Introduction to Reverse Osmosis
Reverse osmosis (RO) is a membrane desalination technique employing a semipermeable membrane so tight that is able to hold back a number of monovalent ions. It is most commonly used in desalination due to the ability of the membrane to retain sodium and chloride ions (salt) while allowing water to pass.
Osmosis is the natural 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.
In reverse osmosis, a pressure is applied on the more concentrated side of the membrane in order to overcome the osmotic pressure and push more water into the less concentrated side. This therefore leaves impurities (including salt) on the concentrated side, and pure desalinated water on the other side.
Reverse osmosis can employ a wide range of different membranes. At commercial scale, industry has standardised on a number of membrane housings to contain the spiral wound, flat sheets. This means that it is possible to change membrane types to upgrade or modify existing installations.
Cellulose acetate membranes
Cellulose acetate (CA) membranes are the oldest type of commercially produced RO membranes. They are made of cellulose diacetate, cellulose triacetate or a blend of the two. They form an asymmetric polymer, resulting in a thin dense active layer for desalination, and an increasingly open back which is made of the same polymer.
They benefit from being relatively low cost & oxidant tolerant. In most instances they actually require a chlorine residual in the water entering the membrane. Microbes may otherwise use the cellulose acetate as a carbon source.
Cellulose acetate membranes also require a tight pH operating band (4-8), otherwise they suffer from hydrolysis, which can reduce their salt rejection. They are temperature intolerant (0-35 deg C required). They can also become compacted at high pressure, however their relatively low permeability necessitates high pressure operation, hence high operating costs.
Polyamide is used as the active layer in thin-film composite membranes.
Polyamide membranes are popular due to their increased permeability. Thus requires less pressure (hence energy and operating cost) than CA membranes. They are also able to operate across a wider range of pH, however they are intolerant to oxidants.
Polyamides cary an anionic surface charge, as such they are more susceptible to cationic contaminants. Their intolerance to chlorine means that total dechlorination is required before the water hits the membrane. Omitting chlorination increases the risk of biofouling.
Thin-Film Composite membranes
Thin-Film Composite Membranes (TFC or TFM) are a laminate of two or more membrane materials, normally with one acting as the active layer (rejecting salts and other contaminants) and the others providing mechanical strength. TFCs for desalination typically use three layers comprising:
- Polyamide as the active layer
- Polysulfone (PS) or polyethersulfone (PES) porous layer
- Non-woven fabric support.
Graphene membranes for desalination are still to enter commercial production. Graphene is a single-atom thick layer of carbon, graphene membranes for water treatment are more commonly made of graphene oxide.
While graphene is stronger than steel, graphene membranes are still susceptible to tearing. As graphene itself is water impermeable, they have to be perforated in order to allow the passage of water.
The thinness of the membranes has been demonstrated to reduce friction, allowing significantly greater permeability than traditional membranes. Their non-reactive surfaces also give a lower propensity for fouling.
Currently high production costs make graphene membranes unfeasible at commercial scale.
Aquaporin and biomimetic membranes
In multiple stage RO systems the concentrate from one stage is passed to a second stage of RO for further concentration. The permeate from each of the stages are combined. This is used to increase the water recovery.
Concentrate can also be recycled to the feed water to increase recovery.
In multiple pass RO systems the permeate from one pass is passed through a second pass.
This is used to increase rejection (typically rejection of difficult to remove ions like boron). Caustic can also be added between passes to remove permeate carbon dioxide concentrations.
Closed circuit desalination
Closed circuit desalination (CCD) is a proprietary semi-batch RO configuration. It is primarily focused on maximising water recovery, but can also reduce fouling and energy consumption.