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.

Arsenic in groundwater can constitute a persistent nuisance for water treatment facilities when it exceeds the admissible limit of 10 μg/L. Recently, a stricter limit has been set as a new challenging target by many companies in the Netherlands, which is below 1 μg/L. However, most of the groundwater treatment plants have been conventionally designed solely for the removal of the most common undesirable groundwater constituents, namely iron, manganese and ammonium. The current research aimed at the investigation of the operational conditions facilitating As removal in biological rapid filters simultaneously with the required Fe removal. This improved As retention should be correlated with an extended length where its adsorption takes place, thus with the deeper Fe penetration inside the bed. Therefore, this was attempted in this research. The process water used throughout the experimental tests contained As(III) and Fe(II) in order to simulate a typical anoxic groundwater quality. For the most part, a triple-layer filter bed was used, consisted of anthracite, sand and garnet. The different settings under examination involved a range of pH values (7.8, 7.1 and 6.4), two filtration velocities (2.5 m/h and 5 m/h) as well as the recirculation of the filtrate back to the feed stream. Finally, the multimedia bed was compared with a single-layer sand filter.
The results of the conducted pilot column filter experiments revealed that high pH values were accompanied with high oxidation rates and thus with the creation of Fe flocks, already in the supernatant water. Due to this, at pH 7.8 and 7.1 lower Fe concentrations were detected in the effluent, denoting a higher Fe retention as compared to pH 6.4. Interestingly enough, a relatively deep Fe penetration was observed for every pH value tested. Regarding As removal, it was evidently favored by high pH values owing to the oxidation-floc formation removal mechanism of Fe (homogeneous reaction), which prevailed under those conditions. On the other hand, at pH 6.4 the adsorption-oxidation mechanism was predominant (heterogeneous reaction), which obstructed the AsOB activity. As far as the tested filtration velocities is concerned, they didn’t seem to significantly impact both Fe spread over the bed as well as As removal at the higher pH levels. Nonetheless, this was not the case for pH 6.4, in which the slow flow rate enabled the generation of more and larger Fe flakes (due to sufficient residence time in the supernatant water), which were then retained in the top part of the bed. The high flow rate on the other hand allowed Fe to reach deeper in the filter. Surprisingly, As removal seemed to be improved at 2.5 m/h, despite the Fe flocks accumulation in the upper layers. Possibly, the short experimental times not allowing equilibrium to be reached could comprise a reasonable explanation of this unexpected result. Furthermore, the filtrate recirculation stream didn’t seem to positively influence As removal. The induced dilution effect resulted in a relatively large dispersion of Fe inside the filter bed, however the essentially halved incoming Fe concentration was not sufficient to adsorb the oxidized As(V). Finally, the comparison between the multimedia bed with the single-layer filter reveals a considerably wider Fe dispersion over the bed height in the former case, which in its turn promotes a more efficient As removal. The overall conclusion of the current study is that triple-layer bed filters facilitate a more gradual Fe removal and its deeper penetration in the bed as compared to single-layer filters. This fact stimulates As removal and additionally allows for longer filter run times. Moreover, heterogeneous Fe removal seems to obstruct As oxidation by AsOB and therefore homogeneous reaction can be considered as more favorable in terms of As removal. This specific removal mechanism becomes predominant at pH levels above 7, when sufficient oxygen is available. Lastly, the operational setting of filtrate recirculation back to the filter inlet, displays a negative impact regarding As removal.