Introduction to forward osmosis

Forward osmosis (FO) is the reversal of the more common reverse osmosis (RO) to allow water to pass through a membrane from a less concentrated solution to a more concentrated solution.  FO is also known as engineered osmosis (EO), manipulated osmosis (MO) and simply osmosis.

Osmosis is the passage of a solvent (water) through a semipermeable membrane from a less concentrated solution into a more concentrated solution.  Given time osmosis will continue until sufficient solvent has passed through the membrane such that the concentrations on each side of the membrane are equal.

How FO works

By adding sufficient draw solute to the product water side of the membrane, water will be drawn from the less concentrated sea water, to the more concentrated draw solution.  The water and draw solute are then separated in a second stage, which may involve membranes (often RO), thermal or physical removal.

Advantages of FO

One of the main advantages of FO is that it has a much lower propensity for fouling.  This means that the membranes suffer less from scale-related pressure drop and require less frequent cleaning.  Lower cleaning requirements lead to increased membrane productivity (uptime).  Less cleaning also means lower chemical costs, and lower impact of the disposal of wastewater from cleaning.

Applications of FO

Emergency hydration

FO emergency drink bags use an edible nutrient (typically glucose or fructose) as the draw solute within a membrane bag.  This bag can then be immersed in most water sources, including sea water.  With time, water will be drawn into the bag to dilute the nutrient, resulting in a nutrient drink made with desalinated/purified water.

Municipal desalination

There are a handful of examples of commercial scale municipal desalination plants employing forward osmosis.  Modern Water designed and operated a pilot plant in Gibraltar (2008) followed by a demonstration plant in Al Khaluf, Oman (2009) and a competitively tendered plant in Al Najda, Oman (2011).

Produced water & landfill leachate treatment

FO’s low propensity for fouling has lead to niche use with difficult-to-treat landfill leachate and oil & gas industry produced water.

Brine concentration and zero liquid discharge (ZLD)

High osmotic pressure draw solutions can be used to draw water from desalination brine and other waste waters.  When this is taken to the extreme (often in combination with crystallisation or other thermal processes) it can result in zero liquid discharge (ZLD).

Pressure retarded osmosis (PRO)

Pressure retarded osmosis (PRO) is generally considered a power generation technology.  It harnesses the osmotic pressure exerted when a salinity gradient is applied across a semipermeable membrane.  Due to the

Cooling

Evaporative cooling is another application in which draw solute need not be removed from the product water.  An osmotic agent that can be maintained in an evaporative cooling system is used.  Evaporation then concentrates the draw solution.  This results in water being drawn across the membrane from the saline or wastewater feed.

Challenges in FO

Second stage separation

The most obvious challenge with FO is that the product water normally needs to be separated from the osmotic agent.  In conventional FO/RO systems this can result in a total energy requirement that exceeds that of the most efficient RO systems.

Reverse diffusion

It is possible with some draw solute / membrane combinations for the draw solute to diffuse across the membrane in the reverse direction to the intended flow.

Concentration polarisation

Concentration polarisation is another major issue with forward osmosis.  Concentration polarisation occurs with any membrane.  It occurs when the flux of water across the membrane leads to increased solute concentration where membrane holds the solute back.  With forward osmosis the polarisation results in reduced local concentration gradient across the membrane, hence reduced draw across the membrane.