Membrane Filtration

Pressure-driven separation processes using semi-permeable membranes for particle, pathogen, and dissolved solids removal across the spectrum from microfiltration to reverse osmosis.

Overview

Membrane processes provide absolute barriers to contaminants based on size exclusion and, in some cases, charge-based (Donnan) exclusion. Ranging from microfiltration (bacteria/protozoa removal) to reverse osmosis (desalination), membranes enable advanced water treatment including drinking water production, water reuse, and industrial process water applications.

Membrane Hierarchy

Membranes are classified by pore size, operating pressure, and rejection characteristics:

Coarse → Fine                                        Low Pressure → High Pressure
┌─────────────┬──────────────┬──────────────┬─────────────────┐
│     MF      │      UF      │      NF      │       RO        │
├─────────────┼──────────────┼──────────────┼─────────────────┤
│ 0.1-10 μm   │ 0.01-0.1 μm  │  0.5-2 nm    │   <0.001 μm     │
│             │  (10-300 kDa)│ (200-1000 Da)│    (<100 Da)    │
├─────────────┼──────────────┼──────────────┼─────────────────┤
│ 0.1-0.5 bar │  0.5-2 bar   │   5-20 bar   │   10-70 bar     │
├─────────────┼──────────────┼──────────────┼─────────────────┤
│ Size only   │  Size only   │ Size + charge│  Solution-      │
│             │              │              │  diffusion      │
└─────────────┴──────────────┴──────────────┴─────────────────┘

Rejection:
Particles       ████████████████████████████████████████████
Bacteria        ████████████████████████████████████████████
Protozoa        ████████████████████████████████████████████
Viruses         ░░░░░░░░░░░░░███████████████████████████████
Macromolecules  ░░░░░░░░░░░░░░░░░░░░░███████████████████████
Divalent ions   ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░███████████████
Monovalent ions ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░████████
TDS             ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░████

Legend: ████ = Rejected (>90%)  ░░░░ = Passes through (<10% rejection)

Available Membrane Technologies

Microfiltration

Particle and bacteria barrier

• Pore size: 0.1-10 μm (typically 0.2-0.4 μm)
• Operating pressure: 0.1-0.5 bar (very low)
• Permeate flux: 50-150 LMH
• Recovery: 90-98%

Rejection:

• Particles, bacteria, protozoa: >99.9% (4-6 log validated)
• Viruses: 0-2 log (unreliable, size-dependent)
• Dissolved solids (TDS): None (0%)
• Macromolecules: Minimal (large molecules only)

Applications:

• Tertiary polishing (TSS <1 mg/L, turbidity <0.1 NTU)
• RO pretreatment (reduce fouling potential, SDI ❤️)
• Giardia/Cryptosporidium barrier (absolute, >4 log)
• MBR (membrane bioreactor) configuration

Best for:

• TSS removal without virus requirement
• RO pretreatment (superior to conventional filtration)
• Lowest energy membrane option (0.02-0.05 kWh/m³)

Ultrafiltration

Virus barrier

• Pore size: 0.01-0.1 μm (10-100 nm) or MWCO 10-300 kDa
• Operating pressure: 0.5-2 bar
• Permeate flux: 30-100 LMH
• Recovery: 90-98%

Rejection:

• Particles, bacteria, protozoa: >99.9% (6+ log validated)
• Viruses: 4-6 log (validated LRV, absolute barrier)
• Dissolved organics: 20-50% (macromolecules >MWCO only)
• TDS: None (0%)

Applications:

• Drinking water (virus barrier, 4-log minimum)
• Direct/indirect potable reuse (multi-barrier treatment)
• RO pretreatment (superior to MF for high-fouling feeds)
• Color removal (partial, humic/fulvic acids)

Best for:

• Virus barrier required (drinking water, reuse)
• Tight pretreatment for RO/NF
• Energy: 0.05-0.10 kWh/m³ (still low compared to NF/RO)

Nanofiltration

Softening and partial desalination

• Pore size: 0.5-2 nm (MWCO 200-1000 Da, typically 300-400 Da)
• Operating pressure: 5-20 bar
• Permeate flux: 15-50 LMH
• Recovery: 70-85%

Rejection:

• Particles, bacteria, viruses, protozoa: >99.9% (6+ log)
• Divalent ions (Ca²⁺, Mg²⁺, SO₄²⁻): 85-98%
• Monovalent ions (Na⁺, Cl⁻): 20-70%
• Organic molecules (>300 Da): 90-99%
• TDS: 40-80% (partial desalination)

Mechanism: Size exclusion + Donnan (charge) exclusion

Applications:

• Water softening (hardness removal without salt)
• Color removal (humic/fulvic acids, 90-99%)
• Micropollutant removal (pharmaceuticals, pesticides, 60-95%)
• Partial desalination (brackish water, TDS 1,000-5,000 mg/L)
• RO pretreatment (reduce scaling potential, higher RO recovery)

Best for:

• Hardness removal (Ca²⁺/Mg²⁺) without full desalination
• Moderate TDS reduction at lower pressure than RO
• Micropollutant removal (water reuse)

Reverse-Osmosis

Desalination and ultrapure water

• Pore size: <0.001 μm (<1 nm, molecular-scale)
• Operating pressure: 10-25 bar (brackish), 55-70 bar (seawater)
• Permeate flux: 15-25 LMH (brackish), 10-15 LMH (seawater)
• Recovery: 75-85% (brackish), 40-50% (seawater)

Rejection:

• All particles, pathogens: >99.9% (essentially 100%)
• Divalent ions: 99-99.9%
• Monovalent ions: 95-99.5% (brackish), 99-99.7% (seawater)
• Organics: 95-99%
• TDS: 95-99.5%

Mechanism: Solution-diffusion (water permeates, salts rejected)

Applications:

• Seawater desalination (TDS 35,000 mg/L → <500 mg/L)
• Brackish water desalination (TDS 1,500-10,000 mg/L → <250 mg/L)
• Direct/indirect potable reuse (wastewater → drinking water)
• Ultrapure water (pharmaceutical, electronics, power generation)
• Industrial process water (boiler feed, manufacturing)
• Zero liquid discharge (ZLD) concentration step

Best for:

• High TDS removal (>95%)
• Drinking water from saline sources
• Water reuse requiring very low TDS

Technology Comparison

Membrane	Pore Size	Pressure (bar)	Flux (LMH)	Energy (kWh/m³)	Virus Removal	TDS Removal	Capital Cost ($/m³/d)	Application
MF	0.2-0.4 μm	0.1-0.5	50-150	0.02-0.05	0-2 log	None	$400-800	TSS, RO pretreat
UF	0.02-0.1 μm	0.5-2.0	30-100	0.05-0.10	4-6 log ✓	None	$500-1,000	Drinking water, virus barrier
NF	0.5-2 nm	5-20	15-50	0.40-0.80	6+ log	40-80%	$800-1,500	Softening, micropollutants
RO	<1 nm	10-70	10-25	0.5-5.0	6+ log	95-99.5% ✓	$1,000-3,000	Desalination, reuse

Energy notes:

• MF/UF: Very low energy (low pressure)
• NF: Moderate energy (medium pressure)
• RO: Higher energy, but with ERD (energy recovery device): Brackish 0.5-1.5 kWh/m³, Seawater 3-5 kWh/m³

Selection Guide

By Water Quality Goal

TSS Removal (<1 mg/L):

• Primary: MF or UF (both achieve <1 mg/L easily)
• Selection: MF if no virus requirement (lower cost/energy), UF if virus barrier also needed

Virus Barrier (4-log LRV):

• Required: UF (validated 4-6 log LRV)
• Alternative: MF is NOT reliable for viruses
• Note: RO/NF also remove viruses, but typically used for other purposes (TDS, etc.)

Cryptosporidium Barrier:

• All membranes (MF/UF/NF/RO) provide absolute barrier (>4 log)
• MF is sufficient and most economical if only protozoa concern

Hardness Removal (>85% Ca²⁺/Mg²⁺):

• Primary: NF (selective for divalent ions, 85-98% removal)
• Alternative: RO (100% removal, but higher energy/cost)

TDS Removal (>95%):

• Required: RO (only option for high TDS removal)
• NF insufficient (40-80% TDS removal only)

Micropollutants (pharmaceuticals, pesticides):

• Best: RO (>95% removal for most compounds)
• Moderate: NF (60-95%, molecular weight dependent)
• Poor: MF/UF (minimal removal, only very large molecules)

Color Removal:

• Best: RO (>95%), NF (90-95%)
• Moderate: UF (20-50%, macromolecules only)
• Poor: MF (minimal, <10%)

By Application

Tertiary TSS Removal (discharge compliance):

Secondary Effluent (TSS 10-20 mg/L) → MF → TSS <1 mg/L, Turbidity <0.1 NTU
- MF sufficient (lowest cost membrane option)
- No virus barrier needed (discharge, not reuse)
- Energy: 0.02-0.05 kWh/m³ (very low)

RO Pretreatment (reduce fouling):

Brackish Water → MF or UF → RO → Potable Water
- MF typically sufficient (TSS, SDI reduction)
- UF if feed water has high organic/biological fouling potential
- Protects RO from particulate fouling, extends membrane life

Drinking Water (surface water):

Filtered Surface Water → UF → Chlorination → Distribution
- UF for 4-log virus removal (regulatory compliance)
- Lower cost than RO (no TDS removal needed)
- Energy: 0.05-0.10 kWh/m³

Indirect Potable Reuse (IPR):

Secondary Effluent → MF/UF → RO → UV/Chlorine → Aquifer or Reservoir
- MF/UF: Virus barrier + RO pretreatment (SDI <2)
- RO: TDS removal, organics removal, additional pathogen barrier
- UV/Chlorine: Final disinfection
- Multi-barrier: >12 log virus reduction (regulatory compliance)

Direct Potable Reuse (DPR):

Secondary Effluent → UF → RO → UV-AOP → GAC → Chlorination → Distribution
- Extreme multi-barrier treatment
- UF: 6 log virus, pretreatment
- RO: TDS, organics, 6 log virus (additive)
- UV-AOP: Advanced oxidation (trace organics, 4 log virus)
- GAC: Polishing (organics adsorption)
- Total: >12 log virus, >10 log Crypto

Water Softening:

Brackish Water (Hardness 200-400 mg/L) → NF → Softened Water (Hardness <50 mg/L)
- NF removes 85-98% hardness (Ca²⁺, Mg²⁺)
- Lower energy than RO (5-15 bar vs. 15-25 bar)
- Partial TDS removal (bonus, 40-80%)

Seawater Desalination:

Seawater (TDS 35,000 mg/L) → DAF → UF → RO (two-stage) → Remineralization → Chlorination
- UF: Critical pretreatment (SDI <2, rigorous)
- RO: Two-stage with interstage booster pump (high recovery)
- Energy recovery essential (ERD, 95% efficiency)
- Final TDS <500 mg/L (potable water standard)
- Energy: 3-5 kWh/m³ (state-of-the-art with ERD)

Brackish Water Desalination:

Brackish Source (TDS 2,000-5,000 mg/L) → Multimedia Filter → Cartridge Filter → RO → Remineralization
- Conventional pretreatment acceptable (multimedia or MF)
- Single-stage RO (lower pressure than seawater)
- Recovery: 75-85%
- Energy: 0.5-1.5 kWh/m³ (with ERD)

Zero Liquid Discharge (ZLD):

RO Concentrate → Additional RO → Brine Concentrator → Crystallizer → Solid Salts
- Multiple RO passes to maximize recovery (90-95%)
- Thermal evaporation (brine concentrator) for final concentration
- Crystallizer for solid salt production
- Achieve zero liquid discharge (all water recovered or evaporated)

By Constraint

Limited Energy Budget:

1. MF (0.02-0.05 kWh/m³, lowest)
2. UF (0.05-0.10 kWh/m³)
3. Avoid: RO (0.5-5.0 kWh/m³, high)

Limited Space:

• All membranes are compact (vertical configuration, small footprint)
• MF/UF: Slightly larger membrane area (higher flux)
• NF/RO: Smaller area (lower flux), but higher pressure equipment

Low Operating Complexity:

1. MF (simple backwash, minimal chemicals)
2. UF (similar to MF, automated)
3. Avoid: RO (scaling management, antiscalant, CIP, concentrate disposal)

High Fouling Feed (poor pretreatment):

1. UF (better tolerance than MF, smaller pores resist internal fouling)
2. MF (acceptable if good pretreatment impossible)
3. Avoid: NF/RO (very sensitive, require excellent pretreatment)

Common Design Features

Membrane Configurations

Hollow Fiber (HF):

• Most common for MF/UF
• High packing density (>10,000 m²/m³)
• Inside-out or outside-in flow
• Backwashable
• Area per module: 30-60 m² (MF), 40-80 m² (UF)

Spiral Wound:

• Standard for NF/RO
• Moderate packing density (~800 m²/m³)
• Feed flows axially, permeate spirals to center tube
• Not backwashable (CIP only)
• Standard element: 8"×40", 37 m² (NF/RO)

Flat Sheet:

• Less common, some MF/UF applications
• Easy to clean, low fouling
• Lower packing density
• Used in MBR (submerged configuration)

Tubular:

• Rare, high-cost
• Very low fouling (large diameter, 5-25 mm)
• Easy mechanical cleaning
• Industrial applications (food processing, high-solids)

Operating Modes

Dead-End Filtration (MF/UF):

• All feed becomes permeate (no concentrate stream during filtration)
• Solids accumulate on membrane (cake layer)
• Periodic backwash removes cake
• Recovery: 90-98% (accounting for backwash)

Crossflow Filtration (NF/RO):

• Continuous concentrate stream (tangential flow)
• High crossflow velocity sweeps membrane surface (reduces concentration polarization)
• No backwash (CIP only)
• Recovery: 40-85% (depending on feed TDS, scaling potential)

Fouling Control

Backwash (MF/UF only):

• Frequency: 15-60 minutes
• Duration: 30-90 seconds
• Reverse flux: 150-250 LMH (higher than forward flux)
• Removes cake layer, restores 80-95% of flux

CEB - Chemically Enhanced Backwash (MF/UF):

• Frequency: Daily (every 12-48 backwash cycles)
• Chemicals: NaOCl (50-200 ppm) for organics/biofilm, citric acid (1000-2000 ppm) for scaling
• Contact time: 5-10 minutes
• Restores flux between CIP intervals

CIP - Clean-In-Place (all membranes):

• Frequency: Monthly to quarterly (MF/UF), 3-6 months (NF/RO)
• Chemicals:
  • Caustic (NaOH 0.1-0.5%) + Surfactant: Organic fouling
  • Acid (HCl or citric acid 1-2%): Mineral scaling
  • Oxidizer (NaOCl 500-1000 ppm): Biological fouling
• Procedure: Recirculation (not backwash), 1-4 hour soak
• Target: >90% flux recovery

Antiscalant (NF/RO only):

• Dose: 2-10 mg/L (feed water)
• Purpose: Prevent CaCO₃, CaSO₄, BaSO₄, silica precipitation
• Allows higher recovery (typical: 75-85% vs. 60-70% without)
• Annual cost: $50,000-200,000 for large plants

Pretreatment Requirements

Membrane	SDI Requirement	Turbidity	TSS	Dechlorination	Scaling Control
MF	N/A (pretreat)	<30 NTU	<50 mg/L	Not critical	Not needed
UF	N/A (pretreat)	<30 NTU	<50 mg/L	Not critical	Not needed
NF	❤️ (preferably <2)	<1 NTU	<5 mg/L	Required (<0.1 mg/L free Cl₂)	Antiscalant required
RO	❤️ (preferably <2)	<1 NTU	<5 mg/L	Critical (<0.05 mg/L free Cl₂)	Antiscalant + pH control

Pretreatment sequence for RO:

Raw Water → Coagulation/Flocculation → Sedimentation → Multimedia Filter → Cartridge Filter (5 μm) → Dechlorination → RO

OR (better):

Raw Water → UF → Cartridge Filter (security) → Dechlorination → RO

Performance Monitoring

Normalized Parameters

Why normalize: Flux and salt passage vary with temperature, pressure, and feed concentration. Normalization allows comparison to baseline.

Normalized Permeate Flow (NPF):

$$NPF = \frac{Q_{permeate,actual} \times TCF}{\Delta P_{actual} / \Delta P_{design}}$$

Where $TCF$ = temperature correction factor (e.g., 1.03^(T-25))

Normalized Salt Passage (NSP):

$$NSP = \frac{(C_p / C_f){actual}}{(C_p / C_f){design}} \times \frac{TCF_{design}}{TCF_{actual}}$$

Action triggers:

• NPF decline >10-15%: Plan CIP
• NSP increase >10%: Investigate membrane damage or seal leak
• Differential pressure increase >15%: Fouling, CIP required

Integrity Testing

Pressure Decay Test (MF/UF, offline):

• Pressurize membrane (0.5-2 bar, below operating pressure)
• Isolate and monitor pressure decay
• Acceptance: ❤️-5 mbar/min (indicates no fiber breaks or seal leaks)

Bubble Point Test (offline):

• Detect largest pore size (fiber break detection)
• Air pressure gradually increased until bubbles appear in permeate
• Compare to specification (should exceed design pore size)

Online Monitoring:

• Permeate turbidity: Continuous monitoring, alarm at 0.1 NTU (MF) or 0.05 NTU (UF)
• Particle counting: >2 μm particles in permeate, alarm at >10 counts/mL
• LRV challenge testing: Periodic (virus surrogate spiking for UF)

Advanced Membrane Systems

Two-Pass RO (Ultrapure Water)

Configuration:

• Pass 1: Standard BWRO (TDS 1,000-5,000 mg/L → 20-100 mg/L)
• Pass 2: Polish RO (TDS 20-100 mg/L → <1 mg/L)

Applications:

• Pharmaceutical water (USP Purified Water)
• Semiconductor manufacturing (resistivity >15 MΩ·cm)
• Power plant boiler feed (prevent scaling/corrosion)

Membrane Bioreactor (MBR)

Configuration:

• Submerged hollow fiber MF/UF membranes in activated sludge basin
• Replaces secondary clarifier
• Operates at very low flux: 15-30 LMH (high MLSS, 8,000-12,000 mg/L)

Advantages:

• Absolute solid-liquid separation (TSS <1 mg/L)
• Small footprint (high MLSS allows smaller aeration basin)
• Excellent effluent quality (ready for disinfection or reuse)

Challenges:

• Higher energy (aeration + membrane)
• Fouling (requires frequent backwash, CEB, CIP)
• Membrane replacement cost

Hybrid Systems

NF + RO:

• NF removes hardness (prevent RO scaling)
• RO operates at higher recovery (no CaSO₄ limit)
• Lower overall energy (NF is lower pressure)

UF + RO:

• UF provides excellent RO pretreatment (SDI <1, turbidity <0.05 NTU)
• Allows higher RO flux (less fouling risk)
• Multi-barrier for water reuse (virus + TDS removal)

Ozone + Ceramic MF:

• Ozone pretreatment (oxidize organics, reduce biofouling)
• Ceramic membranes (higher flux, easier cleaning, long life)
• Drinking water applications

Process Unit Links

• Microfiltration - Particle and bacteria barrier
• Ultrafiltration - Virus barrier and RO pretreatment
• Nanofiltration - Softening and partial desalination
• Reverse-Osmosis - Desalination and ultrapure water

Related Processes

• Rapid-Sand-Filter - Conventional pretreatment alternative
• UV-Disinfection - Post-membrane disinfection
• Chemical-Dosing - Antiscalant, dechlorination, coagulation
• Sludge-Handling - Concentrate and backwash waste management

References

1. Crittenden, J.C. et al. (2012). MWH's Water Treatment: Principles and Design, 3rd Ed., Wiley, Ch. 11-12.
2. AWWA (2011). Water Quality and Treatment, 6th Ed., McGraw-Hill, Ch. 11: Membranes.
3. Baker, R.W. (2012). Membrane Technology and Applications, 3rd Ed., Wiley.
4. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 17: Membrane Filtration.
5. US EPA (2005). Membrane Filtration Guidance Manual, EPA 815-R-06-009.
6. Judd, S. (2011). The MBR Book: Principles and Applications, 2nd Ed., Elsevier.
7. Voutchkov, N. (2013). Desalination Engineering: Planning and Design, McGraw-Hill.