Capacitive deionization employs an electric field to attract ions and trap them at electrode surfaces. When the surfaces are saturated, they are electrically regenerated, driving ions into a small, concentrated reject stream. This technology is particularly effective for treating non-traditional waters of relatively low ionic strength (i.e., dissolved solids <5 g/L), because it operates at ambient pressure and temperature and does not require the use of membranes. Although capacitive deionization systems are becoming more popular, technological innovations are needed to drive market penetration for treatment of unconventional waters. This research project will develop the technology for the next generation of enhanced capacitive deionization systems. In the initial phase, we will develop low-cost mass-manufacturable electrodes, made of charge-trapping polymers. These new electrodes will rely on in situ electrochemical regeneration in place of traditionally used chemical regenerants. Coincident with these low-cost electrodes, the overall efficiency of the system will be enhanced through the use of pulsed electrical fields (i.e., the Wien effect). Our preliminary research demonstrates that the use of short, high-voltage pulses can improve performance by temporarily increasing ion mobility without causing redox reactions. After prototype development, we will test the new treatment module on authentic brackish groundwater and other non-traditional water sources.
To support the use of non-traditional waters in cooling systems with minimal scaling and brine production, we will develop a novel stacked graphene oxide (GO) membrane that enables effective removal of divalent cations as well as in-situ regeneration for ultimate fouling control. The two-dimensional (2D) GO nanosheets to fabricate these membranes can be mass-produced from inexpensive graphite and is capable of enabling ultrafast water flux, low pressure operation, and selective removal of divalent cations. As schematically illustrated in Figure 1, we can tune the charge properties and interlayer spacing of the stacked GO membrane to achieve high removal of divalent cations, anions, and organics while only allowing monovalent ions to go through. The stacked membranes also have a unique property of in-situ regeneration that provides an effective new strategy for scaling/fouling control.
Wei, Y.; Wang, J.; Li, H.; Zhao, M.; Zhang, H.; Guan, Y.; Huang, H.; Mi, B.; Zhang, Y., “Partially reduced graphene oxide and chitosan nanohybrid membranes for selective retention of divalent cations,” RSC Advances 2018, 8, 13656-13663.
Context: Forward osmosis can achieve water quality treatment objectives at a fraction of the energy cost of conventional reverse osmosis systems but its efficiency needs to be improved to enable wide-scale technology deployment.
Objective: Create ionic liquids-based forward osmosis systems to treat non-traditional waters that can take advantage of waste heat sources common to many energy production processes e.g., geothermal. Design and test a prototype reactor that can use low-grade waste heat to achieve its cost objective.
Forward osmosis is a membrane-based separation process that uses the osmotic pressure gradient between a concentrated draw solution and a feed stream to drive water flux across a semi-permeable membrane. Forward osmosis can achieve water quality treatment objectives at a fraction of the energy cost of conventional reverse osmosis systems, especially for high ionic strength waters. While forward osmosis desalination has achieved some market success, its efficiency needs to be improved prior to wide-scale technology deployment. The main challenges are to identify appropriate draw solutions that can create enough osmotic potential to power the trans-membrane transfer and that can be easily removed in a second process. Building upon recent work, we will develop a new approach to desalination that uses ionic liquids as draw solutions for forward osmosis coupled with the use of low-grade heat in the subsequent ionic liquid separation process. This allows us to create forward osmosis systems to treat non-traditional waters that can take advantage of waste heat sources common to many energy production processes (i.e., many ionic liquids exhibit a lower critical solution temperature phase transition with water in which the separated ionic liquids and water phases become miscible upon cooling). During the project’s initial phase, we will identify ionic liquids that are suitable for applications with non-traditional waters. We will then focus on developing a dual-channel reactor that can take advantage of ionic liquid-based phase change materials. The prototype reactor will be tested with well-defined solutions and non-traditional waters collected before and after pre-treatment using the methods described for project 2.7. As indicated above, this project not only addresses desalination, but also is “tunable” and can use low-grade waste heat to achieve its objective.
Context: Desalination results in the production of brines that are difficult to manage and expensive to treat.
Objective: Use natural treatment systems to remove contaminants prior to additional water recovery or crystallization of brines from different sources.
The use of desalination processes to treat non-traditional waters typically produces a concentrate stream that contains high concentrations of salts, organic contaminants, and toxic inorganic constituents (e.g., boron, copper). When deep-well injection or ocean disposal of such streams is infeasible, energy-intensive, zero-liquid discharge processes are needed that offer cost-effective alternatives for removing contaminants from the streams while simultaneously reducing the volume of concentrate. We will adapt a previously developed, managed natural treatment system that efficiently removes organic contaminants, nitrate, and other contaminants from municipal wastewater effluent to address the conditions encountered during the treatment of concentrates generated from desalination of non-traditional waters. This research will use well-defined solutions, as well as concentrate streams produced by actual treatment systems as well as streams produced by the modular technologies developed in other CERC-WET projects. We will begin by assessing the growth and activity of bacteria and photosynthetic diatoms in waters that contain high concentrations of salts and toxic constituents. We will assess the potential to recover minerals that precipitate at the elevated pH conditions that develop as the algae remove carbon dioxide from the water. Later, our research will focus on integrating real-time controls to optimize system performance and recover minerals, nutrients, and organic materials.
Context: Large-scale management of non-traditional water sources (water reclamation/reuse, industrial water, etc.) is critical in arid and semi-arid regions (e.g., Southwester US, Western China, MENA, etc.).
Objective: Develop and test structured dynamic models for energy, carbon footprint, and costs to compare scenarios of non-traditional water management, identify metric scenarios, understand the role of regional factors, and support technology selection.
Salts are a limiting factor in the management of non-traditional waters. The presence of salts at elevated concentrations in cooling water or municipal water supplies can incur significant costs (e.g., through enhanced corrosion or scaling). In agriculture, salts diminish agricultural productivity and limit the types of plants that can be grown. Our previous work has shown that structured dynamic models provide a basis for decision-support in the development of management strategies in the water sector. To adapt these tools to conditions relevant to the management of non-traditional waters, we will evaluate spatially distributed data related to water quality and use, energy intensity of water, power generation, water intensity of crops, unit costs, and tariff structures. The resulting dynamic process models will be used to quantify different scenario indicators (e.g., energy footprint, carbon footprint, life-cycle cost, life-cycle assessment) at a systems level and to critically evaluate the differences in their adoption and results. We will first target the development of an integrated model that will inform the technology development projects (2.1-2.4) and identify strategies to overcome key barriers to efficient use of unconventional waters in California and other Western states. After initial model development, we will team with our Chinese partners to extend the model to conditions encountered in China.
Context: Application of appropriate pretreatment steps can reduce the cost of desalination and improve operational sustainability.
Objective: Characterize the biogeochemistry of unconventional waters and develop predictive models and other tools needed to inform treatment train design and avoid negative outcomes such as membrane fouling.
Non-traditional waters often contain complex mixtures of dissolved ions, natural organic matter including petroleum hydrocarbons, and anthropogenic chemicals such as biocides and surfactants. Treating non-traditional waters requires a fundamental understanding of geochemistry in order to optimize the efficiency of treatment processes and minimize negative outcomes such as membrane fouling. To inform the development of the treatment modules (projects 2.1-2.4) and the systems optimization efforts (project 2.5), we will characterize the composition of unconventional waters from locations throughout the U.S. and China. After a literature review and analysis of existing databases, analyses and testing of unconventional waters collected from field sites will be used to characterize the geochemistry of non-traditional water sources and identify candidate pre-treatment approaches. After our screening studies, we will design and test biological and chemical pre-treatment approaches using non-traditional water collected from different regions with in USA and China. In close coordination with projects 2.1-2.4 and our Chinese collaborators, we will identify approaches for optimizing the overall performance of the treatment trains for different applications that are relevant to the expected uses of the treated water.
Context: Recovery of fresh water from brackish water desalination by reverse osmosis (RO) technology is limited by mineral scale formation.
Objective: We propose to inhibit RO membrane scaling by altering membrane surface chemistry to enable higher recoveries to be achieved.
The goal of this subtask is to advance and field-demonstrate the use of novel surface nano-structured membranes to improve operations of high recovery desalination, particularly with respect to membrane fouling/scaling resistance and membrane cleaning effectiveness. Relevant to high recovery desalting, the research will focus on high salinity brackish water with high fouling/mineral scaling propensity, building on UCLA’s technology of surface nanostructured membranes for low salinity brackish water sources.
Context: The performance of membrane distillation and other desalination technologies can be improved through a detailed understanding of the solution-membrane interface.
Objective: We will test the wetting mechanism of hydrophobic membranes at the atomic scale using molecular dynamics simulation methods.