Solute rejection in the presence of a deposited layer during ultrafiltration

Abstract

During ultrafiltration deposited layers are often formed on the membrane surface. These layers not only reduce the volumetric flux through the membrane, but also may influence the rejection of other solutes in the feed. In the present paper we will show that besides an increase in the rejection, a decrease in rejection may also occur, which can completely alter the aimed selectivity of the separation process. The influence of deposited layers has been studied experimentally by two types of depositing components: silica sol and the protein BSA. In the presence of a relatively open silica deposit a strong drop in the rejection of PEG and dextran was found compared to the rejection on a clean membrane. For thick deposit layers the rejection even decreased to zero, thus resulting in a total permeation of a normally partially rejected solute. On the other hand an increase in PEG rejection occurred in the presence of a BSA deposit. Due to the compressibility of the protein deposit the highest rejections were measured at the highest pressures. The effects were the most pronounced at the isoelectric point of BSA. A model is presented to describe the underlying phenomena.

Coupled effects of colloidal deposition and salt concentration polarization on reverse osmosis membrane performance

“…However, when silica colloids were also present in the feed, the rejection of NaCl was shown to remain unchanged with respect to that when no fouling occurred [21]. Similarly, for suspensions containing iron oxide colloids and humic substances, the rejection of NaCl was shown to improve with respect to NaCl rejection observed when humic substances were the sole foulant [20]. In a study of effects of nanofiltration membrane fouling on rejection, Shäfer et al [22] reported that rejection of cations and negatively charged low molecular weight acids depended on the charge of ferric chloride precipitate deposited on the membrane surface.…”

“…RO membrane fouling by humic substances alone has been shown to result in a slight increase [20] or a marginal decrease in NaCl rejection [21]. However, when silica colloids were also present in the feed, the rejection of NaCl was shown to remain unchanged with respect to that when no fouling occurred [21].…”

Improving Specific Activity of Aspergillus carbonarius Polygalacturonase Using Polymeric Membranes

“…The 10 kDa retentate when processed after suitable dilution using 450 nm MF membrane improved the enzyme purity to 1.39-fold owing to the elimination of 40% larger contaminating proteins ( Table 3). The higher purity obtained could be due to secondary layer formation on the membrane surface due to adsorption and deposition of substances contained in the feed [18,19] that aid in the rejection of non-enzymatic proteins while allowing smaller molecular enzymes. The exclusion of CF reduced the permeate flux during MF, probably due to concentration polarization, fouling and secondary layer formation.…”

The influence of thermodynamic activity on the solute rejection in multicomponent systems

“…For 10 kDa membranes the dextran rejection coefficients were not affected, but a significant increase in rejection was found for 40 kDa membranes. The influence of the presence of open and compact deposit layers on the rejection of other solutes has been discussed by van Oers [8]. It was shown that compact deposit layers caused an increase in rejection, whereas open deposit layers led to a decrease of the rejection to almost zero.…”