5.1 Low-Pressure Membranes: Microfiltration & Ultrafiltration

ABC Water Treatment Operator Class II Certification Study Guide Exam

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1 Chapter 1: Source Water Quality & Characteristics

1.1 Hydrologic Cycle, Watersheds & Water Sources 1.2 Physical, Chemical & Biological Source Water Parameters 1.3 Source Water Protection, Limnology & Raw Water Intakes 1.4 Wellhead Protection, Source Water Protection & Watershed Management Plans 1.5 Raw Water Storage, Release Scheduling & Demand Forecasting

2 Chapter 2: Coagulation & Rapid Mixing

2.1 Coagulation Chemistry & Colloidal Destabilization 2.2 Primary Coagulants, Polymers & Alkalinity Demand 2.3 Rapid Mix Facilities & Process Control 2.4 Pretreatment: Presedimentation, Pre-Oxidation & Algae Control

3 Chapter 3: Flocculation & Sedimentation

3.1 Flocculation Hydraulics & Tapered Floc Formation 3.2 Clarification Zones & Conventional Sedimentation 3.3 High-Rate Clarification Technologies

4 Chapter 4: Granular Media Filtration

4.1 Filter Media Selection, Underdrains & Hydraulic Loading 4.2 Filter Operation, Ripening & Particle Removal Mechanisms 4.3 Backwash Hydraulics, Air Scour & Filter Maintenance 4.4 Slow Sand, Direct, Pressure & Cartridge Filtration

5 Chapter 5: Membrane Filtration & Advanced Separation

5.1 Low-Pressure Membranes: Microfiltration & Ultrafiltration 5.2 High-Pressure Membranes: Nanofiltration & Reverse Osmosis 5.3 Membrane Integrity Testing & Troubleshooting

6 Chapter 6: Softening, Demineralization & Special Treatment

6.1 Lime-Soda Ash Softening Chemistry & Stoichiometry 6.2 Lime Softening Clarification, Recarbonation & Sludge Handling 6.3 Ion Exchange Softening & Demineralization Resins 6.4 Iron & Manganese Removal & Sequestration

7 Chapter 7: Chlorination & Alternative Disinfection

7.1 Chlorine Chemistry & Breakpoint Chlorination 7.2 Chloramination & Monochloramine Residual Maintenance 7.3 Alternative Disinfectants: Ozone, Chlorine Dioxide & Ultraviolet

8 Chapter 8: CT Disinfection Compliance & Disinfection Byproducts

8.1 Surface Water Treatment Rule & CT Disinfection Compliance 8.2 Disinfection Byproduct Regulations & Stage 2 D/DBP Compliance 8.3 Enhanced Coagulation & Total Organic Carbon Precursor Removal

9 Chapter 9: Corrosion Control, Fluoridation & Plant Residuals

9.1 Corrosion Chemistry & Lead and Copper Rule Compliance 9.2 Fluoridation Chemistry, Chemicals & Feed Systems 9.3 Plant Residuals Management & Filter Backwash Recycling

10 Chapter 10: Pumps & Hydraulic Machinery

10.1 Centrifugal Pump Principles, Volutes & Impellers 10.2 Pump Hydraulics: Head, Pressure & Cavitation Prevention 10.3 Positive Displacement & Chemical Metering Pumps 10.4 Pump Maintenance: Packing, Mechanical Seals & Bearings

11 Chapter 11: Motors, Electrical Systems & Instrumentation

11.1 Electric Motors, Motor Starters & Variable Frequency Drives 11.2 Process Instrumentation, Pressure, Level & Flow Sensors 11.3 Automated Process Control, SCADA & PLCs

12 Chapter 12: Chemical Feed Equipment & Piping Systems

12.1 Dry Chemical Feeders, Gravimetric/Volumetric Systems & Slakers 12.2 Liquid Chemical Feed Systems & Drawdown Calibration 12.3 Gas Chlorination Equipment, Regulators, Injectors & Evaporators 12.4 Plant Piping, Valve Types & Water Hammer Surge Control 12.5 Maintenance Management, Work Orders, Asset Management & Plant Air Systems

13 Chapter 13: Physical & Chemical Water Quality Testing

13.1 Turbidity Measurement & Nephelometer Calibration 13.2 pH Measurement, Buffer Calibration & Alkalinity Titration 13.3 Chlorine Residual Analysis: DPD Colorimetry & Amperometric Titration 13.4 Hardness Titration, Conductivity & Jar Testing Protocol

14 Chapter 14: Microbiological Analysis & Laboratory Quality Control

14.1 Microbiological Methods: Colilert, Membrane Filtration & MTF 14.2 Inorganic, Nutrients & Metals Analysis 14.3 Laboratory QA/QC, Chain of Custody & Compliance Records

15 Chapter 15: Chemical Safety & Hazardous Materials

15.1 Chlorine Gas Hazards, Cylinders, Ton Containers & Repair Kits 15.2 Caustic, Acid, Polymer & Chemical Handling (HazCom & SDS) 15.3 Personal Protective Equipment & SCBA Operations

16 Chapter 16: Plant Safety, Confined Space & Electrical Hazards

16.1 Confined Space Entry Regulations (OSHA 29 CFR 1910.146) 16.2 Lockout/Tagout (LOTO) & Electrical Safety (OSHA 29 CFR 1910.147) 16.3 Facility Safety, Eyewashes, Showers & Fire Protection

17 Chapter 17: Regulations, Compliance & Administrative Operations

17.1 Safe Drinking Water Act (SDWA) Framework, MCLs & MCLGs 17.2 Surface Water Treatment Rules & Revised Total Coliform Rule 17.3 Chemical Standards, Public Notification Tiers & CCR 17.4 Emergency Response Planning & Plant Physical/Cyber Security 17.5 Administrative Operations: SOPs, Records, Procurement, Complaints & Optimization

18 Chapter 18: Flow, Volume, Detention Time & Velocity Calculations

18.1 Tank Volume & Storage Capacity Calculations 18.2 Flow Rate Unit Conversions & Pipe/Channel Velocity 18.3 Theoretical Detention Time & Contact Time Calculations

19 Chapter 19: Chemical Dosage & Feed Rate Calculations

19.1 The Pounds Formula & Mass Loading Calculations 19.2 Solution Dosing, Specific Gravity & Pump Drawdown Math 19.3 Fluoride Chemical Feed Calculations & Solution Dilutions

20 Chapter 20: Clarification, Filtration & Disinfection Math

20.1 Surface Overflow Rate & Weir Loading Rate Calculations 20.2 Filter Loading Rate, Backwash Rise Rate & Washwater Volume 20.3 CT Disinfection Calculations & Log-Inactivation Compliance

21 Chapter 21: Pump, Power & Hydraulic Pressure Calculations

21.1 Pressure and Head Conversion Calculations 21.2 Water Horsepower, Brake Horsepower & Electrical Power Math 21.3 Pump Affinity Laws & Impeller Performance Calculations

5.1 Low-Pressure Membranes: Microfiltration & Ultrafiltration

Key Takeaways Explain This Topic AI

Microfiltration (MF) utilizes pore sizes of 0.1 to 0.2 µm, delivering >4-log physical removal of Giardia and Cryptosporidium but negligible virus rejection due to smaller viral dimensions (0.02 to 0.08 µm).

Ultrafiltration (UF) features pore ratings of 0.01 to 0.05 µm (10,000 to 100,000 Daltons MWCO), securing verified >4-log removal credits for protozoan cysts, bacteria, and enteric viruses.

Hollow fiber modules dominate municipal facilities, configured as either enclosed pressurized vessels (10 to 40 psi driving head) or submerged vacuum basins (-3 to -12 psi suction).

Transmembrane Pressure (TMP = [(Pfeed + Pconcentrate) / 2] - Pfiltrate) and operational flux (20 to 50 gfd) must be normalized to 20°C to distinguish true fouling from fluid viscosity shifts.

Automated hydraulic backwashing occurs every 15 to 60 minutes for 30 to 90 seconds, combining vigorous air scouring with reverse permeate backflushing.

Last updated: September 2026

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 / m o l g/mol g / m o l ). 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).

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.

Pathogen Class Physical Dimensions Microfiltration (MF) Removal Credit Ultrafiltration (UF) Removal Credit Operational / Regulatory Caveat

Cryptosporidium 4 to 6 µm (spherical oocyst) ≥ \ge ≥ 4.0-log (99.99%) ≥ \ge ≥ 4.0-log (99.99%) Requires daily Direct Integrity Testing (DIT) with resolution ≤ \le ≤ 3 µm.

Giardia lamblia 8 to 14 µm (oval cyst) ≥ \ge ≥ 4.0-log (99.99%) ≥ \ge ≥ 4.0-log (99.99%) Physical barrier fully excludes intact cysts when integrity is verified.

Enteric Bacteria 0.5 to 3.0 µm ( E. coli , Salmonella ) ≥ \ge ≥ 4.0-log (99.99%) ≥ \ge ≥ 4.0-log (99.99%) Complete physical rejection under intact membrane operations.

Enteric Viruses 0.02 to 0.08 µm (20 to 80 nm) 0.0 to 0.5-log (Uncredited) ≥ \ge ≥ 4.0-log (99.99%) MF pores (0.1 µm) are larger than viruses; UF pores (0.01 µm) physically retain viruses.

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.

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

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.

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.

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 |
• +-------------------------------+ +---------------------------------+

Operational Feature Pressurized Membrane Systems Submerged (Vacuum-Driven) Systems

Vessel Construction Fibers enclosed in cylindrical PVC, fiberglass, or stainless steel pressure vessels. Modules assembled in unpressurized stainless steel cassettes submerged in open concrete basins.

Driving Pressure Positive feed pressure: +10 to +40 psi (0.7 to 2.8 bar) supplied by high-head feed pumps. Negative suction pressure: -3 to -12 psi (-0.2 to -0.8 bar) pulled by permeate vacuum pumps.

Design Flux Rate Higher flux: 30 to 50 gfd ( g p d / f t 2 gpd/ft^2 g p d / f t 2 ). Moderate flux: 20 to 35 gfd ( g p d / f t 2 gpd/ft^2 g p d / f t 2 ).

Footprint & Capital Compact footprint; higher structural piping and pressure vessel costs. Larger civil footprint; lower module vessel costs; utilizes open basins similar to conventional filters.

Turbidity Tolerance Sensitive to solids; rapid TMP rise during river spikes unless pre-clarified. Highly tolerant of high solids (handles spikes up to 200–500 NTU); air scour easily suspends solids.

Visual Inspection Modules are opaque; requires offline isolation and acoustic or pressure testing. Direct visual observation of bubbling cassettes during backwash in open basins.

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: T M P = ( P f e e d + P c o n c e n t r a t e 2 ) − P f i l t r a t e TMP = \left(\frac{P_{feed} + P_{concentrate}}{2}\right) - P_{filtrate} TMP = ( 2 P f ee d ​ + P co n ce n t r a t e ​ ​ ) − P f i lt r a t e ​

For submerged systems operating under vacuum, TMP is calculated from the basin hydrostatic liquid level ( P s t a t i c P_{static} P s t a t i c ​ ) and the negative vacuum pressure on the permeate suction header ( P s u c t i o n P_{suction} P s u c t i o n ​ ): T M P = P s t a t i c − ( − P s u c t i o n ) = P s t a t i c + ∣ P s u c t i o n ∣ TMP = P_{static} - (-P_{suction}) = P_{static} + |P_{suction}| TMP = P s t a t i c ​ − ( − P s u c t i o n ​ ) = P s t a t i c ​ + ∣ P s u c t i o n ​ ∣

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 J 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 = Q p A m J = \frac{Q_p}{A_m} J = A m ​ Q p ​ ​

Where:

• Q p Q_p Q p ​ = Permeate flow rate (gallons per day, gpd)
• A m A_m A m ​ = Total active membrane surface area ( f t 2 ft^2 f t 2 )
• 3. Temperature-Corrected Flux ( J 20 J_{20} J 20 ​ )

Water viscosity varies inversely with temperature. As water cools, viscosity ( μ \mu μ ) 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 × ( μ T μ 20 ) = J T × T C F J_{20} = J_T \times \left(\frac{\mu_T}{\mu_{20}}\right) = J_T \times TCF J 20 ​ = J T ​ × ( μ 20 ​ μ T ​ ​ ) = J T ​ × TCF

Where T C F TCF 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.

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

Filtration Phase : Forward production runs for 15 to 60 minutes , building a thin surface cake of captured solids.

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.

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.

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.

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.