5.4 Advanced Treatment: Membranes, AOP & PFAS Adsorption

Advanced Treatment: Membranes, AOP & PFAS Adsorption

Driven by severe climatic droughts, groundwater over-drafting, and the discovery of persistent anthropogenic contaminants—such as Per- and Polyfluoroalkyl Substances (PFAS), 1,4-Dioxane, and N-Nitrosodimethylamine (NDMA)—California has pioneered the deployment of Advanced Water Purification Facilities (AWPF) for Groundwater Replenishment Reuse Projects (GRRP) and Direct Potable Reuse (DPR).

Advanced water treatment relies on three core unit operations: pressure-driven membrane separation, Advanced Oxidation Processes (AOP), and specialized adsorption media (GAC and IX).

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The Membrane Filtration Spectrum

Membrane filtration utilizes semi-permeable polymeric or ceramic materials to separate particulate, colloidal, biological, and dissolved constituents from water under applied hydrostatic pressure.

1. Low-Pressure Membranes: Microfiltration (MF) & Ultrafiltration (UF)

• Module Configurations: Typically manufactured as hollow-fiber bundles (inside-out or outside-in flow) configured in submerged vacuum basins or pressurized vertical vessels.
• Separation Mechanism: Pure physical size exclusion (sieving).
• Applications: Clarification and pathogen barriers upstream of reverse osmosis. MF reliably removes Giardia and Cryptosporidium ($>4$-log credit) and suspended solids, while UF additionally removes viruses ($>4$-log credit) due to its smaller molecular weight cut-off (MWCO).

2. High-Pressure Membranes: Nanofiltration (NF) & Reverse Osmosis (RO)

• Module Configurations: Manufactured as spiral-wound elements containing thin-film composite (TFC) aromatic polyamide membranes flat-sheet layers wrapped around a central perforated permeate collection tube.
• Separation Mechanism: Solution-diffusion (water molecules dissolve into the non-porous polymer matrix and diffuse across under pressure) combined with electrostatic Donnan exclusion (repelling charged ions).
• Applications: Desalination, softening, total dissolved solids (TDS) removal, nitrate rejection, and elimination of organic micro-pollutants and PFAS.

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Membrane Operating Metrics & Mathematical Equations

Certified operators monitor several critical mathematical parameters to assess membrane fouling, scale formation, and hydraulic performance:

1. Membrane Flux ($J$)

Flux represents the volumetric filtration rate normalized per unit of active membrane surface area:

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

• Expressed in LMH (Liters per square meter per hour, $\text{L}/(\text{m}^2\cdot\text{hr})$) in metric systems, or gfd (gallons per square foot per day, $\text{gal}/(\text{ft}^2\cdot\text{day})$) in US customary units.
• Conversion: $1\text{ gfd} = 1.70\text{ LMH}$.

2. Temperature-Corrected Flux ($J_{20^\circ\text{C}}$)

Because water viscosity increases as temperature decreases, cold water requires higher feed pressure to maintain identical flux. Operators normalize flux to $20^\circ\text{C}$ ($68^\circ\text{F}$) using the Temperature Correction Factor ($TCF$):

$J_{20^\circ\text{C}} = J_{\text{measured}} \times TCF$

3. Transmembrane Pressure (TMP)

TMP is the net pressure driving force across the membrane barrier:

$\text{TMP} = \left(\frac{P_{\text{feed}} + P_{\text{concentrate}}}{2}\right) - P_{\text{permeate}} \quad (\text{Crossflow Filtration})$

$\text{TMP} = P_{\text{feed}} - P_{\text{permeate}} \quad (\text{Dead-End Hollow Fiber})$

An increase in TMP at constant flux indicates membrane fouling or mineral scaling.

4. Recovery Rate (%)

The percentage of feed water converted into treated permeate product:

$\text{Recovery } (\%) = \left(\frac{Q_{\text{permeate}}}{Q_{\text{feed}}}\right) \times 100$

• Low-pressure MF/UF systems typically operate at 90% to 95% recovery.
• Brackish water and water reuse RO systems operate at 75% to 85% recovery (typically in 2-stage or 3-stage array configurations).
• Seawater desalination RO operates at 40% to 50% recovery due to extreme osmotic pressure limitations.

5. Silt Density Index ($SDI_{15}$)

The Silt Density Index is an empirical standard test (ASTM D4189) measuring the particulate fouling potential of RO feed water. Water is passed through a $0.45\text{ }\mu\text{m}$ membrane filter at a constant pressure of $30\text{ psi}$ ($207\text{ kPa}$), measuring the time required to collect a $500\text{-mL}$ sample initially ($t_0$) and after 15 minutes of continuous flow ($t_{15}$):

$\text{SDI}_{15} = \frac{100 \times \left(1 - \frac{t_0}{t_{15}}\right)}{15}$

• Target Limit: RO manufacturers require an $\text{SDI}_{15} < 3.0$ (absolute maximum $< 5.0$). An $\text{SDI}_{15} > 3.0$ causes rapid, irreversible colloidal fouling of RO elements.

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Membrane Maintenance, Cleaning & Integrity Testing

┌──────────────────────────────────────────────────────────────────────────┐
│                     Membrane Cleaning & Integrity Cycle                  │
├──────────────────────────────────────────────────────────────────────────┤
│  Normal Filtration (TMP Rises Gradually)                                │
│        │                                                                 │
│        ▼ Every 15 - 45 Minutes                                           │
│  [Hydraulic Backwash / Air Scour] (Removes surface cake solids)          │
│        │                                                                 │
│        ▼ Every 1 - 6 Months (or when TMP increases by 15-20%)            │
│  [Clean-In-Place (CIP)]                                                  │
│        ├─► Low-pH Acid Wash (pH 2-3: Citric/HCl) ── Dissolves Scale      │
│        └─► High-pH Caustic Wash (pH 11-12: NaOH/EDTA) ── Removes Biofilm │
│        │                                                                 │
│        ▼ Daily Mandatory Compliance Check                                │
│  [Direct Integrity Testing (DIT)]                                        │
│        └─► Pressure Decay Test (PDT) ── Verifies No Broken Fibers        │
└──────────────────────────────────────────────────────────────────────────┘

Clean-In-Place (CIP) Chemical Protocols

When normalized membrane flux drops by 10% to 15%, or when normalized TMP increases by 15% to 20%, the membrane skid is taken offline for chemical CIP:

1. Low-pH Acid Wash (pH 2.0 to 3.0): Uses citric acid, hydrochloric acid ($HCl$), or phosphoric acid to dissolve inorganic mineral scaling (calcium carbonate $CaCO_3$, calcium sulfate $CaSO_4$, barium sulfate $BaSO_4$) and metal oxide precipitates (iron, manganese).
2. High-pH Caustic Wash (pH 11.0 to 12.0): Uses sodium hydroxide ($NaOH$) combined with sodium dodecyl sulfate (SDS surfactant) or EDTA chelating agents to hydrolyze and dissolve natural organic matter (NOM), humic acids, grease, and microbial biofilms.

Direct Integrity Testing (DIT): Pressure Decay Test (PDT)

To maintain regulatory pathogen log reduction credits under Title 22, operators must perform automated Direct Integrity Tests (DIT) at least once every 24 hours on each MF/UF membrane train:

• Procedure: The filtrate side of the hollow-fiber membrane module is pressurized with air to approximately 10 to 15 psi ($70\text{–}100\text{ kPa}$), displacing water. The air supply is isolated, and the rate of pressure drop (decay) is monitored over 5 to 10 minutes.
• Mechanism: In an intact, fully wetted hydrophilic membrane, surface tension prevents air from passing through the microscopic sub-micron pores (bubble point phenomenon). If a single fiber is severed or broken, air rapidly streams through the open lumen, causing a rapid pressure decay rate ($\Delta P / \Delta t$) that triggers an automated skid alarm and shutdown.

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Advanced Oxidation Processes (AOP)

Advanced Oxidation Processes (AOP) are designed to destroy recalcitrant, low-molecular-weight organic chemical contaminants that pass through microfiltration and partially penetrate reverse osmosis membranes.

The Hydroxyl Free Radical ($\bullet\text{OH}$)

All AOP systems operate by generating the Hydroxyl Free Radical ($\bullet\text{OH}$) in situ. With an extraordinary standard oxidation potential of 2.80 V, the hydroxyl radical reacts non-selectively at near-diffusion-controlled rates ($k > 10^9\text{ M}^{-1}\text{s}^{-1}$) by abstracting hydrogen atoms and cleaving stable carbon-carbon bonds, mineralizing complex organics into harmless carbon dioxide ($CO_2$), water ($H_2O$), and inorganic salts.

Primary AOP Configurations:

1. $UV / H_2O_2$ (Ultraviolet / Hydrogen Peroxide):
   • High-intensity UVC light photolytically cleaves the oxygen-oxygen single bond in hydrogen peroxide: $\text{H}_2\text{O}_2 + h\nu (\text{UVC}) \longrightarrow 2\text{ }\bullet\text{OH}$
   • Highly effective for destroying 1,4-Dioxane (an industrial stabilizer and carcinogen) and N-Nitrosodimethylamine (NDMA) (a potent chloramine-induced DBP with a California Notification Level of 10 ng/L).
2. Ozone / $H_2O_2$ (Peroxone Process):
   • Hydrogen peroxide accelerates the decomposition of aqueous ozone into hydroxyl radicals: $2\text{O}_3 + \text{H}_2\text{O}_2 \longrightarrow 2\text{ }\bullet\text{OH} + 3\text{O}_2$
   • Requires no high-energy UV lamps and operates effectively on large volumetric flows.

[!TIP] Radical Scavenging: Background carbonate and bicarbonate alkalinity ($HCO_3^- / CO_3^{2-}$) and dissolved organic carbon (DOC) act as radical scavengers, consuming hydroxyl radicals before they can destroy target contaminants. In direct potable reuse trains, locating AOP immediately downstream of reverse osmosis eliminates $>98%$ of bicarbonate and TOC, maximizing AOP radical efficiency.

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PFAS Adsorption Engineering (GAC, IX & RO)

Per- and Polyfluoroalkyl Substances (PFAS)—including Perfluorooctanoic Acid (PFOA), Perfluorooctane Sulfonate (PFOS), Perfluorohexane Sulfonate (PFHxS), and GenX—are synthetic organic chemicals containing exceptionally strong carbon-fluorine ($C\text{-}F$) bonds ($485\text{ kJ/mol}$). Because these "forever chemicals" resist biological degradation, chemical oxidation, and conventional coagulation, specialized adsorption or membrane processes are required.

In April 2024 the US EPA finalized a National Primary Drinking Water Regulation (NPDWR) for PFAS, setting Maximum Contaminant Levels (MCLs) of 4.0 ng/L (ppt) for PFOA and PFOS, 10 ng/L each for PFHxS, PFNA, and GenX (HFPO-DA), and a Hazard Index of 1.0 for mixtures of PFHxS, PFNA, HFPO-DA, and PFBS.

[!IMPORTANT] This rule is actively in flux - verify the current federal status before relying on any number. Under the PFAS strategy EPA announced on May 18, 2026, the agency is keeping the 4.0 ppt MCLs for PFOA and PFOS but has proposed (a) a rescission of the MCLs for PFHxS, PFNA, HFPO-DA, and the Hazard Index, and (b) an extension of the PFOA/PFOS compliance deadline from April 26, 2029 to April 26, 2031 for systems that request it. The comment period on the proposal closed July 20, 2026. California's own notification levels and response levels for PFOA and PFOS remain in force independently of the federal rule, and California has consistently regulated PFAS more aggressively than the federal floor - so a California operator answers PFAS questions from the state requirements first.

┌──────────────────────────────────────────────────────────────────────────┐
│                     Lead-Lag Adsorption Column System                    │
├──────────────────────────────────────────────────────────────────────────┤
│  Raw Water Feed (PFAS Contaminated)                                      │
│        │                                                                 │
│        ▼                                                                 │
│  ┌──────────────────────────┐                                            │
│  │   LEAD COLUMN (GAC/IX)   │ ──► Adsorbs >95% of incoming PFAS mass     │
│  └─────────────┬────────────┘                                            │
│                │                                                         │
│                ▼ Intermediate Compliance Sample Point (Detects Breakthrough)
│  ┌─────────────┴────────────┐                                            │
│  │   LAG COLUMN (GAC/IX)    │ ──► Polishing Column (Captures Slippage)   │
│  └─────────────┬────────────┘                                            │
│                │                                                         │
│                ▼ Finished Treated Water to Distribution (PFAS Non-Detect)│
│                                                                          │
│  *Rotation Operation:* When Lead column breaks through, spent media is   │
│   replaced; Lag column becomes new Lead, and fresh vessel becomes Lag.   │
└──────────────────────────────────────────────────────────────────────────┘

1. Granular Activated Carbon (GAC)

• Adsorption Mechanics: Hydrophobic and van der Waals adsorption onto high-surface-area porous carbon media (virgin bituminous coal or coconut shell carbon with specific surface area $> 1,000\text{ m}^2/\text{g}$).
• Design Metric — Empty Bed Contact Time (EBCT): $\text{EBCT (minutes)} = \frac{\text{GAC Bed Volume } (V_b)}{\text{Flow Rate } (Q)}$
  • PFAS removal requires an EBCT of 10 to 20 minutes (significantly longer than standard taste/odor removal).
• Bed Volumes Treated (BVT): Metric tracking the volume of water treated before target contaminant breakthrough occurs ($BVT = \text{Total Volume Filtered} / V_b$). Long-chain PFAS (PFOS/PFOA) exhibit high adsorption capacity ($30,000\text{–}80,000\text{ BVT}$), whereas short-chain PFAS (PFBS/PFBA) break through much earlier ($5,000\text{–}15,000\text{ BVT}$).

2. Single-Use Selective Anion Exchange (IX) Resins

• Mechanics: Synthetic macroporous or gel-type polymer beads functionalized with positively charged quaternary amine exchange sites ($-\text{N}^+(\text{CH}_3)_3$). IX achieves dual removal: electrostatic attraction of negatively charged PFAS sulfonate/carboxylate heads, plus hydrophobic adsorption of fluorinated carbon tails.
• Advantages over GAC: Shorter required contact time (EBCT = 2 to 5 minutes), much smaller equipment footprint, and superior removal capacity for short-chain PFAS.
• Disposal: PFAS binds so tightly to selective resins that chemical regeneration is impractical; spent resin is thermally incinerated ($>1,000^\circ\text{C}$) or super-critically oxidized off-site.

3. High-Pressure Reverse Osmosis (RO)

• Rejects $>99%$ of all PFAS compounds via size exclusion and Donnan electrostatic repulsion. However, the rejected PFAS accumulates in the concentrate brine stream (15% to 25% of total flow), requiring dedicated brine treatment or deep well injection.

Comparison of Best Available Technologies (BAT) for PFAS Removal