‌Membrane Technology in Water Treatment Applications

Forward Osmosis (FO) Technology

1. Principle of Forward Osmosis (FO)

A semi-permeable membrane, which allows solvent molecules to pass through but blocks solute molecules, separates the solvent and solution. Under osmotic pressure, solvent molecules spontaneously move from the solvent side to the solution side through the membrane. This phenomenon is termed ‌forward osmosis‌.

‌2. Applications of Forward Osmosis Membranes in Water Treatment

‌Seawater Desalination‌:

FO is widely studied for seawater desalination. Early applications were documented in patents, but most lacked maturity and feasibility.

‌Industrial Wastewater Treatment‌:

Early studies explored FO for low-concentration heavy metal wastewater treatment. However, severe fouling and rapid flux decline in RO membranes hindered further development.

‌Landfill Leachate Treatment‌:

The Coffin Butte Landfill in Corvallis, Oregon, USA, generates ‌(2–4)×10⁴ m³/year‌ of leachate. To meet land-use water quality standards (effluent TDS <100 mg/L), reverse osmosis (RO) is required.

1. Principle of Reverse Osmosis (RO)‌ RO is a pressure-driven membrane separation process. A pump applies pressure to saline water or wastewater to overcome natural osmotic pressure and membrane resistance, enabling water to pass through the RO membrane while retaining dissolved salts or contaminants.

2. Applications of Reverse Osmosis Membranes in Water Treatment

General Water Treatment‌: With freshwater scarcity, global RO capacity has reached ‌millions of tons/day‌.

Municipal Wastewater Reuse‌:

RO is critical for advanced treatment of secondary effluent and reclaimed water.

Heavy Metal Wastewater Treatment‌:

Conventional methods merely transfer pollution (e.g., converting dissolved metals into sludge for landfill). RO avoids secondary contamination risks.

Oily Wastewater Treatment‌:

Oily wastewater, if discharged, forms surface oil films that deplete oxygen and harm ecosystems. RO treats oilfield produced water (oil content: 3.5 mg/L, TOC: 16–23 mg/L) to boiler feedwater standards.

Microfiltration (MF) andUltrafiltrationTechnologies

1. Principles of UF and MF

Both are pressure-driven liquid separation processes based on sieving. Solvent and small solutes (e.g., salts) pass through the membrane, while larger molecules (e.g., organic colloids) are retained. ‌UF‌: Removes molecules with molecular weights ‌500–1,000,000‌. ‌MF‌: Removes larger particles (e.g., suspended solids).

2. Applications of UF and MF

Effectively remove particles, microorganisms (e.g., Cryptosporidium, Giardia, bacteria, viruses), and reduce disinfection byproduct precursors. Limited organic removal (<20%). Widely applicable across diverse water qualities.

Nanofiltration (NF) Technology

‌1. Principle of Nanofiltration (NF)

NF is a molecular-level membrane separation technology with pore sizes ‌1–2 nm‌. It selectively removes divalent/trivalent ions, organics (MW ≥200), microbes, colloids, and viruses. NF membranes are charged, enabling low-pressure (0.5 MPa) operation, high salt rejection, and no acid/alkali waste. Water recovery rates: ‌75–85%‌ (general), ‌30–50%‌ (seawater).

‌2. Applications of NF Membranes

‌Drinking Water Purification‌:

NF reduces DOC (to 0.7 mgC/L), residual chlorine (to 0.1 mg/L), and trihalomethanes (THMs) by ‌50%‌. It improves biological stability and softens water

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Seawater Desalination‌:

Reduces salinity from ‌35,000 mg/L‌ to ‌<500 mg/L‌.

Wastewater Treatment‌:

(1)Domestic Sewage:

Combined ‌micro-flocculation fiber ball filtration + UF + NF‌ processes meet reuse standards for toilet flushing, laundry, etc.

(2)Textile/Dyeing Wastewater:

Removes stubborn dyes.

(3)Tannery Wastewater:

Recovers sulfates and chlorides.

(4)Electroplating Wastewater:

Concentrates heavy metals for recovery.

(5)Paper Industry Wastewater:

Enables closed-loop water systems.

Dialysis and Electrodialysis (EDI)

‌1. Dialysis (D)

Solutes migrate across a membrane under their own concentration gradient.

Used primarily for removing low-MW solutes (e.g., urea in hemodialysis).

‌2. Electrodialysis (EDI)

Under a DC electric field, ion-exchange membranes selectively separate cations and anions for solution concentration, desalination, or purification.

Bipolar Membrane (BPM) Technology

‌1. Introduction to Bipolar Membranes

A BPM consists of an anion-exchange layer, a cation-exchange layer, and an intermediate layer for water dissociation. In a DC field, it splits water into ‌H⁺‌ and ‌OH⁻‌.

2. Applications of BPMs

‌Fluoride Waste Treatment‌:

Converts KF into HF and KOH, recovering fluorine and reducing radioactive waste.

Acid/Alkali Waste Recycling‌:

Treats industrial effluents (e.g., ion exchange resin regeneration, battery, and paper mill waste).

Domestic Sewage‌:

Integrates NF to separate biodegradable and non-biodegradable organics, reducing oxidant use.

Drinking Water Purification‌:

Removes disinfection byproducts, pesticides, heavy metals, and hardness.

Heavy Metal Wastewater‌:

Concentrates metals (Ni, Cu, Zn) for recovery.

‌Food Industry Wastewater‌:

Reduces COD via N-P composite NF membranes.

‌3. Future of BPMs

Enhancing membrane performance, lowering costs, and expanding applications (e.g., environmental engineering) remain critical goals.