How EIS Identifies the Early Stages of Fouling
During RO operations, as water is moved from the feed to the permeate side, the semi-permeable properties of the membrane causes a build-up of solutes (including salt ions) and particulate matter at the membrane surface. This is known as concentration polarisation (see diagram). In steady-state operations the further build-up of material at the surface is mostly removed by the brine crossflow.
There is a critical point of fouling development where build-up of material at the membrane surface can no longer be removed by cleaning from crossflow. This point is the formation of a stationary cake due to agglomeration of material in the concentration polarisation layer.
Once a stationary cake has formed on the membrane surface, removal of salts by back diffusion and cross-flow is reduced, leading to an enhancement of salt concentration polarisation; referred to as cake-enhanced-concentration-polarisation (CECP). With the onset of CECP, mineral scaling, agglomerated colloidal particles and biofilm can only be treated by taking the affected membranes off-line and treating them with aggressive cleaning chemicals.
The earliest stage of cake formation is characterised by a change in the nature of the material accumulating in the concentration polarisation layer. As shown in the diagram, this layer is near to the membrane-solution interface, within the concentration polarisation layer established by the flux through the membrane. Diffusion polarisation exists only during filtration when a current is applied to the membrane and is observable only at very low frequencies.
CMS’s EIS technology captures real-time data on the electrical properties of the concentration polarisation layer. This data provides distinct signatures for early stage incidence of in-organic, organic and biological fouling as well as the accumulation of mineral scale.