Separating Undesirable Molecules From Urban Wastewater Even Smaller Than 450 Daltons

Ceramic membranes by the Fraunhofer Institute for Ceramic Technologies and Systems IKTS (Fraunhofer IKTS)

In order to separate dissolved substances and fine particles from municipal wastewater, membrane filter applications have proved itself in the past. Especially the considerably increasing usage of ceramic membranes for water treatment in recent years can be considered as objectively justified. Despite their brittleness, they work reliable as a semipermeable and selective barrier and enable to segregate particles based on molecular or physical sizes. Their remarkable mechanical stability makes them suitable for elevated pressures during the cleansing process. Furthermore, the average lifetime is higher than in the case of polymer membranes. And last, but definitely not least: they are particularly energy-efficient.

However, there are downsides, too. According to researchers from the Fraunhofer Institute for Ceramic Technologies and Systems IKTS in Hermsdorf, Germany, there are limitations when it comes to the molecular size which is able to separate from the water. Molecules smaller than 450 Daltons can’t be removed with ceramic membranes. Allegedly, some experts were of the opinion that it is an impossibility to go below this lower limiting value.

Molecules With the Size of 200 Daltons Able to Filter

Recently, the German researchers announced a breakthrough: “With our ceramic membranes, we have achieved, for the first time, a molecular separation limit of 200 Daltons – and, thereby, a whole new quality,” says Voigt, Deputy Institute Director of the IKTS and Site Manager in Hermsdorf. In order to achieve this objective, it was needed to eliminate some barriers.

First of all, it is anything but easy to produce a ceramic membrane with pores so small that not even a molecule is able to pass through. Furthermore, all of them have to be in the same size. One larger pore is enough to let pass a certain number of molecules. In this case, the reliable filter separation is endangered. “We achieved these results by refining sol-gel technology,” says Richter, Head of Department at the IKTS.

Separating Molecules With Small Diameters Requires Ultra Small Pores

But there is more to it than that. The question was how to create larger surfaces with defect-free membrane layers? “Whereas only a few square centimeters of surface are usually coated, we equipped a pilot system with a membrane area of 234 square meters, which means that our membrane is several magnitudes larger,” explains Puhlfuerss, scientist at the IKTS.

According to Fraunhofer IKTS, the project isn’t at the laboratory stage anymore. Since 2016, a newly built and by Shell commissioned pilot system in Alberta, Canada, purifies waste water with the above-described features. Right now, the researchers plan to set up a plant with a membrane area larger than 5,000m².