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Annual sales of filtration membranes worldwide were worth US$8.3 billion in 2024 and with a healthy CAGR of 7% will grow to a value of $14.2 billion in 2032, according to analyst Fortune Business Insights, headquartered in Pune, India.

In a paper presented at the Filtech Conference in Cologne, Germany, on November 12, 2024, Professor Steffen Schütz, senior manager of membrane development at Ludwigsburg, Germany-headquartered MANN+HUMMEL, outlined the key markets and applications for membrane technologies in liquid filtration, emerging new developments resulting from intensive R&D and some of the challenges involved in balancing performance requirements and environmental legislation.

Natural Benchmark

As semi-permeable and thin structures used for filtration and separation, membranes have a high transport selectivity for separating single components from multi-component mixtures.

“With human cell membranes, nature provides us with an unbeatable benchmark for design, multi-functionality and selectivity,” Professor Schütz said.

He listed the nine major market areas for membrane separation in liquid processing as in wastewater treatment plants, clean and drinking water generation, food, dairy and sweetener processing, the chemical industry, textiles production, pharmaceuticals and biotech, electronics, automotive and metals manufacturing.

There are four main types of membrane in general, with the most widely used be- ing flat sheet polymer types and for special applications, ceramic or even metal and glass alternatives are employed.

The other main membrane structures are extruded tubular, hollow fiber and multichannel types and options include micro-, ultra- and nanofiltration flat sheet membranes, usually with nonwoven supporting structures, and hollow fiber membranes which both have polymeric porous membrane layers. Reverse osmosis (RO) flat sheet membranes meanwhile combine a non-porous top layer and a porous membrane layer.

There are also a range of different modules for hollow fibers, spiral wound elements, capillary, and membrane bioreactor modules (MBRs).

Operational modes in these modules can be via either crossflow or dead-end filtration.

“A typical water desalination plant will contain between ten and 20,000 membrane modules,” Professor Schütz observed.

Filter Assembly

In membrane module production, initial economic investment must be considered in balance with energy efficiency and key performance parameters to be achieved include a high fouling resistance, well-defined separation characteristics, process robustness and efficient cleanability.

Spiral wound filter modules in particular, can employ a range of membranes including RO, nanofiltration (NF), ultrafiltration (UF), microfiltration (MF) or customized layers integrated with a range of feed and permeate spacer materials which are integrated to control the fouling or blocking of the individual layers and also ensure cleaning efficiency.

MF membranes have a separation range of 10-0.1µm, while with UF membranes the range is 100-10 nm and NF membranes 10-1nm. RO membranes remove bivalent and especially monovalent cations and anions for water desalination.

Effective spacer and element placement and design plays a decisive role in filter assembly, with reproducibility of the membranes, elements and modules all critical. Spacer materials between the engineered

 MANN+HUMMEL Bio-Cel membrane bioreactor module (MBR). MANN+HUMMEL

membrane layers have a significant impact on desirable separation characteristics.

Polymeric Membrane Manufacturing

A common process for polymeric membranes manufacturing is non-solvent induced phase separation (NIPS) in which they are cast from a polymer solution and run through a coagulation water bath to achieve the spontaneous solidification of the porous membrane structure, followed by post-treatments including rinsing, annealing and drying.

There are a number of other phase inversion process types for achieving specific membrane structures including TIPS (thermally induced phase separation), VIPS (vapor induced phase separation) and partial solvent evaporation in combination with NIPS. Other membranes are also produced by common processes such as extrusion.

Molecular Grafting

Comprehensive R&D work is ongoing in order to improve the specific performance properties of membrane systems and Professor Schütz covered a range of project focus areas in his presentation.

Much work has been done, for example, on the modification of UF and NF membranes via the grafting of functional macromolecular layers onto their surfaces