Reverse osmosis dominates desalination because it uses mechanical energy and does so efficiently. But there is one scenario in which the comparison changes: when there is surplus low-temperature heat on site.
An industrial process with waste heat, a power plant or even a solar thermal collector field offers energy that would otherwise be lost. That is where these three technologies come in, as they operate at much lower temperatures than conventional thermal distillation.
Adsorption desalination (AD)
It uses an adsorbent material, usually silica gel or zeolite, that captures water vapour. The cycle works in two phases: the adsorbent takes up vapour from an evaporator, which maintains the low pressure that allows water to evaporate at a reduced temperature; the adsorbent is then regenerated with heat and the released vapour condenses as product water.
Its advantage: it runs on very low-temperature heat, in the order of 50–80 °C, which is precisely the range of waste heat usually considered unusable. The equipment has very few moving parts, which reduces maintenance.
Its limitation: capacity per unit volume is low, so the equipment is bulky for the flow rate it produces. It requires a stable heat source.
Maturity: proven at pilot scale and in installations coupled to processes with available heat. It is not an off-the-shelf technology for a generic project.
Membrane distillation (MD)
It uses a hydrophobic membrane that blocks liquid water but lets vapour through. Maintaining a temperature difference between the two sides creates a vapour pressure difference: water evaporates on the hot side, the vapour passes through the pores and condenses on the cold side.
As only vapour passes through, the product water is practically free of salts and of non-volatile compounds.
Its advantage: it operates at atmospheric pressure and with small temperature differences, which makes it compatible with waste heat and solar thermal energy. It tolerates very high-salinity water, where reverse osmosis is no longer viable, which makes it of interest for concentrating brines and reducing reject volume.
Its limitation: membrane wetting, which occurs when liquid water ends up penetrating the pores and ruins the separation. It is the problem that has held back its industrial uptake.
Maturity: pilot scale and niche applications, with active development in membrane materials.
Forward osmosis (FO)
A common misconception should be cleared up here: forward osmosis does not apply pressure. Quite the opposite, it makes use of the natural process.
Salt water is placed on one side of a semi-permeable membrane and, on the other, a draw solution of even higher concentration. The osmotic pressure difference makes water pass through the membrane spontaneously, from the less concentrated side to the more concentrated one, without any applied pressure.
The water is then diluted in the draw solution, and this is the key to the process: it has to be separated again. That second stage is the one that consumes energy, and it is addressed by choosing a draw solute that is easy to remove; for example, one that breaks down into gases when heated, so that it can be recovered with low-temperature heat.
Its advantage: the separation stage does not need high pressure, which reduces membrane fouling and mechanical energy consumption. It works well with heavily loaded water.
Its limitation: the overall energy balance depends entirely on regenerating the draw solute. If that stage is not efficient, the complete process does not compete with reverse osmosis.
Maturity: specific applications, such as effluent concentration and treatment of heavily loaded water, rather than conventional desalination.
When they are worth evaluating
The short answer: when stable waste heat is available on site, or when the water to be treated has a salinity so high that reverse osmosis is no longer viable.
Outside those two cases, reverse osmosis with energy recovery remains the option with the best balance of energy consumption, maturity and availability of supply.
It is also worth being precise about expectations: no desalination technology removes all salts, and all of them produce a reject stream that has to be managed. What differs between them is energy consumption, tolerance to the feed water and the type of energy they need.
You can explore our desalination solutions or contact us with your site data, including the availability of waste heat.
