Research at the Nexus

Cross-sectional images of FO membranes with the optimal finger-like pore structure in the membrane support layer (left) and with electrospun nanofibers incorporated into the support layer (right).

Forward Osmosis for Water Production

Forward osmosis (FO) is a membrane-based separation process that utilizes osmotic pressure as the driving force for water permeation. FO has the potential to produce water more sustainably and use less energy than conventional desalination processes. Our ongoing research in the application of FO for water production builds upon work from our lab on the use of a recyclable ammonia-carbon dioxide draw solution to generate the osmotic driving force for FO. Our current FO research emphasizes membrane design, surface modification, and characterization; expanded applications of FO for water production; and FO for resource recovery.

FO Membrane Design, Surface Modification, and Characterization. We study improvements in the design of thin-film composite membranes that translate to better membrane performance in forward osmosis. Our work has optimized traditional phase inversion membrane casting techniques to promote mass transfer in the membrane support layer, and we are investigating the use of electrospun fibers for fabricating stronger and thinner membranes.

We research different approaches for modifying the surface of FO membranes to improve their resistance to fouling. Organic fouling, biofouling, and inorganic scaling reduce membrane performance, consume energy for cleaning and removal, and shorten membrane life. Our work has demonstrated the effectiveness of membrane surface modifications using poly(ethylene glycol) polymers and nanomaterials to reduce fouling.

Accurately characterizing membrane water and salt transport properties is a crucial part of our research to improve FO membrane design and performance. We have developed several membrane characterization techniques that allow us to assess membrane properties under realistic operating conditions and to gain new insights into the effects of membrane design improvements and surface modifications on membrane performance.

Expanded Applications of Forward Osmosis for Water Production. FO is advantageous for pretreating the feed water in a conventional desalination process because of its low fouling propensity, high contaminant rejection, and low energy consumption. FO may also be applied to treat the waste streams generated from desalination processes to reduce their environmental impact. We study the potential benefits of integrating FO into conventional water production processes for improved water quality, reduced energy consumption, and improved sustainability.

Schematic illustrating the combined forward osmosis and membrane distillation process for recovering phosphorus-containing struvite from wastewater solids (collaborative work with Professor Long Nghiem, Wollongong University, Australia).

We also investigate the application of FO for water production from unconventional feed waters, such as production of recycled water from wastewater and treatment of high-salinity produced water from oil and gas operations. With Professor Hokyong Shon, our research collaborator in Australia, we study fertilizer-drawn forward osmosis, or fertigation, which uses fertilizer as a draw solution to treat brackish groundwater and produce nutrient-rich water suitable for agricultural irrigation.

Forward Osmosis for Resource Recovery. In addition to producing water, FO may also be used to recover valuable mineral and nutrient resources. In collaboration with Professor Long Nghiem at Wollongong University in Australia, we are studying the application of FO for “sewer mining,” which is the direct treatment of municipal wastewater to recover water for recycling and to concentrate wastewater solids for subsequent processing and disposal. We are also researching the recovery of valuable phosphorus minerals from wastewater solids using a process that combines forward osmosis and membrane distillation.

Membrane-Based Processes for Energy Generation

Natural Salinity Gradients are an Untapped Energy Resource. When fresh water mixes with the sea, free energy equal to a 270 m high waterfall is released. With an annual global river discharge of 37,300 km3, natural salinity gradients represent an enormous source of renewable energy that can potentially meet 13% of global electricity demand. Pressure retarded osmosis (PRO) uses a semipermeable membrane to harness this free energy of mixing. PRO can also be used with anthropogenic waste streams, such as concentrated brine from a desalination plant, to reclaim energy and reduce the environmental impact of discharged water.

We investigate membrane-based energy generation through both experimentation and modeling. The fabrication of advanced membranes tailored for PRO is critical for the success of the process. By optimizing the permeability and structural properties of membranes, we have shown dramatic improvements in performance. Fouling from organic matter can severely reduce the effectiveness of the system. We examine the mechanisms of fouling in PRO and develop methods to mitigate fouling.

The Osmotic Heat Engine. PRO can be used in a closed-loop system to generate power using low-temperature waste heat from power plants, industrial facilities, and geothermal wells. In this system, called the osmotic heat engine, waste heat is used as the energy source in membrane distillation (MD) to separate an input salt water stream into two output streams, one with high salt concentration and one with low salt concentration. After separation, the two streams are recombined and the energy released upon mixing is captured using PRO. The mixed saltwater stream is then sent back to the MD system utilizing the waste heat source, allowing the process to begin again. This system can considerably increase the efficiency of power generation systems.

We are working with Dr. Tzahi Cath at the Colorado School of Mines to develop a pilot closed-loop PRO-MD plant. For this project, we fabricate robust membranes for PRO and MD, investigate innovative module designs, and explore the use of different engineered draw solutions. We also conduct modeling of the entire osmotic heat engine system to understand the cost and energy efficiency of the process.

Severe biofilm formation in membrane modules often leads to their disposal.

Membrane Biofouling

Biofouling: An Inescapable Truth. Bacteria will always attach to and grow on membrane surfaces, forming a biofilm. This process, termed biofouling, often deteriorates membrane performance by decreasing separation efficiency and product water yield. We study biofouling dynamics on the surfaces of membranes used for reverse osmosis, forward osmosis, and membrane distillation.

We have shown that biofouling of reverse osmosis membranes occurs primarily through two mechanisms. First, the attached bacteria cells themselves increase the osmotic pressure near the membrane surface through the phenomenon of “biofilm-enhanced osmotic pressure”. Second, bacteria produce extracellular polymers that inhibit water flow through the membrane. Both of these mechanisms work in tandem to decrease the efficiency of reverse osmosis and increase the cost of the process.

Although biofouling in reverse osmosis has been studied for several years, biofouling in forward osmosis and membrane distillation are not as well characterized. We are working to characterize these biofilms and compare them to the well-studied biofilms in reverse osmosis. To do this, we use natural microbial communities and model organisms. We characterize biofilms with, among other techniques, confocal laser scanning microscopy and atomic force microscopy. These techniques allow us to observe hydrated biofilms in their natural environment. We hope to use the information gained in these studies to mitigate membrane biofouling.

Combating Membrane Biofilms: Developing a Toolkit. Biofouling may be mitigated using a variety of strategies. These strategies may employ specific operating conditions, membrane surface modifications, or optimized cleaning protocols. We have used several of these strategies to prevent biofouling of reverse osmosis and forward osmosis membranes.

Biodegradable capsules covalently bind to a membrane surface. Modified membranes show less biofouling (biovolume).

Nanoparticles may be attached to the membrane surface using either electrostatic forces or covalent bonds. We have developed methods for attaching a variety of inorganic (for example, silver, copper, and silica) nanoparticles to the surface of membranes. These particles either change the surface chemistry or release biocides to prevent biofouling. We have also fabricated biodegradable capsules that release biocides in a controlled fashion. These types of modifications could aid in combating several types of membrane fouling.

Schematic of a thin-film composite polyamide membrane functionalized with graphene oxide nanosheets. The graphene oxide layer inactivates bacterial cells upon contact.

Environmental Applications of Nanomaterials

Nanomaterials have unique properties that can differ significantly from bulk materials of the same chemistry. We are interested in utilizing these properties, through the engineered application of nanomaterials, to address environmental challenges. Specifically, we study the incorporation of nanomaterials into membrane technologies for fouling control, performance enhancement, and fundamental transport studies.

Through post-fabrication modification of membranes with nanomaterials, properties such as membrane cytotoxicity and hydrophilicity can be augmented without disrupting other membrane properties. These types of fine property modifications can be highly advantageous for reducing membrane propensity for organic and biological fouling.

Schematic of an aligned carbon nanotube membrane.

Our research in this area includes membrane functionalization with biocide-releasing nanomaterials (silver, copper, polymer particles) for antimicrobial effect, superhydrophilic surface-tailored nanoparticles for reduced organic fouling, and carbon nanomaterials (carbon nanotubes, graphene oxide) for non-depleting, contact-action bacterial inactivation. Ongoing research is being conducted on the applications of nanomaterials for biofilm mitigation in membrane technologies.

We also work in collaboration with Professor Chinedum Osuji on the fabricationof aligned carbon nanotube (CNT) membranes by magnetic field directed assembly of polymerizable lyotropic liquid crystalline templates. In these membranes, nanotubes serve as the pores for size-based separations. Aligned CNT membranes are of interest for the study of fluid transport in confined, rigid geometries. Additionally, CNTs have atomically smooth cores, which enable enhanced water transport properties.

Water, Sanitation, and Health in Developing Countries

Nearly 1 billion people lack access to safe water and nearly 2 billion are without adequate sanitation. Over 2 million deaths a year are attributed to unsafe water, mostly due to waterborne diarrheal diseases. Ninety percent of those who die from diarrheal diseases are children in developing countries.

Whereas centralized systems have not been able to grow rapidly enough to serve rapidly growing urban populations, nor become economically feasible in rural areas, decentralized filtration technologies offer an alternative. We have investigated decentralized membrane-based water depots and point-of-use filtration technologies that can be used in developing countries to offer a short-term solution, while taking advantage of rapidly developing technologies. To determine their feasibility in these contexts, technologies need to be more than appropriate technically solutions; they need to work within the local economic contexts to reduce disease.

We have also carried out research to analyze the effectiveness of water, sanitation, and hygiene interventions on reducing water borne diseases, and to determine which environmental indicators are most closely associated with the diseases. In addition, we monitored urban children to understand the associated reduction in diarrhea prevalence associated with consumption of water from decentralized membrane-based water refill stations.

Last updated: 7/21/2014