Antiscalants are chemicals used in membrane-based water desalination processes to prevent the scaling of salts on the membrane. Previous studies suggested that antiscalants could lead or contribute biological fouling by providing growth-promoting factors such as source of biodegradable carbon or phosphorus. However, the test in previous studies were conducted using drinking water and pure cultures of bacteria isolated from freshwater. These conditions do not reflect those of desalination systems. In this study, we determined the microbial growth potential of eight antiscalants under conditions relevant to desalination plants. To this end, autochthonous microbial communities from the Red Sea were used, and we exposed them to chemically diverse antiscalants. The chemical characterization of the antiscalants showed that their carbon content ranged from 0.02 to 0.15 mg C/ mg antiscalant. Fourier transform infrared and nuclear magnetic resonance spectroscopy allowed us to classify the eight tested antiscalants into three types based on their chemical structure. These types were phosphonate-based, natural inulin-based, and polyacrylate-based antiscalants. The growth potential of the antiscalants was determined by incubating seawater with antiscalants (50 mg/L) and an initial bacterial cell content of 20,000 cells/ml at 30$^o$C. A reference without antiscalant addition was used for comparisons. The microbial growth was followed by measuring the cell concentrations over time by flow cytometry. Four phosphonate-based antiscalants promoted microbial growth to different degrees, while distinctively one phosphonate did not increased growth significantly under all tested conditions. At the same time, inulin and polyacrylate-based antiscalants did not increase bacterial growth compared to the reference. The growth potential of 8 antiscalants used presently in practice desalination plants ranges between 1,479,793 and 2,074,106 cells/ml based on the results of the growth potential test, strongly suggesting that antiscalant selection and optimization contributes to biofouling prevention and control.

Thermal desalination technology especially, multi-effect distillation MED is of great importance to oil producing countries such as those in the gulf region owing to its efficacy in processing seawater with the minimum pre-treatment of the feed and cheap energy input available from waste heat. One of the main drawback of the current MED processes is the susceptibility of scaling when operate above 70 ºC. This limitation deprives the technology to be energy efficient and reduce its optimal productivity. An optimized pre-treatment of the seawater feed by NF membranes can enhance its efficiency significantly. In this work, the possibility of applying a tailored feed quality using thermodynamic speciation chemistry of the feed water and prediction of the scale propensity based on the Saturation Index of the scale minerals was investigated. Different NF membranes with different properties were used and compared experimentally with each other and theoretically with predictions that are based the saturation index. Moreover, new generation of the polymeric antiscalants promoted by the main manufacturers of the inhibitors industry to control scale formation has been investigated. In addition, as part of planned work for future studies, design and construction of a pilot scale based on NF membrane process was carried out and meant to be a potential extension of this work.

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Resource recovery has become essential to compensate for costly and complex technical requirements to implement zero-liquid discharge (ZLD) policies. Antiscalant removal, especially phosphonate-based antiscalants, is a clear example of resource and potential subsequent valorization. The global market for phosphonate-based antiscalants is expected to grow in the coming years along with increasing membrane desalination projects. It implies the disposal in surface waters of tons of phosphonates and trace elements, present in wastewater and concentrates, every day. Removing the phosphonates and trace elements allows their subsequent recovery and valorization, minimizing squeezing produc-tion/extraction procedures and saving the environment from suffering any im-pact because of them. The first part of this thesis focuses on phosphonate removal with iron and aluminum-based adsorbents. Porous iron and aluminum (oxi)hydroxides can remove phosphonates from concentrates completely. The main limitation of this process is the diffusion of the phosphonates through adsorbent particles. As proved in this thesis, temperature significantly improves the adsorption kinetics of the phosphonates on both adsorbents as a result of the variation of the diffusion coefficient. The presence of calcium also plays an important role, since accelerates the adsorption at the first stages of the process, but limits and saturates the capacity of the adsorbent surface for further adsorption. More research on the role of calcium is needed in this regard to better understand how the adsorption/diffusion of the phosphonates is affected by this common element present in concentrates. Electrocoagulation was studied in the second part of this thesis as a potential approach for phosphonate removal. Using pure iron electrodes, the applied current density can be easily optimized. As a result, dissolved phosphonates are quickly removed from a large concentrate volume at a relatively low cost and minimal sludge production. The benefits of this technique lie in the possibility of producing the substrate (adsorbent) in situ for the phosphonate to be adsorbed. Furthermore, the time required to completely remove the phosphonates is remarkably shorter compared to adsorption, which, as pointed out above, is limited by diffusion phenomena. Alkaline washing was relatively successful at recovering the phosphorus from the sludge, depending on the dissolved phosphonate in the concentrate. Although experimental results may look promising, further research on finding the optimum working conditions has to be addressed. The process is open to improvement in terms of new electrode materials, reactor design, phosphorus recovery, or optimal working temperature. In the third part of the thesis, the adsorption potential of previously tested adsorbents for the removal of elements at trace levels. The iron-based adsorbent, commercialized for phosphate and arsenic removal, turned out to be excellent at removing transition metals (TM) and rare earth elements (REE). The study was carried out in parallel with the exploration of the capabilities of a high-resolution inductively coupled plasma mass spectrometry (HR ICP-MS) instrument. The potential of this analytical procedure allows the detection and quantification of all the isotopes at the ultra-trace level (in the range of a few ng·L-1) and in only one measurement round. Furthermore, interferences from polyatomic species, formed during the ionization in the plasma, are easily resolved due to the high-resolution mode. As a result, the detection of the targeted element is easily discriminated from the potential interferences. This feature makes a remarkable difference regarding ordinary ICP-MS, which requires different analytical procedures to properly resolve overlapping signals. This analytical procedure opens new possibilities to test the adsorbents in new conditions and develop analytical methods for water speciation.