Desalination Process Moves toward Commercialization
Article Posted On 3/10/2011
The need for sea water desalination has been driven by dwindling freshwater sources, population growth and urbanization. One of the currently available desalination technologies - reverse osmosis (RO) membrane processes - have made significant strides during the last three decades and are being widely used globally. However, there is an urgent need to improve the technology to address the problems of susceptibility to fouling and lack of durability of RO membranes; low recovery from seawater and disposal of reject brine; and high energy consumption.
Under partial funding from PITA, Pennsylvania-based industrial partner Rohm and Haas, Inc., and the National Science Foundation (NSF), a current research program at Lehigh University, under the direction of Professor Arup K. Sengupta, has led to a significant modification of universally accepted RO processes.
All of the existing membrane-based desalination processes suffer from the inevitable phenomenon of "concentration polarization" that causes the saline water in the feed side to get further concentrated inside the boundary layer formed at the surface of the membrane. At the elevated concentrations inside the boundary layer, some of the ions, particularly sulfate, in the sea water tend to form precipitates (e.g., calcium and barium sulfate) as their concentrations exceed the corresponding solubility product values.
In order to resolve this problem, two measures are practiced in RO seawater desalination plants: 1) continuous dosing of anti-scaling or sequestering agents and 2) restricting the permeate recovery in a single stage process to less than 30%. These two processes adversely affect the economics of the reverse-osmosis based desalination process.
This PITA-funded process is able to completely eliminate the possibility of sulfate precipitation without needing any anti-scaling agent and attains over 50% permeate recovery in a single-stage RO process with lower energy requirement. Furthermore, the process modifications can be readily incorporated or retrofitted into an existing system.
This project has resulted in the formal approval and imminent issuing of a United States patent application. Additionally, a second Pennsylvania-based industrial partner, Advanced Geoservices Corp., located at West Chester, has been awarded NSF Small Business Innovation Research funding for further work leading to commercialization of the process. Lehigh University has also been recently invited by the NSF to submit a proposal under the Accelerated Innovation Research program for additional funding for rapid commercialization of the RIX-M process.
In their PITA process, the scale formation potential of seawater at the membrane surface of a desalination process is eliminated by changing the chemistry of the feed water through a reversible cation exchange step before the reverse osmosis process. The scale-forming cations - like calcium, barium, and strontium - are exchanged for non-scale forming magnesium ions when contacted with a bed of cation exchanger pre-loaded with magnesium ions. The concentrate from the membrane process is used for regeneration of the ion exchanger. Elimination of scaling potential enables them to achieve a higher product water recovery compared to the conventional reverse osmosis process. The most significant finding is that the ion exchange process can be sustained without needing any external regenerant.
The successful demonstration and field-scale validation of the technology provide an opportunity for Pennsylvania-based industrial partners to be at the forefront of technologies that relate to the area defined by water-energy nexus.