Appearance
Microfiltration (MF)
Pressure-driven membrane separation using 0.1-10 μm pore size for particle, bacteria, and protozoa removal, operating at 0.1-0.5 bar with fluxes of 50-150 LMH and achieving turbidity <0.1 NTU.
Overview
Microfiltration (MF) membranes provide an absolute barrier to particles, bacteria, and protozoa through size exclusion, producing high-quality effluent for water reuse or as pretreatment for reverse osmosis systems. MF operates at low transmembrane pressure (TMP), making it energy-efficient compared to higher-pressure membrane processes.
When to Use
- Tertiary polishing (TSS removal to <1 mg/L)
- Water reuse (non-potable to restricted potable)
- RO pretreatment (reduce fouling potential)
- Removal of Giardia and Cryptosporidium (4+ log)
- Turbidity reduction for UV disinfection (<0.1 NTU)
- Oil and grease removal (industrial wastewater)
- MBR (membrane bioreactor) component
- Drinking water treatment (coagulation + MF)
Key Characteristics
- Pore size: 0.1-10 μm (typically 0.2-0.4 μm for wastewater)
- Operating pressure: 0.1-0.5 bar (1-5 psi)
- Permeate flux: 50-150 LMH (L/m²·h) for wastewater
- Recovery: 90-98%
- Turbidity removal: >99.9% (to <0.1 NTU)
- Bacteria removal: >4 log (validated >6 log)
- Protozoa removal: >4 log (Giardia, Cryptosporidium)
- Virus removal: 0-2 log (size-dependent, not reliable barrier)
- Backwash frequency: 15-60 minutes
- CIP (clean-in-place): Weekly to monthly
Operating Modes
Basic Mode
- Constant flux operation
- Time-based backwash trigger
- Default resistance model (clean membrane)
- Simple TMP calculation
- Fixed recovery ratio
Advanced Mode
- Resistance-in-series fouling model
- Backwash optimization (minimize water loss)
- TMP-based backwash trigger (adaptive)
- CEB (chemically enhanced backwash) scheduling
- Flux stepping protocols
- Fouling prediction and CIP planning
Mathematical Model
Darcy's Law (Flux Equation)
Permeate flux driven by transmembrane pressure:
$$J = \frac{\Delta P}{\mu \times R_{total}}$$
Where:
- $J$ = permeate flux (L/m²·h or LMH)
- $\Delta P$ = transmembrane pressure (TMP) (bar or Pa)
- $\mu$ = dynamic viscosity (Pa·s), temperature-dependent
- $R_{total}$ = total hydraulic resistance (m⁻¹)
Unit conversion: $$J [LMH] = \frac{\Delta P [bar] \times 10^5}{R_{total} [m^{-1}] \times \mu [Pa \cdot s]} \times 3600$$
Resistance-in-Series Model
Total resistance comprises multiple components:
$$R_{total} = R_m + R_c + R_{irr} + R_{rev}$$
Where:
- $R_m$ = intrinsic membrane resistance (m⁻¹), constant
- $R_c$ = cake layer resistance (m⁻¹), removed by backwash
- $R_{irr}$ = irreversible fouling (m⁻¹), removed by CIP only
- $R_{rev}$ = reversible fouling (m⁻¹), removed by backwash
Typical values for MF (wastewater):
- $R_m$ = 1-5 × 10¹¹ m⁻¹ (clean membrane)
- $R_c$ = 5-20 × 10¹¹ m⁻¹ (before backwash)
- $R_{irr}$ = 2-10 × 10¹¹ m⁻¹ (accumulates over weeks)
Flux Decline During Filtration Cycle
Cake filtration model:
$$\frac{dR_c}{dt} = k_f \times C_b \times J^2$$
Where:
- $k_f$ = fouling coefficient (s/m², depends on particle properties)
- $C_b$ = bulk particle concentration (kg/m³)
- $J$ = instantaneous flux (m/s)
Integration (constant TMP operation):
$$J(t) = \frac{J_0}{1 + \alpha \times t}$$
Where:
- $J_0$ = initial flux (clean membrane)
- $\alpha$ = fouling rate parameter
Temperature Correction
Viscosity varies strongly with temperature:
$$\mu_T = \mu_{20} \times \exp\left[\beta \times (20 - T)\right]$$
Where $\beta$ ≈ 0.025/°C for water
Flux correction:
$$J_T = J_{20} \times \frac{\mu_{20}}{\mu_T} \approx J_{20} \times 1.03^{(T-20)}$$
Example:
J₂₀ = 80 LMH @ 20°C
J₁₀ = 80 × 1.03⁻¹⁰ = 59 LMH @ 10°C (26% reduction!)Backwash and Recovery
Backwash removes cake layer:
$$R_{total,after_BW} = R_m + R_{irr}$$
Recovery of flux:
$$\text{Flux recovery} = \frac{J_{after_BW} - J_{before_BW}}{J_{clean} - J_{before_BW}} \times 100%$$
Typical recovery: 80-95% per backwash
Net water production:
$$Recovery = \frac{V_{permeate}}{V_{feed}} = \frac{t_{filtration} \times J - t_{backwash} \times J_{BW}}{t_{filtration} \times J} \times 100%$$
Typical: 92-97% (3-8% backwash loss)
Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| Pore size | $d_p$ | μm | 0.1-10 | 0.4 | Nominal pore diameter |
| Design flux | $J$ | LMH | 40-150 | 80 | Net flux (averaged over cycle) |
| Operating TMP | $\Delta P$ | bar | 0.1-0.5 | 0.2 | Transmembrane pressure |
| Temperature | $T$ | °C | 5-30 | 15 | Feed water temperature |
| Filtration cycle | $t_f$ | min | 15-60 | 30 | Time between backwashes |
| Backwash duration | $t_{BW}$ | s | 30-90 | 60 | Backwash time |
| Backwash flux | $J_{BW}$ | LMH | 150-300 | 200 | Reverse flux for cleaning |
| CEB frequency | - | cycles | 1-48 | 24 | Backwashes between CEB |
| CIP frequency | - | days | 30-180 | 90 | Days between CIP |
| Feed TSS | - | mg/L | 5-30 | 15 | Influent suspended solids |
| Feed turbidity | - | NTU | 2-10 | 5 | Influent turbidity |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| Membrane area | m² | Required membrane surface area |
| Number of modules | - | Membrane cassettes or units |
| Permeate turbidity | NTU | Effluent turbidity (<0.1 typical) |
| TMP @ end of cycle | bar | Maximum TMP before backwash |
| Net water production | % | Recovery accounting for backwash |
| Backwash volume | m³/d | Daily water for backwash |
| CIP chemical cost | $/year | NaOCl + citric acid |
| Energy consumption | kWh/m³ | Pumping energy |
| Log removal bacteria | log₁₀ | Validated microbial removal |
Configuration Guide
Step-by-Step Setup
Select Membrane Pore Size
Based on application:
TSS removal only: 0.4-1.0 μm (higher flux, lower cost) Bacteria removal: 0.2-0.4 μm (standard) RO pretreatment: 0.1-0.2 μm (minimize fouling) Oil/grease: 0.4-0.8 μm (larger pore acceptable) Typical selection: 0.4 μm (balance of flux and removal)Determine Design Flux
Conservative design for long membrane life:
Secondary effluent (TSS 10-20 mg/L): J = 60-80 LMH Tertiary effluent (TSS <10 mg/L): J = 80-120 LMH RO pretreatment (low fouling): J = 100-150 LMH MBR configuration (submerged): J = 15-25 LMH (different regime) Recommendation: J = 80 LMH (conservative for wastewater)Calculate Membrane Area
$$A = \frac{Q}{J \times R_{net}}$$
Where $R_{net}$ = net production ratio (account for backwash)
Example: Q = 10,000 m³/d = 417 m³/h J = 80 LMH R_net = 0.95 (5% backwash loss) A = 417 / (80 × 0.95) = 5,487 m²Select Membrane Modules
Common module types:
- Hollow fiber (HF): 20-50 m² per module
- Flat sheet: 10-30 m² per module
- Spiral wound (less common for MF): 30-40 m² per module
Using hollow fiber @ 40 m²/module: Number of modules = 5,487 / 40 = 137 modules Organize into trains: 4 trains × 35 modules = 140 modules total (allow 1 train for maintenance)Design Backwash System
Backwash frequency:
t_filtration = 30 minutes (typical) t_backwash = 60 secondsBackwash volume per cycle:
A = 5,487 m² J_BW = 200 LMH (reverse flux) t_BW = 60 s = 1 min V_BW = A × J_BW × (t_BW / 60) = 5,487 × 200 × (1/60) = 18,290 L = 18.3 m³Daily backwash:
Cycles per day = 24 × 60 / (30 + 1) = 46.5 cycles/d Daily BW volume = 46.5 × 18.3 = 851 m³/d Backwash % = 851 / 10,000 = 8.5% Net recovery = 91.5% ✓CEB (Chemically Enhanced Backwash) Design
Frequency: Every 24 backwash cycles (once per day typical)
Chemicals:
- NaOCl: 50-200 ppm (oxidative cleaning, organics/biofilm)
- Citric acid: 500-2000 ppm (mineral scale removal)
Procedure:
- Backwash with NaOCl solution (5-10 min contact)
- Rinse with permeate
- Backwash with citric acid (5-10 min contact)
- Rinse with permeate
CIP (Clean-in-Place) Design
Frequency: Every 90 days (or when TMP increases 30%)
Chemicals:
- NaOCl: 500-1000 ppm (1-2 hour soak)
- Citric acid or HCl: 0.5-2% (1-2 hour soak)
- Caustic (NaOH): 0.1-0.5% (optional, for organic fouling)
Procedure:
- Recirculation cleaning (not backwash)
- Soak steps with each chemical
- Thorough rinsing
- Integrity test before return to service
Calculate Energy Consumption
Feed pump:
ΔP_total = TMP + ΔP_piping ≈ 0.2 + 0.3 = 0.5 bar Q_pump = Q_permeate / Recovery = 10,000 / 0.915 = 10,929 m³/d = 455 m³/h Power = (Q × ΔP) / (η × 3600) = (455 × 0.5 × 10⁵) / (0.75 × 3600) = 8.4 kW Daily energy = 8.4 × 24 = 202 kWh/d Specific energy = 202 / 10,000 = 0.02 kWh/m³ (very low!)Backwash pump:
Q_BW = A × J_BW = 5,487 × 200 / 1000 = 1,097 m³/h (peak) ΔP_BW = 1.0 bar (higher for reverse flow) Power_BW = (1,097 × 1.0 × 10⁵) / (0.75 × 3600) = 40.6 kW (when running) Daily hours = 46.5 cycles × (60s / 3600s/h) = 0.78 hours Daily energy_BW = 40.6 × 0.78 = 31.6 kWh/d Total specific energy = (202 + 31.6) / 10,000 = 0.023 kWh/m³
Typical Design Values
| Application | Flux (LMH) | TMP (bar) | Recovery (%) | Backwash Freq (min) | CIP Interval (days) |
|---|---|---|---|---|---|
| Tertiary TSS removal | 80-100 | 0.15-0.25 | 93-97 | 30-60 | 60-120 |
| RO pretreatment | 100-120 | 0.2-0.3 | 95-98 | 30-45 | 90-180 |
| Water reuse | 70-90 | 0.15-0.25 | 92-95 | 20-40 | 60-90 |
| Industrial (high fouling) | 50-70 | 0.2-0.4 | 90-93 | 15-30 | 30-60 |
Performance Criteria
Removal Efficiency
| Parameter | Influent | Permeate | Removal |
|---|---|---|---|
| Turbidity | 2-10 NTU | <0.1 NTU | >99% |
| TSS | 10-30 mg/L | <1 mg/L | >98% |
| Bacteria (E. coli) | 10⁴-10⁶ cfu/100mL | <1 cfu/100mL | >4 log (validated >6 log) |
| Giardia cysts | Present | Not detected | >4 log (absolute barrier) |
| Cryptosporidium | Present | Not detected | >4 log (absolute barrier) |
| Viruses | Variable | Partial removal | 0-2 log (not reliable) |
| Dissolved organics | No removal | No removal | 0% (MF is size exclusion only) |
| TDS | No removal | No removal | 0% |
LRV (Log Removal Value) validation:
- Challenge testing with surrogate microorganisms
- Integrity testing (pressure decay, bubble point)
- Direct removal testing (spiking studies)
Membrane Integrity Monitoring
Methods:
- Pressure decay test (offline): Detect pore size >3 μm
- Bubble point test (offline): Detect fiber breaks
- Turbidity monitoring (online): Permeate turbidity spike indicates breach
- Particle counting (online): Continuous LRV verification
Acceptance criteria:
- Permeate turbidity: <0.15 NTU (alarm at 0.1 NTU)
- Pressure decay: <5 mbar/min
- Particle count (>2 μm): <10 counts/mL
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| Rapid TMP increase (fouling) | High TSS in feed | Improve upstream treatment, reduce flux |
| Backwash ineffective | Increase BW flux/duration, optimize CEB | |
| Irreversible fouling (R_irr) | Perform CIP, review chemical dosing | |
| Temperature drop | Reduce flux proportionally with T | |
| High permeate turbidity | Fiber break or seal leak | Integrity test, isolate faulty module |
| O-ring failure | Replace seals, check assembly | |
| Bypassing (channeling) | Inspect module installation | |
| Low flux even after CIP | Membrane aging (permanent fouling) | Replace membrane elements |
| Operating beyond design life | Plan membrane replacement | |
| Incompatible chemical damage | Review CIP chemicals vs. membrane material | |
| High backwash frequency | Fouling rate too high | Reduce flux, improve pretreatment |
| Particulate spikes in feed | Equalization, upstream filtration | |
| Excessive backwash water use | BW duration too long | Optimize to minimum effective time |
| BW flux too high | Reduce to 150-200 LMH | |
| CIP not restoring flux | Wrong chemical selection | Organic: NaOCl; Inorganic: Acid; Both: Sequential |
| Inadequate contact time | Increase soak time to 2-4 hours | |
| Chemical concentration too low | Increase NaOCl to 1000 ppm, acid to 2% | |
| Membrane fiber breakage | Excessive TMP | Limit TMP to <0.5 bar, check control system |
| Mechanical stress (flow surges) | Flow smoothing, pressure relief valves | |
| Chemical attack | Compatible chemicals only (check manufacturer) |
Operation & Maintenance
Continuous Monitoring (Automated)
- Permeate flow rate (each train)
- TMP (transmembrane pressure)
- Permeate turbidity (online)
- Feed and permeate pressure
- Backwash cycle counting
- Chemical tank levels (CEB/CIP)
Daily Checks
- Review TMP trend (flag if increasing >10%/week)
- Verify permeate turbidity <0.1 NTU
- Check backwash effectiveness (TMP drop after BW)
- Inspect for leaks or unusual noises
- Verify CEB chemicals available
- Review flow balance (feed vs. permeate + backwash)
Weekly Maintenance
- Perform integrity test (pressure decay or bubble point)
- Calibrate turbidity meters
- Inspect membrane modules (visual for fiber integrity)
- Review TMP vs. flux trends
- Check for air binding in modules
- Clean strainers and pre-filters
Monthly Tasks
- Detailed performance audit (flux recovery, TMP trends)
- Particle count testing (permeate quality)
- CEB optimization (review chemical effectiveness)
- Inspect backwash pumps and valves
- Review chemical consumption vs. design
- Plan CIP if TMP increased >20-30%
Quarterly/CIP Events
- Full CIP execution (NaOCl + acid sequence)
- Post-CIP integrity testing
- Document flux recovery (target: >90% of clean membrane)
- Replace damaged modules if needed
- Calibrate all pressure and flow instruments
- Review membrane life (plan replacement if >5-7 years)
Annual Tasks
- Comprehensive membrane performance review
- Replace O-rings and seals (preventive)
- Inspect all modules for fiber integrity
- Review and optimize operating conditions (flux, BW, CEB)
- Cost analysis: chemical, energy, membrane replacement
- Regulatory compliance review (LRV validation)
Advanced Features
Flux Stepping Protocol
Gradually increase flux during filtration cycle to maintain constant TMP:
Benefits:
- Constant energy consumption
- Extended cycle time
- Reduced backwash frequency
Implementation:
t = 0-10 min: J = 70 LMH (low fouling rate)
t = 10-20 min: J = 80 LMH
t = 20-30 min: J = 90 LMH (maximize production before BW)Membrane Aeration (Submerged MF)
For submerged hollow fiber MF (common in MBR):
- Air scour from below membranes
- Air flow: 0.3-0.6 Nm³/m²·h
- Reduces cake layer formation
- Enables lower TMP operation
Two-Stage Backwash
- Air backwash (30 s): Dislodge cake layer
- Water backwash (30 s): Rinse out solids
Improves cleaning efficiency, reduces water use.
Ultrafiltration Hybrid (MF/UF)
Operate 0.1 μm MF membranes in UF pressure regime:
- TMP: 0.5-1.0 bar (higher than typical MF)
- Flux: 80-120 LMH
- Virus removal: 2-3 log (improved over standard MF)
Worked Example
Given
- Flow: 8,000 m³/d average
- Feed: Secondary effluent, TSS = 15 mg/L, Turbidity = 5 NTU, Temperature = 12°C
- Application: RO pretreatment (require turbidity <0.1 NTU)
- Target recovery: 95%
Solution
Step 1: Select Membrane and Flux
Pore size: 0.2 μm (fine pore for RO pretreat)
Design flux @ 20°C: J₂₀ = 100 LMH (aggressive for RO feed quality)
Temperature correction:
J₁₂ = 100 × 1.03⁻⁸ = 78.9 LMH (use 80 LMH)Step 2: Calculate Membrane Area
Q = 8,000 m³/d = 333 m³/h
Recovery = 0.95
A = Q / (J × Recovery) = 333 / (80 × 0.95) = 4,382 m²
Use 4,400 m² (allow 0.4% margin)Step 3: Select Configuration
Hollow fiber modules @ 50 m²/module:
Modules = 4,400 / 50 = 88 modules
Organize: 4 trains × 22 modules = 88 total
(Each train = 1,100 m², can operate with 3 trains = 75% capacity)Step 4: Design Backwash
t_filtration = 40 minutes
t_backwash = 60 s
J_BW = 200 LMH
Backwash volume = 4,400 × 200 × (1/60) = 14,667 L = 14.7 m³/cycle
Cycles/day = 24 × 60 / 41 = 35.1
Daily BW = 35.1 × 14.7 = 516 m³/d
Backwash % = 516 / 8,000 = 6.5%
Net recovery = 93.5% (slightly below 95% target, acceptable)Step 5: CEB Design
Frequency: Once per day (35 backwash cycles)
NaOCl consumption:
Soak volume = membrane internal volume ≈ 0.1 × A = 440 L
Concentration = 100 ppm
NaOCl (12% solution) = 440 × 100 × 10⁻⁶ / 0.12 = 0.37 kg/d = 135 kg/year
Cost @ $500/tonne = $67/year (negligible)Step 6: Energy Consumption
Feed pump:
ΔP_total = 0.25 + 0.25 = 0.5 bar (TMP + piping)
Q_feed = 8,000 / 0.935 = 8,556 m³/d = 356 m³/h
Power = (356 × 0.5 × 10⁵) / (0.75 × 3600) = 6.6 kW
Daily energy = 6.6 × 24 = 158 kWh/d
Backwash pump:
Q_BW = 4,400 × 200 / 1000 = 880 m³/h (peak)
ΔP_BW = 1.0 bar
Power_BW = (880 × 1.0 × 10⁵) / (0.75 × 3600) = 32.6 kW
Daily hours = 35.1 × (60/3600) = 0.58 h
Daily energy_BW = 32.6 × 0.58 = 19 kWh/d
Total energy = (158 + 19) / 8,000 = 0.022 kWh/m³
Annual energy cost @ $0.15/kWh = 177 × 365 × 0.15 = $9,686/yearStep 7: Verify Performance
Permeate turbidity: <0.1 NTU ✓ (MF specification)
TSS removal: 15 mg/L → <1 mg/L ✓
Bacteria removal: >6 log ✓
Recovery: 93.5% (close to 95% target) ✓Related Units
- Ultrafiltration - Finer pores, virus removal
- Rapid-Sand-Filter - Conventional alternative
- UV-Disinfection - Downstream disinfection
- Reverse-Osmosis - Final TDS removal (MF pretreatment)
- Chemical-Dosing - Coagulation pretreatment (optional)
References
- Crittenden, J.C. et al. (2012). MWH's Water Treatment: Principles and Design, 3rd Ed., Ch. 11: Membranes.
- WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 17: Membrane Filtration.
- Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed., pp. 1203-1241.
- AWWA (2011). Water Quality and Treatment, 6th Ed., Ch. 11: Microfiltration and Ultrafiltration.
- Judd, S. and Judd, C. (2011). The MBR Book: Principles and Applications of Membrane Bioreactors, 2nd Ed., Elsevier.
- US EPA (2005). Membrane Filtration Guidance Manual, EPA 815-R-06-009.
- Baker, R.W. (2012). Membrane Technology and Applications, 3rd Ed., Wiley.