Forward Osmosis: A great leap forward for water treatment or a false start?

Two years ago I got really excited about forward osmosis. I was at the Water Reuse Association’s Annual WateReuse Symposium in Dallas where I saw several presentations on the potential to use FO for high quality effluent reuse applications at significantly lower energy consumption than RO. At that time energy prices were through the roof, population growth was exploding in the arid U.S. West and I thought that this was just the technology we needed to resolve what has become known as the “energy-water nexus”!

And I wasn’t the only one jumping on the FO train. Over the next 12 months at least two start-up companies (Oasys Water Inc. and QuantumSphere Inc.) filed patents and claimed to have uncovered desalination applications for FO that would solve the water crises of the future and analysts covering the water sector were hailing FO as the next big thing.

Two years later, as far as I can tell, the hype over FO has died down. Why?

How it Works
In my excitement about the prospects for FO at the 2008 WRA Conference, I bought two Water Reuse Research Foundation reports to learn more. For those unfamiliar with FO, a simplified description as to how the technology works is as follows:

Forward osmosis is an osmotic process that uses a semi-permeable membrane to effect separation of water from dissolved solutes. The driving force for this separation is an osmotic pressure gradient, where a “draw” solution of high concentration is used to induce a net flow of water through the membrane from a solution of a lower concentration into the draw solution thus separating the feed solution from its solutes (Figure 1). In contrast, reverse osmosis (RO) uses hydraulic pressure as the driving force for separation, counteracting the osmotic pressure gradient that would otherwise favor water flux from the permeate to the feed.

Driven by an osmotic pressure gradient, FO does not require significant energy input other than low energy pumping of the process streams. Potential advantages of FO are high rejections of a wide range of contaminants at a lower membrane fouling potential than pressure driven membrane processes such as RO. All of this is achieved at significantly lower energy consumption than RO. So why wouldn’t you be excited about the potential for this technology??

Drinking Water Fool’s Gold?
To date, the only commercial drinking water application for FO has been developed by a company called Hydration Technology Innovations (HTI) who has developed a small water purification bag (hydration bag) used mostly by the military. The bag is made of a semi-permeable FO membrane and contains a solid glucose draw solution. When immersed in contaminated water from puddles, ponds or even urine in emergency situations, purified water diffuses into the bag, slowly diluting the solid draw solution to produce a sweet drink. A nice application but hardly economical on a large scale to put Powerade in the distribution system…..

One of the start-up companies, Oasys Water Inc., uses FO technology developed at Yale University and states that its technology can produce desalinated water at less than half the cost and using 90% less energy than RO. If that is the case, this technology should be taking off! But if seawater is the feed, what is the draw solution? A strong draw solution is formed using highly soluble ammonia and carbon dioxide gases which produces a strong enough osmotic pressure to extract water from seawater. The diluted draw solution is heated to about 60 deg C to decompose the ammonia and carbon dioxide and the pure product water is then recovered after degassing via column or membrane distillation.

Now I have no doubt that the Oasys technology works, but it sounds like a pretty complicated treatment train to me. I believe that for this process to be economically feasible it should be located near the ocean where there is a waste heat source, so preferably at a power plant with a seawater cooling system. A lot of ‘ifs and buts’, so to me this is a very specialized application that will fit when all the right stars align, rather than having the potential for large scale adoption.

QuantumSphere Inc. states that it uses ‘a special organic solution’ as a draw solution for seawater desalination. The diluted organic solution is then heated to cause the specially formulated organic solute to drop out. The purified water requires a final purification step through activated charcoal. Process energy costs are 70% less than traditional RO the company claims. This last purification step suggests to me that there could be some issues with traces of organics in the purified water after removal of the organic solute. If so, there will be some challenges in getting approval for the process from regulatory authorities plus there will be resistance from water utilities to use a process that adds then removes a possibly hazardous chemical to their water supply.

Unfortunately, I believe the use of FO for low energy desalination is still a long way off and in the meantime, gradual improvements in RO energy efficiency will lessen the benefits that FO has to offer.

A Promising Application

There is another very intriguing application for FO, where if you could convince the public to accept direct potable reuse of treated sewage, there are some real benefits that could be realized. This one application I saw presented in 2008 based on research carried out by the Colorado School of Mines (Cath). The application utilizes treated sewage or another impaired water source in a FO system to dilute seawater prior to desalination with RO (Figure 2).

With RO seawater desalination, most of the energy is used to overcome the osmotic pressure of the salt water and this high osmotic pressure limits the amount of pure water that can be recovered. In coastal areas, treated wastewater is being discharged to ocean, wasting a valuable resource. By using seawater as the draw solution and passing wastewater through the FO membrane prior to ocean discharge, purified wastewater can be used to dilute seawater prior to feeding to an RO system for desalination. The osmotic pressure of the seawater is then reduced which lowers the energy required for desalination and improves water recovery. Disposal of RO concentrate back to the ocean has always caused some environmental concerns, so a more dilute combination of FO and RO wastes may also overcome many of these concerns. An elegant solution, but with one big problem; this is essentially direct potable reuse!

Those with experience in the water and wastewater industry understand the public resistance to indirect potable reuse let alone direct reuse, no matter how elegant the solution is...

The Future?
If FO really did have potential for widespread application in the near to medium future, water tech savvy goliaths such as Veolia, Siemens or GE would have gobbled up the start-ups quicker than a rat up a drainpipe!

Don’t get me wrong though, I think FO has a future in specialized applications such as coastal areas in California where it is necessary to treat wastewater to a high standard, including desalination, for reinjection into the groundwater supply as a barrier to salt water intrusion (i.e. Orange County’s Water Factory 21). But, FO will not be the silver bullet to provide low energy desalinated drinking water unless there is a major shift in public attitude towards direct potable reuse.

Cath, T.Y., et al, “A novel hybrid forward osmosis process for drinking water augmentation using impaired water and saline water sources”, Water Reuse Association Annual WateReuse Symposium, Dallas, TX, 2008.