Advances in membrane technology provide one of the most promising areas in water and wastewater treatment today. While the brackish and seawater desalination market has seen a rapid uptake in membrane technology in the past, using membranes for treatment and advanced separations has now become more common across municipal and industrial markets. Membranes are passive elements that provide a barrier against contaminants, but a key advantage that technology providers are trying to exploit today is their ability to provide an alternative to high-energy separations such as distillation, for instance, to separate salts, concentrate brine, or dehydrate solvents. A good example of this is the desalination market, where thermal desalination capacity has seen a steady decline in comparison to membrane-based technologies that dominate with over 62% of the market.
Innovations across membrane technologies can be seen in both organic- and inorganic-based materials. Organic, or polymer-based, membranes remain the dominant technology in the market. Manufacturers generally use polymers, including polyamide (PA), polyethersulfone (PES), PVDF, polyacrylonitrile (PAN), or, in some cases, cheaper polymers such as polyethylene, for most ultrafiltration (UF) and microfiltration (MF) membranes. The first seawater reverse osmosis (RO) membranes were made from hollow fiber cellulose acetate, which proved robust to cleaning chemistries and oxidants such as chlorine, but had a lower specific flux and salt rejection than thin-film composite polyamides. Nanofiltration (NF) membranes are also made from similar polymers with some modifications to improve hydrophilicity for low-pressure operation, ion removal, and to improve biofouling.
Advances in membrane materials
Today, several technology providers are working with novel materials that boast properties like higher durability and wider operating temperatures and pH range, and are more robust to cleaning. While these material can be several times more expensive than existing membranes, they do open up new markets for the technology or in some cases provide a stronger differentiator in crowded ones. Below are examples of such startups:
- Hydrop – Performance-enhancing additives for desalination membranes. We recently interviewed Israel-based Hydrop. The company was established in 2015, with seed funding from the Hutchison Kinrot and the Office of the Chief Scientist to develop and test new RO membranes. It uses a novel peptoid-based material embedded between the rejection layer and the membrane support, which can increase membrane flux by 30% without any decrease in salt rejection. The process can fit in existing roll-to-roll manufacturing processes. Hydrop is working with traditional PES-based substrates to produce 50 cm2 prototypes but hopes to find a strategic partner to pilot test at larger scale. The company is also testing different formulations (see patent) to improve the membrane’s robustness to oxidizing chemicals commonly used for cleaning and operating at temperatures above 45 ºC, common for existing RO membranes.
- Whitefox Technologies offers hollow fiber polyimide (PI) and polybenzimidazole (PBI) membranes for removing water from solvents and chemicals. They can operate at high pressure and temperatures as high 150 ºC (for PI) and 450 ºC (for PBI). The company has had success in replacing molecular zeolite sieves used in dehydration of biofuels and pharmacuetical ingredients, providing savings in water and energy as well as opening up plant capacity upstream by reducing the need to recycle streams back into distillation columns.
- Saltworks Technologies, which is known for its patented crystallizer technology, has recently boosted its manufacturing capabilities to produce 100,000 m2 per year of ion exchange membranes. It developed a modified inorganic polymer membrane – polyamide acrylate functionalized with a acrylamide coating – that is highly selective to monovalent ions. While the company did about $20 million in revenues for 2016, largely from crystallizer sales, CEO Ben Sparrow expects a majority of future revenues to arise from its electrodialysis-reversal (EDR) and EDR-RO system and membrane sales. The company is also targeting a unique application in the municipal wastewater market, where its EDR system can remove ammonia from wastewater centrate and convert it to nitrogen gas. This opens up wastewater capacity upstream and helps treatment plants remain in compliance for nutrient discharge. While the process can be energy intensive, the technology is applicable to most high-concentration ammonia wastewaters like those found in refining, chemicals, and landfill leachate treatment, which currently require complicated biological dentrification steps.
- Trevi Systems recently signed a distribution agreement in the chinese market for its polyglycol-based, high-osmotic-potential draw solution for forward osmosis (FO). While the company has traditionally used TFC membranes from supplier Toyobo, it is now on the verge of commercializing its own PBI membranes that can operate with fewer cleaning cycles for difficult-to-treat industrial wastewaters and the zero-liquid-discharge market. CEO John Webley claims the membranes are likely to cost 15% more than alternative TFCs, but will foul less often.
- New Zealand-based Hydoxysys is working with a much lower cost substrate – polyethylene – upon which it grafts a thin layer of SPEEK or sulfonated poly(ether ether ketone). The company has developed a roll-to-roll manufacturing process that can add a thin rejection layer of different pore sizes to produce – UF, NF, RO, and FO membranes. While technology development has been slow at Hydroxsys, it is bypassing the low-profit water markets and targeting specialized applications in fats, proteins, whey separation, and dewatering in the dairy industry.
The water industry is conservative and a slow adopter of new technology, particularly the municipal market where most operators prefer to trade costs over incremental performance improvements. However, the large volumes and a global push towards nonpotable reuse makes this market very hard to ignore for a membrane company. Advancements in pretreatment for desalination and overall recovery improvements are key areas to watch in this market, whereas the demand for next-generation membranes in the industrial markets is being driven by water sourcing issues, lower energy consumption, smaller footprint, and the economics of small-scale wastewater reuse.
By: Abhirabh Basu