Water purification

Membranes for water purification

Membranes are now widely used in the purification of water. In particular, filtration membranes (micro-, ultra-, nanofiltration) are used to separate different particles (bacteria, viruses) and substances by size exclusion. Fraunhofer IGB is working on the development of new, green manufacturing processes for filtration membranes (Green Membranes).

In filtration, the pressure to be applied increases with decreasing pore size and thus the energy input increases considerably. We are therefore developing membrane adsorbers to retain even smaller molecules by adsorption such as micropollutants at low pressures (Micropollutants).

Reverse osmosis (RO) now also plays the most important role in the desalination of seawater. Thin-film composite (TFC) membranes produced by interfacial polymerization, which are permeable to water but retain salts, are used for this purpose. Pressures higher than the osmotic pressure of the solution to be desalinated must be used.

Membranes for forward osmosis (FO)

In forward osmosis (FO), on the other hand, two liquids with different osmotic pressures are brought together via a semipermeable membrane. In this process, water flows towards the chamber with high osmotic pressure, while substances dissolved in the liquid are retained.

The main applications of forward osmosis are the recovery of potable water from contaminated water sources, the use of salt concentration differences (e.g., in the transition from river to seawater) to generate energy using pressure-retarded osmosis (PRO), or the reduction of fouling in the concentration of landfill leachate.

Solvent-resistant FO membranes for the purification of complex mixtures

Forward osmosis experiments in a stirred cell
© Fraunhofer IGB
Forward osmosis experiments in a stirred cell: RO-1 and -2 are commercial RO membranes; IGB-CA cellulose acetate membranes, from right to left post-treatment temperature 50°C, 70°C, 90°C; IGB-TFC thin-film composite membranes, from right to left reaction times 30 s, 120 s, 480 s.

Fraunhofer IGB is developing FO membranes that can be used for the purification of surface waters containing sulfates. In addition, particularly stable FO membranes have been produced that are resistant to solvents such as acetone, butanol and ethanol and can be used for butanol enrichment from ABE fermentation broths. For second-generation fermentation-derived fuels such as butanol, downstream processing in particular still needs to be significantly improved to make them competitive and environmentally friendly.

Meanwhile, forward osmosis membranes are available both cellulose acetate-based (CA) and thin-film composite (TFC) membranes. With CA membranes, the separation efficiency of the membrane can be controlled by the temperature of the post-processing bath. The higher the temperature, the lower the water flux and the higher the retention. For the TFC membranes, the separation performance can be controlled by the reaction time of the interfacial polymerization. It was shown that with forward osmosis it is technically possible to concentrate butanol solutions to such an extent that phase separation occurs.

Cellulose acetate membrane under the electron microscope.
© Fraunhofer IGB
Cellulose acetate membrane under the electron microscope.

Services

  • Development of flat and hollow fiber membranes with optimized properties for forward osmosis
  • Adaptation of thin-film-composite and cellulose acetate membranes to different separation tasks
  • Feasibility studies for the purification of complex mixtures

Publications

  1. Touati, K.; Hänel, C.; Tadeo, F.; Schiestel, T. (2015) Effect of the feed and draw solution temperatures on PRO performance: Theoretical and experimental study, Desalination 365: 182-195.
  2. Touati, K., & Schiestel, T. (2013). Evaluation of the Potential of Osmotic Energy as Renewable Energy Source in Realistic Conditions. Energy Procedia, 42, 261-269. doi:https://doi.org/10.1016/j.egypro.2013.11.026.