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Abstract
Gas sparging has been shown to significantly affect the performance of nanofiltration of molasses wastewater using a hydrophilized polyamide membrane (molecular weight cut-off 250) in a flat sheet module. Sparging of nitrogen at two different gas velocities was capable of appreciable alleviation of permeate flux decline over a period of time under the present experimental conditions when compared with the conventional unsparged nanofiltration. Critical flux (Jcrit), limiting flux (Jlim), and shear stress number (Ns) were determined at different cross-flow velocities for both the cases in the presence and absence of gas sparging. Increase in Ns with increasing cross-flow velocity was more pronounced in presence of gas sparging. Almost 1.5-2 fold increase in Ns was observed using cross-flow velocity of 0.8 ms−1 with gas sparging, compared to the case of “no sparging.” However, there was no significant improvement of rejection behavior of the solutes in presence of gas sparging. The value of mass transfer coefficient was considerably higher with increasing cross-flow velocity, while gas sparging was on, as against the case of “no sparging.” In terms of percentage increase in permeate flux with increase in trans-membrane pressure (TMP), gas sparging could achieve higher enhancement where concentration polarization is expected to be severe, that is, at high TMP and high feed concentrations. Some practical issues and technical limitations of gas sparging are also discussed.
ACKNOWLEDGEMENTS
The authors are thankful to Mr. Kishore Bajaj, Technodesign, VU Nagar, Gujarat and the Sophisticated Instrumentation Centre for Applied Research and Testing (SICART), Vallabh Vidyanagar, Gujarat, for their valuable help to conduct certain analysis. Suggestions by the anonymous reviewers to improve the manuscript are also gratefully acknowledged.