3.2 Membrane Filtration & Ion Exchange Softening

Membrane Filtration & Ion Exchange Softening

Public water utilities utilize advanced separation processes to control pathogens, disinfection byproduct precursors, dissolved hardness, and emerging contaminants such as per- and polyfluoroalkyl substances (PFAS). Membrane filtration and cation exchange softening provide engineered barriers with selective removal capabilities exceeding conventional granular media.

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1. Membrane Filtration Categories & Cutoffs

Membranes are semipermeable synthetic barriers that separate impurities under hydraulic pressure. They are categorized into four classes based on pore diameter, molecular weight cutoff (MWCO), and separation mechanisms:

Microfiltration (MF)

• Pore Size & MWCO: 0.1 to 0.2 microns (100 to 200 nm); MWCO > 100,000 Daltons.
• Operating Pressure: 5 to 30 psi.
• Target Removals: Suspended solids, turbidity, bacteria, and protozoan cysts (Cryptosporidium, Giardia) via physical sieving. Ineffective against enteric viruses or dissolved salts.

Ultrafiltration (UF)

• Pore Size & MWCO: 0.01 to 0.05 microns (10 to 50 nm); MWCO = 10,000 to 100,000 Daltons.
• Operating Pressure: 10 to 50 psi.
• Target Removals: Complete protozoan/bacterial barrier, high log-removal of enteric viruses (Hepatitis A, Norovirus; up to 4-log credit), and macromolecules via sieving. Does not remove dissolved minerals.

Nanofiltration (NF)

• Pore Size & MWCO: 0.001 to 0.01 microns (1 to 10 nm); MWCO = 200 to 1,000 Daltons.
• Operating Pressure: 50 to 150 psi ("membrane softening").
• Target Removals: Divalent ions (Ca2+, Mg2+, SO4 2-), color, and DBP precursors via sieving and Donnan charge exclusion. Rejects 40% to 60% of monovalent salts.

Reverse Osmosis (RO)

• Structure & MWCO: Dense, non-porous polyamide thin-film composite; pore size < 0.001 microns (< 1 nm); MWCO < 100 Daltons.
• Operating Pressure: 150 to 800+ psi to overcome feed osmotic pressure.
• Target Removals: Monovalent ions (Na+, Cl-), nitrate (NO3-), heavy metals, radionuclides, dissolved minerals (> 99% TDS rejection), and PFAS via solution-diffusion.

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2. Membrane Operational Parameters & Hydraulics

Membrane plant operation depends on four key hydraulic parameters:

1. Flux (J): Permeate flow rate per unit active membrane area, expressed in gallons per square foot per day (gfd) or liters per square meter per hour (LMH): $J = \frac{Q_p}{A_m} = \frac{\text{Permeate Flow (gpm)} \times 1,440 \text{ min/day}}{\text{Active Membrane Surface Area (sq ft)}}$ Design flux ranges from 15 to 35 gfd for MF/UF and 10 to 20 gfd for spiral-wound NF/RO.
2. Transmembrane Pressure (TMP): Net driving pressure forcing water through the membrane: $\text{TMP} = \frac{P_{\text{feed}} + P_{\text{concentrate}}}{2} - P_{\text{permeate}}$ For vacuum submerged fibers, TMP = P_tank - P_suction. Increasing TMP at constant flux indicates fouling.
3. Recovery Rate (%): Percentage of feed water converted to permeate: $\text{Recovery (\%)} = \frac{Q_{\text{permeate}}}{Q_{\text{feed}}} \times 100$ MF/UF achieves 90% to 95% recovery. NF/RO recovery is limited to 75% to 85% by mineral scaling in the concentrate stream.
4. Temperature Correction Factor (TCF): Water viscosity increases as temperature drops (~2.5% to 3.0% per °C). At constant pressure, flux drops in winter; to maintain production, TMP must increase. SCADA tracks temperature-normalized specific flux (J at 20°C) to isolate true fouling.

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3. Membrane Integrity Testing & Log Credits

Under the SDWA Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR), plants receive up to 4-log (99.99%) Cryptosporidium credit by verifying membrane integrity.

• Direct Integrity Testing: A daily automated physical challenge on an isolated skid. In the Pressure Decay Test (PDT), the skid is drained and pressurized with air to 10 to 30 psi. Intact wetted pores hold air behind their capillary bubble point. A broken fiber (> 3 µm) allows air to escape, causing a rapid pressure drop (psi/min) recorded by digital sensors. Direct testing must resolve defects ≤ 3 microns to confirm the Log Reduction Value (LRV).
• Indirect Integrity Testing: Continuous filtrate monitoring via turbidimeters (≤ 0.15 NTU) or laser particle counters (2 to 5 µm) to detect sudden fiber breaks between daily PDTs.

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4. Fouling Mechanisms & Clean-In-Place (CIP)

Membranes accumulate particulate cakes (silt/flocs), inorganic scale (CaCO3, CaSO4, BaSO4, SiO2), biofouling (bacterial slime/EPS), and organic adsorption (humic acids). Routine backwashes with air scour clear reversible cake layers.

When normalized TMP rises 15% to 20% or normalized flux drops 10% to 15%, a Clean-In-Place (CIP) is conducted:

• Acid Wash: 1% to 2% citric acid or dilute hydrochloric acid (pH 2.0 to 3.0) at 35°C to 40°C dissolves mineral scale and metal oxides (iron/manganese).
• Caustic/Oxidant Wash: Sodium hydroxide (NaOH, pH 11.0 to 12.0) with sodium hypochlorite (100 to 500 mg/L NaOCl) cleans organics and biofilms from MF/UF membranes.

[!CAUTION] Polyamide thin-film composite RO membranes cannot tolerate free chlorine (< 1,000 ppm-hours exposure causes permanent damage). Hypochlorite must never be applied to polyamide RO systems; use non-oxidizing biocides instead.

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5. Cation Exchange Water Softening Chemistry & Blending

Cation exchange softening utilizes synthetic polystyrene-divinylbenzene resin beads functionalized with negative sulfonate groups (R-SO3-) charged with mobile sodium (Na+) ions.

Stoichiometric Exchange

Divalent hardness cations carry higher charge and affinity than sodium. As hard water filters through the resin, each divalent cation displaces two sodium ions: $\text{Ca}^{2+} + 2 \text{ R-SO}_3\text{Na} \rightleftharpoons (\text{R-SO}_3)_2\text{Ca} + 2 \text{ Na}^+$ $\text{Mg}^{2+} + 2 \text{ R-SO}_3\text{Na} \rightleftharpoons (\text{R-SO}_3)_2\text{Mg} + 2 \text{ Na}^+$

Split-Stream Blending

Ion exchange produces zero-hardness effluent (0 mg/L as CaCO3). Pure softened water is corrosive and strips pipe scales. Plants bypass a calculated portion of raw water and blend it with softened effluent to maintain a finished target hardness of 80 to 120 mg/L as CaCO3: $Q_{\text{blend}} = Q_{\text{total}} \times \left(\frac{H_{\text{target}}}{H_{\text{raw}}}\right) \quad \text{and} \quad Q_{\text{soft}} = Q_{\text{total}} - Q_{\text{blend}}$

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6. Four-Step Ion Exchange Regeneration Sequence

When resin sites become exhausted, hardness breaks through. The unit is regenerated through a 4-step sequence:

1. Backwash (5 to 10 min): Upflow of filtered water at 5 to 8 gpm/sq ft expands the bed by 50% to 75%, scrubbing trapped solids and reclassifying resin beads.
2. Brine Injection (20 to 30 min): Downflow of 10% to 14% NaCl solution (6 to 15 lbs salt per cubic foot of resin). High sodium concentration drives mass action, displacing bound Ca2+ and Mg2+ from resin sites back into solution.
3. Slow Rinse / Displacement (20 to 40 min): Clean water at the brine injection rate pushes the brine plug slowly through the bed, maximizing contact time.
4. Fast Rinse (10 to 15 min): Downflow at service rate (1.5 to 3.0 gpm/cu ft) flushes spent chlorides and residual salt to waste until effluent conductivity reaches baseline potable levels.

Waste Brine Disposal

Spent softener brine is a concentrated, high-chloride waste stream. Under IEPA regulations, brine cannot be discharged to unlined lagoons or freshwater streams without an NPDES permit or industrial sewer discharge approval.