Distillation: Desalination and Water Purification

Water is essential for life, yet many parts of the world struggle with access to safe drinking water. While the UK primarily relies on rainwater collected in rivers and reservoirs, other regions do not receive as much rainfall, necessitating alternative sources. This is where Distillation: Desalination and Water Purification becomes a vital process, transforming undrinkable water into a clean supply.

Over 70% of the Earth's surface is covered by water, but most of it is seawater, which is unsafe to drink due to its high salt content. Drinking seawater can actually lead to increased thirst by removing water from your body's cells. To combat this, processes like desalination are used to remove most of the salts, making the water potable.

Distillation: Desalination and Water Purification Explained

Desalination is the process of removing salt and other dissolved substances from water to make it suitable for drinking. While it requires expensive equipment and significant space, it's a crucial technology in many regions. For example, the Jebel Ali desalination plant in the United Arab Emirates produces over 800 million liters of drinking water daily.

One of the most effective methods for desalination is distillation. This process mimics the natural water cycle on a smaller scale to separate pure water from dissolved impurities.

How Does Distillation Purify Water?

The principle of distillation relies on the fact that water evaporates at a lower temperature than the salts and other solutes dissolved in it. Here's a breakdown of the process:

1. Heating: Seawater (or any impure water) is heated, causing the water to turn into steam.
2. Evaporation: As water evaporates, it leaves behind the dissolved solids, such as salts, because they do not evaporate with the water.
3. Collection and Cooling: The steam is then collected and directed into a cooling apparatus, such as a condenser.
4. Condensation: Inside the condenser, the steam is cooled down, causing it to condense back into liquid water.
5. Pure Water: The resulting liquid is pure, distilled water, free of the original solutes.

Laboratory Distillation Apparatus

In a laboratory setting, an apparatus called a still is used to distil mixtures like salty water. A common setup involves:

• Flask: Holds the solution to be distilled. Anti-bumping granules are often added to prevent violent boiling.
• Heating Source: Applies heat to the flask.
• Liebig Condenser: This key component has an inner tube where steam flows and an outer tube through which cold water circulates. The cold water keeps the inner tube cold, causing the steam to condense.
• Collecting Beaker: Pure (distilled) water runs into this beaker.

Safety When Heating Liquids

Heating liquids to boiling point in a flask can pose a hazard, primarily due to scalding from hot liquid or steam. This risk can be reduced by:

• Using anti-bumping granules to ensure smooth boiling.
• Wearing safety goggles to protect eyes from splashes.
• Ensuring the apparatus is securely set up to prevent accidental spills.

Solar-Powered Water Stills

Not all distillation requires complex industrial plants. Solar-powered stills offer a simpler, cost-effective way to obtain clean water, especially in remote or less developed areas. Charles Wilson invented the solar-powered water still in 1872 to supply a mining community in Chile.

How a Basin Solar Still Works

Diagram D illustrates a basin solar still:

1. Dirty Water Basin: Impure water is placed in a black basin under a transparent cover.
2. Solar Heating: Sunlight penetrates the cover, heating the water in the basin.
3. Evaporation: Water evaporates, forming water vapor, leaving impurities behind.
4. Condensation: The water vapor rises and condenses on the cooler inner surface of the transparent cover.
5. Collection: The condensed pure water runs down the sloped cover into a collecting chamber.

Advantages of Solar Stills

Solar-powered stills are incredibly useful:

• On a broken-down ship at sea: They can provide essential drinking water from seawater using only available sunlight, crucial for survival.
• In countries with bacterial contamination: They can purify water containing disease-causing bacteria, as bacteria do not evaporate with the water, providing a safe alternative to contaminated sources.

Comparing Lab Stills and Solar Stills

Both laboratory stills (like diagram C) and basin solar stills (like diagram D) utilize distillation for purification, but they have distinct differences:

• Similarities:

• Both heat water to create steam.

• Both cool the steam to condense it back into liquid water.

• Both leave dissolved solids/impurities behind.

• Both produce pure water.

• Differences:

• Energy Source: Lab stills typically use external heaters (e.g., Bunsen burner), while solar stills use sunlight.

• Scale/Control: Lab stills offer precise control over heating and cooling for experimental purposes. Solar stills are generally simpler and less controlled, relying on natural solar intensity.

• Equipment: Lab stills involve glassware like flasks and condensers. Solar stills often use a basin and a transparent cover.

• Cost/Complexity: Solar stills are often cheaper and simpler to build and operate, making them suitable for remote or emergency use. Lab stills can be more complex and expensive.

For obtaining the most pure water from salty water, the laboratory still apparatus (Diagram C) is generally better. This is because it allows for more controlled heating and efficient cooling through the Liebig condenser, ensuring a more complete separation of water from solutes. Solar stills, while effective, can be less efficient due to varying sunlight intensity and potentially less rapid cooling.

Even inflatable solar stills are part of emergency supplies on life rafts, highlighting the critical role distillation plays in ensuring access to safe drinking water worldwide.

Frequently Asked Questions About Distillation and Desalination

What does desalination mean and why is it important?

Desalination means removing salts and other dissolved substances from water, typically seawater, to make it safe to drink. It's crucial in regions with limited freshwater sources, providing a vital supply for human consumption and agriculture.

Why are desalination plants often built next to the sea?

Desalination plants are built next to the sea because seawater is their primary raw material. Locating them close to the source minimizes the cost and energy required to transport the vast quantities of water needed for processing, making the operation more efficient.

What is another product of distilling seawater besides drinking water?

Another product of distilling seawater is concentrated salt brine. This is the highly concentrated solution of salts that remains in the heating flask after the pure water has evaporated. This brine is often a waste product but can sometimes be processed further to extract valuable minerals.

How does distillation make water pure?

Distillation purifies water by separating it from dissolved solids. When water is heated and evaporates, only the water turns into steam, leaving behind impurities like salts and bacteria. The steam is then collected and condensed back into liquid water, which is now pure because the contaminants could not evaporate.

Can distillation remove bacteria from water?

Yes, distillation is an effective method for removing bacteria and other microorganisms from water. Since bacteria cannot evaporate and turn into steam with the water, they are left behind in the boiling vessel, ensuring the condensed water is free from these biological contaminants.