4.3 Membrane Filtration: MF, UF, Nanofiltration & Reverse Osmosis
Key Takeaways
- Microfiltration and ultrafiltration are porous membranes that strain particles and, for UF, viruses; nanofiltration and reverse osmosis are semi-permeable and remove dissolved constituents.
- Nanofiltration preferentially rejects divalent ions, which is why it is used as membrane softening and for removing color and disinfection byproduct precursors.
- Reverse osmosis permeate is aggressive and low in alkalinity, so post-treatment stabilization and degasification of carbon dioxide and hydrogen sulfide are mandatory.
- Recovery is permeate divided by feed flow, rejection is the percentage kept out of permeate, and flux is permeate flow per unit membrane area in gfd.
- Polyamide thin-film composite NF and RO membranes are destroyed by free chlorine and require feed dechlorination, while many PVDF MF and UF membranes tolerate chlorine cleaning.
4.3 Membrane Filtration: MF, UF, Nanofiltration & Reverse Osmosis
Low-Pressure Membrane Filtration: Microfiltration & Ultrafiltration
Low-pressure membrane technologies serve as advanced physical separation barriers, replacing or augmenting conventional sedimentation and granular filtration.
Pressure-Driven Membrane Spectrum
┌─────────────────┬─────────────────┬─────────────────┬──────────────────┐
│ Microfiltration │ Ultrafiltration │ Nanofiltration │ Reverse Osmosis │
│ (MF) │ (UF) │ (NF) │ (RO) │
├─────────────────┼─────────────────┼─────────────────┼──────────────────┤
│ 0.1 to 0.2 µm │ 0.01 to 0.05 µm │ 0.001 to 0.002µm│ < 0.001 µm │
│ Operating Press:│ Operating Press:│ Operating Press:│ Operating Press: │
│ 5 to 30 psi │ 10 to 45 psi │ 50 to 150 psi │ 150 to 800+ psi │
├─────────────────┼─────────────────┼─────────────────┼──────────────────┤
│ Removes: │ Removes: │ Removes: │ Removes: │
│ • Protozoa │ • Protozoa │ • Divalent ions │ • All dissolved │
│ (Crypto/ │ (Crypto/ │ (Hardness: │ ions/salts │
│ Giardia) │ Giardia) │ Ca²⁺, Mg²⁺) │ • Nitrates, │
│ • Most bacteria │ • All bacteria │ • NOM / Color │ PFAS, sodium │
│ • Suspended TSS │ • True Viruses │ • PFAS compounds│ • Complete │
│ │ (2 to 4-log) │ │ demineral- │
│ (NO VIRUSES) │ │ │ ization │
└─────────────────┴─────────────────┴─────────────────┴──────────────────┘
Hollow-Fiber Membrane Architecture
Drinking water membrane facilities predominantly deploy hollow-fiber modules containing thousands of microscopic straw-like semi-permeable polymeric fibers (made of PVDF [polyvinylidene fluoride] or polyethersulfone [PES]):
- Outside-In Flow: Raw feedwater surrounds the exterior of the hollow fibers, and vacuum (suction) or positive pressure drives filtrate into the central lumen. High solids tolerance; easy to air-scour during backwash.
- Inside-Out Flow: Feedwater is pumped directly into the central lumen of each fiber, and permeate filters outward through the skin. Provides uniform fluid dynamics but requires strict upstream strainers (200 to 500 µm) to prevent lumen plugging.
Key Membrane Operating Metrics
- Flux ($J$): The filtration rate expressed as volume per unit membrane surface area per unit time, standardly defined as Gallons per Square Foot per Day (GFD) or Liters per Square Meter per Hour (LMH):
- Typical municipal design flux: 30 to 80 GFD.
- Transmembrane Pressure (TMP): The net driving pressure pushing water across the membrane barrier:
- As foulants accumulate on the membrane surface, TMP rises. When TMP reaches terminal operating levels (typically 20 to 35 psi), automated backwash and Clean-in-Place (CIP) chemical cleaning cycles are triggered using sodium hypochlorite (for organic fouling) and citric acid (for mineral scaling).
Direct Integrity Testing (DIT): The Pressure Decay Test (PDT)
Under the EPA Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR), water systems receive significant pathogen removal credit (e.g., 4.0-log removal for Cryptosporidium) ONLY if they verify membrane integrity daily.
Daily Pressure Decay Test (PDT) Protocol
Pressurized Air Supply (15 to 30 psi)
│
▼
┌───────────────┬────────────────┐
│ Isolated Membrane Module │
│ Liquid-Filled Shell (Feed) │
│ ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░ │
│ ┌──────┐ ┌──────┐ ┌──────┐ │
│ │Fiber │ │Fiber │ │Broken│◄───┼── Air leaks through breached fiber
│ │Lumen │ │Lumen │ │Fiber │ │ (Bubble stream detected)
│ └──────┘ └──────┘ └───┬──┘ │
│ Pressurized Air │ │
└────────────────────────┼───────┘
▼ Pressure Decay Monitored via Digital Sensor
- Principle of the Test: The membrane pores are filled with water. Because water surface tension holds liquid inside microscopic pores, air cannot penetrate the pores below the bubble-point pressure (typically >45 to 60 psi). An applied air pressure of 15 to 30 psi will NOT pass through an intact membrane fiber.
- Testing Execution: The module is isolated from service. The filtrate side is pressurized with clean air to a baseline pressure (e.g., 20 psi), the air supply valve is sealed, and a precision digital pressure transmitter logs the rate of pressure decay over a 5- to 10-minute hold period.
- Integrity Criteria: If the pressure decay rate remains below the calculated critical decay limit (typically <0.05 to 0.10 psi/min), the membrane barrier is 100% intact. If the decay rate exceeds the threshold, a broken fiber is present. The module is opened, visual bubble testing or ultrasonic acoustic sensors locate the damaged fiber, and the operator inserts a stainless steel or elastomeric pin into both ends of the fiber lumen to seal it permanently.
Nanofiltration & Reverse Osmosis (High-Pressure Membranes)
Where microfiltration and ultrafiltration are porous membranes that strain particles, nanofiltration (NF) and reverse osmosis (RO) are semi-permeable, non-porous membranes that separate by diffusion through a dense polyamide film. They remove dissolved constituents that low-pressure membranes pass straight through.
| Property | MF | UF | NF | RO |
|---|---|---|---|---|
| Nominal pore size / cutoff | 0.1 µm | 0.01 µm | ~200–1,000 Da | Non-porous |
| Typical feed pressure | 5–30 psi | 10–40 psi | 50–150 psi | 150–1,000+ psi |
| Removes | Particles, protozoa, most bacteria | Above plus viruses | Above plus hardness, color, DBP precursors, many organics | Above plus monovalent salts, nitrate, sodium, most PFAS |
| Does not remove | Dissolved solids | Dissolved solids | Most monovalent salts | Dissolved gases (CO2, H2S) |
- NF is often called "membrane softening." It rejects divalent ions (calcium, magnesium, sulfate) far more effectively than monovalent ions, which makes it an alternative to lime softening with the added benefit of removing color and total organic carbon, thereby cutting disinfection byproduct formation potential downstream.
- RO rejects better than 95 to 99 percent of dissolved salts and is used in New Jersey for brackish groundwater, nitrate reduction, and PFAS removal. Element arrays are staged (for example 2:1) so that concentrate from the first stage becomes feed to the second, maintaining crossflow velocity as permeate is removed.
- Key definitions: Recovery is permeate flow divided by feed flow; rejection is the percentage of a constituent kept out of the permeate; flux is permeate flow per unit membrane area (gfd); concentrate or reject is the waste stream. Raising recovery raises concentrate concentration and scaling risk.
- Post-treatment is mandatory. RO permeate is aggressive, nearly free of alkalinity and hardness, and will attack cement linings and metallic plumbing. It must be stabilized by blending with bypass water, adding lime or caustic soda and carbon dioxide, or passing through calcite contactors. RO also passes dissolved gases, so hydrogen sulfide and carbon dioxide require degasification.
Membrane Fouling, Cleaning & Integrity Verification
Fouling is the accumulation of material on or in the membrane that raises transmembrane pressure and lowers flux at a given pressure.
| Fouling type | Cause | Typical cleaning |
|---|---|---|
| Particulate / colloidal | Silt, clay, iron floc | Backwash, air scour, low-pH acid clean |
| Organic | Natural organic matter, oils | High-pH caustic clean with surfactant |
| Biological (biofouling) | Biofilm growth on the membrane | Caustic clean plus approved biocide; chloramine or chlorine tolerance depends on membrane material |
| Scaling | CaCO3, CaSO4, BaSO4, silica precipitating in the concentrate | Antiscalant dosing, lower recovery, acid clean |
Normalize operating data (temperature-corrected specific flux and differential pressure) before deciding to clean. Clean-in-place (CIP) is triggered when normalized specific flux declines about 10 to 15 percent, when differential pressure rises about 15 percent, or when salt passage rises about 10 percent from baseline.
Direct integrity testing (DIT) verifies that a low-pressure membrane still meets its pathogen log removal credit. The pressure decay test pressurizes the air side of a drained module and measures pressure loss over a fixed interval; a decay rate above the control limit indicates a broken fiber or a seal leak, and the rack must be taken offline until the breach is located (typically by bubble testing) and pinned. Continuous indirect integrity monitoring by filtrate turbidity or particle counting runs between direct tests.
Exam Trap Alert: Chlorine tolerance is membrane-specific. Polyamide thin-film composite RO and NF membranes are destroyed by free chlorine and require dechlorination of the feed, while many PVDF microfiltration and ultrafiltration membranes tolerate chlorine cleaning. Never assume a cleaning chemical approved for one membrane is safe for another.
During the daily Direct Integrity Testing (DIT) of an ultrafiltration membrane rack, a Pressure Decay Test (PDT) is performed at 20 psi. The recorded pressure decay rate is 0.45 psi/min, which significantly exceeds the critical regulatory compliance limit of 0.08 psi/min. What does this test result indicate, and what is the proper operator response?
A reverse osmosis system treating brackish groundwater is operated at 80 percent recovery instead of its design 70 percent. What is the most likely consequence?