Traditionally, industrial processes produce wastes that, even though often containing useful materials, are discarded, contributing to environmental pollution and depletion of natural resources. An example of such wastes are brines, flows of concentrated salts, produced in water treatment processes, which are now routinely discharged into receiving water bodies. Brines however can also be considered as flows of reusable materials which should be recovered, and the Zero Brine cooperation project aims to develop processes for that purpose. For a demineralized water production plant in the port of Rotterdam (the Netherlands), a closed water processing cycle was proposed to treat the large volume of spent Ion Exchange (IEX) regenerant brine which, apart from recovering demineralized water, is also intended to produce magnesium (Mg2+) and calcium (Ca2+) salts, with the highest purity possible, from the otherwise discharged brine. The process scheme includes nanofiltration (NF) for separating mono- and multivalent ions, followed by sequential chemical precipitation of Mg2+ and Ca2+ ions from the NF concentrate, and production of demineralized water by evaporation of the NF permeate. The concentrate of monovalent ions produced in the evaporator, essentially a concentrated sodium chloride solution, in its turn might be reused for IEX regeneration. Part of the supernatant of the sequential precipitation may be fed to the evaporator as well, but bleeding the other part of this supernatant is essential in order to maintain process stability, avoid accumulation of minor pollutants, and reduce scaling. In this study, various scenarios to operate the process were modeled, using PHREEQC and Excel. According to the simulation results, recovery of ≈97% of Mg2+ and Ca2+ is possible, the latter with a higher purity than the former. The main factors affecting the results are the concentration of carbonate present in the spent IEX regenerant, as well as characteristics of the NF membrane and the dosing of sodium hydroxide in the sequential precipitation steps. The results of the simulations were used for the design and operation of a pilot plant, comprising all mentioned process steps.

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Silica scaling is one of the major scaling challenges in Reverse Osmosis (RO). The safe operation practice is to keep the silica concentration below 150 mg/L in RO concentrate. This study addresses the effects of divalent cations such as calcium and magnesium on silica scaling in a seawater RO installation used as a pretreatment to Eutectic Freeze Crystallisation (EFC). Results showed that in the absence of antiscalant and divalent cations a sustained silica concentration of approximately 280 mg/L in concentrate is possible without declining membrane permeability. At a higher concentration of divalent cations, the membrane permeability decreased. Membrane autopsy and analysing destructed membrane showed a relatively low magnesium and a high calcium concentration on the membrane after adding divalent ions into the solutions. It is concluded that in absence of divalent cations and without antiscalant the limits of 150 mg/L silica can be extended to 280 mg/L for 6–8 h.

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This work presents a joint experimental and simulation campaign aimed at characterizing two nanofiltration membranes (TS80 and NF270) in the presence of a multi-ionic water solution simulating the spent regenerant of cationic ion exchange resins employed for water softening. We identified the membrane parameters, which allowed for predicting the performances through the Donnan Steric Pore Model with Dielectric Exclusion. A good agreement between model and experimental trends of rejection as a function of the applied pressure was observed (error < 15%). The analysis of trans-membrane fluxes and exclusion coefficients showed that dielectric exclusion was the crucial mechanism for the ionic partition. In fact, the lower pore dielectric constant found for TS80 justified the higher rejections to divalent cations with respect to NF270. Moreover, negative charge densities were found for both membranes, because of the high concentration of chloride ions in the feed, which likely adsorbed onto the membrane. However, it was observed that the experimental rejections did not change significantly with the feed pH. This result, in line with the minor role of the Donnan exclusion resulting from the model, suggested that the membrane performances were not much affected by the charge density at high feed ionic strengths (~1 M).

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Imbalance between water supply and demand is increasing in many parts of the world. Lack of water security is attributed to climate change, population growth and unsustainable abstraction of groundwater. Managed Aquifer Recharge (MAR) can increase groundwater recharge, facilitate higher water availability and accessibility, and hence reduce water supply deficit. This paper investigates the potential of MAR in a karstic and semi-arid region with the use of GIS Multi Criteria Decision Analysis (GIS-MCDA). While previous studies have focused on MAR techniques and MAR suitably mapping separately, this paper combines technique selection and suitability mapping to emphasize the importance of considering both. The methodology was applied for a karstic and semi-arid region, using the Ramotswa Transboundary Aquifer Area (RTAA) as case study. MAR suitability mapping of the selected MAR technique was carried out with six thematic layers using Weighted Linear Combination (WLC) decision rule. The objective was to identify suitable areas for implementation of spreading methods and generate MAR suitability maps to guide decision-making in such implementation. The suitable areas were identified at the border between Botswana and South-Africa. The study develops a methodology for GIS-MCDA considering spreading method for a karstic aquifer, but more research is needed to assess the methodological transferability to other karstic and semi-arid regions.

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Scaling problems are more frequently observed in hot water equipment fed with remineralised reverse osmosis permeate compared to hot water systems fed with conventionally purified water. These problems occur when permeate from reserve osmosis is remineralised to a hardness to meet the regulations. The effects of temperature, initial calcium concentration, and the addition of two types of scaling inhibitors (humic substances and phosphate) on scaling formation were investigated using synthetic water to mimic remineralised RO permeate and tap water. No scaling was observed at room temperature for solutions with an initial concentration of about 40–60 mg/L Ca²⁺. At higher temperature, the saturation index increased, and scaling occurred. In conventional hot tap water, scaling occurred after prolonged exposure time but was about 50% less compared to synthetic water, probably due to the presence of scaling retarding compounds in the tap water such as humic substances. In hot synthetic water, scaling was also retarded when humic substances were dosed. Humic substances at its optimum dosage (3 mg/L C) was more efficient for inhibition of scaling than phosphate at its optimum dosage (1 mg/L P).

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Feed spacer orientation affects the velocity pattern and pressure drop of spacer-filled channels such as those encountered in Spiral-wound Membrane (SWM) modules of Reverse Osmosis (RO). However, there are only limited numbers of experimental studies on this topic. This study sets out to reveal more detailed information on the pressure drop and velocity patterns of spacer-filled channels. Particle Image Velocimetry (PIV) is used to provide high-resolution velocity maps for three commercial feed spacers of different thicknesses at a flow attack angle of 45° and 90°. The pressure drop is measured for the applied operational conditions (Re < 250). Results showed higher pressure losses, a better mixing of flow, a lower variation of temporal velocity, and a smaller variation of velocity over the channel height in the orientation with a flow attack angle of 45° as compared to 90°. The results presented here can be used to validate numerical studies, determine the fouling-sensitive regions in a spacer-filled channel and consequently, design the optimal spacer with respect to its orientation and thickness.

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Making improvements to feed spacers of spiral-wound membrane (SWM) modules of reverse osmosis (RO) is a necessary step towards a wider application of these modules. This study sets out to evaluate the performance of six commercial feed spacers by comparing their actual velocity profiles and their pressure drop. Velocity profiles are obtained from Particle Image Velocimetry (PIV). Comparing images from PIV with corresponding friction losses revealed that the transition from steady to unsteady flow occurs at the moment when the incline of the friction factor changes from steep to slight. From the two types of spacers used, zigzag spacers showed a better distribution of flow than the cavity spacers did, but at the cost of higher pressure drop. The flow was in a straight line from inlet to outlet with zigzag spacers only at low Reynolds numbers and with cavity spacers for the entire studied range of Reynolds numbers. Additionally, results showed that hydraulic conditions in channels with cavity spacers are mainly affected by geometric characteristics of transverse filaments. The results from this study can be used to understand the effects of spacer geometry on the hydraulic conditions inside the feed channel and as a validation tool for computational modeling.

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Spiral-wound membrane (SWM) modules are the most common membrane configuration utilized in reverse osmosis (RO) and nanofiltration. The enhancement of SWM module design, particularly in the geometric design of the feed spacer, can play a crucial role in the cost and the potential for wider application of these modules. The feed spacer influences the flux, pressure losses and fouling in the membrane process and consequently the product water unit cost. Despite the shift in the application of SWM modules of RO toward low salinity sources and the resulting higher sensitivity performance using these waters, the configuration and orientation of feed spacers have not significantly changed since the original design. A wider use of SWM modules, therefore, requires the adaptation of geometric parameters of the feed spacer to the water source. Improving the feed spacer's design according to the feed water type requires the knowledge of previous studies conducted in spacer-filled channels as well as further needed investigations in future. This paper reviews the role of the feed spacer in SWM modules and provides an overview of studies conducted in narrow spacer-filled channels to determine the effect of different geometric characteristics of the feed spacer on hydraulic conditions.

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Depletion of fresh groundwater sources as the result of overdraft, salinization and pollution becomes a major problem in parts of the world. Desalination of brackish groundwater by membrane technology, e.g. reverse osmosis (RO), seems to be a promising solution to water scarcity problems. However, energy consumption and concentrate disposal are considered as the main reasons for avoiding RO application. In order to overcome these drawbacks, the PURO concept, which consists of vertically-configured RO unit in an especially drilled well is designed, installed and is going to be tested. The installation operates without any chemical pretreatment and therefore, the concentrate can be injected into a deeper confined aquifer that contains water of similar concentration. To avoid chemical pretreatment, the system operates at lower recovery (50%) than conventional brackish groundwater reverse osmosis (BWRO). Higher energy consumption, as the results of lowering the recovery, is avoided by using natural hydrostatic pressure at the depth that RO is installed and by extracting the permeate water only. PURO consumes about 39% less energy when compared to a conventional BWRO installation of the same capacity. This article describes the PURO concept and discusses its advantages and disadvantages. It also provides a rough calculation of water cost for PURO and conventional BWRO with emphasizing on the energy costs.

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This study focuses on spiral-wound membrane (SWM) modules, which are the most common commercially available membrane modules for reverse osmosis (RO) and nanofiltration (NF). While RO membranes can remove almost all kinds of substances from the feed water, they are usually equipped with pretreatment steps for conditioning and modifying the feed water to prevent clogging and fouling of these modules. Energy consumption, fouling and concentration polarization are considered as the primary challenges in these types of RO modules. These challenging factors depend of the feed water quality and they are related directly or indirectly to the design of applied feed spacer in SWM modules. A feed spacer provides a channel between two envelopes from entrance to outlet of a module to let the water flows tangentially over the membrane surfaces from the feed to the concentrate side. Additionally, feed spacers are designed to destabilize the concentration polarization layer; and thereby increase the mass transfer in SWM modules of RO. However, application of feed spacers to efficiently mix the flow comes at the expense of higher energy consumption and fouling formation. Therefore, it is important to understand the hydraulic conditions inside the spacer-filled channels such as those encountered in SWM-modules of RO. Previous RO-studies related to production of drinking water are primarily performed with the assumption that RO-elements are used for desalination. In contrast to this assumption, the most commercially available configuration RO elements, SWM modules, gained more attention to be used for purification of freshwater resources currently. The main reason of using RO for freshwater purification is that it provides an effective barrier against the continuously emerging micro- and nano-contaminants, which cannot be (easily) removed by conventional treatment technologies. The energy use, scaling and retention in RO are influenced by the concentration polarization. When RO is applied on freshwater, the effects of concentration polarization on the energy use become significantly smaller because the osmotic pressure difference is negligible. This thesis is divided into two parts. In the first part, this thesis describes a unique brackish water pilot study, which operates without chemical pretreatment. Such pilot study is referred to as one step membrane filtration (OSMF) system. An OSMF-system is an NF/RO-system in which membranes are applied directly on the feed water and operate without chemical pretreatment. In the second part, this thesis focuses on the hydraulic conditions of spacer-filled channels and the role of spacer geometry and orientation thereon. The effect of feed spacers on hydraulic conditions is investigated by studying the actual velocity profiles occurring in the spacer-filled channel and the relation between energy losses and spacer geometry. Particle image velocimetry (PIV) technique is used to determine the orientation and configuration effects of spacers on the actual velocity profiles. PIV is a non-invasive and powerful tool to achieve high-resolution velocity profiles experimentally, which can be used for verification and validation of numerical studies related to spacer-filled channels. 2D and 3D numerical studies have contributed significantly to our understanding of hydraulic conditions in SWM modules of RO. However, these numerical simulations are usually validated with low-resolution experimental methods. PIV measurements from this study, thus, can provide high-resolution experimental data (7.4£7.4¹m2) for validation of these numerical studies. The first results in PIV technique is a simultaneous velocity profile, which is obtained by using each two taken frames at a determined time interval. In this thesis, the simultaneous velocity profiles are used to investigate the variation of temporal velocity at certain locations (points) inside a mesh of a spacer. The spatial velocity profiles that are discussed in this thesis obtained by computing the average of related simultaneous velocity profiles. Chapter 3 describes a detailed explanation about the experimental methods used in this thesis. Summary of chapter 3 is repeated in the experimental section of chapters 5, 6 and 7. The effect of feed spacers on hydraulic conditions is investigated by comparing an empty channel with a spacer-filled channel, which was filled with a commercial feed spacer (chapter 5). The flow in the empty channel was in a straight line from inlet to outlet and it was steady compared to the flow in the spacer-filled channel. The spatial velocity profiles of spacer-filled channels showed a bimodal shape with a peak at low-velocity ranges and a peak at high-velocity ranges. The low-velocity regimes occurred mostly in regions close to the filaments and a high-velocity regimes occurred at the narrowed parts of the channel or directly after the narrowing parts. The difference between low and high velocity regimes with regard to the velocity magnitude and frequency was higher at a higher flow. Although the increase in velocity magnitude causes generation of a higher shear, it is not necessarily beneficial. That is because the optimal flux as the results of destabilization of the concentration polarization layer will be achieved at a specific velocity. A further increase of the velocity from this optimal velocity will have only a marginal effect on destabilization of the concentration polarization layer, enhancing the flux and consequently production increase. The hydraulic conditions inside a spacer-filled channel are influenced by configuration as well as orientation of the spacer. The pressure drop, which is measured during this thesis inside the channels with commercial feed spacers was in a good agreement with pressure drop from previous mathematical models. Previous studies reported a lower pressure inside the channels with the cavity spacers than the channels with zigzag spacers. The zigzag or net-type configuration is the common configuration used in SWM of RO. In the zigzag configuration, two layers of filaments with equal average diameter lay on top of each other and make an angle of 45o with each other (hydraulic angle) and with the flow (flow attack angle). In the cavity configuration, the diameter of transverse filaments is smaller than longitudinal filaments. Cavity spacers in this study had a flow attack angle of 135o. The biggest ratio of transverse filaments’ diameter to channel height was about 0.6 with cavity spacers. With cavity spacer used in this thesis, the flow was mainly in a straight line from inlet to outlet. The flow disturbance in channels with cavity spacers was at down- and upstream of transverse filaments. The flow acceleration over the transverse filaments of examined spacers was greater for cavity spacers with bigger transverse filaments. In channels with cavity spacers, the velocity was clearly higher at the channel side without transverse filaments than the channel side with transverse filaments. In channels with zigzag spacers, the flow close to the membrane was in the direction of filaments attached to the membrane, i.e. the direction of flow at the top of channel was perpendicular to the direction of flow at the bottom. The flow pattern at the middle of the channel was a combination of flow patterns at the top and bottom. The greater friction losses that were found for spacers with bigger relative height and smaller aspect ratio indicate that the orientation and geometry of transverse filaments contribute to pressure losses. However, it was not possible to find a reliable correlation for predicting the pressure losses based on the geometric characteristics of feed spacers in experimental conditions. Effect of feed spacer orientation on the flow investigated by using the same commercial spacer at two different flow attack angles. For this purpose, three feed spacers are used with different thickness. The thickness of the top and bottom filaments was the same for each spacer. Pressure drop in the channel with the thinnest spacer was clearly higher at normal orientation (with a flow attack angle of 450) than at ladder orientation (with a flow attack angle of 90o). The difference between two orientations in the channels with thicker spacers was insignificant. The difference between the lowest and highest the velocity was greater in ladder orientation than zigzag orientation. Commercial ladder spacers with the characteristics described in this thesis are more sensitive to fouling than zigzag spacers because in ladder spacers the velocity becomes virtually zero at the side of the channel where transverse filaments are attached to the membrane. @en