Fouling mechanisms of alginate solutions as model extracellular polymeric substances

Extracellular polymeric substances (EPS) are viewed as major fouling components during filtration of biological feeds in industry. However, very few studies investigated the fouling mechanisms of polysaccharides, one major group of macromolecules in EPS. In this work, dead end unstirred filtration of sodium alginate, a model polysaccharide, was carried out and analysed using classic filtration laws and the combined pore blockage and cake formation model. It was found that the cake model appears to fit the entire range of the ultrafiltration data while the consecutive standard pore blocking model and cake model are more applicable to microfiltration membranes (0.2 µm track-etched and 0.22 µm PVDF membranes). The specific cake resistance (αc) (approximately 1×1016 m/kg) is similar to αc of bovine serum albumin (BSA) and actual EPS in MBR systems reported in the literature, and higher than that of many colloidal particles. αc increases with the increase of alginate concentration but slightly decreases with the presence of BSA in the feed solution. The alginate cake obtained during filtration features both low compressibility and low porosity despite some differences between different membranes. The lack of a full flux recovery following filtration interruption implies the existence of strong intermolecular attractive interactions within the alginate fouling layer.

This work introduced a systematic description of the fouling behaviour of alginate solution (fouling mechanisms, specific cake resistance and compressibility) during dead end filtration with different operating parameters like pore size, pore size distribution and feed concentration being varied. The cake model fitted throughout the duration of UF while the consecutive standard pore blocking and cake filtration models appear to apply to alginate MF by PVDF and TE membranes. This was also confirmed by a combined pore blocking-cake filtration model. In general, the cake formed by alginate on the membrane surface was found to be slightly porous, nearly incompressible, with a relatively high αc. The main effects of shifting the operating conditions on the fouling properties are summarised in Table 5. 1. The increase of four operating parameters (TMP, pore size, pore size distribution and feed concentration) results in higher pore blocking propensity. 2. The influences of these four operating parameters on the cake property are different. The increase of pore size and its distribution leads to the formation of a cake with lower αc, but higher porosity and compressibility. However, the rise of TMP increases the final cake height and slightly decreases cake porosity. 3. αc increases with alginate feed concentration.