Colloidal Fouling of Ultrafiltration Membranes: Impact of Aggregate Structure and Size

Abstract

A close coupling between the structure and size of hematite flocs formed in suspension and the permeability of the cake that accumulates on ultrafiltration membranes is observed. Specific resistances of cakes formed from flocs generated under diffusion limited aggregation (DLA) conditions are at least an order of magnitude lower than cakes formed from flocs generated under reaction limited aggregation (RLA) conditions. Similar effects are observed whether the aggregation regime is controlled by salt concentration, pH or added organic anions. This dramatic difference in cake resistance is considered to arise from the size and fractal properties of the hematite assemblages. The ease of fluid flow through these assemblages will be influenced both by the fractal dimension of the aggregates and by their size relative to primary particle size (since for fractal aggregates, porosity increases as the size of the aggregate increases). The size and strength of aggregates are also important determinants of the relative effects of permeation drag, shear induced diffusion and inertial lift and result, in the studies reported here, in relatively similar rates of particle deposition for both rapidly and slowly formed aggregates. The results presented here suggest that control of cake permeability (and mass) via control of aggregate size and structure is an area with scope for further development though the nature and extent of compaction effects in modifying the fractal properties of aggregates generated in suspension requires attention.

Characterizations of Nanofluid Heat Transfer Enhancements

“…Hence, the change of inter porosity during cake layer collapse appears more significant for estimating specific resistance values than accounting for differences between intra and intra porosities within cake layers. Comparing findings with other studies, fitted porosities are higher than the reported value of ~ 0.4 for C60 nanoparticles and Bentonite colloids [76,77], and specific resistances for both of DLA and RLA are slightly higher than what is reported for aggregated nanoparticles and SiO2 colloids of 460 nm [72,77,78]. Nevertheless, specific resistances of DLA and RLA are lesser than that of Bentonite colloids of 5000 nm; this is in line with predications of equations 19 and 20.…”

“…In addition, DLA resistance is found to be lesser than that of RLA. This is different from reports for C60 nanoparticles [76], but in agreement with others [72,[77][78][79] and equations 19 and 20.…”

“…Measured values exceeded predicted values by nearly a factor of 19 for both DLA and RLA scenarios. Inaccuracies of specific resistance predictions are often reported [53,72] and may be attributed to estimates of the Kozeny constant. It is likely that the fractal dimension of aggregates measured free of pressure cannot be used to estimate the porosity of the cake layer with applied pressure due to the compressible nature of cake layers [57,73].…”

Application of cross‐flow filtration for determining the solubility of iron species in open ocean seawater

“…Note that this equation holds only for the instantaneous permeate concentration (ic P ) flowing out of the ultra filter and not the concentration (c P ) in the bottle collecting the permeate (see also Appendix 1). Colloidal fouling of cross-flow filtration membranes-Iron colloids are well known to cause fouling of ultrafiltration membranes (Soffer et al 2004(Soffer et al , 2002Waite et al 1999) leading to a reduction of permeate flow. In studies of Fe solubility, this also presents a problem as colloidal Fe will be adsorbed on the filter membrane and not in the retentate, resulting in an apparent loss of Fe from the system when considering a simple mass balance.…”

Hydraulically irreversible membrane fouling during coagulation–microfiltration and its control by using high-basicity polyaluminum chloride

“…Liu, Chen, Yu, Shen and Gregory [10], in contrast, report that floc particles of a high fractal dimension as well as a large size formed by two-stage coagulant dosing mitigated TMP development more than those formed by a single dose. The strength (resistance toward shear stress) of floc particles formed by coagulation also plays an important role in the permeability of the cake layer [11][12][13]. The increase in transmembrane pressure (TMP) in an ultrafiltration (UF) system is lower with floc breakage, which lowers the fractal dimension of flocs, than without breakage [14].…”