Reverse osmosis (RO) is considered the most reliable and cost-effective membrane desalination technologyworldwide. However, it suffers significant performance limitations due to mainly inorganic foulinggenerated in the highly concentrated brine. Especially, scaling caused by silica and silicates depositionsresults in irreversible damages with considerable economic implications. Recently, a different ROconfiguration, termed as closed-circuit reverse osmosis (CCRO), has been claimed to exhibit substantialbenefits over conventional RO in terms of both energy savings as well as higher scaling resilience.CCRO is operated in batches, during which the generated brine is continuously recycled inside theclosed loop until a desired recovery has been accomplished, after which the brine is released and replacedby fresh feed. Regarding CCRO scaling resistance superiority, an experimental-based proofis missing from the relevant literature. The current thesis was realized in collaboration with LenntechB.V., aiming at investigating the intrinsic propensity of CCRO to withstand and delay silica scaling. Tothat end, a campaign of filtration tests was carried out by means of a single-module CCRO pilot setup,during which two scaling indicators were periodically monitored. The used indicators were the masstransfer coefficient (MTC) and the applied feed pressure (Pfeed). Prior to the filtration trials, preliminarybatch tests, of 4-hour duration each, were carried out in order to simulate and more thoroughly examinethe circulated brine conditions. Various synthetic brines were prepared and silica polymerization wasmonitored. The effects of silica supersaturation level, pH and hardness ions were investigated. Of greatimportance was whether silica existed in its monomeric or polymeric form, since this greatly impactsthe scaling occurrence probability. Batch tests results revealed that at high pH conditions (pH>10)monomeric silica concentration remained unchanged in pure silica solutions (even at high supersaturationlevels), owing to the great silica solubility level. Nevertheless, when Mg2+ and/or Ca2+ werepresent in the solution, the quantity of silicic acid rapidly reduced. This was the result of the instantaneousformation of metal-silicate precipitates. Batch tests at pH 7 were also performed. In that case,monomeric silica concentration in pure silica solutions remained constant up to initial concentrations ofabout 450 mg/L SiO2 for the examined 4-hour duration. However, at higher SiO2 concentrations, suchas at 750 mg/L, rapid polymerization occurred. When hardness cations were included in the neutral pHsolutions, they showed an accelerating effect on silica polymerization process, but they did not reactwith either monomeric or polymeric silica. This effect relates to the suppression of the silica colloidsdiffuse double layer by the hardness cations, which subsequently facilitates colloids agglomeration.Regarding the CCRO filtration tests, they were conducted in sequences with duration of 20 or 40 min,which in its turn determined the achieved sequence recovery. For most of the carried out sequencesthe initial feed composition was: 120 mg/L SiO2 and 24 mg/L Mg2+. Only the final 5 out of the total40 sequences were realized in the absence of magnesium in the feed solution. All the filtration runswere performed at pH 7, at ambient temperature and at constant flux 15 L/m2h. The outcome was ascaling-free desalination process for a total cumulative operational period of approximately 11 hours,during which recoveries as high as 90.9% were reached, whereas severe scaling took place only afterabout 14 hours of total operation. The obtained results were contrasted with filtration tests results ofconventional RO received from literature resources and in that way the higher efficiency of CCRO towithstand and delay silica scaling was proved. Additionally, through silica mass balance calculations itwas shown that during all filtration tests significant silica polymerization took place. Also, cations analysisbe means of IC excluded the participation of Mg2+ ions in the formed scale layer. It was concludedthat the scale development was the result of an initial attachment of silica colloids to the membranesurface followed by monomeric units adsorption onto them. Finally, a simple customized method forthe prediction of silica scaling potential in CCRO operations based on batch tests was proposed.

Synthesis of new perylene-3,4,9,10-tetra-carboxylic acid (PTCA) derivates is a research topic that has rapidly developed during the last decades. Current synthetic efforts are largely focussed on functionalizing the perylene core, via either bay- or ortho-substitution. The aim of this research project was to synthesize novel perylene mono-imide di-ester derivates by attaching different substituents at the bay-positions. The first synthetic route that was attempted in this research project was a route via a perylene mono-anhydride mono-imide derivate. The anhydride was attached to this molecule to activate the relevant bay-positions. It was found that the anhydride is not stable enough to withstand the reaction conditions that were used in an attempt to substitute the relevant bay positions. The next synthetic route that was attempted in this research project was a route via a perylene di-ester mono-imide derivate. Reactions were performed to explore whether the di-ester was able to activate the relevant bay-positions. It was found that at the used reaction conditions, phenol exchange takes place on the bay-positions of the perylene core. Due to the challenges that arose, the target compounds of this project have not been synthesized. However, three other new PTCA derivates have been synthesized during this project.

With the growing population and industrial development, there is more stress on natural water resources. Additionally, environmental laws make the disposal of waste streams from industries difficult. In this scenario, it is crucial to treat waste streams to recover water and possibly minerals, for reuse in the industry, reducing the dependency on new resources. Reverse osmosis (RO) is a membrane-filtration technology that has been in use for decades. RO systems can waste up to 30% of the feed water through the production of brine, containing the rejected minerals. This rejection is high when dealing with saline waters. Treatment of this brine stream for reclamation of water would increase the overall efficiency of RO systems. In this research, the treatment of RO brine by a closed-circuit configuration of RO – closed-circuit desalination (CCD) - is investigated. In CCD, filtration is done in batches, with the recirculation of the concentrate stream back into the feed stream. The high cross-flow velocity and short filtration batches result in prevention of scaling in these systems.
This research focused on proving the resilience of the CCD system to silica scaling despite supersaturations of silica. Lab-scale experiments were performed on a single-element CCD system, specially built with relevant measurement instruments. Silica scaling was monitored by mass transfer coefficient (MTC) calculations, silica mass balance calculations, and membrane autopsies. Synthetic feed water was used, containing only NaCl (10 mg/L) and varying concentrations of silica (70 – 120 mg/L as SiO2). No anti-scalants were used. Longer experiments were performed, i.e. 20 consecutive cycles of 1 hour each with 120 mg/L of silica in the feed. The cycle duration was increased, 2 cycles of 3-hour each were performed, with 120 mg/L of silica in the feed.
The MTC curves in all the experiments had a gradual decline with the progression of each cycle, but always recovered at the beginning of the next cycle. This decline was probably due to the increasing osmotic pressure in the recirculation loop. There was no permanent decline in the MTC, leading to the observation that there was no scaling in the system. This was supported by the silica in the mass balance calculations, that showed there was no loss of silica from the brine. The reactive silica concentration in the brine was as high as 1800 mg/L in the 3-hour cycle, attaining a recovery of 93%, without signs of scaling. The membrane autopsy showed that the membranes used for the experiments with 120mg/l silica in the feed had higher silicon content compared to the blanks. However, there were too few samples to compare against and make strong conclusions whether there was scaling.
Thus, the designed CCD system was resistant to silica scaling in these conditions, of high pH and in the absence of other components such as iron, aluminium, calcium and magnesium. The results of this study proved that, despite high concentrations of silica in the feed, CCD can improve the total efficiency of RO systems (with regard to the water wastage) by recovering water from the brine produced by RO installations. The extremely high recoveries attained by the system would result in small volumes of very concentrated brine, making the extraction of minerals more cost-effective, because lesser volumes must be treated to obtain the same amount of minerals.

The need to treat waste streams produced during the water treatment processes is becoming a challenge nowadays due to the industries’ disposal expenditures, the limitations imposed by laws and the environmental impacts. The spent regenerant from ion exchange (IEX) processes is an example of such streams. This study investigated the use of Nano-filtration technique for separation as a part of the treatment step for IEX spent regenerant. This research is focused on the parameters such as membrane characteristics, ionic composition and operating conditions that affect the ion rejection of the IEX spent regenerant. This research aims to select a membrane that contributes to a maximum separation of monovalent and divalent ions of the IEX spent regenerant.
During this research six nanofiltration membranes and two reverse osmosis membranes were tested, of which the molecular weight cut-off (MWCO) was measured. Each membrane was placed at a flow Cell unit and fed with artificial water.
The MWCO of the nanofiltration membranes was varied from 115 to 508 Da. The purpose of testing different membranes was to assess how the pore size of the membrane influences the ion rejection of the IEX spent regenerant. In addition, the ion rejection was evaluated in different ionic strengths and different molar ratios. The aim was to investigate how the ionic composition influences the ion rejection. Finally, operating parameters such as permeate flux and temperature were tested to research their impact on the ion rejection of the IEX spent regenerant.
Results showed that the ion rejection of the IEX spent regenerant was strongly affected by the pore size of the membrane as the charge effect was insignificant due to the high ionic strength of the feed solution (1 M). The main exclusion mechanisms were the steric and the dielectric exclusion. Loose membranes with MWCO greater than 500 Da rejected less than 10% of both monovalent and divalent ions. Membranes with pore size from 200 Da to 300 Da rejected 30-60% of Ca2+ and contributed to negative rejections of Na+. NF membranes with MWCO of approximately 150 Da, rejected 90% of Ca2+ and 40% of Na+. Tighter NF membrane and RO membranes rejected more than 95% of divalent ions and more than 70% of monovalent ions.
It was also observed that the ionic composition did not influence the rejection of divalent cations as it remained almost constant in different ionic strengths and molar ratio of the solution. Contrary, the rejection of monovalent cations was greatly influenced by the molar ratio of the solution due to the presence of predominant amounts of ion of higher charge and the need of the ions to maintain electroneutrality. Higher molar ratio resulted in lower rejection of monovalent cations.
The permeate flux affected the ion rejections. Higher fluxes resulted in higher ion rejection and different ion separation. To keep high rejection of bivalent and low rejection of monovalent ions a compromise would be desired. Temperature had a great influence on the ion rejection of the IEX spent regenerant as a 5oC increase in the temperature cased 19% decrease in divalent cation rejection.
TS80, one of the nanofiltration membranes, performed better than other membranes as it rejected 90% of Ca2+ and 40% of Na+. Double Pass NF with TS80 membrane was proposed as the first treatment step of the IEX spent regenerant stream as it resulted in 97% Ca2+ and 45% Na+ rejection.

Ion exchange membranes (IEMs) are used to selectively transport ions between two solutions. Most membranes are not selective to certain ion species. This study investigates the possibility of increasing phosphate selectivity (in the form of H2PO4 – ) of anion exchange membranes (AEM) by modifying commercial AEMs with a Layer-by-Layer (LbL) assembly of a few promising candidate polyelectrolytes (PE): Poly(diallyldimethylammonium chloride) (PDADMAC), poly(allylamine hydrochloride) (PAH), and poly(allylamine hydrochloride)-Guanidinium (PAH-Gu). The anionic layer was composed of poly(sodium 4-styrene sulfonate) (PSS). Membranes were modified with 4 layers. This was done on only one side, the asymmetric modification ((PE/PSS)4), or both sides of the membrane, the symmetric modification ((PE/PSS)2). The membranes were then characterized using low-frequency impedance spectroscopy to obtain, among other parameters, the resistance of the membrane. Asymmetrically modified membranes showed non-reproducible impedance data, while their symmetric counterpart was in good agreement with a theoretical model. Compared to the bare membrane, a decrease in the membrane’s resistivity was found for all the modified membranes in the studied salt concentrations (0.1 to 0.4 M). In the low concentration region, the PAH-Gu modified membrane had the lowest resistance. This result is most likely caused by a high affinity between Guanidinium and phosphate. The electric properties of other parameters, such as diffusive boundary layer and electrical double layer resistance, were found to be independent from the membrane modification. We then performed permselectivity measurements with a competing anion, sulfate. To this end, electrodialysis was performed in the underlimiting current regime. Modified membranes showed no increase in discriminatory function for phosphate. This could be due to several reasons. For example, there might be a pH-change near the surface of the membrane, changing the charge of the phosphate anion. Additionally, the PAH-Gu membrane did show an increase in overall anion flux compared to the bare membrane. These results are promising and warrant further research into phosphate selective receptors in combination with IEMs.