In membrane filtration special attention is paid to an effect which influences productivity and operational costs: membrane fouling. Membrane fouling occurs when a particle deposits on the membrane surface or in the membrane pores, thereby decreasing the performance of the membrane. Membrane fouling can be controlled by targeted surface modifications. This article examines how surface charge analysis contributes to the optimization of membrane performance and how it can best be performed.

Membranes are used for selective separation in a variety of filtration processes. Applications of membrane technologies and different membrane types are manifold: Reverse osmosis membranes are used e.g. in seawater desalination, nanofiltration membranes are applied for hardness removal in water purification, and ultrafiltration membranes are used in blood dialysis and microfiltration for cold sterilization of beverages – to name only a few examples. Depending on what is best suited for the specific separation process, membranes are available in different geometries: as tubular, flat sheet, spiral wound or hollow fiber configurations. Polymer-based membranes are applied in the most common applications, whereas ceramic filters are used for specific filtration tasks under more aggressive conditions.

The ultimate goal of all membrane applications is to achieve optimum separation and productivity. The filter material and process conditions thus need to be selected carefully in order to achieve highest rejection and highest flux – while keeping the filtration process as economical as possible.

During membrane operation, dissolved compounds in the feed solution interact with the membrane’s surface and inevitably lead to membrane fouling, a major challenge in membrane technology. Membrane fouling is caused by the deposition of dissolved particles and organics on a membrane’s surface and within its pores during the filtration process. Fouling leads to reduced performance of a membrane, as it affects a membrane’s selectivity as well as the permeate flux through the membrane pores. In order to maintain a membrane’s productivity, pretreatment of the feed water and chemical cleaning of the membrane are often required. In case of irreversible fouling or membrane degradation after aggressive cleaning, membranes need to be replaced frequently, which further increases the operational costs. As a consequence, a lot of effort is put into understanding and ultimately reducing the effects of membrane fouling by precise engineering of membrane surface characteristics.

Membrane structure parameters such as roughness, as studied by means of Atomic Force Microscopy (AFM), and porosity are known to have an effect on membrane fouling. Membranes with reduced surface roughness as well as membranes with larger pores are typically less affected by fouling.

A membrane’s fouling tendency is further influenced by its wettability and surface charge, i.e. parameters which describe the interaction of membranes with their aqueous environment. A surface exposed to an aqueous environment assumes an electric surface charge which arises either from dissociation or protonation of surface functional groups or from selective adsorption of ions. Regardless of the membrane material, the surface charge of a membrane inevitably has an effect on the filtration process.

A membrane’s surface charge gives direct information on electrostatic interactions between a membrane’s surface and compounds in the feed water. It is a key parameter describing the interface between a membrane and its environment.

The surface charge of a membrane can be tuned to favor or suppress certain interactions with components in the feed water – directly influencing membrane fouling. Materials with the same surface charge are known to repel each other. As such, negatively charged particles are less likely to stick to negatively charged surfaces and fouling is reduced.

Furthermore, surface charge analysis can be used to monitor the early stages of membrane fouling. This will help to identify when a membrane is due for cleaning. Surface charge analysis can also be used to optimize membrane cleaning by evaluating the effects of different cleaning agents on a membrane’s surface. Membrane ageing and degradation due to cleaning is another major cause of process failure.

Surface charge is related to the zeta potential at the solid/liquid interface. The surface zeta potential can be derived from measurements of the streaming potential, which arises from the motion of a liquid phase relative to the solid membrane surface. SurPASS™ 3 for instance employs the streaming potential method for a fully automated surface charge analysis by means of zeta potential. It provides surface charge analysis from reverse osmosis to nano-, ultra-, and microfiltration membranes, from polymers to ceramics, from flat sheet to hollow fiber membranes. Furthermore, it gives access to information on membrane surface characteristics under environmental conditions, thus simulating the behavior of a membrane in the technical process.