Forward Osmosis: An overview

What is Osmosis?

The natural flow of water through a semi-permeable membrane from a solution of lower concentration of solute to a higher concentration of solute until the concentration of solute is same on either side of the membrane is called Osmosis.

What is Osmotic Pressure?

The pressure exerted by water on the membrane during its natural flow from low solute concentration [permeate side] to high solute concentration [feed side] is called osmotic pressure.

What is Reverse Osmosis?

Reversing Osmosis by applying external pressure on high solute concentration side [ feed side ] in excess of Osmotic pressure to prevent movement of water from low concentration side [ permeate side ] to high concentration side[ feed side ] is called Reverse Osmosis.

What is Forward Osmosis [FO]?

In FO, the driving force for solute / solvent separation is an osmotic pressure while in RO it is the hydraulic pressure [ pump pressure ] , which serves to counteract the osmotic pressure gradient between the feed and permeate side.

In FO, a "draw" solution of higher concentration relative to that of the feed solution is used to induce a net flow of water through the membrane from the feed side into the draw solution. which serves to counteract the osmotic pressure gradient that would otherwise favor water flux from the permeate to the feed. Hence significantly more energy is required for reverse osmosis compared to forward osmosis.

The simplest equation describing the relationship between osmotic and hydraulic pressures and water (solvent) flux is:

Jw = A (Δπ - ΔP)

where Jw is water flux, A is the hydraulic permeability of the membrane, Δπ is the difference in osmotic pressures on the two sides of the membrane, and ΔP is the difference in hydrostatic pressure. Negative values of Jw indicates reverse osmotic flow.

The draw solution on the permeate side pulls water from the feed side by the natural process of Osmosis, thus effectively separating the feed water from its solutes without using any high pressure pump. In FO water moves from feed side to permeate side along the direction natural Osmotic pressure, water does not require to move against Osmotic pressure across the semipermeable membrane from feed to permeate side like RO. Therefore, this process of separation of water from solutes is called ' forward osmosis ' For the same reason, FO does not require a high pressure pump to push the water across the membrane and this gives a substantial benefit to FO in terms of a much lower energy consumption compared to RO.

The draw solution can consist of a single or multiple simple salts or can be a substance specifically tailored for forward osmosis applications. The feed solution can be a dilute product stream, a waste stream or seawater. Therefore the composition final purified water would depend on the composition of the draw solution used to induce flow of water from feed to the draw side of the membrane.

In the reverse osmosis , water resulting from the RO process is in most cases fresh water ready for use. In the FO process, this is not the case. The membrane separation of the FO process in effect results in a "trade" between the solutes of the feed solution and the draw solution. The draw water may need to be dewatered in some cases.

What does FO do ?

• It can dilute a solution of higher osmotic pressure with a solution of lower osmotic pressure. It can concentrate a solution of lower osmotic pressure with a solution of higher osmotic pressure.

One key element is the dilution/concentration process takes place across a selectively permeable membrane, at low pressure and the ions are rejected in both the direction of forward flow and reverse flow. In a similar way to reverse osmosis we talk about salt passage in the direction of forward flow, but ion mass transfer also takes place in the reverse direction which is often termed as back diffusion.

Application of FO

The process has considerable potential across a wide variety of applications, like hydration drinks , power generation, enhanced oil recovery, produced water treatment, fluid concentration,  and desalination, However only a few of these applications have been currently commercialised; hydration drink is one of them. In such applications, only the first step of the forward osmosis process is required — where a concentrated sugar syrup is diluted to a desirable level — placing forward osmosis at an advantage to reverse osmosis. The other development is , desalination process using combined FO and RO is being tried out at a number of sites.

Key challenges of FO

There are two key steps in FO [ 1 ] forward osmosis step [ 2 ] regeneration step to re-concentrate the osmotic agent / draw solution for reuse and to separate the treated permeate water, before any post-treatment. The regeneration step is normally either thermal or RO.

 Following are the key points in FO

• A nontoxic osmotic agent / draw solution
• A constant osmotic pressure for the concentrated osmotic agent
• No contamination of the osmotic agent with salts from the feed solution
• Minimum back diffusion of the osmotic agent to the feed solution
• A membrane that can satisfactorily handle flow on both sides
• Working on a continuous basis

 Some of the advantages of FO

•  FO does not have the fouling limitations inherent in pressure driven membrane separations. This is a big advantage.
• Since the suspended solids are not pushed under pressure to the semipermeable membrane FO can process feed streams with high levels of suspended
• In separations driven by hydraulic pressure (such as RO) all components of a feed water are unselectively forced against the membrane surface. In FO, osmotic pressure, which is hydraulic pressure created by Osmosis as water passes through a semi-permeable membrane into a more concentrated stream, selectively draws molecules through the membrane avoiding membrane fouling and compaction.

To summarize, RO typically requires high pumping pressures, which require high energy usage to force the feed solution against a very tight membrane material and force the filtered water through the membrane to the clean water side. During that process all solutes and pollutants on the dirty water side are forced against the membrane material, under high pressure, which compounds RO’s sensitivity to fouling and need for relatively clean feed streams. It is just the opposite in FO, it operates at very low pressure, simply adequate to circulate the fluids, and the natural osmotic pressure in the draw solution pulls water through the membrane, leaving solids and foulants behind in the concentrated feed solution.

On the energy front, a major advantage with FO is it does not require the high pressure pump. The next step in FO, the regeneration draw water , that involves , either thermal separation of the FO treated water or RO step, the bulk of the energy is consumed in this step in FO. The picture of energy usage in FO is not very clear. One argument that goes against FO being highly energy efficient when compared with RO is that even if the second step of draw regeneration — in which the concentrated salt solution is dewatered, can achieve the same level of efficiency as the reverse osmosis process, the actual energy consumption of forward osmosis will consistently surpass that of reverse osmosis. This is because the salt solution that results from the first step of forward osmosis will necessarily be more concentrated than standard seawater, if we consider FO’s application in sea water desalination , meaning it will always require a higher level of energy for regeneration.

 Conclusion:

FO is no doubt a remarkable new technology. But it is still going through the development phase. The technology is yet to be tested with challenging water in the field – we do not see many reports. It would be premature to draw a clear conclusion about its potential to compete with RO in applications like waste water reuse and sea water desalination

Forward Osmosis: Hype vs Reality update (2019)

Forward osmosis has attracted a lot of recent attention over the past few years but with limited market success. The FO addressable market now ranges from a low of the zero-liquid discharge (ZLD) market of less than $300 million in 2017 and a high of $6 billion if it includes product concentration, ZLD and water reuse. Within ZLD, the opportunity for FO is in minimising pre-treatment needs, concentrating the total dissolved solids (TDS) higher than reverse osmosis (RO) can, and thus pulling market share away from evaporators. While there is little reason to consider FO over RO except for more challenging feed waters, FO combined with RO does have potential promise to enhance recovery in seawater desalination to pull more water out of the RO concentrate and increase plant capacity.FO has failed to gain anywhere near the same success as RO, with some commercial failures giving the technology a reputation for being “over-hyped”, with a disproportionate share of academic and industry attention.

The FO market

The market is dominated by FTSH2O, Porifera, Oasys and Modern Water. However, the FO market is small. None of the companies have more than a few commercial installations

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