5.1 Low-Pressure Membranes: Microfiltration & Ultrafiltration

Low-Pressure Membrane Fundamentals

Low-pressure membrane filtration has emerged as a premier technology for treating surface water and Groundwater Under the Direct Influence of Surface Water (GWUDI). Unlike granular media filters (sand and anthracite) that rely on depth filtration, chemical coagulation, and attachment mechanisms, low-pressure membranes operate primarily via size exclusion (sieving). Suspended particles, flocs, algae, and microorganisms larger than the membrane pore geometry are physically intercepted at the membrane boundary.

                      Low-Pressure Membrane Spectrum
                      
   Coarse Solids        Bacteria        Cryptosporidium / Giardia       Viruses
   (10 - 100 µm)      (0.5 - 3.0 µm)           (4 - 14 µm)          (0.02 - 0.08 µm)
         |                  |                       |                      |
---------+------------------+-----------------------+----------------------+---------
         |   Microfiltration (MF: 0.1 - 0.2 µm)     |                      |
         |   [Retains Turbidity, Bacteria, Protozoan Cysts]                |
         |                                                                 |
         |   Ultrafiltration (UF: 0.01 - 0.05 µm / 10k - 100k Daltons)     |
         |   [Retains Turbidity, Bacteria, Protozoa, AND Enteric Viruses] -/

Microfiltration (MF) Pore Classification

• Pore Size Spectrum: Nominally spans 0.1 to 0.2 µm (absolute ratings up to 0.2 µm).
• Separation Mechanism: Pure mechanical sieving. Particulates and colloids exceeding the pore throat are physically blocked on the feed side.
• Primary Target Contaminants: Suspended silt, precipitated iron and manganese hydroxides, biological floc, algae, bacteria (Escherichia coli, Legionella), and protozoan parasites (Giardia lamblia cysts and Cryptosporidium oocysts).
• Dissolved Constituents: MF membranes provide no direct barrier to dissolved natural organic matter (NOM), color compounds, salts, or synthetic organic chemicals unless combined with pre-coagulation or powdered activated carbon (PAC).

Ultrafiltration (UF) Pore Classification

• Pore Size Spectrum: Nominally spans 0.01 to 0.05 µm.
• Molecular Weight Cut-Off (MWCO): Because UF pores approach macromolecular dimensions, they are conventionally rated by MWCO, spanning 10,000 to 100,000 Daltons ($g/mol$). The MWCO represents the molecular mass of a globular solute (typically dextran or polyethylene glycol) that is 90% rejected by the membrane matrix.
• Separation Mechanism: Sieving combined with minor surface electrostatic exclusion.
• Pathogen Removal: In addition to turbidity, bacteria, and protozoa, UF physically excludes enteric viruses (such as Poliovirus, Rotavirus, Norovirus, and Hepatitis A), whose physical dimensions range from 0.02 to 0.08 µm (20 to 80 nm).

Membrane Material Properties

Commercial hollow fibers are engineered from resilient synthetic polymers designed to withstand hydraulic stress and aggressive chemical cleaning:

• Polyvinylidene Fluoride (PVDF): The industry standard for municipal applications due to its high tensile strength, wide pH tolerance (1 to 12), and exceptional tolerance to chlorine oxidants (cumulative exposure >1,000,000 mg/L·h).
• Polyethersulfone (PES) / Polysulfone (PS): Highly hydrophilic polymers offering high flux rates at lower operating pressures, but displaying moderate sensitivity to continuous high-dose chlorine.
• Polypropylene (PP): Highly hydrophobic and mechanically durable, frequently used in dry-stored modules but requiring specialized wetting agents.
• Cellulosic Polymers (Cellulose Acetate): Highly hydrophilic with low fouling tendencies, but susceptible to biological degradation and limited to a narrow operating pH window (4 to 8).

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Microbial Pathogen Removal & EPA Regulatory Credits

Under the EPA Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) and the Surface Water Treatment Rule (SWTR), public water systems must achieve certified log-removal credits for regulated pathogens.

Class II Exam Pointer: Regulatory agencies award microfiltration plants no formal virus credit because viral particles readily pass through 0.1 µm pores. MF facilities must achieve their mandatory 4-log virus inactivation through downstream chemical disinfection (free chlorine, chloramines, or ozone). Ultrafiltration plants, however, can receive up to 4-log virus removal credit if manufacturer challenge testing and continuous site verification demonstrate integrity.

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Module Configurations & Flow Regimes

Municipal drinking water plants employ hollow fiber module geometries. A single full-scale module contains between 10,000 and 50,000 flexible, straw-like hollow fibers (outer diameter 0.5 to 2.0 mm, inner lumen diameter 0.3 to 1.0 mm) bundled together and sealed inside polyurethane potting headers.

                  Inside-Out Flow vs. Outside-In Flow
                  
       Inside-Out Flow Regime                 Outside-In Flow Regime
       
        Feed In ---> [ Lumen ]                 Feed Pressurizes Shell
                         |                                |  
                 (Pores in Wall)                          v  
                         |                     [ Outer Fiber Surface ]
                         v                                |  
                  Filtrate Exits                   (Pores in Wall)
                  to Outer Shell                          |  
                                                          v  
                                                   Filtrate Enters
                                                  Lumen ---> Out

Flow Regimes: Outside-In vs. Inside-Out

1. Outside-In Flow: Raw feed water surrounds the exterior of the fibers under pressure or vacuum. Clean filtrate permeates across the fiber wall into the central hollow core (lumen) and exits through the module manifold.
   • Advantages: Can tolerate high suspended solids (turbidity spikes >100 NTU) because solids accumulate in the spacious interstitial area between fibers. Vigorous air scour bubbles can easily agitate fiber exteriors to dislodge cake solids.
   • Dominance: Represents over 85% of municipal hollow-fiber installations.
2. Inside-Out Flow: Raw water enters the internal lumen of each fiber under positive pressure, forcing clean water through the fiber wall to collect in the outer housing shell.
   • Disadvantages: Vulnerable to lumen plugging. If raw water contains large debris, hair, or fibers, the tiny 0.7 mm bores plug permanently. Mandatory upstream strainers (100 to 200 µm) are required.
   • Advantages: Uniform fluid hydraulics and lower feed volume hold-up.

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Pressurized vs. Submerged (Vacuum-Driven) Systems

Municipal low-pressure membranes operate under two primary physical configurations:

          Pressurized System                     Submerged (Vacuum) System
          
   +-------------------------------+       +---------------------------------+
   |  Steel/Fiberglass Cylinder    |       |   Open Concrete Basins          |
   |  Feed In: +10 to +40 psi      |       |   Feed: Atmospheric Pressure    |
   |  Filtrate Out: Positive Head  |       |   Suction Pump: -3 to -12 psi   |
   +-------------------------------+       +---------------------------------+

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Operating Hydraulics & Core Calculations

Operators must monitor hydraulic parameters continuously to detect fouling, verify pump efficiency, and initiate automated cleaning sequences.

1. Transmembrane Pressure (TMP)

Transmembrane Pressure represents the net hydraulic force driving water across the membrane barrier. It is the average pressure on the feed/concentrate side minus the pressure on the filtrate side:

$TMP = \left(\frac{P_{feed} + P_{concentrate}}{2}\right) - P_{filtrate}$

For submerged systems operating under vacuum, TMP is calculated from the basin hydrostatic liquid level ($P_{static}$) and the negative vacuum pressure on the permeate suction header ($P_{suction}$):

$TMP = P_{static} - (-P_{suction}) = P_{static} + |P_{suction}|$

As foulants deposit on the membrane surface, hydraulic resistance increases. To maintain a constant treated water flow rate, the system must increase TMP. A typical clean membrane starts with a baseline TMP of 2 to 5 psi; when TMP reaches 20 to 30 psi, chemical cleaning is mandatory.

2. Membrane Flux ($J$)

Membrane flux defines the volumetric production rate per unit of active membrane surface area, expressed in gallons per square foot per day (gfd):

$J = \frac{Q_p}{A_m}$

Where:

• $Q_p$ = Permeate flow rate (gallons per day, gpd)
• $A_m$ = Total active membrane surface area ($ft^2$)

3. Temperature-Corrected Flux ($J_{20}$)

Water viscosity varies inversely with temperature. As water cools, viscosity ($\mu$) increases substantially, requiring more driving pressure to push the same volume through the pores. Water at 0°C (1.79 cP) is nearly twice as viscous as water at 25°C (0.89 cP). To assess whether an escalating TMP is caused by physical foulant accumulation or simply seasonal water cooling, operators must normalize flux to a standard temperature of 20°C (68°F):

$J_{20} = J_T \times \left(\frac{\mu_T}{\mu_{20}}\right) = J_T \times TCF$

Where $TCF$ is the Temperature Correction Factor (approximately a 2.5% to 3.0% change in flux per 1°C change in temperature). If raw water cools from 20°C to 5°C, an uncorrected membrane will experience a ~35% drop in flux at constant TMP, even with zero surface fouling.

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Routine Hydraulic Cleaning Cycles

To control reversible cake layer buildup without taking skids offline for extended hours, membrane units execute automated hydraulic cleaning cycles:

                      Automated Backwash Sequence
                      
 [ Filtration Mode ] ---> [ Air Scour ] ---> [ Permeate Backflush ] ---> [ Forward Flush ]
    15 - 60 min               30 s                 30 - 60 s                  30 s
   Forward Flow         Violent Bubbles       Reverse Flow (Permeate)     Sweep Solids
   Collect Filtrate     Loosen Surface Cake   Dislodge Bound Pores        Purge Waste

1. Filtration Phase: Forward production runs for 15 to 60 minutes, building a thin surface cake of captured solids.
2. Air Scour: High-volume, oil-free compressed air (10 to 30 psi) is injected into the bottom of the module or cassette. Rising air bubbles agitate the flexible hollow fibers, causing them to rub against one another and mechanically sheer loose the compacted particulate cake.
3. Permeate Backflush: High-quality filtered permeate water is pumped in the reverse direction (from the lumen outward into the shell or basin) at a rate 1.5 to 2.5 times the forward filtration flux. This lifts dislodged debris out of the pore throats.
4. Drain / Forward Flush: The dislodged solids are swept out through the waste drain manifold using raw feed water. The entire backwash sequence consumes 30 to 90 seconds and utilizes 3% to 7% of the total plant production volume.
5. Maintenance Cleans (Chemically Enhanced Backwash - CEB): Conducted once every 24 to 72 hours, CEB injects low concentrations of sodium hypochlorite (50 to 200 mg/L) or citric acid (500 to 2,000 mg/L) into the backwash stream. The chemical solution soaks inside the fibers for 10 to 20 minutes before flushing, removing organic foulants and mineral films before irreversible fouling establishes.