Question

Describe the two main methods used to desalinate seawater. Why is the drinking water produced by desalination relatively expensive?

Short answer

The two main desalination methods are reverse osmosis and thermal distillation. Desalination water is expensive due to high energy and technology costs.

Step-by-step solution

Reverse Osmosis Desalination

Reverse osmosis is a method where seawater is forced through a semi-permeable membrane. The membrane blocks salts and other impurities, allowing only water molecules to pass through. This process reduces the salinity and provides fresh water suitable for drinking.

Thermal Desalination

Thermal desalination involves heating seawater until it evaporates, leaving salts behind. The vapor is then condensed back into liquid form, resulting in desalinated water. Techniques such as multi-stage flash distillation are commonly used.

Cost Factors in Desalination

Desalination, especially reverse osmosis and thermal methods, is energy-intensive. The cost of energy, the complexity of technology, maintenance of equipment, and environmental considerations contribute to the higher price of desalinated water.

Key concepts

Reverse Osmosis

Reverse osmosis is a popular method for removing salt from seawater, thereby turning it into drinkable water. The term "osmosis" refers to the natural movement of water across a membrane from an area of low salt concentration to high salt concentration. Whereas, reverse osmosis flips this process. In this method, pressure is applied to push water through a semi-permeable membrane from the side of higher salt concentration.

The membrane acts like a very fine sieve, allowing only water molecules to pass through while blocking out over 95% of dissolved salts and impurities. This process requires a significant amount of energy to create the necessary pressure. However, the water produced is of very high quality and safe for human consumption.

• High efficiency in salt removal
• Requires less space compared to other methods
• Energy-intensive, thus impacting operating costs

Thermal Desalination

Thermal desalination is another technique used to remove salt from seawater, but it leverages heat rather than pressure. It involves the process of heating seawater until it turns into vapor, which leaves salt and other impurities behind.

The vapor, now free from salts, is then condensed back into liquid, resulting in pure water. There are several variations of thermal desalination, with multi-stage flash distillation and multiple effect distillation being the most common methods.

• Multi-Stage Flash Distillation: Water is boiled in stages, and the vapor is collected to produce freshwater.
• Multiple Effect Distillation: The vapor produced in one stage condenses to heat the next stage, improving energy efficiency.

This method is particularly well-suited for regions with access to cheap energy or where seawater is abundant. While effective, it consumes a lot of energy, contributing to its high operational costs.

Cost of Desalination

The production of drinking water through desalination is generally more expensive than conventional water sources. The high cost is primarily due to energy consumption. Both reverse osmosis and thermal desalination require substantial energy to operate, whether that's in creating the pressure necessary for reverse osmosis or the heat needed for thermal desalination.

Factors contributing to the cost include:

• High energy consumption, often from fossil fuels
• The need for specialized and durable equipment
• Maintenance and operation expenses
• Environmental impacts, such as brine disposal

In addition, desalination plants require sophisticated technology to ensure efficient operation and regular maintenance to prevent membrane fouling or equipment corrosion. All these elements lead to increased initial and operational costs, making desalinated water more expensive than water from traditional sources.