a b s t r a c tOrganic 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.
Initial deposition of bacteria is a critical stage during biofilm formation and biofouling development in membrane systems used in the water industry. However, the effects of hydrodynamic conditions on spatiotemporal deposition patterns of bacteria during the initial stages of biofilm formation remain unclear. Large field epifluorescence microscopy enabled in situ and real-time tracking of Bacillus subtilis in a forward osmosis system with spacers during the first 4 h of biofilm formation. This study quantitatively compares the spatiotemporal deposition patterns between different hydrodynamic conditions: high and low permeate water flux (6 or 30 L m −2 h −1 ) as well as high and low crossflow velocity (1 or 14 cm s −1 ). Low crossflow velocity and high permeate water flux maximized bacterial attachment to the membrane surface, which was 60 times greater (6 × 10 3 cells mm −2 ) than at high crossflow velocity and low permeate water flux (<100 cells mm −2 ). Imaging at 30 s intervals revealed three phases (i.e., lag, exponential, and linear) in the development of deposition over time. Quantification of spatial deposition patterns showed that an increase in the ratio of permeate water flux to crossflow velocity led to a homogeneous deposition, while a decrease had the opposite effect. The insights of this research indicate that an appropriate choice of hydrodynamic conditions can minimize bacteria accumulation prior to biofilm formation in new and cleaned FO membrane systems treating water of high fouling propensity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.