Colloidal fouling mitigation using pulsating flows in osmotic membrane processes

Abstract

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.

Stormwater Treatment Using an Ultrafiltration Membrane and Pulsatile Fluid Flow

“…According to integrity test no leakage was observed which showed that using pulsatile flow in dead-end filtration did not damage the membrane. Kastl et al (2019) have shown that Womersley numbers in the range of 0 to 7 and transient inertial forces, can overcome viscous effects existing in the boundary layer of the membrane, which prevents cake layer formation on the membrane surface and leads to a longer experimental run time and more permeate volume. In this study, using a pulsatile flow with pulse frequencies between 0.8-4 Hz, the Womersley number varied between 0.7 and 1.5, which is in the quasi steady and then for frequencies of 3.2 and 4 Hz in intermediate region (Özahi and Çarpınlıoğlu 2015).…”

“…Higher productivity shows a higher net permeate flow rate and while investigating pulse frequencies higher than 4 Hz may be useful, it was not possible with the laboratory set-up used here because the maximum TMP was limited to 300 kPa. Kastl et al (2019) investigated pulse frequencies as high as 11.57 Hz when filtrating silica in water as a colloidal solution and reported that the formation of a fouling layer was totally prevented. However, using such high pulse frequencies also increases the energy demand of the process and might not be energy efficient in stormwater treatment, considering the large size of the volumes to be treated.…”

“…Pulsatile flow is explained as a flow with a steady component with time-dependent oscillation (Zhuang, Tan, and Yuan 2016) and increases the fluctuations in the flow, resulting in better mixing of the feed solution near the membrane surface. Thus, this method can be used to reduce boundary layer effects and increase mass transfer (Li, Bertram, and Wiley 1998;Pontrelli 1998;Rodrigues et al 2015;Kastl et al 2019). Furthermore, depending on the quality of the stormwater, a suitable pre-treatment strategy should be considered to improve the quality of imposing feed to the membrane.…”