Why do forward osmosis membranes face less scaling?

[MartinLe]

"Forward osmosis (FO) is an osmotic process that, like reverse osmosis (RO), uses a semi-permeable membrane to effect separation of water from dissolved solutes. The driving force for this separation is an osmotic pressure gradient, such that a "draw" solution of high concentration (relative to that of the feed solution), is used to induce a net flow of water through the membrane into the draw solution, thus effectively separating the feed water from its solutes." (wikipedia)

In a second step, the diluted draw solution is pumped through RO membranes, leading to desalinated water and a thickened draw solution to be recycled. The stated benefit is that the FO membrane faces far less scaling than in an RO process.

Now the electrolyte used as draw solution can be optimized so that no scaling on the RO membranes occurs. Since the manufactuers of these plants don't publish the composition of the draw solutions, I presume a lot of research went into that direction.

However, I have conceptual question on why FO is advantageuos at all: when water diffues through a membrane, the concentration of the solutes is higher near the membrane and this ultimately leads to scaling if hardness builders etc. are present. It is stated that this occurs far less when the diffusion is driven by osmotic pressure instead of pressure from a pump. Why is that so?

 

[ashtree]

The reason that FO experience less scaling is that normally the product that you have labelled as the Osmotic agent is selected on the basis that it has a high solubility threshold ie; it can be very conecentrated with out precipitating out of solution onto the RO membrane which you have called the Regeneration system. On the forward osmosis membrane scaling would be possible but unlikely on the feed side if you could concentrate sufficently. Using FO the flux rate is likely to be low , so without a large membrane area the concentration of the reject is likely to still be fairly close to the feed.But minimal energy will be required to make the FO work and the RO can be optimised around a single chemical solution without worrying about all the minor scaling elements.FO has its limitations but also advantages when dealing with particularly difficult chemistry. Potentially FO may offer energy reductions over RO and may ultimately be a cheaper membrane to produce.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[MartinLe]

This sounds plausible. I'll look, maybe this can even be confirmed from public sources.

A year or two back I read a publication by modern water, according to which their seawater desalniation plant using FO used significantly less enery (over one year), but it's no longer online.

 

[IRstuff]

Seems to me that FO is fine for simply diluting solutes, but RO is most systems is used to remove solutes. For drinking water, it's unclear whether the quoted >90% removal rate can be met by FO, since that's equivalent to passing 9x the volume of clean water through the membrane.

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[itsmoked]

The Gov'mt just released a $100M project to figure out how to reduce the cost of desalinization.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MartinLe]

IRstuff: I don't understand your comment, what 90% are you referring to?

 

[IRstuff]

The purpose for installing an RO system is to remove "contaminants," and the quoted performance for such systems is >90% for those that RO can remove.

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[bimr]

The big benefit of forward osmosis is the energy savings.

There will be less scaling because the forward osmosis may be operated at lower temperatures than reverse osmosis. Scaling increases with temperature.

Link