Colloidal fouling mitigation using pulsating flows in osmotic membrane processes

Organic and inorganic fouling continues to be the major limiting factor in membrane processes. It is expected that increasing the wall shear stress by application of pulsating flows will help to reduce fouling and therefore allow treatment of highly polluted water. Furthermore, this can reduce pre-treatment and the quantity of chemicals required, as well as increase the water recovery. This study theoretically and experimentally investigates pulsating flows for mitigation of colloidal fouling in osmotic membrane processes. It is the aim to quantify the potential of pulsating flows to prevent the build-up of a so-called cake layer. An analytic solution in an empty channel, 2-D CFD simulations based on a preliminary study, and experimental results provide insight into the interrelation of Womersley number, amplitude ratio and the hydrodynamic phenomena in pulsating flows. The theoretical investigations show that not only the frequency but also the amplitude ratio has a strong influence on the wall shear stress. The higher the amplitude ratio, the higher the increase in mean wall shear stress relative to the steady-state value. The CFD simulations also indicate that an increasing Womersley number increases the wall shear stress near spacer filaments, which correlates to the area where particles accumulate. The experiments were conducted with a forward osmosis test rig that included a pulsation generator and a corresponding measurement application. A siren was used to reach the high Womersley numbers at which a high increase in wall shear stress was expected. For low frequencies, a solenoid valve was applied. The amplitude ratio was measured based on the differential pressure across an orifice. Experiments showed that the fouling propensity of the process is frequency and amplitude dependent. It could be shown that pulsating flows can mitigate colloidal fouling and therefore increase the permeate flux by up to 20% compared with operation without pulsations.