π Lesson 12
D5
Transport Mechanisms in RO, NF, UF & MF
Transport mechanisms in membrane processes describe how water and dissolved substances move across filters (like RO, NF, UF, MF) β driven by pressure, concentration, or electrical forces.
π― Learning Objectives
- β Explain the dominant transport mechanism for each membrane process (RO, NF, UF, MF) using molecular-scale reasoning
- β Calculate solvent flux and solute rejection using the solution-diffusion and pore-flow models
- β Analyze concentration polarization effects on observed rejection and permeate flux
- β Design membrane system operating conditions to minimize fouling while maintaining target separation performance
π Why This Matters
Understanding transport mechanisms is the foundation for selecting the right membrane process, predicting performance under real plant conditions, and diagnosing failures like sudden flux decline or poor salt rejection. In mining, these processes treat acid mine drainage, recover metals from leachates, and produce process water β where misjudging transport behavior leads to premature membrane failure, excessive energy use, or noncompliant effluent.
π Core Principles
Membrane transport is governed by three primary mechanisms: (1) Solution-diffusion dominates in dense polymeric membranes (RO/NF), where water and solutes dissolve into the membrane matrix and diffuse across under chemical potential gradients; (2) Sieving/pore flow dominates in asymmetric or porous membranes (UF/MF), where separation relies on size exclusion and convective flow through pores; (3) Coupled phenomena β including concentration polarization (CP), osmotic back-transport, and Donnan exclusion β modify observed performance. CP forms a boundary layer that reduces effective driving force and increases fouling risk; Donnan exclusion enhances multivalent ion rejection in NF due to fixed charge groups in the membrane polymer.
π Solution-Diffusion Flux Model
The solution-diffusion model quantifies water flux (J_w) and solute flux (J_s) in RO/NF membranes. It separates permeation into dissolution (partitioning) and diffusion steps, enabling prediction of rejection and pressure dependence.
π‘ Worked Example
Problem: A spiral-wound RO membrane has A = 4.2 L/mΒ²Β·hΒ·bar (water permeability), B = 0.85 L/mΒ²Β·h (solute permeability), applied ΞP = 25 bar, osmotic pressure difference ΞΟ = 8.2 bar, and bulk feed concentration C_b = 3,500 mg/L NaCl. Calculate J_w, J_s, and % rejection.
1.
Step 1: Compute water flux: J_w = A Γ (ΞP β ΞΟ) = 4.2 Γ (25 β 8.2) = 4.2 Γ 16.8 = 70.56 L/mΒ²Β·h
2.
Step 2: Compute solute flux: J_s = B Γ C_b = 0.85 Γ 3.5 = 2.975 g/mΒ²Β·h (convert mg/L β g/mΒ³ β g/mΒ²Β·h via flux units)
3.
Step 3: Compute rejection R = 1 β (J_s / J_w Γ C_b)^β1? Wait β correct form: R = 1 β (C_p / C_b); but C_p β J_s / J_w = 2.975 / 70.56 β 0.0422 g/L = 42.2 mg/L β R = 1 β (42.2 / 3500) = 0.988 β 98.8%
Answer:
J_w = 70.6 L/mΒ²Β·h; J_s = 2.98 g/mΒ²Β·h; Rejection = 98.8% β consistent with high-rejection seawater RO performance.
ποΈ Real-World Application
At the Antamina copper mine (Peru), a 2,200 mΒ³/d NF system treats acidic mine drainage (pH ~2.8, [CaΒ²βΊ] = 420 mg/L, [SOβΒ²β»] = 2,100 mg/L). Engineers selected NF over RO because Donnan exclusion and steric hindrance enabled >95% sulfate removal at lower pressure (12β15 bar vs. 25+ bar for RO), reducing OPEX by 32% and avoiding gypsum scaling in downstream evaporation. Transport modeling confirmed pore radius (~0.5 nm) and surface charge density (β12 mC/mΒ²) were optimal for divalent anion rejection while permitting monovalent ion passage to control osmotic load.
π Case Connection
π Bioethanol Dehydration Using Pervaporation Membranes
Azeotropic limitation of conventional distillation causing 30% energy penalty
π Wastewater Reclamation for Semiconductor Fab Using RO-NF Hybrid
High silica, boron, and trace metals (Cu, Ni) exceeding ultrapure water (UPW) specs (<0.1 ppb metals)