Desalination: Fresh Water from Seawater

Introduction

About 97% of Earth's water is in the oceans, yet it is too salty to drink or to use on crops. In dry, coastal regions such as the Middle East, parts of Australia and southern Spain, rivers and groundwater cannot meet demand, so fresh water is made from seawater by desalination . The ordinary water treatment stages cannot do this, because screening, settling and filtering remove only insoluble particles; the salt is dissolved. Two main methods separate water from dissolved salts: distillation and reverse osmosis.

Core explanation

Why seawater is unsafe to drink. Seawater contains about 35 g of dissolved salts per kilogram — several times more concentrated than the salts in body fluids. Drinking it would make the body lose more water through the kidneys than it gained, causing dehydration.

Method 1: Distillation. Seawater is heated until the water evaporates. The steam contains only water molecules, because the ions stay behind in the liquid. The steam is then cooled and condenses into fresh water. Large plants use a series of chambers at lower and lower pressures (multi-stage flash distillation), so that the water boils at lower temperatures and heat released by condensing steam is reused to warm incoming seawater. Even so, a great deal of energy is needed, because water's latent heat of vaporisation is large — about 2.3 kJ per gram.

Method 2: Reverse osmosis. Normally, in osmosis , water moves through a partially permeable membrane from a dilute solution into a more concentrated one. In reverse osmosis , high pressure (around 55–80 times atmospheric pressure for seawater) is applied to the salty side, forcing water molecules through the membrane in the opposite direction. The membrane's tiny pores let water through but hold back most of the ions. The seawater is first filtered to stop particles clogging the membrane. Reverse osmosis needs no change of state, so it uses much less energy than distillation, and it is now the most common method worldwide.

Feature Distillation Reverse osmosis --- --- --- Principle change of state (evaporate, condense) pressure through a membrane Energy use very high (heating) lower (pumping) Pre-treatment little needed thorough filtering to protect membranes Product purity very high high, small traces of salt remain Main cost fuel for heating electricity and membrane replacement

Environmental issues. Both methods produce brine , very salty waste water, which can harm sea life if discharged carelessly. Both use energy, often from fossil fuels, adding to carbon dioxide emissions — unless solar or other renewable energy is used. Water from desalination may also need minerals added back so that it is not too corrosive to pipes.

Step-by-step reasoning

To choose a desalination method:

1. Check available energy: cheap heat (such as waste heat from a power station) favours distillation. 2. Consider cost of electricity and membranes: reverse osmosis usually costs less overall. 3. Plan safe disposal of brine to avoid harming coastal ecosystems. 4. Consider how pure the water must be for its use.

Visual explanation

Picture two diagrams side by side. On the left, a flask of seawater over a heat source; steam rises, travels through a cooled tube and drips as fresh water into a beaker, while salt crusts at the bottom of the flask. On the right, a tube divided by a thin membrane; a piston pushes on the salty side, and water molecules (small circles) pass through while ions (larger, charged circles) are held back.

Real-world analogy

Reverse osmosis is like pushing a crowd through a turnstile that only lets children through. Pressure from behind squeezes the small ones (water molecules) through, while the larger people (ions) are kept back on the crowded side.

Real-world example

Israel now obtains a large share of its household drinking water from reverse-osmosis plants on its Mediterranean coast. Countries on the Arabian Peninsula, where fuel has historically been cheap, still operate many large distillation plants alongside newer reverse-osmosis facilities.

Why?

Why does distillation separate salt from water so completely? Water molecules can escape from the liquid into the vapour at its boiling point, but the ions in salt are held in the liquid by strong attractions and would need temperatures of over 1400 °C for sodium chloride to boil. So only water enters the vapour.

Common misconception

"Filtering seawater makes it drinkable." Ordinary filters remove only insoluble particles. Dissolved ions are far smaller than the gaps in sand or paper filters, so they pass straight through. Only special membranes under high pressure, or distillation, can remove them.

Worked example

Question: Evaporating 1 g of water needs about 2.3 kJ. Roughly how much energy would simple distillation need to produce 1 m³ (1000 kg) of fresh water, if no heat were recovered?

Reasoning: 1000 kg = 1 000 000 g. Energy = 1 000 000 × 2.3 kJ = 2 300 000 kJ = 2300 MJ.

Answer: About 2300 MJ — which is why real plants recycle the heat released when the steam condenses.

Quick check

1. Why is reverse osmosis usually cheaper than distillation? Answer: It does not require water to be boiled, so it uses much less energy.

Exam focus

Be able to describe both methods in a few clear steps and compare them using energy, cost and environmental impact. A typical six-mark question asks you to evaluate desalination for a particular country — mention energy source, brine disposal and cost compared with other water supplies.

Advanced insight

The minimum energy needed to separate fresh water from seawater, set by thermodynamics, is about 1 kWh per cubic metre. Modern reverse-osmosis plants use around 3–4 kWh/m³, partly because they recover energy from the high-pressure brine stream using pressure exchangers. Research into new membrane materials aims to bring this closer to the theoretical limit.

Summary

Desalination removes dissolved salts from seawater. Distillation boils seawater and condenses the steam, leaving the salts behind; it gives very pure water but needs a lot of energy. Reverse osmosis forces water through a partially permeable membrane under high pressure; it uses less energy and is now more common. Both produce brine and use energy, so they are costly and have environmental impacts.

Practice questions

1. Explain why ordinary sand filtration cannot remove salt from seawater. Answer: The salt is dissolved as ions, which are much smaller than the gaps between sand grains, so they pass straight through. 2. Describe how distillation produces fresh water from seawater. Answer: The seawater is heated so water evaporates; the vapour, containing no salt, is cooled and condenses to liquid fresh water, and the salt stays behind. 3. What is the role of high pressure in reverse osmosis? Answer: It forces water molecules through the partially permeable membrane from the salty side to the fresh side, against the natural direction of osmosis. 4. Give two environmental concerns about large desalination plants. Answer: Discharging concentrated brine can harm marine life, and the large energy use often comes from fossil fuels, releasing carbon dioxide.