Tailor-made membranes

Economic efficiency and environmental protection are often perceived as conflicting goals. Nevertheless, research shows that they can be combined if scarce resources are used efficiently or if raw materials are recovered. In the case of phosphorus, researchers at the HLS are already on the way to achieving this with the help of a multi-layer membrane that can be customised for specific objectives. In laboratory tests they have recovered 90 per cent of the phosphorus from acid waste solutions. The new technology will be used to extract phosphorus from sewage sludge.

As a country with limited natural resources, Switzerland depends on imports of raw materials. This reliance can be offset by recovering raw materials from waste products such as the sludge produced during water treatment in sewage plants. In addition to phosphorus, this sludge also contains large quantities of metals and heavy metals. Since phosphorus is economically vital as a basic component of fertilisers, its recovery from sewage sludge will be compulsory in Switzer- land from 2026. However, many existing recycling technologies are still expensive or inefficient. The engineer Kirsten Remmen from the Institute for Ecopreneurship at the HLS has now developed an improved method for phosphorus recovery by nanofiltration using a modified layered membrane.

Nanofiltration is a well-known technology applied in the pharmaceutical and food industries. Unlike microfiltration and ultrafiltration, in which pore size determines which particles pass through the membrane, nanofiltration uses the charge of the ions. “Nanofiltration membranes are charged, enabling them to differentiate between polyvalent and monovalent charged ions,” says Remmen. “Mono- valent ions can pass through the membrane but polyvalent ions are retained.” “Before conducting the nanofiltration, sewage sludge must pass a multi-stage process in which metals, heavy metals and phosphorus, among other things, are dissolved using acid,” explains the researcher. After that, individual components are separated using one of several possible technologies. “In Switzerland liquid – liquid extraction is applied, although nanofiltration could be used as the final purification stage to further improve the quality of the product.” The problem is that commercially available nanofiltration membranes are not very efficient at sewage sludge phosphorus recovery. Nanofiltration should retain metals and only allow the important phosphorus to pass through the membrane. In order to achieve this, Remmen and her team have modified the nanofiltration mem- brane properties layer by layer (LbL), in particular the charge. “Unlike most con- ventional negatively charged nanofiltration membranes, an LbL membrane can be charged positively; in this way we can also vary the charge strength,” explains the engineer. The more positive the charge of the LbL membrane, the more metals it retains.

The researchers coated a hollow fibre membrane with charged polymer polyelectrolytes, some with positive and some with negative charges. Plus and minus layers are applied alternately, together forming a so-called bi-layer. “By altering the type and number of the bilayers, I can change the properties of the membrane and hence its filtering properties,” explains Remmen. The big advantage of this membrane is the many parameters that can be individually adjusted and adapted to specific applications, such as the solution concentration or the polyelectrolytes used. Remmen and her team have run laboratory tests on the LbL membrane’s suitability for phosphorus recovery. Their experiment with an acidic model solution showed that the membrane allows permeation of 90 per cent of the phosphorus, which can then be processed for fertiliser, while unwanted metals were retained by the membrane. The researcher is pleased with this success: “There is still no satisfactory solution on the market. The LbL membrane has great potential for decontaminating acidic waste flows and recovering raw materials. Phosphorus is just one example.”

Another advantage points to the future use of LbL membranes: the flow rate in Remmen’s phosphorus recovery experiments was 16 times higher than conventional nanofiltration membranes, making this technology attractive for industry. Due to the acidic, viscous and highly osmotic leachate from sewage sludge, conventional membranes require either high pressure to force the liquid through the membrane or a great deal of time, both of which add to the expense. These high costs are one reason why recovery from acid waste products is not more widespread; until now it has been cheaper to neutralise and dispose of the acid. With the LbL membrane, smaller systems can be used for recovery, thus reducing the investment necessary. Remmen is convinced of the potential of customised LbL membrane technology; she and her team are working to make it suitable for industrial use and are developing a prototype with Pentair.