membrane-nf

What this skill does

# Nanofiltration (NF)

Reference for NF process design and diagnostics — what distinguishes NF from RO/UF, where it earns its place, and how to operate it.

Convention markers:
- `⚠️` — likely bug or incorrect assumption in calling code
- `💡` — enhancement opportunity (NF-specific monitoring, selectivity tracking)

---

## §1 — NF Positioning vs RO and UF

| Parameter | UF | NF | RO |
|---|---|---|---|
| Effective pore size | 10–100 nm | ~0.5–2 nm | < 0.5 nm (dense, solution-diffusion) |
| MWCO | 10–1 000 kDa | 150–2 000 Da (NF90 ~200; NF270 ~200–400; Suez D-series 150–300) | < 100 Da |
| Operating pressure | 1–3 bar | 4–15 bar | BWRO 10–25 bar; SWRO 55–83 bar |
| Rejection mechanism | Size exclusion | Steric + Donnan + dielectric | Solution-diffusion |
| Divalent ions (Mg²⁺, SO₄²⁻) | pass freely | 90–99 % (MgSO₄ ≥ 99 % on NF270) | > 99 % |
| Monovalent ions (Na⁺, Cl⁻, K⁺) | pass freely | 20–70 % (NF270 NaCl ~46 %; NF90 NaCl 90–96 %) | > 99 % |
| Organics > 200 Da | partial | 90–99 % (NOM, pesticides, dyes) | > 99 % |
| Permeate flux (typical) | 50–200 LMH | 15–30 LMH | 10–25 LMH |
| Retentate osmotic pressure | negligible | lower than RO at same recovery | high — sets pressure floor |

**Positioning insight**: NF sits between RO and UF. It runs at 4–15 bar (vs RO 10–83) because monovalent ions pass → osmotic backpressure on the retentate side is smaller. Choose NF when the goal is partial desalination, divalent removal, or color/NOM removal — not when full demineralization is required.

---

## §2 — Separation Mechanisms

NF rejection is the superposition of three mechanisms:

### 2.1 Steric (size) exclusion
Solutes larger than the effective pore are rejected. Governs neutral organic rejection and monovalent ion rejection at high ionic strength (charge screened). Quantified by the Donnan Steric Pore Model (DSPM).

### 2.2 Donnan exclusion
Polyamide NF membranes carry fixed charges (carboxyl, amine groups). Co-ions are electrostatically repelled; counter-ions follow them to maintain electroneutrality, so the salt as a whole is rejected. Divalent co-ions rejected more strongly (∝ z²).

- A negatively-charged NF membrane at pH 7 rejects SO₄²⁻ ≫ Cl⁻
- For asymmetric salts (e.g. MgSO₄), divalent rejection is governed by Donnan + dielectric; monovalent rejection (NaCl) is dominated by steric hindrance

### 2.3 Dielectric exclusion
Inside a nanometric pore, confined water has reduced dielectric constant. The Born solvation energy that an ion pays to enter the pore is higher than in bulk. Divalent ions pay 4× the energy (∝ z²), so divalent rejection is enhanced even when pore diameter > hydrated ion size.

The combined model is **DSPM-DE** (Donnan Steric Pore Model with Dielectric Exclusion) — the standard framework for predicting mixed-ion NF rejection.

### 2.4 pH effect on membrane charge

Polyamide NF is **amphoteric** — both protonatable amine and deprotonatable carboxyl groups. Surface charge depends on pH.

- **Isoelectric point (IEP)**: pH at net zero charge
- **NF270 IEP ≈ 3.1–3.3** (zeta potential ≈ −16 mV at pH 7.7)
- Below IEP: net positive (rejects cations less; the membrane attracts SO₄²⁻ slightly, lowering net divalent rejection in some cases)
- Above IEP: net negative (rejects anions more, especially SO₄²⁻)

**Process implication**: feed pH adjustment tunes selectivity. For sulfate removal, **mildly alkaline pH (7–9)** maximizes SO₄²⁻ rejection. Lowering pH below IEP can actually *decrease* Mg²⁺ rejection.

### 2.5 Concentration polarization

Same physics as RO but generally less severe due to moderate NF flux and lower per-stage recovery. At high recovery (> 80 %) it elevates divalent scaling species near the surface and accelerates BaSO₄ / CaCO₃ precipitation.

---

## §3 — NF KPIs and Formulas

### 3.1 Standard metrics (same algebra as RO)

| Metric | Formula | Notes for NF |
|---|---|---|
| Recovery Y | `Y = Qp / Qf` | 70–90 % typical |
| Observed rejection per ion | `R_obs = 1 − Cp / Cf` | **Must be reported per species** — NF rejection is ion-specific |
| Real (intrinsic) rejection | `R_real = 1 − Cp / C_wall` | Corrects for concentration polarization |
| Salt passage | `SP = 1 − R` | Trending baseline |

### 3.2 NF-specific KPI — Selectivity Ratio

```
Selectivity = R_divalent / R_monovalent
            = R_solute_A / R_solute_B
```

The design number for softening or selective separation.

| Application | Ratio | Membrane |
|---|---|---|
| Ca²⁺/Na⁺ softening | ~ 10:1 | NF270 |
| SO₄²⁻/Cl⁻ for offshore injection | ~ 10:1 to 100:1 | NF245, DK |
| Dye/Na₂SO₄ (textile recovery) | > 100, up to 250+ | Loose-NF dye-recovery membranes |

### 3.3 Net Driving Pressure

Same form as RO:
```
NDP = (Pf + Pc)/2 − Pp − Δπ
```
but Δπ is smaller because monovalents pass and don't concentrate as steeply.

### 3.4 Temperature Correction Factor

```
TCF = exp[ K · (1/298 − 1/(273 + T)) ]
K ≈ 2 640 for polyamide NF (close to RO Ke ≈ 2 700)
```

Flux rises ≈ 3 % per °C around 25 °C.

```
Q_normalized = Q_measured · (NDP_ref / NDP_actual) · TCF
```

### 3.5 Permeate flux
```
Jw = Qp / Am   [LMH]
```
Typical 15–30 LMH at standard test conditions. NF270 is high-flux ("low-pressure NF"); NF90 is lower-flux but higher-rejection.

⚠️ For NF reporting, aggregate conductivity-based normalization is **insufficient** — track per-ion rejection (SO₄²⁻, Ca²⁺, Cl⁻, conductivity, TOC). A 10 % drift on a 70 % NaCl rejection means a measurable downstream impact (3 % more salt → hundreds of ppm in product); the same 10 % drift on RO 99.5 % rejection is invisible.

---

## §4 — Vendor Matrix

| Series | Vendor | MWCO | Test conditions | MgSO₄ rej. | NaCl rej. | Typical application |
|---|---|---|---|---|---|---|
| **FilmTec NF270** | DuPont | ~200–400 Da | 2 000 ppm MgSO₄, 4.8 bar (70 psi), 25 °C, 15 % Y | > 97 % | ~40–50 % | Surface/ground water softening, TOC and THM-precursor removal |
| **FilmTec NF90** | DuPont | ~200 Da | 2 000 ppm MgSO₄ or NaCl, 4.8 bar, 25 °C | > 97 % | 85–96 % | High-rejection NF: nitrate, iron, pesticides, herbicides |
| **FilmTec NF200** | DuPont | ~200 Da | per DuPont PDS | High | ~50–70 % | General purpose: high atrazine + TOC, medium hardness passage |
| **FilmTec NF245** | DuPont | rejects > 300 Da | per PDS | > 99 % SO₄²⁻ | Variable | Sulfate removal (offshore SWRO injection); FDA-compliant materials |
| **Suez/Veolia DK** | Veolia | 150–300 Da | 2 000 ppm MgSO₄, 7.6 bar (110 psi), 25 °C, pH 8, 15 % Y | > 98 % | ~30–50 % | Industrial high-rej. NF, dairy demin |
| **Suez/Veolia DL** | Veolia | 150–300 Da | per fact sheet | High | Concentration-dependent | Dye removal/concentration, NaCl diafiltration |
| **Suez/Veolia HL** | Veolia | 150–300 Da | per fact sheet | High | Low–medium | Water softening, color removal, THM precursor |
| **Toray SUL** | Toray | varies (SU-220S) | per PDS | High | Variable | Specialty NF / low-pressure RO crossover |
| **LANXESS Lewabrane NF** | LANXESS | per series | per PDS | High | series-dep. | Industrial RO/NF supplier; smaller NF catalog |
| **Pentair X-Flow tubular NF** | Pentair | per element | per PDS | High | Low | **Dairy whey** concentration, fouling-tolerant tubular geometry |
| **Synder NFX** | Synder | 150–300 Da | per PDS | ≥ 99.3 % | Low–medium | OEM spiral-wound NF |
| **Synder NFS / NFG / NFW** | Synder | 200–300 / 600–800 / 300–500 Da | per PDS | Varies | Low | Looser NF for specialty separations |

⚠️ Vendor MgSO₄ rejection is at 15 % recovery — **not** the rejection you see in a full installation. Use as relative benchmark only.

---

## §5 — Applications

### 5.1 Water softening (alternative to ion exchange)

- **Membrane**: NF270, NF90, Suez HL
- **Process**: single-stage spiral-wound, 6–10 bar, 70–85 % recovery
- **Result**: ≥ 90 % hardness removal, Na⁺ passes (no taste flattening), no brine regen waste
- **vs IEX**: no brine, lower OPEX in high-throughput; works on color and organics too

### 5.2 Color / NOM removal (surface water)

- **Membrane**: NF270, NF200, Suez HL
- **Driver**: hu