Abstract
Scaling and (bio)fouling phenomena can severely alter the performance of the reverse osmosis process during desalination of seawater. Pretreatments must be applied to efficiently remove particles, colloids, and also precursors of the organic fouling and biofouling. Ceramic membranes offer a lot of advantages for micro and ultrafiltration pretreatments because their initial properties can be recovered using more severe cleaning procedure. The study focuses on the interaction between metal oxides and marine organic matter. Experiments were performed at laboratory scale. The first series of experiments focus on the filtration of different fractions of natural organic matter and model compounds solutions on flat disk ceramic membranes (47 mm of diameter) characterized with different pore size and composition. Direct filtration experiments were conducted at 0.7 bar or 2 bars and at room temperature (20 +/- 0.5 degrees C). The efficiency of backflush and alkaline cleaning were evaluated. The second series of experiments were carried out with metal oxide particles of the same composition than the disk ceramic membranes. Isotherm adsorption and adsorption kinetics of different fractions of organic matter with different concentration and physico-chemical characteristics were studied (pH, temperature, and ionic strength). Metal oxides studied are aluminum, zirconium, and titanium oxides. Each metal oxide corresponds to a specific pore size for the disk ceramic membranes: 80, 60, and 30 nm. Different sizes of metal oxide particles are used to measure the impact of the surface area on the adsorption of the organic matter. Seawaters from the Arabian Gulf and from the Red Sea were collected during algal blooms. Cultures of algae were also performed in the laboratory and in cooperation with woods hole oceanographic institute. Solutions of algal exudates were obtained after a couple of weeks of cultivation followed by sonication. Solutions were successively filtered through GFF (0.7 mu m) and 0.45 mu m membrane filters before use. The dissolved organic carbon (DOC) concentration of final solution was between 1 and 4 mg/L and showed strong hydrophilic character. These various solutions were prepared with the objective to mimic the dissolved organic matter composition of seawater subjected to algal bloom. Characterization of the solutions of filtration experiments (feed water, permeate water, and backwashes) and batch experiments (raw solution and supernatant solutions) included fluorescence, UV, total organic carbon, total nitrogen, and Liquid Chromatography - Organic Carbon Detection measurements. In comparison to seawater under normal condition, seawaters collected during algal bloom and algal exudates solutions led to very severe fouling of all ceramic membranes (with some DOC removal of 10-20%). It is important to indicate that algal exudate permeates still exert fouling properties when filtered through similar new (virgin) membrane. These results indicate that both size exclusion and adsorption phenomena occur (on surface, inside pores). In all cases, the backflush (performed with MilliQ [MQ]) showed minor or no effect and the cleaning backwash was not able to recover the initial flux (30-60%). Analyses performed on the backflush solution (performed with MQ) and on the alkaline cleaning solution revealed the retention of both high and low molecular organics on the different metal oxides.
Unfortunately, analyses of membrane surface using imaging various tools are not providing informative results in term of foulant/adsorbed material composition.