Desalination: Drinking Water from Seawater

Introduction

About 97% of the water on Earth is in the oceans, but it is far too salty to drink or to use for crops. In dry countries with long coastlines, such as those around the Persian Gulf, much of the drinking water comes from the sea. Turning seawater into fresh water is called desalination . The oldest large-scale method is simply distillation, the same process used in the laboratory, carried out in enormous plants. Its main drawback is the energy it needs.

Core explanation

What is in seawater? Seawater contains about 35 g of dissolved salts in every kilogram, mostly sodium chloride, with smaller amounts of magnesium, calcium, potassium and sulfate ions. Drinking water must contain far less — well under 1 g per kilogram. Filtration cannot help, because the salts are dissolved and pass through any ordinary filter.

Thermal desalination. Seawater is heated so that water evaporates, leaving the salts behind. The water vapour is condensed on cool surfaces, giving almost pure water. The leftover concentrated salt solution, called brine , is returned to the sea. Large plants use many chambers in series, and some work at reduced pressure so that water boils below 100 °C.

The energy problem. Turning liquid water into vapour needs a great deal of energy: about 2260 kJ for every kilogram, on top of the energy to warm it up. This is the latent heat of vaporisation. Without any recovery, evaporating one cubic metre (1000 kg) of water would need over 2 million kJ. Real plants recover much of this energy, because the vapour condensing in one chamber releases heat that warms the incoming seawater for the next. Even so, thermal desalination uses a lot of fuel and is most common where energy is cheap.

Reverse osmosis. Most new plants use reverse osmosis instead. Seawater is pumped at high pressure against a special membrane. Water molecules are forced through the membrane, but dissolved ions are held back. No change of state is needed, so the energy use is much lower — typically around 3 to 4 kWh per cubic metre of fresh water — but the membranes must be protected by filtering the seawater first and cleaned regularly.

After treatment. Water from either method is so pure that it tastes flat and can corrode pipes. Small amounts of minerals are added back, and the water is disinfected before it is supplied as potable water.

Environmental issues. Desalination produces large volumes of brine, which is denser than seawater and can harm sea life near outlets if not spread out. Burning fossil fuels to power the plants adds carbon dioxide to the atmosphere, although solar-powered plants are increasingly used.

Step-by-step reasoning

To evaluate thermal desalination:

1. Identify the separation: water must be removed from dissolved salts. 2. Recognise that distillation works because salts do not boil. 3. Note that each kilogram of water must absorb a large amount of latent heat. 4. Conclude that the method is reliable but energy-hungry.

Visual explanation

Picture a row of large steel chambers by the coast. Warm seawater flows in at one end; clouds of vapour rise inside each chamber and condense on pipes carrying cooler seawater. Fresh water collects in trays and flows to storage tanks, while a pipe carries dense brine back out to sea.

Real-world analogy

Desalination by distillation is like drying wet laundry with a tumble dryer rather than on a washing line. It works in any weather, but it needs a lot of electricity because every drop of water must be turned into vapour.

Real-world example

On ships and some small islands, compact desalination units supply fresh water where no rivers or wells exist. Large coastal cities in the Middle East, Australia and Spain also rely on desalination plants, especially during droughts.

Why?

Why does distillation need so much more energy than warming water? To boil, water particles must overcome the strong attractions that hold them together in the liquid. Separating the particles completely takes far more energy than simply making them move faster, so the latent heat of vaporisation is very large.

Common misconception

"Seawater can be made drinkable by filtering it." Filters trap only undissolved particles. The salt in seawater is dissolved as ions far smaller than filter pores, so it passes through. Distillation or reverse osmosis is needed.

Worked example

Question: A small plant produces 500 kg of fresh water per hour by distillation. Using 2260 kJ/kg, calculate the energy needed per hour just to vaporise the water.

Reasoning: Energy = mass × latent heat = 500 × 2260 = 1 130 000 kJ.

Answer: About 1.1 × 10⁶ kJ (1.1 GJ) per hour, before any energy is recovered.

Quick check

1. What is the name of the concentrated salt solution left after desalination? Answer: Brine.

Exam focus

You may be asked to evaluate desalination. Balance the benefit (fresh water where it is scarce) against the costs: high energy use, greenhouse gas emissions if fossil fuels are used, and the brine produced. Mention that it is used mainly where fresh water sources are limited.

Advanced insight

Multi-stage flash plants run a series of chambers at gradually lower pressures. Hot seawater entering a lower-pressure chamber partly "flashes" into vapour instantly, because its boiling point there is below its temperature. The condensing vapour pre-heats incoming seawater, so the same energy is reused many times, greatly improving efficiency.

Summary

Desalination produces drinking water from seawater, which contains about 35 g of salts per kilogram. Thermal desalination distils the water, leaving brine behind, but vaporising water requires about 2260 kJ/kg, so energy costs are high. Reverse osmosis pushes water through a membrane under pressure and uses less energy. Desalinated water is remineralised and disinfected before use.

Practice questions

1. Why can filtration not be used to remove salt from seawater? Answer: The salt is dissolved as tiny ions that pass through the filter paper. 2. Explain why thermal desalination has a high energy cost. Answer: Every kilogram of water must be vaporised, which requires a large amount of latent heat (about 2260 kJ/kg). 3. Give one environmental problem caused by desalination. Answer: Brine returned to the sea can harm marine life, or burning fossil fuels for energy releases carbon dioxide. 4. Why does reverse osmosis use less energy than distillation? Answer: It does not involve a change of state; water is pushed through a membrane rather than being boiled. 5. Why are minerals added back to desalinated water? Answer: Very pure water tastes flat and can corrode pipes, so small amounts of minerals improve taste and protect the supply system.