Water. Desalination + reuse
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A new paper by a team of researchers brought together by the US Nanotechnology- Enabled Water Treatment (NEWT) Research Centre has investigated the potential for membrane distillation (MD) for desalination. The main driver of research into MD so far has been its ability to use low-grade heat such as solar, geothermal, or waste heat from cooling tow - ers. The downside is that the process, in which water vapour diffuses through a membrane leaving the salt on the warmer side, is also a low performer in terms of energy efficiency. "There has been interest in MD because potentially you can run it off-grid and using sustainable heat sources, but if the efficiency of the process is too poor, people won't use it," explains Akshay Deshmukh, PhD candidate in the Elimelech Research Group, at the Department of Chemical and Environmental Engineering, Yale University. Industrial application That said, at smaller system sizes, MD is more energy efficient than other thermal processes such as multi-effect distillation (MED) and multi- stage flash (MSF). Partly, this is because MED and MSF are more efficient at large scale, and partly because MD is modular, enabling greater flexibly in operation. Another potential benefit of MD is that as an evaporation-based process it can handle high-salinity feed-waters more easily than reverse osmosis (RO). This combination of flex - ibility in operation, ability to handle hyper-saline water, and potential to run off-grid lends itself to applications in treating fracking-produced water. A further reason why the MD process might be suitable for treating industrial wastewater is because there is no hydraulic pressure involved and therefore fouling tends to be reduced. "In principle, you can take down the operation for a short period and a cross-flow of clean water gives the sheer force needed to take foulants off the mem - brane," says Chanhee Boo, PhD candidate in the Department of Chemical and Environmental Engineering, Yale University. As yet, there has been no real-world demonstration of MD in such an industrial application, however the paper points to its potential applicability. The process may also be useful in combination with RO, perhaps as a brine treatment system. The researchers further inves - tigated whether developments in membrane or system design might improve performance. "The ideal membrane would be highly porous and have the right thickness. Performance rises consistently as membrane poros- ity increases, but the membrane can't be so porous that liquid goes through, so this sets an up- per boundary," says Deshmukh. The paper also considers novel designs such as solar MD, which provides energy efficiency gains, but overall "there is work still to be done on process design to improve efficiency," Deshmukh says. Another potential devel- opment is a next generation "omni-phobic membrane," says Boo. "This could repel oil and water, preventing the membrane from blocking or wetting." Such a membrane might have proper- ties more advanced than Teflon, the polytetrafluoroethylene (PTFE) product used in non-stick saucepans. Deshmukh concludes that while there are likely benefits from future research into mem - brane and process design, MD is niche. "We don't think it could ever compete with RO. The real- istic applications are small scale, off-grid, perhaps using cheap solar collectors, or geothermal, to get the heat. As well, indus - trial wastewater from fracking, again in off-grid areas where volumes vary significantly," he says. "However, the questions re - main: Is that waste heat actually available; can these new process designs hit the efficiencies that are needed; and will new materi - als be able to keep out the bad stuff in shale water?" Membrane distillation: where are we now? Water Research Foundation seeks potable water technologies The Water Research Foundation (WRF), US, is calling for drinking water technology providers to submit their latest innovations for inclusion in the Leaders Innovation Forum for Technology (LIFT) Technology Scan programme. Proposers are invited to highlight the key aspects of their technology or process for review by a panel of experts through LIFT. Accepted technologies will be invited to present to utilities, consultants and other WRF subscribers, and will be included in the LIFT Link online technology innovation and collaboration platform. Areas of interest include, but are not limited to: Advanced oxidation processes, asset management, bio- filtration, coagulation, disinfection, distribution networks, energy efficient devices, filtration, lead and copper management, leak detection, disinfection byproducts, membranes, microbiology, nitrate recovery, PFAS/PFOA removal, remote monitoring, scale Metric Energy efficient High-salinity feed-waters Utilising low- grade energy Fouling resistance MD RO MED MSF MVC How membrane distillation measures up against other desalination processes 26 Far Site June 2018 Water. desalination + reuse FAR SITE