Characterisation of nanofiltration membranes for sulphate rejection

Nanofiltration (NF) membranes are used for a range of industrial applications one of which is for the removal of the sulphate constituent in seawater. This is a mature activity for the treatment of seawater that is to be injected into oil reservoirs in the offshore oil/gas industry. Such sulphate removals have also been the subject of much interest, as a pretreatment strategy, in seawater desalination plants that is either utilising thermal technology or reverse osmosis. Nevertheless, there is a need for robust criteria, such as the comparative permeate flux and sulphate rejection, of selecting the optimum NF membrane. There is a major difficulty in the assessment of the comparative filtration performance and the role of membrane structure because the data from manufacturers and also the information from the scientific literature emanates from different testing protocols. This can result in an enigmatical situation for obtaining the optimum NF membrane for a particular application. Against the above background this PhD project has focused on undertaking a fundamental study of different commercially available NF membranes in order to facilitate improved assessment of their filtration performance for sulphate rejection applications in relevant standardised testing conditions. Moreover, on the basis that those variations in membranes’ functioning are attributed to membrane structure and characteristics, a major segment of the research was focused on correlating filtration performance and membrane features. The research comprised two main phases; the first phase involved determining the comparative filtration performance of eight commercially available NF membranes supplied from four manufacturers. The second main phase was to undertake detailed characterisation studies on the NF membranes in order to obtain a clear understanding of their sulphate separation mechanism and permeate flux. The first phase involved assessments of the permeate flux and selectivity of the eight membranes. The experimental protocol in the second, characterisation part of the work was directed to the identification and evaluation of NF active surface layer characteristics:  Pore characterisation by porosity factor calculations,  Hydrophilicity/Hydrophobicity nature by contact angle measurements,  Surface Free Energy calculations,  Surface roughness measurements by AFM,  Membrane potential measurements and average charge density calculations. This approach is an acknowledged strategy for NF membrane scientific research assessment and, in the current work provided key data of membrane features that facilitated a systematic understanding of membrane functioning. These characterisation features were also linked successfully to the membrane performance parameters to yield a characterisation/performance envelope which represents a useful basis for NF membrane selection and utilisation to optimise membrane usage and consequent economic advantage. The general discussion includes a summary of the interface between the role of NF and the operational and economic features of the two main types of desalination processes. It includes an outline of a process scheme for the incorporation of NF pretreatment into an MSF plant from the conceptual design stage as opposed to the application employed hitherto where the emphasis has been on attaching NF pretreatment equipment on to an existing unit. As a result, it is expected that NF usage should increase performance ratio, reduce energy consumption, hence the running cost, and increase recovery.