Fighting bacteria, viruses & more
More than ten billion cubic meters of wastewater run through the German sewer system every year. In the last stage of water treatment, known as ultrafiltration, hollow fibre membranes can be used to filter out impurities. However, the membranes are already full of organic substances and must be cleaned after a short while. To make this step unnecessary in the future, an extra component with a dirt-repellent effect is added to the membranes. Lara Grünig has joined the fight against “bacteria & more”. She is a doctoral researcher at the Institute of Polymer Research in Geesthacht.
"We have shown in numerous tests that this principle works. We must now investigate "the perfect additive"." - Lara Grünig. Photo: HZG/ Rolf Otzipka
For many years, hollow fibre membranes have been well established in wastewater treatment. Pore sizes of ultrafiltration membranes range from one to twenty-five nanometres. Thus, due to size exclusion, even bacteria and viruses are retained by the membranes. The wastewater thereby flows through the module in which the hollow fibres are located. The water that passes through the separation layer of the membranes is cleaned and purified; it is called “the permeate”.
However, after only a few days, a filter cake that contains the retained impurities, blocks the open pores. Organic material and microorganisms ‒ that is, viruses and macromolecules such as sugar ‒ attach themselves to the porous structure. This process is called fouling.
Lara Grünig explains: “Until now, there have been two standard possibilities for cleaning the fibres: One of them is to run the membrane backwards. In this step, water is pressed through the membrane from the outside, and hopefully the filtration cake is removed by the pressure of incoming water. However, this process can only remove the substances attached to the very inner surface of the hollow fibres. Those particles that are deeply stucked into the pore structure, remain in the membrane. Therefore, additional chemical cleaners are needed to clean the membranes entirely. The chemical cleaners that commonly contain chlorine can reduce the polymer matrix of the membranes. As a result, the material turns brittle. Additionally, the chemical treatment is also harmful to the environment. Membranes with a lower tendency for fouling are more economical, as fewer membranes are necessary for the same amount of cleaned water. The modules are thus smaller and cheaper. Therefore, we must find another solution.”
The basic principle of wastewater treatment using hollow fibre membranes is represented in the diagram on the left. The middle section shows that viruses and bacteria are deposited in a hollow fibre membrane without additives (fouling). Less water therefore flows through the pores of the membrane. The diagram on the right shows a hollow fibre membrane with the additive, a block copolymer. This custom-made polymer prevents bacteria and viruses from clogging the pores, whereby increasing permeate flow. Graphic: RoseFlohr Kommunikation
Plastic additives prevent deposits
In the context of her PhD work in the Department of Material Characterization and Processing (PMM) at HZG, Lara Grünig cooperates with BASF SE, the University of Duisburg-Essen, inge GmbH and the IWW Water Centre to improve the antifouling properties of the membrane. The overall objective is to avoid agglomeration of organisms such as viruses on the membrane separation layer. The project “MABMEM” (“Material-Auswahl-Box für Hochleistungsmembranen”, i.e., a materials toolbox for high-performance membranes), funded by the German Federal Ministry of Education and Research, has been running since May 2016.
Until now, the standard hollow fibre membranes, such as those studied in the project, consist of two components: a matrix polymer and a “pore former”, which is a polymer additive that interconnects the pores of the membrane. The matrix polymer forms the support structure; the pore former creates, as the name indicates, the interconnected pores and provides the membrane with a spongy structure. In the project, the scientists are adding a second additive to the standard mixture. This is what is known as the dope solution: An antifouling additive or, more precisely, an amphiphilic block copolymer.
Photo: HZG/ Rolf Otzipka
This additive polymer consists of hydrophobic (water-repellent) as well as hydrophilic (water-attracting) components. The hydrophobic anchor is built into the actual fibres; the hydrophilic group sticks out of the surface of the membrane, similar to bristles of a brush. Hence, the surface of the membrane has a hydrophilic function and prevents viruses and other microorganisms from attaching themselves to the membrane structure.
“We have shown in numerous tests that this principle works. We must now investigate “the perfect additive” that repels the organic components in the most efficient way,” says Grünig. Therefore, she is testing different variants in the laboratory, trying various types of additives or changing the block length of the polymers.
She has already produced fifty different kinds of membranes. This requires approximately three hours of “spinning” time in the laboratory for each of them. She then ends up with two hundred metres of hollow fibres, that are cut into 50-centimetre pieces. As soon as the fibres are produced, they are characterised with the utmost precision.
First implementation tests planned for 2018
Each type of membrane is integrated by inge GmbH into one hollow fi bre module. A module looks like a large tube that contains all hollow fibre membranes affixed inside. The modules are then subjected to laboratory tests at the University of Duisburg-Essen, where they are examined under extreme conditions. When all additive variants are analysed, four of the best performing membranes with the least observed fouling will then be subjected to a practical test. In April 2018, these pilot modules shall be implemented by the project partner IWW in at least two locations: one waste water treatment plant and one reservoir, where they will be tested under real conditions for several months.
Water treatment in developing countries
If the membranes pass the tests, they can be implemented in professional water treatment modules in the future. “This would be a major step, particularly for developing countries who suffer from water shortages.” says Grünig. “Generally, the problem is not that there’s too little water available; it is that there is not enough access to the drinking water. Drinking water can be extracted from saltwater by a process called reverse osmosis. But this step can only take place before the saltwater was ultrafiltrated with, for example, hollow fibre membranes for ultrafiltration. In this way, only salt remains to be extracted from the particle-free water.”
Author: Gesa Seidel (HZG)
Published in in2science #5 (December 2017)