Fouling Behavior Of Polyssachride During Microfiltration At Various Solution Conditions:xDLVO Approach

Microfiltration (MF) membrane fouling by sodium alginate (SA), which is ubiquitous in membrane bioreactor (MBR) wastewater treatment operation, usually exerts significant effect on the process efficiency and cost. Therefore, research efforts on the microscopic mechanisms of MF membrane fouling by SA appear critical to fouling control and hence the improvement of membrane process performance. In this study, the xDLVO approach was adopted to quantitatively assess the short-range interfacial interactions involved in SA fouling of MF membrane at various solution ionic conditions. The major factor was also studied via which solution ionic conditions affect SA fouling of MF membrane. Physicochemical properties of hydrophobic and hydrophilic MF membranes and SA were characterized under different solution ionic strength and in the presence of divalent ions. Based on the xDLVO approach, MF membrane-SA and SA-SA interfacial free energies were calculated and MF membrane fouling behaviors were predicted accordingly at the corresponding ionic conditions. The theoretical predictions were verified by fouling results and cleaning efficiencies.Results showed that ionic conditions influence MF membrane fouling by SA mainly through the alteration of acid-base interaction. The important role of acid-base interaction was further proved by the higher cleaning efficiency of SDS than that of NaOH. In comparison, van der Waals interfacial free energy did not change with the different ionic conditions; although double layer electric interfacial free energy changed with ionic conditions, its absolute value was much smaller than those of the acid-base and van der Waals interfacial free energies. Therefore, effect of double layer electric interaction was negligible on the overall interfacial interactions when SA contact with MF membrane surface.As ionic strength of SA solution increased, the interfacial interactions became more attractive or less repulsive, and the permeate flux decreased consequently. When Ca2+presented in the SA solution, the increased concentration of Ca2+could also make interfacial interactions more attractive or less repulsive, causing permeate flux reduced; although both increased Na+concentration or presence of Mg2+could lead to the less attractive or more repulsive interfacial interactions and therefore increase permeate flux compared to that when Ca2+presented, they were unable to enhance the reversibility of membrane fouling. At different ionic conditions, the initial and subsequent fouling potentials for hydrophobic and hydrophilic membranes strongly correlated with the MF membrane-SA and SA-SA interfacial free energies, respectively. As a result, the xDLVO approach could successfully predict SA fouling potential of MF membrane at different ionic conditions.At various ionic strengths, physical cleaning efficiency (i.e. reversibility of membrane fouling) increased with the increasing interfacial free energies; whereas chemical cleaning efficiencies of NaOH and SDS decreased with the increasing interfacial free energies. When divalent ions presented, no relation was found between physical cleaning efficiency and interfacial free energy; however, chemical cleaning efficiencies of NaOH and SDS decreased as interfacial free energy increased. Hence, the xDLVO approach could successfully predict fouling reversibility at different ionic strengths, but failed to predict fouling reversibility in the presence of divalent ions.Under all the tested ionic conditions, hydrophobic MF membrane interacted attractively with SA, whereas hydrophilic MF membrane interacted repulsively with SA. Accordingly, the hydrophobic MF membrane suffered more severe fouling than the hydrophilic MF membrane during the initial filtration stage. In later filtration stage, however, fouling behaviors of the two membranes were almost the same because it was the identical SA-SA interfacial interactions that control fouling behaviors of the two membranes at this stage.