Collision-attachment simulation of membrane fouling by oppositely and similarly charged colloids.

The fouling propensity of oppositely charged colloids (OCC) and similarly charged colloids (SCC) on reverse osmosis (RO) and nanofiltration (NF) membranes are systematically investigated using a developed collision-attachment approach. The probability of successful colloidal attachment (i.e., attachment efficiency) is modelled by Boltzmann energy distribution, which captures the critical roles of colloid-colloid/membrane interaction and permeate drag. Our simulations highlight the important effects of ionic strength Is, colloidal size dp and initial flux J0 on combined fouling. In a moderate condition (e.g., Is =10 mM, dp=50 nm and J0= 100 L/m2h), OCC mixtures shows more severe fouling compared to the respective single foulant owing to electrostatic neutralization. In contrast, the flux loss of SCC species falls between those of the two single foulants but more closely resembles that of the single low-charged colloids due to its weak electrostatic repulsion. Increased ionic strength Is leads to less severe fouling for OCC but more severe fouling for SCC, as a result of the suppressed electrostatic attraction/repulsion. At a high Is (e.g., 3-5 M), all the single and mixed systems show the identical pseudo-stable flux Js. Small colloidal size leads to the drag-controlled condition, where severe fouling occurs for both single and mixed foulants. On the contrary, better flux stability appears at greater dp for both individual and mixed species, thanks to the increasingly dominated role of energy barrier and thus lowered attachment efficiency. Furthermore, higher J0 above limiting flux exerts greater permeate drag, leading to elevated attachment efficiency, and thus more flux losses for both OCC and SCC. Our modelling gains deep insights into the role of energy barrier, permeate drag, and attachment efficiency in governing combined fouling, which provides crucial guidelines for fouling reduction in practical engineering.

Relating critical and limiting fluxes to metastable and long-term stable fluxes in colloidal membrane filtration through collision-attachment theory.

In membrane technology for water/wastewater treatment, the concepts of critical flux (JC) and limiting flux (JL) suggest the existence of a threshold flux below which no fouling occurs. However, their important roles on stable flux duration have not been sufficiently understood. This work adopts a collision-attachment approach to clarify the relationship of JC, JL to metastable (i.e., short-term stable) and long-term stable fluxes based on their dependence on initial flux (J0), foulant-clean-membrane energy barrier (Ef-m), and foulant-fouled-membrane energy barrier (Ef-f). When J0 is below JL, water flux remains stable over a long time even for the case of J0 over JC, thanks to the strongly repulsive Ef-f. At J0 > JL and J0 > JC, the water flux is unstable at the beginning of filtration, and the flux ultimately decreases to JL as the long-term stable flux. Under the condition of JL < J0 ≤ JC, an initial metastable flux appears owing to the high Ef-m, with longer metastable period observed at lower J0 and for more hydrophilic/charged membrane or colloids. Nevertheless, rapid flux decline occurs subsequently due to the energy barrier shifting to weak Ef-f, and the water flux eventually degenerates to JL in long-term fouling duration. Our results provide significant guidelines for fouling control strategies with respect to membrane design, feedwater pretreatment, and operational optimization.