Advanced characterisation techniques to assess seawater organic matter removal by dissolved air flotation (DAF)

Membrane fouling in reverse osmosis (RO) systems remains a significant operational issue during seawater treatment for potable applications. Specifically, organic and biological fouling caused by the deposition of dissolved organic matter (DOM) and/or growth of microorganisms in RO systems is a major problem, despite the low concentration of these compounds (at 1-4 mg L-1 as C), compared with the other chemical constituents present in seawater (e.g. dissolved salts at 25-50 g L-1). Hence, pre-treatment of DOM remains a key issue influencing operation and maintenance costs in RO systems. Dissolved air flotation (DAF) has recently received attention as a potential pretreatment option for seawater, since this process has been used extensively in freshwater situations to remove low density particles such as algae, colloids and DOM. However, there have been limited investigations in its application to seawater. Furthermore, such investigations have been hampered by the difficulty in characterising DOM in seawater, due to the high salt content and low carbon concentration. In this study, a model seawater solution was prepared, using six model organic compounds that have been shown to be present in seawater, to develop and validate fluorescence excitation emission matrix (FEEM) and liquid chromatography with organic carbon detection (LC-OCD) analytical methods. These methods were then used to characterise real seawater. Subsequently, these analytical techniques were applied to assess the performance of DAF under varying coagulation conditions for removing DOM from the model seawater and a number of real seawater samples. Once the methods were optimised, both LC-OCD and FEEM were able to provide detailed information on the concentration and character of the low concentrations of DOM in saline environments and to assess the DAF performance. On DAF treatment of model solution and seawater samples, the biopolymer and humic substance fractions were the major organic fractions removed at 42-98% and 20-95%, respectively. Humic acid removal increased at low pH (5-6) which was observed generally to be the optimal operating pH. The normalised coagulant dose (Fe3+/DOC ratio) required for successful coagulation was observed to be between 0.5-4 at pH 5.5, increasing to 4-12 at pH 7.5