6 - Advances in electrodialysis for water treatment

Both brackish water desalination and seawater desalination processes are well established and in common use around the globe to create new water supply sources. The farther the location of the source water from the ocean or seashore, the lower the salinity (TDS) of the water and the lower the osmotic pressure that needs to be overcome when desalinated water is produced. This is one of the major reasons that brackish desalination is often considered less costly than seawater desalination. A number of project considerations, however, indicate that seawater desalination can be beneficial and more cost-effective than brackish water desalination. To make a fair comparison, we need to properly compare all major aspects of both types of projects to define the best and most appropriate desalination technology. While brackish water has less feed water TDS, it is more challenging to dispose of the produced concentrate. Also, although brackish water desalination needs less energy to overcome osmotic pressure, it usually requires more energy to draw the water from the well than it takes to pump seawater from the open ocean intake. Another factor is that the temperature of the brackish well water may be lower than the temperature of ocean water, giving seawater desalination an advantage in energy demand. In comparing brackish to seawater desalination, these major aspects should be evaluated: (1) Locations of seawater and brackish water plants, relative to the major consumers of the desalinated water, (2) Transportation (pumping and disposal) costs of the feed water and produced water, (3) Potential colocation of a seawater plant with a large industrial user (e.g., power plant) of the seawater for cooling or other purposes, (4) Produced quality of brackish water and seawater desalination in terms of major minerals and emerging contaminants, (5) Sustainability of the water source: capacity and depth of the brackish water wells, as well as the type of soil. (6) Technical and economic aspects of produced concentrate disposal, (7) Permitting process costs for brackish and seawater desalination, and (8) The economics of both brackish and seawater desalination treatment processes: capital costs, operational and maintenance (O&M) costs, lifetime water cost, and total water cost (TWC). This paper discusses the major evaluation criteria and considerations involved in properly comparing the economic and technical aspects of brackish and seawater desalination to determine the more favorable desalination technology for a given desalination project.

Exfoliated MoS2 nanosheets loaded on bipolar exchange membranes interfaces as advanced catalysts for water dissociation

Recovery of organic acids with high molecular weight using a combined electrodialytic process

Desalination and disinfection of inland brackish ground water in a capacitive deionization cell using nanoporous activated carbon cloth electrodes

In the essence of the green biorefinery concept an interest in further implementations of wood compounds has gained a lot of attention. Therefore, it is crucial to identify and develop efficient and eco-friendly extraction processes. In particular in a lignin biorefinery plant, electrochemical acidification of Kraft black liquor via electrodialysis with bipolar membrane is considered as a sustainable avenue to acidify the Kraft black liquor and subsequently extract lignin. Even though the application of this acidification technique results in less chemical consumption than the acid precipitation method, the colloidal fouling of the ion exchange (bipolar and cation exchange) membranes, adversely affects its performance. This study was performed to determine the influence of the colloidal fouling and chemical cleaning process on the integrity of the membranes. Four commercially available cation exchange membranes and one bipolar membrane were examined. Membrane analyses, such as thickness, contact angle, io...

The reagent-free electromembrane process of removing carbonates, bicarbonates, and carbonic acid from softened natural carbonate water using an electrodialysis synthesizer EDS-01 with a two-cell unit cell formed by a bipolar membrane and a cation-exchange membrane has been studied. MB-2M membranes modified with an ionopolymer containing phosphoric acid groups catalytically active in a water-splitting reaction have been used as bipolar membranes, while heterogeneous membranes Ralex CMH (Mega a.s., Czech Republic) have been used as cation-exchange membranes. The decarbonization process has been carried out in two stages. At the first stage, a reagent-free correction of pH of the solution treated has been carried out. The value of pH in acid compartments has been adjusted to be 2.5–4.0. At the second stage, this solution has been deaerated with air purified from carbon dioxide. For a quantitative description of the process, a previously developed model has been adapted to describe the electrodialysis process with bipolar and cation-exchange membranes. It is shown that the electrodiffusion transfer of anions through the cation-exchange membrane and bipolar membrane is practically absent, and the change in the concentrations of carbonate ions, bicarbonates, and carbonic acid is due to the quasi-equilibrium chemical reactions. The deaeration of acidified softened water reduces the total carbon content from 5 to 1 mmol/L. The decarbonization of softened water is accompanied by a decrease in the concentration of sodium cations and total mineralization. With an EDS-01 electrodialysis synthesizer performance of 100 L/h, the specific energy consumption is in the range from 0.16 to 6.12 kW h/m3 depending on the current density.

Brackish water desalination using reverse osmosis and capacitive deionization at the water-energy nexus

As the world produces more renewable energy in the need to transition away from fossil fuels, the use of low-temperature electrochemical reactors is an increasingly important topic of research. An enabling component of electrochemical reactors is an ion exchange membrane, which selectively transports either cations, in a cation exchange membrane (CEM), or anions, in an anion exchange membrane (AEM), from one compartment of a reactor to another. When a CEM and AEM are physically combined they can create a bipolar membrane (BPM), which can generate protons and hydroxide ions from water dissociation at the junction of the AEM and CEM when operated in reverse bias, while also reducing unwanted ion crossover in electrodialysis and electrolysis applications. Maintaining the junction interface is critical to the operation of a BPM, however due to the small size of the junction and the similarities of appearance between the AEM and CEM, it is difficult to obtain spectroscopic information about the BPM junctions. Additionally, in the literature the mechanism behind BPM failures are not well reported. These gaps in knowledge make rational design for improved membranes hard to achieve. Here, we use Confocal Raman Spectroscopy (CRS) to spatially observe spectral data throughout the AEM, CEM and AEM/CEM junction of a novel 3D spun bipolar membrane. CRS uses a monochromatic light to initiate and observe Raman scattering from the polymers in each membrane, showing characteristics of the polymer backbone structure, linkers, and ion exchange functional groups, throughout the membrane thickness. The confocal Raman microscope enables spectrum collection from a micron scale voxel, as opposed to the bulk volume, resulting in a nominally non-destructive technique for making spatiotemporal measurements. Using CRS, we identify the BPM junction thickness and homogeneity through 3-dimensional spatial mapping. We also demonstrate the ability of CRS to analyze the method of BPM failure when used in an electrodialysis reactor by obtaining spectral maps of failed membranes, showing junction delamination, ionic buildup in either the CEM or AEM, and ionic polymer breakdowns of the CEM or AEM due to contaminants. In collaboration with experimental collaborators, 3D BPM fabrication techniques were assessed for their impact on overall BPM performance and durability, while demonstrating the non-destructive usage of the CRS technique.

Photovoltaic electrodialysis system for brackish water desalination: Modeling of global process

The desalination of brackish ground water using cascade Rankine cycle is proposed. A pair of a Rankine cycle like steam Rankine cycle (SRC) and organic Rankine cycle (ORC) as a waste heat recovery. The single stage steam turbine for the SRC unit while the scroll expander for ORC unit is selected. Simulation of cascade RO system performance is considered using R245fa as a working fluid for ORC unit. The saturated steam from solar Scheffler disc will expand into steam turbine, where the reject heat from steam turbine will utilize for evaporation of ORC working fluid. The high-pressure RO pumps integrated with SRC and ORC turbines to provide net driving pressure to the RO module. This type of system is well suitable for desalination of brackish water due to moderate working temperature & pressure. Result shows that the pair of Rankine cycle will increase the overall (cascade) efficiency of the system. The basic input parameters are optimised with Taguchi approach. The performance of the system shows a good agreement with variation of mass flow rate of the steam in which the permeate flow rate from RO will increase along with the cycle efficiencies.

Relationships between transport and physical–mechanical properties of ion exchange membranes

Electrodialysis is a membrane technology that is used increasingly in the food sector to concentrate, purify, or modify the properties of foods. Electrodialysis can be performed in a dilution-concentration mode (conventional electrodialysis) when anion exchange and cation exchange membranes are used, or for pH modification when bipolar membranes are used (electrodialysis with bipolar membranes). Advantages of the technology include a modular design, product purification with no dilution, efficiency, pH variation and adjustment with no addition of external solutions, and the lack of requirements for additional thermal treatment. Today, the most important use of conventional electrodialysis is the desalination of brackish water for the production of potable water. However, other applications are gaining increasing importance including large-scale industrial installations in the food industry, whey and molasses demineralization, tartaric stabilization of wine, and deacidification of fruit juices). Electrodialysis using bipolar membranes is also used at the industrial scale to produce organic acid. Other applications currently under development that have potential for future industrial applications include the production of plant protein isolates, production of acid caseinates, fractionation of whey proteins, regeneration of wastewater resulting from food processing, and separation of peptides using an ultrafiltration–electrodialysis integrated process. The chapter summarizes some of the key industrial scale applications of both electrotechnologies in the food sector. Furthermore a few selected applications under development are briefly presented, with emphasis on the applications having the most potential to help in environmental protection.

Desalination of brackish water by nanofiltration and reverse osmosis

Novel ecofriendly cation exchange membranes for low-cost electrodialysis of brackish water: Desalination and antiscaling performance

Most solar energy conversion technologies generate power through transport of energized electron particles; however, the physics that describes these technologies only requires that the particles be charged and not specifically that they are electrons. My research group studies solar energy conversion technologies that generate power from sunlight through ion transport. In my presentation I will report on my research group’s recent demonstration of ion transport against concentration gradients driven by solar illumination of dye-sensitized ion-exchange materials. Mechanistically, visible light was used to drive endergonic excited-state proton transfer from a covalent photoacid-functionalized polymer membrane. Photoacid molecules convert the energy in light into a change in the chemical potential of protons via a weakening of protic functional groups on the photoacid, i.e. a drop in its pK a. As a model system for ion-channels in polymer-electrolyte ion-exchange membranes, dye-sensitized conical nanopores in poly(ethylene terephthalate) (1 – 108 pores/cm2) were investigated. Remarkably, in a region occupied by ~20 zeptoliters (~2 x 10-20 L) of aqueous electrolyte, electrochemistry was used to determine the number of binding groups of the dyes and the ground-state pK a of the dyes, and fluorescence microscopy was used to determine the conditions where excited-state proton transfer occurred. These data were consistent with a hypothesis that pK a values of the photoacids in the ground-state and excited-state were significantly smaller than those measured for dye molecules in solution, likely due to incomplete screening of surface charges in the confined nanopores. Dye-sensitized Nafion monopolar ion-exchange membranes and bipolar ion-exchange membranes were also investigated. Under sunlight-simulated illumination these materials were found to exhibit photovoltaic action, i.e. generation of a photocurrent and a photovoltage. Bipolar membranes are a class of polymeric ion-exchange materials that consist of a monopolar cation-exchange membrane that is in intimate contact with a monopolar anion-exchange membrane. They are unique among the ion-exchange membranes in that they that separate and maintain pH differences across the membrane even during passage of ionic current. Moreover, the physics of ion equilibration processes within these membranes resembles that which occurs during equilibration of semiconductor pn-junctions. This body of work represents an underexplored solar energy conversion process that is being pioneered by my research group to operate via a mechanism similar to that in semiconductor pn-junctions. The applicability and practicality of these materials as standalone ionic photoelectrochemical devices will also be presented.