0.37 - 0.09} = 3.6 \text{ days}$$

Design SRT = 3 x SRT_min = 11 days Select SRT = 20 days (conservative for MBR, low F/M)

Step 4: Calculate Bioreactor Volume

Select MLSS = 10,000 mg/L

Sludge production: $$P_x = Y_{obs} \times Q \times S_0 = \frac{0.67}{1 + 0.4 \times 20} \times 10,000 \times 0.22 = 163 \text{ kg/d}$$

$$V = \frac{P_x \times SRT}{X} = \frac{163 \times 20}{10} = 326 \text{ m}^3$$

Wait - this is too small. Using mass balance approach:

$$V = \frac{Q \times S_0 \times SRT}{X \times (1 + b \times SRT)} = \frac{10,000 \times 0.22 \times 20}{10 \times (1 + 0.4 \times 20)} = 489 \text{ m}^3$$

Check with HRT: HRT = 489 x 24 / 10,000 = 1.2 hours - too short for MBR

Use F/M approach: $$V = \frac{Q \times S_0}{F/M \times X} = \frac{10,000 \times 0.22}{0.08 \times 10} = 2,750 \text{ m}^3$$

HRT = 2,750 x 24 / 10,000 = 6.6 hours - appropriate

Select V = 2,500 m3 (round down, high MLSS provides buffer)

Step 5: Calculate Oxygen Demand

Carbonaceous: $$OD_C = \frac{Q \times (S_0 - S)}{1.6} = \frac{10,000 \times (220 - 10) \times 0.001}{1.6} = 1,313 \text{ kg O}_2\text{/d}$$

Nitrogenous: $$OD_N = 4.57 \times Q \times (TKN - N_e) = 4.57 \times 10,000 \times (45 - 2) \times 0.001 = 1,965 \text{ kg O}_2\text{/d}$$

Total OD = 1,313 + 1,965 = 3,278 kg O2/d

Step 6: Size Aeration System

For MBR, alpha = 0.5 (high MLSS):

$$SOTR = \frac{OD}{\alpha \times 0.95 \times 0.85} = \frac{3,278}{0.5 \times 0.95 \times 0.85} = 8,119 \text{ kg O}_2\text{/d}$$

At SOTE = 25%: $$Q_{air,process} = \frac{SOTR}{0.25 \times 1.2 \times 0.21 \times 24} = 5,378 \text{ Nm}^3\text{/h}$$

Membrane air (SADm = 0.5): $$Q_{air,membrane} = 0.5 \times 24,000 = 12,000 \text{ Nm}^3\text{/h}$$

Total air = 5,378 + 12,000 = 17,378 Nm3/h

Step 7: Calculate Energy Consumption

Membrane aeration (assume 7 kW per 1000 Nm3/h at 5m depth): P_membrane = 12,000 / 1000 x 7 = 84 kW

Process aeration: P_process = 5,378 / 1000 x 7 = 38 kW

Permeate pumping (0.5 kW per 100 m3/h): P_permeate = 417 / 100 x 0.5 = 2 kW

Recirculation (if external): P_recirc = 0 kW (submerged system)

Total power = 84 + 38 + 2 = 124 kW Specific energy = 124 x 24 / 10,000 = 0.30 kWh/m3 (low - membrane aeration dominates)

Step 8: Design Summary

Parameter	Value	Unit
Bioreactor volume	2,500	m3
Membrane area	24,000	m2
MLSS	10,000	mg/L
SRT	20	days
HRT	6	hours
Design flux	20	LMH
Peak flux	32	LMH
Total air flow	17,400	Nm3/h
Specific energy	0.30	kWh/m3

Troubleshooting

Issue	Possible Cause	Diagnostic	Solution
Rising TMP	Membrane fouling	Check permeability trend	Increase cleaning frequency
	High MLSS	Measure MLSS	Increase WAS
	Insufficient aeration	Check SADm	Increase membrane air
Decreasing flux	Fouling	TMP/flux ratio	Chemical clean
	Membrane damage	Integrity test	Replace modules
Poor effluent quality	Fiber breakage	Turbidity spike, integrity test	Isolate/replace cassette
	Insufficient SRT	Check SRT calculation	Reduce WAS
High energy	Excessive aeration	Audit air flows	Optimize SADm
	Inefficient blowers	Check efficiency	Upgrade/maintain
Foaming	High SRT	Foam characteristics	Reduce SRT, anti-foam
	Nocardia/M. parvicella	Microscopic exam	Chlorine addition
MLSS instability	Inconsistent WAS	Review WAS control	Automate SRT control
	Return sludge issues	Check RAS (if applicable)	Adjust recirculation
Low DO	Insufficient process air	Measure OUR	Increase process aeration
	High organic loading	Check influent BOD	Flow equalization

MBR vs Conventional Activated Sludge

Aspect	MBR	Conventional AS
Footprint	30-50% smaller	Baseline
Effluent TSS	<5 mg/L	10-30 mg/L
Effluent turbidity	<0.5 NTU	5-15 NTU
Pathogen removal	4-6 log	1-2 log
Energy	0.4-0.8 kWh/m3	0.2-0.4 kWh/m3
Capital cost	30-50% higher	Baseline
O&M cost	Higher (membranes)	Lower
Clarifier	Not required	Required
Operator skill	Higher	Standard
Sludge quality	Better settleability irrelevant	SVI dependent

Related Topics

• CSTR - Continuously Stirred Tank Reactor
• IFAS - Integrated Fixed-Film Activated Sludge
• SBR - Sequencing Batch Reactor
• Ultrafiltration - Tertiary membrane filtration
• Nutrient-Removal - BNR configurations

References

1. Judd, S. (2011). The MBR Book: Principles and Applications of Membrane Bioreactors for Water and Wastewater Treatment, 2nd Ed. Butterworth-Heinemann.
2. Metcalf & Eddy (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th Ed. McGraw-Hill.
3. WEF (2012). Membrane Bioreactors, WEF MOP 36.
4. Henze, M. et al. (2000). Activated Sludge Models ASM1, ASM2, ASM2d and ASM3. IWA Publishing.
5. Drews, A. (2010). "Membrane fouling in membrane bioreactors - Characterisation, contradictions, cause and cures." Journal of Membrane Science, 363(1-2), 1-28.
6. Kraume, M., and Drews, A. (2010). "Membrane Bioreactors in Waste Water Treatment - Status and Trends." Chemical Engineering & Technology, 33(8), 1251-1259.