The main water purification/desalination processes currently in use fall under two categories: thermal desalination and reverse osmosis. Of the two, reverse osmosis is more efficient and cost effective. However, the process is not without its kinks. The membrane used to filter impurities is susceptible to microbes in the impure water. As a result, a chlorine wash is necessary to combat the biofilm that collects on the membrane surface. Unfortunately, current membranes are significantly weakened by exposure to chlorine.
Recently, Dr. Benny Freeman, a chemical engineering professor at The University of Texas at Austin, and his colleagues developed a chlorine-tolerant membrane that promises to simplify the water desalination process, increasing access to pure water and possibly reducing greenhouse gases.
Scientist Live spoke with Dr. Freeman about the new chemical membrane and its potential for reducing greenhouse gases.
Can you describe the current means of reverse osmosis water purification?
The current membranes are aromatic polyamides that are deposited on top of a porous polysulphone support that provides mechanical structure to the membrane. The desalination is done by the aromatic polyamide layer on top of that. In round numbers, it is about 100 nanometres thick.
How efficient is the process?
The easiest way to judge the process' efficiency is to compare it with its alternative. The membrane amounts to about half the cost of other technologies for desalinating water. The alternative way is through evaporation techniques, basically where you boil the water and the water evaporates but the salt does not.
Essentially a distillation.
That's right. The process generally requires about twice the energy as the membranes do to desalinate the same amount of water. So by that measure, the membranes are the most efficient way to do this. As the price of energy increases, that makes energy efficient techniques more attractive.
SciLive Audio Alert: Listen to audio clip at the end of the article to hear Dr. Freeman discuss how the newly developed membrane affects the desalination process.
What are current membranes made of?
They are made of aromatic polyamides.
Can you explain the formation of biofilm on membranes?
Essentially if you take raw water out of the ground or ocean, it has microorganisms that will basically colonise the surface. Membranes provide a good area on which biofilms like to grow on. This is a natural process because there are microorganisms in water.
What dangers do biofilms pose?
The danger that they pose is that they coat the surface of the membranes, resulting in a loss of flux and, in some cases, a decrease in the ability of the membranes to reject salt. Both of these factors contribute to significantly lower membrane lifetime than one could obtain absent biofouling.
How is the chemical composition of the membrane your laboratory developed different from other membranes currently being used in the purification process?
We have basically removed the amide linkages, which are the weak link in conventional membranes - that is, the amide linkages are highly sensitive to attach by chlorine. We have replaced them with linkages, such as sulfone groups, that are much more tolerant to exposure to aqueous chlorine. That differentiates the chemical structure of our membrane from current generation materials. This is why they are resistant.
How does the new membrane contribute to the reduction of the water purification process' carbon footprint?
Basically, if you do not have any of the bio-fouling problems because you can use adequate amounts of chlorine, this makes the membrane work at higher flux and therefore less energy input is needed to generate a given amount of purified water. Also, if you can use membranes rather than evaporation techniques, the membranes require about half the energy. So if you can use membranes to desalinate water, you have an inherent reduction in the energy consumption required to produce purified water. That leads to reduced carbon admissions.
What is next for your lab's research?
There are two directions along which we are working right now. One is to work with industrial partners to scale up and deploy this platform of membrane. The second is to further optimize the chemistry in our laboratory - higher salt rejection and to attack other ions beside just salt such as boron and arsenic that are also concerns in the purification of water.
