About Forward Osmosis

About Forward Osmosis

In forward osmosis (FO), water molecules migrate by diffusion, without energy input, into a more concentrated “draw solution.” Energy for the process is supplied by osmotic pressure difference of the two solutions. The dynamics of FO is opposite of Reverse Osmosis which requires energy to overcome the osmotic pressure difference. Forward Osmosis mimics water transport by plants, when fresh water is drawn by osmotic pressure into the plant through its roots in the soil.

In FO water molecules migrate by diffusion, without energy input, into a more concentrated “draw solution.” Energy for the process is supplied by osmotic pressure difference of the two solutions. The dynamics of Forward Osmosis is opposite of Reverse Osmosis which requires energy to overcome the osmotic pressure difference. FO mimics water transport by plants.

• FO membranes remove salts, xenobiotics, bacteria, small organics and other contaminants
• FO membranes are low fouling: foulants do not compact on surface like UF & RO (no cake layer)
• FO requires lower energy for certain processes.
• FO can recover useful products from brine or waste streams while also purifying water for reuse
• FO often requires separation of the purified water from the “draw solution.”
• FO can operate in many system configuration

How does forward osmosis (FO) compare to reverse osmosis (RO)?

People often confuse reverse osmosis and forward osmosis. If you think of just osmosis, it’s the use of a semi-permeable barrier or membrane and water diffuses across that membrane from a lower concentration to a higher concentration so that both sides are at equilibrium. That's what osmosis is and that's what forward osmosis is.

Reverse osmosis uses that same semi-permeable barrier or membrane, but instead of allowing fresh water or lower-saline water to move across to a higher concentration to dilute that side, you do the opposite. You have to overcome that osmotic pressure by using hydraulic pressure to essentially force water through the membrane, leaving the salt behind.

When you think about RO, you think of high-pressure pumps, high-pressure vessels, stainless steel, and lots of energy — because you have to overcome that natural osmotic gradient and then have additional pressure to actually force water through. That's a major difference between RO and FO.

What does a forward osmosis system consist of ?

“Forward Osmosis Tech’s definition of a forward osmosis system: “All the components needed to enable forward osmosis membranes to be used in water treatment applications”

“Forward Osmosis Tech’s definition of a forward osmosis system: “All the components needed to enable forward osmosis membranes to be used in water treatment applications”

With this definition in mind, forward osmosis systems typically include the following components:

• The forward osmosis membrane housing (also know as a forward osmosis module)

• Low energy pumps to move the draw and feed stream in a cross-flow configuration across either side of the FO membrane

• Pipes and valves

• Feed stream pre-treatment systems to remove large contaminants

• Various instruments & meters for continuous performance evaluation

• Draw solution reservoir tank

• Feed solution reservoir tank

• Performance enhancing design elements

A draw solution regeneration system (i.e. a system able to separate draw solutes from the water continuously extracted from the feed stream) if one of the end products of the system in question is reusable water

FO fouls less than RO.

In contrast with forward osmosis, the reverse osmosis process uses hydraulic pressure as the driving force for separation, which serves to counteract the osmotic pressure gradient that would otherwise favor water flux from the permeate to the feed. One of the reasons that FO membranes are considerably less prone to fouling than membranes used in pressure driven processes is the absence of external pressure which compacts foulants into the membrane surface restricting flow.

Application of Forward Osmosis ( FO )

Most of the applications of FO, thus fall into three broad categories: product concentration, waste concentration or production of clean water as a bi-product of the concentration process.  The most efficient FO applications combine all three.  At it’s best, FO can concentrate waste, turning waste into a product all while producing clean water.

The Forward Osmosis process has applications in many different industries, including but not limited to: Water Reuse and Desalination; Food and Beverage; Mining; Oil and Gas; and the Power Industry.

• Water Reuse
• Water Desalination
• Brine Concentration
• Product concentration (examples: juice, chemicals)
• Produced water treatment

How does Forward Osmosis Work ?

Forward osmosis is the normal flow of water (or any solvent). The water flows through a semi-permeable membrane, from the the highest to the lowest concentration.

to know where the water is flowing its easier to consider the solute concentration, imagine distilled water with near 100% purity, and a bit of salted water with a 5% concentration of salt. So you have 100% water on one side, and 95% water on the other, so the water will naturally flow until the concentration of water on the 95% side goes up (lowering the concentration of salt which hasn't changed since no salt was added).
It's a bit more complicated then this, but I think it should be enough to understand the whole concept.

On the other side reverse osmosis uses hydraulic pressure to cancel and overcome this natural flow (osmotic pressure) working as a filter. One use is having a membrane with pores only big enough for water (H2O) and using the pressure to push the molecules against it's will through the membrane, leaving the impurities on one side, and pure water on the other.