As part of WWi's technology comparison series, we address the topic of membrane material innovation, particularly graphene. Is this material a game-changer for the energy and economic cost of seawater desalination? We speak to Dow, BASF, LANXESS, NanoH2O and Veolia Water & Solutions Technologies to get their thoughts.
Redesigning membrane microarchitecture
Steve Rosenberg, research fellow, Dow Water & Process Solutions
While seawater desalination is one of the more visible applications, in reality, ultrapure water for production of steam for fossil and nuclear power plants, as well as microelectronics production facilities that are fueling the current mobile device era, have equally led to a quiet revolution of innovation in water purification technologies.
Advances in the design of membrane microarchitecture have enabled precise design of pore structure that can enable water to flow at high rates, while rejecting ionic solutes. At the heart of current advances are novel multilayer structures. Thin film composite technologies that combine a support layer, separation layer, and surface protective layer represent the current state-of-the-art.
At the forefront of fundamental scientific research are new composite structures and new materials, such as graphene. Such materials offer significant potential to improve the precision of separations, while enhancing the durability and breadth of application. From an energy savings perspective, we are very close to the thermodynamic limits of traditional reverse osmosis applications for desalination and need to look into secondary performance attributes and extended applications. Challenges remain for the rejection of neutral solutes, prevention of fouling, longevity in the face of harsh chemicals and applications with extreme temperatures. New composite structures show promise for emerging applications, such as brine concentration.
Unlocking nanotechnology
Dr Graeme Pearce, principal, Membrane Consultancy Associates
Nanotechnology offers enormous potential in a wide variety of applications. Included in the range of nanomaterials, there are examples of porous and dense films with exceptional strength and tailored surface properties which are extremely light or thin, or at a microscopic scale.
These attributes could be of great interest for membranes used in desalination, and the last few years have seen the start of what could be a revolutionary development for desalination and water treatment.
However, commercial progress so far has arguably been slow apart from one notable exception, namely the Nano H2O product. The fear is that membrane media incorporating nanotechnology may be too expensive, or may be impractical from a manufacturing standpoint. Worst of all, it may offer benefits with limited relevance to real world applications.
The key requirement for applying nanotechnology to desalination is to offer selectivity for the separation process of removing salts from water at a significantly lower energy than is currently achieved. Preferably, the technology should be able to tolerate the presence of foulants, both particulate and dissolved organics.
Different approaches have been followed in trying to apply nanotechnology to the needs of desalination. These include the incorporation of zeolite nanoparticles into existing conventional thin film composite RO membranes, and this approach has been successfully commercialised by Nano H2O.
An alternative approach is to use nanotechnology to create novel nano-structures such as graphene and carbon nanotubes, which could be used as a completely novel form of RO membrane with a highly organized ultrathin structure. The idea of these new membranes is to offer very high selectivity since the pore morphology can be precisely controlled. At the same time, permeability is enhanced since the film is very thin, and if water transport through the membrane can be facilitated due to beneficial surface interactions, energy use can be minimized.
Although the nanoparticle approach has made a promising start due to a relatively straightforward path to implementation, it has been difficult to confirm the commercial promise of ultrathin nanotechnology films, since products are challenging to make. However, the importance of energy in desalination provides a driver of overwhelming importance, which will continue to drive these novel product developments.
Cleaning new membrane materials
Jens Lipnizki, head of technical marketing membrane business, LANXESS
Every development for desalination in the future to reduce the total water costs will be based on process optimization, RO element improvement and the development and optimization of the RO membrane. While process optimization was one of the keys to reduce the energy consumption from 12 to around 2 kWh/m3 starting in the seventies, the decline of RO elements costs reduce the maintenance and investment cost.
The robotic manufacturing of the RO spiral wound elements made not only RO more economic it also did improve the quality and the packing density. Still spiral wound elements are far away from a good hydrodynamic design. Also the development of a novel feed channel spacer reduced the pressure drop along the elements. The unequal flow distribution lead to stronger fouling in some region of the element.
Changing the heart of an RO process, with a new kind of membrane, is a critical part. There are composite polyamide RO membranes with extremely high flux on the market, now.
These new element types lead to a trend of mixed installation in one pressure vessel to reduce the energy consumption by an equal workload (recovery rate) per element. New membranes based on carbon nanotube or zeolite technology, which promise much lower operational pressure, may have to work similarly.
Alternatively a new element configuration with lower pressure drop has to be developed for these membranes with a new RO process.
Maybe this will be closer to the design of an ultrafiltration plant, where backwashing and chlorine treatment become an option. This leads to one of the challenging questions: how fouling and cleaning of the new membranes have to be taken in consideration. If a better pre-treatment is necessary, the saving on the energy side may not be compensated.
Therefore only an RO membrane which leads to a lower pre-treatment and energy consumption will be the membrane of the future. Whether these membrane will be successful is dependent on their availability, need of pre-treatment and final costs. Finally, a complete new technology may be the solution to make high quality water available even for the poorest regions. This should be our common task.
Creating smart surfaces
Claudia Staudt, principal scientist, advanced materials & systems research, BASF
Different membrane technologies are well established in drinking water production. The transport mechanism for filtration membranes is typically based on a sieve process whereas for reverse osmosis membranes, water flow is driven by a solution-diffusion mechanism through a dense polymer layer without pores.
Commonly used materials for water membranes are polysulfone, polyethersulfone or polyvinylidene difluoride in ultrafiltration and cross-linked polyaramides in reverse osmosis. In the past, main innovation came from optimization of system design, but innovation on the membrane material can also contribute to address for instance better energy efficiency in ultrafiltration processes.
Surface activation followed by grafting can create smart surfaces with reduced fouling or switchable pore sizes, leading to more effective cleaning. For metal oxides, metal organic frameworks, carbon nanotubes and graphenes it is reported that performance as well as anti-fouling properties can be improved.
These nano-sized materials can be used as additives since suitable formulation technologies exist for homogeneous distribution of inorganic particles in highly viscous polymer solutions. New membrane fabrication processes are also foreseeable.
Isoporous membranes with significant higher fluxes compared to any commercially available membranes can be obtained using self-segregating block-copolymers, aligned carbon nanotubes, two dimensional structures based on graphene oxides or protein structures like aquaporines. However, these materials are still in an early stage and up-scaling as well as large scale membrane production need to be proven.
A question of RO module disposal
Alberto Goenaga, vice president of product and process development, NanoH2O
Forecasting the impact of graphene or any other material on membrane desalination is a difficult task. The road between material discovery and profitable products is fraught with unforeseen challenges and surprises - good and bad.
The history of semiconductors and displays teaches us that membrane desalination will go through periods of steady product performance interspersed with surprising bursts of radical innovation spawned by new materials, processes and product designs. Although new materials (like graphene) have the "potential to revolutionise the industry", these materials will very likely require advances in processing technology for their potential to be realised.
Moreover, the right product with the right value proposition will need to be designed for these materials. As you can see, it becomes difficult to predict the future with specificity.
In addition to materials innovation (and mirroring developments in semiconductors and displays), I believe the membrane industry will evolve towards more precise process technologies.
For example, multilayer thin film formation/deposition can pave the way for more robust and functional membrane architectures. On the product design front, there is ample opportunity to develop more environmentally friendly packages than the standard spiral wound fiberglass shell.
And finally, have you ever asked yourself how will we dispose of millions of spent RO modules?
Ensuring long-term resilience
Jerome Leparc, project manager, desalination research and development, Veolia Water Solutions & Technologies
Beyond the great challenge of developing new materials, a major challenge in bringing these new materials to the final customer lies in their integration into full-scale systems to capture the most savings in energy and cost while maintaining their resilience over time.
While initial laboratory-scale testing on enhanced polymeric membranes or completely new materials may show great promise for reduced energy consumption and/or reduced membrane surface area, the development of a cost-viable product (membrane module and then membrane system) requires many steps, including field testing and optimisation of the hydraulics of the system. Indeed, the foremost priority for a water utility or an industry when investing in a new water production facility is process reliability and resilience over time.
With novel materials allowing higher water flux at low energy, the key to fully transfer the efficiency of these materials into process reliability resides in the hydraulic balancing within the desalination system and in the water quality feeding the system – i.e. optimised process integration.
