The role of hydrodynamic conditions and solution chemistry on protein fouling during ultrafiltration
Introduction
Ultrafiltration (UF) has received increasing popularity in the recent decades. UF membranes have been used in surface water treatment (such as natural organic matter and pathogen removal), in membrane bioreactors (MBRs), in pretreatment processes for reverse osmosis (RO) and nanofiltration (NF), and in many other industrial applications [1], [2], [3], [4]. Similar to other types of pressure-driven membranes, UF membranes are prone to fouling — the accumulation of inorganic colloids, organic macromolecules, and biological entities on membrane surfaces or inside membrane pores, which directly reduces the productivity and increases the operational costs.
Protein has been identified as one of the major membrane foulants in wastewater treatment and reclamation applications [5], [6], [7], [8]. Fouling of non-porous membranes (NF and RO membranes) is likely dominated by cake layer formation in addition to concentration polarization [8], [9], [10]. In contrast, the permeability loss of porous microfiltration (MF) and UF membranes might be attributed to a combination of pore blockage and cake layer formation [11], [12], [13]. Existing studies suggest that initial stages of fouling for porous membranes are likely dominated by pore plugging, while the flux behavior is probably determined by a foulant cake layer when severe fouling occurs at longer filtration duration [11], [12], [13], [14], [15]. Numerous studies have also demonstrated that membrane fouling by organic macromolecules are affected by hydrodynamic conditions such as membrane flux and cross-flow velocity [8], [16], [17], feedwater characteristics including foulant concentration, pH, and ionic compositions [8], [15], [16], [18], [19], [20], as well as membrane properties (membrane hydrophobicity, roughness, and porosity) [7], [18], [21]. In general, severe fouling is observed at the isoelectric point (IEP) of a protein where the electrostatic repulsive force between protein molecules is at the minimum [8], [15], [19], [20]. In addition, protein fouling is typically promoted by increased membrane flux and/or reduced cross-flow velocity [8], [17], [22]. Such phenomena are consistent with the critical flux concept [17], [22] which states that little flux decline occurs when the membrane flux is below a threshold value, i.e., the critical flux. On the other hand, severe fouling can occur above the critical flux [17], [22].
Despite the vast number of studies on protein fouling, the effects of ionic strength and foulant concentration have been controversial in the literature. Studies on non-porous membranes [8], [20] reported that protein fouling was more severe at elevated ionic strength due to electric double layer (EDL) compression. Consistent with the above studies, several research groups [15], [19], [23] also reported greater protein fouling tendency for MF and UF membranes at greater ionic strength. Similar effect of EDL compression has been suggested for natural organic matter, alginate, and inorganic colloids [24], [25], [26]. In contrast, Chan and Chen [16] observed lower fouling rate for MF membranes at greater background salt concentrations, which was attributed to the greater protein solubility at increased salt content. Similar observation has also been documented for UF membranes [27]. The literature on the effect of foulant concentration is equally confusing. While many researchers have suggested that more severe fouling occurred at higher protein concentrations [28], [29], [30], other studies revealed that the quasi-steady flux at long filtration time was independent of the foulant concentration [24], [31], [32]. Despite of the increased fouling rate (the rate approaching to the stable flux) at greater foulant concentrations, Tang and Leckie [24] demonstrated that the long-term stable flux was determined by interaction forces between foulant–membrane and foulant-deposited-foulant which were largely independent of foulant concentration.
The study aims to systematically investigate the influence of hydrodynamic conditions and solution environment on protein fouling during ultrafiltration. The effect of membrane flux, cross-flow velocity, protein concentration, pH, and ionic strength were thoroughly investigated through constant pressure fouling tests. The results from the current study may help us to better understand the coupled effects of solution chemistry and hydrodynamic conditions.
Section snippets
Chemicals and materials
Ultrapure water, supplied from an ELGA water purification system (UK) with a resistivity of 18.2 MΩ cm, was used for preparing all reagents and working solutions. The ionic composition and solution pH were adjusted by drop-wise addition of analytical grade sodium chloride, sodium hydroxide, and hydrochloric acid (Sigma-Aldrich, St. Louis, MO).
Bovine serum albumin (BSA, 98% purity, A7906, Sigma-Aldrich) was used as a model protein foulant. It is approximately ellipsoidal (14 × 4 × 4 nm), with a
Effect of hydrodynamic conditions
The effect of applied pressure and cross-flow velocity on the BSA fouling is systematically investigated. Fig. 2(a) shows the flux performance of the MW membrane at various applied pressures ranging from 20 to 500 kPa. The feed composition was fixed at pH 5.8, 10 mM NaCl, and 20 mg/L BSA. In a typical run, membrane experienced rapid initial flux decline in the first 15 min of the test. Subsequent flux decline was much milder, and stable flux was achieved within 2 h. Increasing applied pressure
Conclusions
The effect of hydrodynamic conditions and solution chemistry on protein fouling during ultrafiltration was systematically investigated. Severe fouling occurred at high initial flux and/or low cross-flow velocity. A limiting flux was observed at high applied pressure, beyond which increase in pressure did not enhance the stable flux. The rate and extent of BSA fouling were strongly dependent on the feedwater composition, such as BSA concentration, pH, and ionic strength. While the long-term flux
Acknowledgement
This research was funded by AcRF Project No. SUG4/07, Nanyang Technological University (NTU), Singapore. We thank GE Osmonics© for providing membrane samples used in this study.
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