Crossflow Sampler Modified Fouling Index Ultrafiltration (CFS-MFI UF)—An alternative Fouling Index

Abstract

A Fouling Index known as the Crossflow Sampler Modified Fouling Index Ultrafiltration (CFS-MFI UF) measured under constant flux is developed in this paper. The method involves a specially designed crossflow cell, the permeate of which flows to a dead-end MFI measuring device. This method has both the capability of capturing small foulant particles as well as simulating the crossflow hydrodynamic conditions of an reverse osmosis (RO) module. The traditional fouling indices, such as MFI, are measured using dead-end filtration and the crossflow hydrodynamic conditions in actual RO membrane filtration are not taken into account. In the current study, colloidal silica and humic acid were selected as representatives of a colloidal foulant and macromolecular foulant, respectively. When humic acid was added to the silica colloids, CFS-MFI UF was found to have a linear relationship with the humics concentration, suggesting that CFS-MFI UF is not only sensitive toward colloids, but also to organic matter. In all of the experiments, CFS-MFI UF was compared with the Modified Fouling Index Ultrafiltration constant flux (MFI-UF const.flux). The results indicated that the conventional MFI-UF const.flux tends to predict a higher fouling propensity by 10–40% when compared to the CFS-MFI UF. CFS-MFI UF was also tested using seawater and pre-filtered seawater. The CFS-MFI UF shows significant improvement after pre-treatment, but this improvement cannot be detected by MFI-UF const.flux. Preliminary RO experiments with silica and humic acid mixture solution indicated that CFS-MFI UF was more sensitive to fouling propensity. The CFS-MFI UF predicted transmembrane pressure (TMP) profile agreed well with the actual RO fouling behaviour with only 11% deviation in the absence of salt. While with the salt in the solution, TMP profile predicted based on CFS-MFI UF plus osmosis pressure due to salt alone is unable to match the actual RO fouling behaviour. This indicates the potential contribution of cake enhanced osmosis pressure in RO fouling.