Final Report: Forward Osmosis Water Purification Membranes for Small OperationsEPA Contract Number: EPD12023
Title: Forward Osmosis Water Purification Membranes for Small Operations
Investigators: Tsai, Chung-Yi A
Small Business: T3 Scientific LLC
EPA Contact: Manager, SBIR Program
Project Period: March 1, 2012 through August 31, 2012
Project Amount: $80,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2012) RFA Text | Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Drinking Water Treatment and Monitoring
According to the World Health Organization, 1 billion people do not have access to clean, piped water. In arid parts of the United States and many other countries, groundwater resources are already dwindling, and supplies that remain are becoming increasingly brackish. Environmental concerns have drastically limited the building of new dams in recent decades. Such concerns have made desalination a fast-growing alternative, but desalination faces its own problems. The two technologies at the heart of conventional desalination plants, evaporation and reverse osmosis (RO), both require huge amounts of energy.
For remote areas where accessibility to water and electricity grid is not available or is cost prohibitive, energy-intensive RO systems do not offer any relief. Other issues, such as discharge, maintenance and residual handling, further complicate the requirements. This sector is definitely underserved by current technologies.
Despite more recent technology advancements, RO systems, by definition, still need to provide enough driving force to overcome the intrinsic osmotic pressure in brackish or seawater. A more energy efficient way is to harness the force of nature — osmotic pressure — the natural pressure that drives water molecules to the saltier side across a semipermeable membrane. Forward osmosis (FO) does just that. The process itself is not new. The advantage comes when the salt in the draw solution can be removed efficiently and cheaply. The process is a simple two-part process. A draw solution with salt content higher than the water to be purified is used to draw water across a semipermeable membrane. Water comes to the draw solution without external energy or pressure. After water is drawn to the saltier side, the draw solution is processed to remove the salts. The salts are subsequently recovered and reused. The amount of heat involved is small, just 20o C above room temperature. This process can desalinate water for about one-half the cost of standard RO desalination. With the help of waste heat, the cost can be as low as one-tenth of the cost of RO.
An FO system with its low energy operation, close to zero liquid discharge operation, ingredients safe to handle and completely recoverable, low capital, maintenance and replacement cost, and the possibility to be completely off the grid makes it an ideal choice for remote and small operations.
The development of a suitable FO membrane became the most critical piece for realizing the potential of the FO desalination technology. There are several functional requirements of an FO membrane. Existing commercial membranes lack one or more of these characteristics, inhibiting their use in osmotically driven membrane processes. Of these requirements, the most challenging one is to increase flux across the membrane. There are two aspects to increasing water flux: the selective layer and the support. Typically, the majority of flow resistance comes from the selective layer, thus reducing the flow resistance of the selective layer and having the greatest impact in reducing the overall membrane resistance. Whereas other FO membrane developers focus on the less impact area of reducing the support resistance, T3 Scientific LLC focuses on the greatest impact area of reducing the selective layer resistance while using a low resistance support. The challenge to the reduction of the selective layer resistance or increasing its permeance is to maintain membrane selectivity simultaneously.
Conventional techniques using phase inversion or interfacial polymerization to fabricate a selective layer produces a loosely packed membrane structure, thus prone to defects. T3 Scientific has developed a proprietary technology to deposit an ultrathin selective layer with extreme control of uniformity and thickness. The resulting membrane provides higher water flux, thus lowering operating material and equipment cost for FO operations. This project uses this proprietary technique to lay down an ultra-thin membrane layer onto a low resistance support designed for the FO process to ensure a high and robust membrane. This membrane is expected to out-perform any existing and development FO membranes, enabling the use of low energy FO process for water desalination. The high efficiency and low energy requirement of this technology is naturally suited for small to very small water purification operations.
Results confirmed that T3 Scientific LLC's propriety technique in making the membrane functional indeed provides higher flux than commercial FO membranes with higher salt rejection rate.
The two objectives of Phase I have been achieved successfully. Phase I has achieved a water flux higher than any commercial FO membranes. Phase I also achieved a salt rejection higher than commercial FO membranes and was comparable to the best-of-class RO membranes. Deposition, the proprietary technique, was demonstrated successfully in Phase I. More importantly, the search for a suitable polymeric support with suitable material, pore structure and surface pore size was proved successful as demonstrated by the membrane performance. These results showed that the membrane layer was practically perfect without major defect. Material stability and compatibility with draw solution was demonstrated. Preliminary cost analysis showed that the cost of this membrane is very cost-effective to the customer as this membrane system will provide at least 50 percent energy savings compared to the current RO systems. In conclusion, feasibility of the technology was largely demonstrated in Phase I. Phase II will focus on process optimization, equipment development and prototype demonstration.
There are many potential applications and markets for this technology. The remote and small operation is definitely an important market that this technology was designed for. This technology will be ideal for emergency and disaster relief as a short-term solution for temporary water shortage. The recovery of produced water from the oil and gas industries' fracking process is emerging as an early adaptor of the FO process, as hauling water long distances to drilling sites is becoming a less viable solution. The food and beverage industry has used FO membranes in a small scale; certainly this will be an attractive target market especially when the problems associated with conventional FO membrane are solved by this technology. Integrated membrane systems utilizing several types of membrane and processes to meet customer demand and lower bottom line is becoming more important as both regulatory standards and energy cost have increased. High flux FO membranes certainly will find their place in an integrated membrane system, for example, to polish the RO brine reject to further reduce disposal volume and cost. Finally, the selective layer developed for the FO system can be transferred easily to the existing RO membrane construction to provide the next generation RO membrane. With this straight-forward replacement, this technology will fit right into all the existing RO applications, such as seawater and brackish water desalination, drinking and wastewater treatments. This technology certainly will benefit many industries and usher in a new era of water processing.