Desalinating the sea

Intake, membranes, pressure, electricity, brine and when it pays off. Theory, formulas, worked examples and applications.

Removing salt means separating molecules, not always boiling

Reverse osmosis: pressure against a selective membrane

The dominant modern desalination technology, reverse osmosis, presses seawater at high pressure against semipermeable membranes designed to let water molecules through while retaining the vast majority of dissolved salts. Before reaching those membranes, the water goes through a capture and pretreatment stage — filtration, chemical adjustment — specifically designed not to damage the membranes, which are the most expensive and most delicate component of the whole facility. After separation, energy-recovery systems reuse part of the residual pressure from the reject stream to reduce the process's overall electricity consumption, the already-desalinated water is chemically adjusted to make it fit for consumption or irrigation, and finally it is distributed to the network. Thermal plants, the dominant technology before reverse osmosis matured, evaporate seawater and then condense the salt-free vapor, a process that typically consumes several times more energy per cubic meter than modern reverse osmosis.

What remains when the water is removed: brine

Fresh water is not the only product of a desalination plant: the other output stream is a concentrated brine, with a salt concentration far higher than the original seawater, plus the chemical reagents added during pretreatment and any substance present in the original intake water. Discharging that brine back into the sea without proper control can significantly alter local salinity conditions and affect marine ecosystems near the discharge point; that is why responsible design requires properly diluting the brine and siting the discharge point specifically to protect sensitive local ecosystems. The seawater intakes themselves can also affect marine organisms trapped or damaged in the intake process. And like any industrial facility, membranes, filters and pumps wear out and are periodically replaced over the plant's life, generating their own waste stream that has to be managed.

A drought-resistant supply, but not cheap in energy

Desalination's great advantage is its almost total independence from local freshwater availability: as long as there is sea nearby and available electricity, desalination can keep producing water even during a prolonged drought that exhausts other conventional sources. That resilience has a structural energy cost: desalination consumes considerably more electricity per cubic meter than treating most conventional freshwater sources. If the destination city sits at a high elevation or a considerable distance from the coast, the additional pumping needed to carry desalinated water there adds an even greater energy cost on top of desalination's already-high one. Using renewable-source electricity to power the plant reduces the emissions associated with the process, but eliminates neither the need for seawater intake, nor the management of the resulting brine, nor the rest of the physical infrastructure desalination requires in any case.

Last resort does not mean bad resource, if used where it belongs

Desalination can be an essential and fully justified tool in coastal regions where water is structurally scarce and household saving measures, treated-water reuse, and reasonable management of other available watershed sources have already been exhausted or are insufficient. Presenting it as a universal solution without comparing alternatives is just as rigorous as dismissing it outright without considering the specific local context. Before evaluating any desalination project, it is worth asking about the real specific energy consumption of that particular plant — not a generic figure for the technology in the abstract —, the exact electricity mix powering it, whether it incorporates energy-recovery systems from the reject stream, the planned destination and dilution of the brine, the state of the local marine ecosystem, the leak rate of the network that will receive that extra water, and who can actually afford to pay the final cost of that water, systematically more expensive than that of most conventional sources.

Last resort does not mean bad resource

In favor: almost total independence from local freshwater availability; can keep supplying water even during a severe drought that exhausts other sources.

To watch: structurally higher electricity consumption than treating conventional fresh water; responsible management of concentrated brine; impact of intakes on marine organisms; additional pumping cost if the destination is far away or at a high elevation.

It can be essential where water is structurally scarce in coastal areas and household saving, treated-water reuse, and reasonable management of other available sources have already been exhausted.

Before evaluating any project: ask about the real specific energy consumption of that particular plant, the electricity mix powering it, whether it recovers energy from the reject stream, the destination and dilution of the brine, the state of the local ecosystem, the leak rate of the network that will receive the water, and who can actually pay that cost — systematically higher than most conventional sources.

The numbers, with their source

From 20 kWh to under 3 kWh per cubic meter in five decades

In the 1970s, desalinating a cubic meter of seawater via reverse osmosis could consume on the order of 20 kWh. Modern, efficient reverse osmosis plants today operate in a range of 2.0 to 3.5 kWh per cubic meter for the membrane process alone, though a large plant's total consumption — including pretreatment, post-treatment and internal pumping — exceeds that isolated process figure. The theoretical minimum thermodynamic limit for separating fresh water from seawater is about 1 kWh/m³, and reverse osmosis's realistic practical limit sits around 1.6 kWh/m³ — meaning today's most efficient plants already operate quite close to the process's physical limit, with little room for further improvement without a deeper technological shift.

Sources: Hannah Ritchie — How much energy does desalinisation use?; Pumps & Systems — Energy Efficiency in Seawater Reverse Osmosis.

Thermal desalination consumes several times more

Thermal desalination plants — which evaporate and condense water instead of filtering it under pressure — consume on the order of 13 kWh per cubic meter, between three and five times more energy than modern reverse osmosis to produce the same amount of fresh water. This difference explains why reverse osmosis has almost completely displaced thermal technology in new plants built in recent decades, except in regions with access to very cheap industrial waste heat that reduces that comparative cost.

Source: Pumps & Systems — Energy Efficiency in Seawater Reverse Osmosis.

What this table is missing

None of these figures include the energy cost of the additional pumping needed if the destination city is at a high elevation or far from the coast, nor the cost of responsibly treating and discharging the resulting brine — both can add a significant fraction to the real total energy cost of a specific desalination project, beyond the membrane process itself.

Sources