Forward Osmosis.

18th October 2007

The WaterReuse Foundation recently released a study of Forward Osmosis. The work was cosponsored by the US Bureau of Reclamation and the California State Water Resources Control Board. The principal investigators were Samer Adham & Montgomery Harza. The projects Advisory Committee included Menachem Elimelech (Yale University), Kerry Howe (U of New Mexico), Scott Irvine (US Bureau of Reclamation) Rich Mills (California State Water Resources Control Board), George Tchobanoglous (UC Davis).

I’ve read about this process from time to time but but the reports have been sketchy. The WaterReuse Foundation fills in the gap. I went to their site recently and ordered a copy of the study.

What is Forward Osmosis?

From the page 1 of the WaterReuse Foundation Report.

When solutions of different solute (salt) concentrations are separated by a semipermeable membrane, the solvent (ie water) will move across the membrane from the lower-solute-concentration side to the higher-concentration-solute (draw side).

How is this different from Reverse Osmosis (RO)?

Reverse Osmosis (RO) uses huge pumps to force salt/briny water against semipermeable membranes. Fresh water passes through the membrane while leaving behind a concentrate. Forward Osmosis places the membrane between two solutions. On one side of the membrane is a lower-solute-concentration. Lower-solute-concentration passes through the membrane to the higher-concentration-solute side (ie draw solution) by way of osmotic pressure.

Why is Forward Osmosis necessary?

The RO concentrate can be diluted and sent back to sea for coastal desalination plants but inland brackish water desalination plants have a concentrate disposal problem. FO offers a cheaper means of dewatering the concentrate because little extra energy is needed for the process to work.

The WaterReuse study examined various draw solutions & membranes and made recommendations of draw solutions & membranes for further study.

The report concluded that forward osmosis is ready for prototyping the dewatering of concentrate after RO for inland brackish water desalination plants.

The FO process has been shown to be economically feasible for RO concentrate minimalization. The costs for implementing FO for dewatering RO concentrate before ZLD processing are lower than those for implementing ZLD on the entire RO concentrate stream, as operational costs are substantially reduced by utilizing the FO train ($2.49/1000 gal) instead of the baseline treatment train ($3.07/1000 gal) for a 10-mgd IMS incorporating an MBR and an RO process.

Curiously the most economical draw solution (for now)was found to be salt.

The use of salt as the draw solution and an IX (ion exchange) process for reconconcentrating the salt from the diluted draw solution was also found to be economically feasible.

Here I think it would be worth mentioning that scaled up version of the ENI OEM-12B3 13.56 MHz RF Generator that generates John Kanzius’ radio waves might offer an even better way to reconcentrate the draw solution (salt) while providing an additional source of power by way of the hydrogen output. The heavily concentrated NaCl in turn might provide further efficiencies to the Kazius effect

Interestingly, a Norwegian company is prototyping forward osmosis too — only they’re working at it from the energy side. According to the article Statkraft is set
“to build world’s first osmotic power plant capable of harnessing process of osmosis to generate electricity.”

From the Statkraft article:

Statkraft plans to harness energy from this phenomenon by passing fresh water through a membrane into salt water and using the ensuing pressure difference to drive a turbine.

The plant would be at the mouths of rivers where fresh water mixes with salt water.

“You need a continual flow of fresh and sea water coming into the system and a continual outflow of brackish water that runs the turbine,” explained Torbjørn Steen, vice president of communications at Statkraft.

Statkraft provides a very good diagram of this process here.

The company, which has invested £9m in developing the technology, said the prototype plant will be completed by the end of 2008 and it expects to have a commercially viable technology ready by 2015.

Statkraft estimates that globally osmotic power could generate 1,600TWh of power, including 200TWh in Norway accounting for 10 per cent of the country’s current energy use.

However, Steen said that the company will need to continue to improve the efficiency of the technology in order to make it commercially viable.

“Improving the efficiency of output per square metre of membrane is the main challenge for the prototype plant,” he explained. “When we started the project we were generating less than one watt per square metre of membrane and now we are up to three watts per square metre. We estimate we need five watts per square metre to make it commercially viable, but we are heading in the right direction.”

Statkraft’s progress maps pretty well over onto forward osmosis for desalination in the US.

I think American desalination people should regularly consult with Statkraft.


Coastal desalination plants mix their RO concentrate with seawater. That mixing might be used to produce energy using Statkraft’s process. Once again check out their designs. As well their membrane issues are similar.


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