The Science of Molecular Filtration

In an increasingly polluted environment, conventional filtration is no longer sufficient. Standard activated carbon filters remove only a fraction of contaminants – they were designed for a water quality that no longer exists. Molecular separation processes operate at the level of individual molecules, opening entirely new possibilities in water treatment.

The Problem: Trace Contaminants in the Water Cycle

Modern wastewater treatment plants were dimensioned in the 1970s for a biologically stable input scenario. At that time, the main tasks were reducing oxygen demand, nitrification, and phosphate elimination. The trace contaminants of the 21st century – hormones, pharmaceutical residues, corrosion inhibitors, biocides, PFAS, and microplastics – were not on the agenda.

Activated carbon does partially remove organic trace substances, but has decisive limitations:

• Loading capacity: Activated carbon becomes increasingly inactive with rising concentration and must be regularly replaced or reactivated.
• Breakthrough behaviour: At high hydraulic loads, contaminants migrate through the filter layer faster than laboratory tests indicate.
• No true removal: Adsorbed substances remain on the carbon – upon reactivation, they pass into the exhaust air or are transferred to a waste stream.
• Selectivity issues: Natural humic substances compete with trace contaminants for binding sites.

The Principle of Molecular Filtration

Molecular separation processes work physically rather than chemically. They use the size, shape, and polarity of molecules to physically separate them through a semipermeable membrane. The most important representatives are:

1. Nanofiltration (NF)

Pore size 0.5–2 nm. Separates multivalent ions, organic molecules > 200 g/mol, and many pharmaceuticals by > 90 %. Increasingly used for the 4th treatment stage at wastewater plants.

2. Reverse Osmosis (RO)

Pore size < 0.5 nm. Near-complete retention of all dissolved substances including monovalent ions. Energy-intensive due to pressure differentials of 10–80 bar. Used for industrial process water treatment.

3. Ultrafiltration (UF)

Pore size 2–100 nm. Removes particles, bacteria, and viruses as well as larger organic molecules. Used as a pre-treatment stage for RO and NF.

What Molecular Filtration Achieves

In contrast to activated carbon, micro-pollutants are actually removed by membrane filtration – the concentrate can be specifically disposed of or oxidized. Typical retentions:

• Hormones (estradiol, ethinylestradiol): > 99 %
• Pharmaceutical residues (diclofenac, carbamazepine, metformin): 90–99 %
• Pesticides and herbicides (atrazine, glyphosate): > 95 %
• PFAS (PFOA, PFOS): 90–99 % depending on chain length
• Microplastics > 0.1 µm: complete

Challenges and Solution Approaches

Molecular membrane processes face three central challenges:

1. Fouling and scaling: Organic deposits and mineral precipitates reduce flux. Solution: regular backwashing, chemical cleaning (CIP), ultrafiltration as pre-treatment.
2. Energy demand: Reverse osmosis requires 0.5–2.5 kWh/m³. Modern energy recovery devices reduce specific consumption to below 0.5 kWh/m³.
3. Concentrate disposal: 15–30 % of the feed is discharged as concentrate. Solution: oxidative treatment, evaporation, crystallization, or Zero Liquid Discharge (ZLD).

Fields of Application in Practice

Air Liquide and its customers deploy molecular filtration across various industries:

• Pharmaceutical industry: Production of Water for Injection (WFI) per USP and Ph. Eur.
• Semiconductor industry: Ultra-pure water (UPW) for wafer rinsing with conductivities < 0.055 µS/cm.
• Food and beverage industry: Demineralization of brewing water, fruit juice concentration.
• Municipal 4th treatment stage: Trace contaminant elimination for water body protection per the EU Urban Wastewater Treatment Directive.

Conclusion

Molecular filtration is a key technology for water supply in the coming decades. It complements oxidative processes such as ozonation or hydrodynamic cavitation (HC) with a physical barrier and achieves retentions that are impossible with classical treatment. The combination of both processes – HC/ozone plus NF/RO – is the state of the art for future-proof water treatment.