How difficult is it to make drinking water from salty ocean water?

 A backpack can desalinate seawater in an emergency; supplying a major city takes vast pumps, membranes, power, and careful environmental engineering.

Turning seawater into drinking water is one of those things that sounds almost magical until you look at the energy bill. It is absolutely possible, done at huge scale in many countries, and technically routine. The difficulty is not “can it be done?” but “can it be done cheaply, reliably, and in large enough quantities?”

The ocean is salty in a very stubborn way. Typical seawater contains about 35 grams of dissolved salts per liter. To make it drinkable, a desalination plant has to separate most of that salt from the water, and nature does not give that separation away for free.

There are two main ways this is done:

1. Reverse osmosis

• Seawater is pushed through extremely fine membranes at very high pressure.
• Water molecules pass through; salts mostly do not.
• This is the dominant modern method because it is usually the most energy-efficient.

1. Thermal desalination

• Seawater is heated so that water evaporates and then condenses as fresh water.
• This works well, but it usually uses more energy than reverse osmosis.
• It is common in places with abundant energy or existing large heat infrastructure.

The real challenges are practical:

• Energy use: Desalination needs a lot more energy than treating river water or groundwater.
• Cost: Plants are expensive to build and maintain.
• Membrane fouling: Reverse osmosis membranes clog with microbes, organic matter, and minerals.
• Corrosion: Saltwater is hard on pipes, pumps, and metal equipment.
• Brine disposal: The concentrated salt waste has to go somewhere, usually back to the sea, which creates environmental concerns if poorly managed.

That said, modern desalination is no longer exotic. Countries such as Saudi Arabia, Israel, the United Arab Emirates, Spain, and Australia rely on it heavily. Some large plants produce hundreds of millions of liters per day. The Sorek plants in Israel are often cited as examples of desalination becoming part of ordinary national water infrastructure rather than a technological curiosity.

So the honest answer is: technically, it is not very difficult anymore; economically and energetically, it is still demanding.

In other words, making drinking water from ocean water is straightforward in principle, hard in scale, and expensive compared with using fresh water that is already available.