"Membranes offering greater specificity in separation could also offer improved water management"
Membranes are playing an increasing role in water and wastewater treatment, desalination and water recycling. Emerging design and fabrication technologies hold the promise of more sustainable membrane-based approaches.
Membrane research for water applications has enabled the development of materials and fabrication methods that have resulted in enhanced properties, leading to improved efficiency of membrane-based processes, with less energy consumption and overall reduced costs. To learn more about the latest advances in membrane technologies and their implications for the use of non-conventional water resources we spoke with Professor Stephen Gray, the Executive Director of the Institute for Sustainable Industries and Liveable Cities at Victoria University, whose research focuses on water and membranes and the impact on current challenges in urban and industrial water.
Can you tell us briefly about your career path and how you became involved in water research?
After completing a PhD in mineral processing and surface chemistry, I was fortunate to receive a post-doctoral fellowship position and then an ongoing position in the CSIRO’s water research group. This team focused on water treatment technologies and developed Magnetic Ion Exchange (MIEX) material that was commercialised by Orica (now IXOM) as well as the Sirofloc technology and the readily available COD meter (RACOD meter). This team was composed of water and polymer chemists, engineers and biological wastewater treatment researchers, so provided great experience in working with other STEM disciplines.
High recovery and resource recovery have become more prominent, and advances in membrane technologies are enabling those objectives
Later, CSIRO set up a broader multi-disciplinary team of researchers that became the “Water for a Health Country” research programme, and I became a member of this programme. This programme had a broader remit than just water treatment and included water resource and asset management, as well as economic and social aspects of water management. It provided experience in working with social scientists as well as STEM scientists, and the applied nature of CSIRO research led to a focus on research impact.
I moved to Victoria University in 2006 to set up a water research programme and established a multi-disciplinary research programme that focuses on research impact, addressing urban and industrial water challenges.
What have been the most significant advances in membrane technologies in the past few years?
Micro (MF)/ultra-filtration (UF) and reverse osmosis (RO) have been present for many years and they have enabled water recycling and seawater desalination to become viable. More recently high recovery and resource recovery objectives have become more prominent and advances in membrane technologies that are enabling these objectives to be more readily met are: ultra-high pressure RO and ultra-high pressure energy recovery devices. These advances have extended the salt concentration range for which membranes can operate economically, and can outcompete alternative non-membrane water recovery approaches. I expect to see many high-recovery RO processes implemented over the coming years.
Small and uniform pore size UF and NF membranes are available on the market, and enable reliable removal of viruses as well as bacteria
As water scarcity leads to the treatment of poorer water quality sources, then membranes that can cope with high solids loads and high foulant loads are required. Ceramic MF/UF membranes are now reaching economic viability and offer greater abrasion resistance and have the ability for more aggressive cleaning to recover from fouling. I expect more ceramic membrane plants installed as costs continue to decrease. A technological advance that is enabling this is the PWN Technologies CerMac housings that can include up to 90 ceramic membrane elements within one housing. Similar housing ideas are now also being used for RO membranes e.g. the Veolia barrel.
Small and uniform pore size UF and nano-filtration (NF) membranes have also become available on the market, and have enabled reliable removal of viruses as well as bacteria. These membranes are finding application for water recycling without RO where salt removal is not required, and potentially to achieve colour removal without chemical addition. The control of pore size is enabling simplified process flowsheets and avoidance of unnecessary production of salty concentrate.
You participate in research teams working on different membrane materials. Could you give us some highlights of that research, and what are your expectations for their commercialisation?
We have undertaken research on increased abrasion resistant polymer UF membranes that led to significantly improved abrasion resistance
We have undertaken research on increased abrasion resistant polymer UF membranes that led to significantly improved abrasion resistance. The focus was on enabling polymer UF membranes to perform better in applications such as seawater RO pre-treatment where sand particles are more abrasive than organic particles found in wastewater streams. The objective was to increase the service life of polymer UF membranes in seawater applications to similar or extended lifetimes for UF in wastewater applications. Our research has indirectly led to one producer offering greater abrasion-resistant UF membranes.
We have also undertaken research on high-flux hydrophilic pervaporation membranes and anti-wetting membrane distillation membranes. The application for such membranes is longer term when thermally driven membrane processes for higher water recoveries (following ultra-high-pressure RO) become economically viable. Such membranes will improve process performance in the drive towards zero liquid discharge processes.
Can you comment on trends in membrane monitoring technologies?
The use of data and artificial intelligence is increasing in the water industry and its use is driving improved operating performance. Software that uses operating performance data to optimise energy use for seawater desalination plants and for control of RO systems dealing with variable salinity water sources is available. However, there is also a trend for more sensors to be installed, with wirelessly connected salinity sensors now available for installation on individual RO elements. Early-stage scaling sensors to identify the need for cleaning prior to detection via operating variables have also been considered, but are yet to reach the commercialisation stage.
We are working on membrane integrity monitoring technology to ensure RO and UF virus rejection during operation that will verify pathogen rejection performance for recycled water applications.
There is a trend for more sensors to be installed, with wirelessly connected salinity sensors now available for individual RO elements
What would be the most pressing research needs for the treatment and use of non-conventional water resources, such as water reuse and desalination?
Municipal water reuse and desalination are widely practiced commercially, so there is no impediment to their immediate application. However, improvements in performance could enable improved economic performance, more reliable treatment and reduced chemical requirements. For instance, online membrane integrity monitoring in water recycling applications would enable high Log Removal Values (LRV) to be claimed for the RO stage, and hence reduce the number of unit processes required for the water recycling process. Similarly, a better understanding of micro-contaminant rejection may enable the operating performance of RO membranes to be used to verify micro-contaminant rejection in a similar manner to what is now used for pathogen rejection.
Membranes that better mitigate silica fouling can enable higher water recoveries and alleviate the need for dangerous cleaning chemicals
The trend towards high recovery RO means the treatment of higher salt concentration streams, and hence there is potential for increased scaling. Silica scaling is particularly problematic and often cleaning of silica scale from membranes requires hydrofluoric acid. Hence, membranes that better mitigate silica fouling could enable higher water recoveries and alleviate the need for handling and managing dangerous cleaning chemicals.
Generally, MF/UF and RO operate well but are not specific in their separations. Membranes that offer greater specificity in separation could also offer improved water management. For example, saline municipal wastewaters are often high in sodium and have high sodium adsorption ratio (SAR). Membranes that are better able to separate monovalent and divalent membranes could enable such wastewaters to be more readily used for agricultural irrigation by lowering the SAR to values suitable for unrestricted land application. Specific separation could also enable industrial recycling of compounds in waste streams at a more localised level – e.g. ammonia specific membranes to enable recycling of ammonia for reuse rather than disposal in wastewater, or the use of membranes for recovery of specific compounds e.g. lithium specific membranes are being developed.
How has the development of new technologies and the evolution of their cost affected the global use of non-conventional water resources? Can you comment on the differences between different world regions?
Seawater desalination and municipal water recycling have been the two most prominent technologies recently implemented to address water scarcity challenges and membranes play a leading role in both these processes. Cost reductions of membranes and their increased energy efficiency and availability of energy recovery devices have enabled the growth of seawater desalination and water recycling.
The implementation of seawater desalination or water recycling in various regions of the globe appears to not just rely on economics and available water sources, but also on local politics. For instance, in Australia, many large seawater desalination processes were implemented during a period of drought around 2010, but no potable water recycling systems were introduced until recently. This is because water recycling plants usually require the community to be accepting of drinking recycled water and this can take years of community engagement to achieve. The time available between recognition of the requirement for a new water source and the need to construct a plant did not allow this to occur in Australia, and desalination plants were installed without the need for community consultation.
The water sector is beginning to focus on resource recovery and energy production, and membranes are likely to play a role
Conversely, in the U.S. the move has been towards potable recycling rather than seawater desalination largely due to environmental concerns restricting the implementation of seawater desalination plants in the U.S. There have been examples where it has taken a decade to get environmental approval for a desalination plant to receive planning approval due to concerns about the impact of seawater desalination plants on marine wildlife.
In the Middle East, seawater desalination has also been widely implemented, while in Israel the approach has been to use desalinated water for municipal purposes and to use recycled wastewater for agricultural purposes.
What do you think will be the impact of evolving membrane technologies on the water treatment market in the coming years?
The number of seawater desalination and water recycling plants will continue to increase as water security challenges remain a challenge with continued climate change. Membranes will continue to play a leading role in these processes, with desalinated water being used for agricultural as well as municipal applications. Additionally, water from industrial uses (e.g. mining, process industries) also has the potential to be incorporated into recycled water streams, so membranes with their ability to reliably make high-quality water will enable water from any source to be considered for any application.
Additionally, the water sector is also beginning to focus on opportunities for resource recovery and energy production, and again membranes are likely to play a role in meeting these objectives.