Appearance
Ultrafiltration (UF)
Pressure-driven membrane separation using 0.01-0.1 μm pores (MWCO 10-300 kDa) achieving virus-level removal at 0.5-2 bar operating pressure with fluxes of 30-100 LMH for advanced water treatment.
Overview
Ultrafiltration (UF) provides a tighter barrier than microfiltration, offering reliable removal of viruses in addition to bacteria and protozoa. UF operates in the transition zone between MF and nanofiltration, using membranes characterized by molecular weight cut-off (MWCO) rather than pore size. Common applications include drinking water treatment, water reuse, and pretreatment for reverse osmosis.
When to Use
- Drinking water production (virus barrier, 4-6 log LRV)
- Direct potable reuse (multi-barrier treatment)
- RO pretreatment (superior to MF for high-fouling feeds)
- Removal of dissolved macromolecules (proteins, polysaccharides)
- Color removal (humic/fulvic acids)
- Pharmaceutical/biotech water systems
- Membrane bioreactor (MBR) for high-quality effluent
- Dairy and food processing wastewater
Key Characteristics
- Pore size: 0.01-0.1 μm (10-100 nm)
- MWCO: 10-300 kDa (kilodaltons), typically 100 kDa
- Operating pressure: 0.5-2 bar (5-30 psi)
- Permeate flux: 30-100 LMH for wastewater
- Recovery: 90-98%
- Turbidity: <0.05 NTU (better than MF)
- Bacteria removal: >6 log (validated)
- Virus removal: 4-6 log (validated, absolute barrier)
- Protozoa removal: >6 log
- Dissolved organics: Partial (macromolecules >MWCO removed)
- Backwash frequency: 20-60 minutes
- CIP frequency: Weekly to monthly
Operating Modes
Basic Mode
- Constant flux operation
- Time-based backwash
- Default resistance model
- Simple TMP monitoring
- Fixed MWCO
Advanced Mode
- Resistance-in-series fouling model with gel polarization
- Critical flux operation (flux < critical to minimize fouling)
- TMP-based backwash trigger
- CEB (chemically enhanced backwash) optimization
- Fouling prediction (Hermia models)
- Molecular weight fractionation modeling
Mathematical Model
Darcy's Law with Osmotic Pressure
For UF, concentration polarization creates osmotic pressure:
$$J = \frac{\Delta P - \Delta \pi}{\mu \times R_{total}}$$
Where:
- $J$ = permeate flux (m/s or LMH)
- $\Delta P$ = transmembrane pressure (TMP) (bar)
- $\Delta \pi$ = osmotic pressure difference (bar), usually <0.1 bar for wastewater
- $\mu$ = dynamic viscosity (Pa·s)
- $R_{total}$ = total hydraulic resistance (m⁻¹)
For wastewater UF, $\Delta \pi$ is often negligible (<0.05 bar), so simplified form applies:
$$J = \frac{\Delta P}{\mu \times R_{total}}$$
Resistance-in-Series Model
$$R_{total} = R_m + R_p + R_{gel} + R_{irr}$$
Where:
- $R_m$ = membrane resistance (m⁻¹), intrinsic
- $R_p$ = pore blocking resistance (m⁻¹)
- $R_{gel}$ = gel layer resistance (m⁻¹), concentration polarization
- $R_{irr}$ = irreversible fouling (m⁻¹), removed by CIP only
Typical values for UF (wastewater):
- $R_m$ = 5-15 × 10¹¹ m⁻¹ (tighter than MF)
- $R_p$ = 2-10 × 10¹¹ m⁻¹ (internal pore fouling)
- $R_{gel}$ = 5-20 × 10¹¹ m⁻¹ (removed by backwash)
- $R_{irr}$ = 3-15 × 10¹¹ m⁻¹ (accumulates over time)
Molecular Weight Cut-Off (MWCO)
MWCO is the molecular weight (in Daltons) at which 90% of solutes are rejected:
$$MWCO = MW_{90% \text{ rejection}}$$
Relationship to pore size (empirical):
$$d_p [\text{nm}] \approx 0.033 \times MWCO^{0.46}$$
Examples:
- MWCO 10 kDa → pore size ≈ 2.5 nm
- MWCO 100 kDa → pore size ≈ 5.5 nm
- MWCO 300 kDa → pore size ≈ 9 nm
Rejection Coefficient
Fraction of solute retained by membrane:
$$R = 1 - \frac{C_p}{C_f}$$
Where:
- $C_p$ = permeate concentration
- $C_f$ = feed concentration
Typical rejection:
- Particles >0.01 μm: R ≈ 1.0 (100%)
- Macromolecules >MWCO: R > 0.9 (>90%)
- Macromolecules <MWCO: R < 0.1 (<10%)
- Dissolved salts (TDS): R ≈ 0 (no rejection)
Concentration Polarization and Gel Layer
Rejected solutes accumulate at membrane surface:
$$C_{wall} = C_{bulk} \times \exp\left(\frac{J}{k}\right)$$
Where:
- $C_{wall}$ = concentration at membrane surface
- $C_{bulk}$ = bulk feed concentration
- $k$ = mass transfer coefficient (m/s), typically 1-5 × 10⁻⁵ m/s
Critical flux: When $C_{wall}$ reaches gel concentration ($C_{gel}$), a gel layer forms, drastically increasing resistance.
$$J_{critical} = k \times \ln\left(\frac{C_{gel}}{C_{bulk}}\right)$$
Design principle: Operate below $J_{critical}$ to minimize fouling.
Hermia Fouling Models
Four mechanisms of flux decline:
- Complete blocking: $\frac{1}{J} = \frac{1}{J_0} + k_b \times t$
- Intermediate blocking: $\frac{1}{\sqrt{J}} = \frac{1}{\sqrt{J_0}} + k_i \times t$
- Standard blocking (pore constriction): $\frac{1}{J^2} = \frac{1}{J_0^2} + k_s \times t$
- Cake filtration: $\frac{1}{J^2} = \frac{1}{J_0^2} + k_c \times t$
For wastewater UF, cake filtration typically dominates.
Temperature Correction
Same as MF:
$$J_T = J_{20} \times 1.03^{(T-20)}$$
Example:
J₂₀ = 60 LMH @ 20°C
J₁₀ = 60 × 1.03⁻¹⁰ = 44.5 LMH @ 10°C (26% reduction)Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| MWCO | - | kDa | 10-300 | 100 | Molecular weight cut-off |
| Pore size (equivalent) | $d_p$ | nm | 2-10 | 5 | Nominal pore diameter |
| Design flux | $J$ | LMH | 30-100 | 60 | Net flux (averaged) |
| Operating TMP | $\Delta P$ | bar | 0.5-2.0 | 1.0 | Transmembrane pressure |
| Temperature | $T$ | °C | 5-30 | 15 | Feed water temperature |
| Filtration cycle | $t_f$ | min | 20-60 | 40 | Time between backwashes |
| Backwash duration | $t_{BW}$ | s | 30-90 | 60 | Backwash time |
| Backwash flux | $J_{BW}$ | LMH | 150-250 | 200 | Reverse flux |
| CEB frequency | - | cycles | 12-48 | 24 | BW cycles between CEB |
| CIP frequency | - | days | 30-120 | 60 | Days between CIP |
| Feed TSS | - | mg/L | 2-20 | 10 | Influent suspended solids |
| Feed COD | - | mg/L | 20-100 | 50 | Chemical oxygen demand |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| Membrane area | m² | Required membrane surface area |
| Number of modules | - | Membrane elements |
| Permeate turbidity | NTU | Effluent turbidity (<0.05 typical) |
| Permeate COD | mg/L | Organics in permeate |
| TMP @ end of cycle | bar | Maximum TMP before backwash |
| Net water production | % | Recovery accounting for backwash |
| Backwash volume | m³/d | Daily backwash water |
| CIP chemical cost | $/year | NaOCl + citric acid + caustic |
| Energy consumption | kWh/m³ | Pumping energy |
| Log removal virus | log₁₀ | Validated viral removal (4-6 log) |
Configuration Guide
Step-by-Step Setup
Select MWCO and Pore Size
Based on application:
RO pretreatment: 100-150 kDa (balance flux and SDI reduction) Drinking water (virus barrier): 30-100 kDa (validated 4-log virus) Direct potable reuse: 30-50 kDa (tight, conservative) Color removal: 10-30 kDa (remove humic substances, low flux) Protein recovery: 10-50 kDa (based on target protein size) Typical wastewater: MWCO 100 kDa (pore ~5 nm)Determine Design Flux
Conservative to minimize fouling:
Secondary effluent (MBR): J = 15-25 LMH (submerged) Tertiary effluent (external pressure): J = 50-80 LMH RO pretreatment: J = 60-100 LMH (good feed quality) High-fouling feed: J = 30-50 LMH Recommendation for wastewater: J = 60 LMH (below critical flux)Calculate Membrane Area
$$A = \frac{Q}{J \times R_{net}}$$
Example: Q = 5,000 m³/d = 208 m³/h J = 60 LMH R_net = 0.93 (7% backwash loss) A = 208 / (60 × 0.93) = 3,727 m²Select Membrane Configuration
Common types:
- Hollow fiber (inside-out): Most common, 30-60 m²/module
- Hollow fiber (outside-in): Better for high-fouling feeds
- Spiral wound: Less common for UF, 30-40 m²/module
- Tubular: High cost, easy to clean, industrial applications
Using hollow fiber (inside-out) @ 50 m²/module: Modules = 3,727 / 50 = 75 modules Organize: 3 trains × 25 modules = 75 total (Allow 1 train offline for maintenance, operate at 66% capacity)Design Backwash System
t_filtration = 40 minutes t_backwash = 60 s J_BW = 200 LMH (reverse flux) Backwash volume per cycle: V_BW = 3,727 × 200 × (1/60) = 12,423 L = 12.4 m³ Cycles per day = 24 × 60 / 41 = 35.1 Daily BW = 35.1 × 12.4 = 435 m³/d Backwash % = 435 / 5,000 = 8.7% Net recovery = 91.3% ✓CEB (Chemically Enhanced Backwash)
Frequency: Every 24 cycles (once per day)
Chemicals:
- NaOCl: 100-200 ppm (organic/biological fouling)
- Citric acid: 1000-2000 ppm (mineral scaling)
- NaOH (optional): 0.1-0.3% (protein/oil fouling)
Procedure:
- Backwash with NaOCl (10 min soak)
- Rinse
- Backwash with acid (10 min soak)
- Rinse
- Return to service
CIP (Clean-in-Place)
Frequency: Every 60 days or when TMP increases 30%
Chemicals:
- NaOCl: 500-1000 ppm (1-3 hour soak)
- Citric acid or HCl: 1-2% (1-3 hour soak)
- NaOH: 0.5-1.0% (1-3 hour soak, for organics)
Procedure:
- Caustic soak (if organic fouling)
- Rinse thoroughly
- Acid soak (mineral scaling)
- Rinse thoroughly
- NaOCl soak (biofouling, final sanitization)
- Rinse and integrity test
Calculate Energy Consumption
Feed pump:
ΔP_total = TMP + ΔP_piping = 1.0 + 0.4 = 1.4 bar Q_feed = 5,000 / 0.913 = 5,477 m³/d = 228 m³/h Power = (228 × 1.4 × 10⁵) / (0.75 × 3600) = 11.8 kW Daily energy = 11.8 × 24 = 283 kWh/d Specific energy = 283 / 5,000 = 0.057 kWh/m³Backwash pump:
Q_BW = 3,727 × 200 / 1000 = 745 m³/h ΔP_BW = 1.5 bar Power_BW = (745 × 1.5 × 10⁵) / (0.75 × 3600) = 41.4 kW Daily hours = 35.1 × (1/60) = 0.585 h Daily energy_BW = 41.4 × 0.585 = 24.2 kWh/d Total specific energy = (283 + 24.2) / 5,000 = 0.061 kWh/m³
Typical Design Values
| Application | MWCO (kDa) | Flux (LMH) | TMP (bar) | Recovery (%) | Backwash (min) | CIP (days) |
|---|---|---|---|---|---|---|
| Drinking water (virus barrier) | 30-50 | 50-80 | 0.8-1.5 | 92-96 | 30-60 | 60-90 |
| RO pretreatment | 100-150 | 60-100 | 1.0-2.0 | 93-97 | 30-50 | 60-120 |
| Direct potable reuse | 30-100 | 40-70 | 0.8-1.5 | 90-95 | 30-60 | 30-60 |
| Industrial wastewater | 50-100 | 30-60 | 1.0-1.8 | 88-93 | 20-40 | 30-60 |
| MBR (submerged) | 100-200 | 15-30 | 0.1-0.5 | 95-98 | 10-20 | 30-90 |
Performance Criteria
Removal Efficiency
| Parameter | Influent | Permeate | Removal |
|---|---|---|---|
| Turbidity | 1-5 NTU | <0.05 NTU | >99.5% |
| TSS | 5-20 mg/L | <0.5 mg/L | >99% |
| Bacteria (E. coli) | 10⁴-10⁶ cfu/100mL | <1 cfu/100mL | >6 log (validated) |
| Viruses | 10³-10⁵ PFU/100mL | <10 PFU/100mL | 4-6 log (validated LRV) |
| Giardia/Crypto | Present | Not detected | >6 log |
| TOC (>MWCO) | Partial | Reduced 20-50% | 20-50% (MW-dependent) |
| COD | Variable | Reduced 10-40% | 10-40% |
| Color (humic acids) | Variable | Reduced 40-80% | 40-80% (MWCO-dependent) |
| TDS | No removal | No removal | 0% |
Virus Log Removal Validation
Critical for drinking water and reuse:
Challenge testing:
- MS2 coliphage (27 nm, MWCO 100 kDa → >5 log)
- ΦX174 bacteriophage (25 nm → >4 log)
- Direct removal testing or surrogate correlation
Integrity testing:
- Pressure decay test: Detect defects >0.05 μm
- Acoustic sensor: Detect fiber breaks (advanced)
Acceptance criteria:
- LRV validation: >4 log for viruses (regulatory requirement)
- Permeate turbidity: <0.05 NTU
- Pressure decay: ❤️ mbar/min
Comparison to MF
| Factor | MF (0.4 μm) | UF (100 kDa, ~5 nm) |
|---|---|---|
| Bacteria removal | >6 log | >6 log |
| Virus removal | 0-2 log | 4-6 log ✓ |
| Turbidity | <0.1 NTU | <0.05 NTU |
| Macromolecule removal | None | Partial (>100 kDa) ✓ |
| TMP | 0.1-0.5 bar | 0.5-2 bar |
| Flux | 80-120 LMH | 50-80 LMH |
| Fouling rate | Lower | Higher |
| Energy | 0.02 kWh/m³ | 0.05-0.08 kWh/m³ |
| Cost | Lower | Higher |
Selection: Use UF when virus removal or macromolecule rejection required, otherwise MF is more economical.
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| Rapid TMP increase (fouling) | Operating above critical flux | Reduce flux to <80% of critical |
| High protein/polysaccharide content | Add caustic to CEB, increase frequency | |
| Gel layer formation | Lower flux, improve crossflow (if applicable) | |
| Irreversible pore blocking | CIP with caustic + oxidizer | |
| Flux decline not recovered by BW | Pore adsorption (irreversible) | CIP with NaOH (protein) or NaOCl (organics) |
| Mineral scaling | CIP with acid (citric or HCl) | |
| Membrane aging | Replace membranes (typically 5-7 years) | |
| High permeate turbidity (>0.05 NTU) | Fiber break or seal failure | Integrity test, isolate faulty module |
| Bypassing around module | Inspect O-rings, reseat module | |
| Poor virus LRV | Membrane integrity loss | Pressure decay test, replace damaged module |
| Flow bypassing membrane | Inspect all connections, manifold seals | |
| Low flux even after CIP | Chemical incompatibility (damage) | Review CIP chemicals vs. membrane material |
| Permanent fouling (aging) | Plan membrane replacement | |
| Operating beyond design life | Replace membranes (5-7 year typical life) | |
| High COD in permeate | MWCO too high for target organics | Use tighter membrane (lower MWCO) |
| Membrane degradation | Integrity test, check for chlorine attack | |
| Color breakthrough | Humic acids <MWCO | Use MWCO 10-30 kDa for color removal |
| Excessive CIP frequency (<30 days) | Flux too high | Reduce design flux by 20-30% |
| Poor pretreatment | Add upstream filtration or coagulation | |
| Biological fouling | Increase CEB frequency, chlorination |
Operation & Maintenance
Continuous Monitoring (SCADA)
- Permeate flow rate (each train)
- TMP (trend analysis critical)
- Permeate turbidity (alarm at 0.05 NTU)
- Feed and permeate pressure
- Temperature (flux correction)
- Backwash cycle counting
- CEB/CIP chemical levels
Daily Checks
- Review TMP trend (flag if >5%/week increase)
- Verify permeate turbidity <0.05 NTU
- Check backwash effectiveness (TMP recovery)
- Inspect for leaks or module damage
- Verify chemical tanks filled (CEB/CIP)
- Review flux vs. design
Weekly Maintenance
- Integrity test (pressure decay or bubble point)
- Calibrate turbidity and pressure sensors
- Inspect membrane modules (visual)
- Review normalized TMP (correct for temperature)
- Particle count testing (if required for reuse)
- Clean pre-filters/strainers
Monthly Tasks
- Performance audit (flux, TMP, recovery)
- Virus LRV testing (if required for potable reuse)
- CEB effectiveness review (optimize chemicals)
- Inspect backwash valves and pumps
- Review chemical consumption
- Plan CIP based on TMP trends
Quarterly/CIP Events
- Full CIP protocol (caustic + acid + NaOCl)
- Post-CIP integrity testing
- Flux recovery validation (target: >90% of clean membrane)
- Replace damaged modules
- Calibrate all instruments
- Membrane life assessment
Annual Tasks
- Comprehensive performance review
- Membrane autopsy (if fouling issues)
- Replace O-rings and seals (preventive)
- Challenge test for LRV (virus surrogate)
- Review operating conditions and optimize
- Cost-benefit analysis (chemical, energy, replacement)
- Regulatory compliance audit (LRV validation)
Advanced Features
Critical Flux Determination
Flux stepping test:
- Start at low flux (J = 20 LMH)
- Increase flux stepwise (20 → 40 → 60 → 80 LMH)
- Monitor TMP at each step (30 min equilibration)
- $J_{critical}$ = flux where TMP begins rapid increase
Operating strategy: Design flux = 0.7-0.8 × $J_{critical}$
Variable Frequency Drive (VFD) Control
Maintain constant TMP throughout filtration cycle:
- As membrane fouls, reduce flux automatically
- Prevents excessive TMP stress on membrane
- Extends membrane life
Forward Flush with Air Scour
For submerged UF (MBR configuration):
- Coarse bubble aeration from below
- Air flow: 0.3-0.5 Nm³/m²·h
- Reduces gel layer, enables lower TMP
- Combined with intermittent relaxation (no permeation, 1-2 min)
Two-Phase Cleaning (Gas-Liquid)
Inject air during backwash:
- Creates turbulence in membrane pores
- Dislodges internal fouling
- More effective than water-only backwash
UF/NF Hybrid
Operate tight UF (MWCO 10-30 kDa) as "loose NF":
- Partial salt rejection (10-30%)
- Hardness removal (Ca²⁺, Mg²⁺): 20-50%
- Color removal: 70-90%
- Lower pressure than true NF (0.5-1.5 bar vs. 5-15 bar)
Worked Example
Given
- Flow: 3,000 m³/d (drinking water application, virus barrier)
- Feed: Filtered surface water, Turbidity = 1 NTU, TOC = 3 mg/L, Temp = 10°C
- Requirement: 4-log virus removal (validated)
- Target recovery: 94%
Solution
Step 1: Select Membrane MWCO
For 4-log virus removal: MWCO ≤ 50 kDa (validated)
Use MWCO = 50 kDa (pore size ≈ 4 nm)Step 2: Determine Design Flux
Drinking water, low fouling: J₂₀ = 70 LMH
Temperature correction:
J₁₀ = 70 × 1.03⁻¹⁰ = 51.9 LMH
Use J = 50 LMH (conservative, below critical flux)Step 3: Calculate Membrane Area
Q = 3,000 m³/d = 125 m³/h
Recovery = 0.94
A = 125 / (50 × 0.94) = 2,660 m²
Use A = 2,700 m² (1.5% margin)Step 4: Module Selection
Hollow fiber UF @ 60 m²/module:
Modules = 2,700 / 60 = 45 modules
Configuration: 3 trains × 15 modules = 45 total
(Operate 2 trains minimum = 66% capacity during maintenance)Step 5: Backwash Design
t_filtration = 50 minutes (extended cycle, low fouling)
t_backwash = 60 s
J_BW = 200 LMH
Backwash volume = 2,700 × 200 × (1/60) = 9,000 L = 9.0 m³/cycle
Cycles/day = 24 × 60 / 51 = 28.2
Daily BW = 28.2 × 9.0 = 254 m³/d
Backwash % = 254 / 3,000 = 8.5%
Net recovery = 91.5% (slightly below 94% target, but typical for UF)Step 6: CEB and CIP
CEB: Daily (28 cycles)
Chemicals: 100 ppm NaOCl, 1000 ppm citric acid
Cost: Negligible (<$50/year)
CIP: Every 90 days
Chemicals: NaOCl (1000 ppm) + HCl (1%) + NaOH (0.5%)
Annual CIP cost: ~$500/yearStep 7: Energy Consumption
Feed pump:
ΔP = 1.2 + 0.5 = 1.7 bar (TMP + piping)
Q_feed = 3,000 / 0.915 = 3,279 m³/d = 137 m³/h
Power = (137 × 1.7 × 10⁵) / (0.75 × 3600) = 8.6 kW
Daily energy = 8.6 × 24 = 206 kWh/d
Backwash:
Q_BW = 2,700 × 200 / 1000 = 540 m³/h
Power_BW = (540 × 1.8 × 10⁵) / (0.75 × 3600) = 36.0 kW
Daily hours = 28.2 × (1/60) = 0.47 h
Daily energy_BW = 36.0 × 0.47 = 16.9 kWh/d
Total specific energy = (206 + 16.9) / 3,000 = 0.074 kWh/m³
Annual energy cost @ $0.12/kWh = 223 × 365 × 0.12 = $9,768/yearStep 8: Verify Performance
Virus LRV: >4 log ✓ (validated for 50 kDa UF)
Turbidity: 1 NTU → <0.05 NTU ✓
Recovery: 91.5% (acceptable for UF)
Energy: 0.074 kWh/m³ (reasonable)Related Units
- Microfiltration - Coarser pores, bacteria/protozoa only
- Nanofiltration - Tighter, partial salt rejection
- Reverse-Osmosis - Final desalination (UF pretreatment)
- UV-Disinfection - Complementary disinfection
- Chemical-Dosing - Coagulation pretreatment (optional)
References
- Crittenden, J.C. et al. (2012). MWH's Water Treatment, 3rd Ed., Ch. 11: Membranes.
- AWWA (2011). Water Quality and Treatment, 6th Ed., Ch. 11.3: Ultrafiltration.
- WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 17.
- US EPA (2005). Membrane Filtration Guidance Manual, EPA 815-R-06-009.
- Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed., pp. 1203-1241.
- Judd, S. (2011). The MBR Book, 2nd Ed., Elsevier.
- Baker, R.W. (2012). Membrane Technology and Applications, 3rd Ed., Wiley, Ch. 4.