What is Reverse Osmosis and How Do Reverse Osmosis Plants Work?

What is Reverse Osmosis and How Do Reverse Osmosis Plants Work?
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The Need for Fresh Water

Water is the most essential ingredient for life on the planet. Availability of fresh water was the reason that almost all of the ancient civilisations thrived on the banks of rivers. With depleting fresh water resources, increase in population and habitats away from the water sources have made it necessary to produce fresh water form the sea by Desalination Plants. More than 13000 desalination plants world wide produce almost 45 billion litres of water every year. Almost 75 % of these plants are concentrated in the Middle East. Low energy cost and the lack of fresh water resources are the main reasons for the concentration in the Middle East. Also, most of the ships use desalination plants for their fresh water needs.

Two most common method of producing fresh water from the sea water is the Multi Stage Flash Distillation (MSF) and the Reverse Osmosis (RO) process.

How Does a Reverse Osmosis Plant Work?

The currently emerging method which is gaining ground for producing fresh water form the sea water or brackish water is by the Reverse Osmosis (RO) process.

In the RO process water at high pressure passes through very fine membranes that allow only the water molecules to pass through.

The plant basically consists of two phases. The first phase is a pre treatment plant. Here the

  • Filtration and coagulation removes the solids and suspended particles.
  • Chlorination and other chemicals removes the biological organisms.
  • Chemical addition controls the pH and hardness.

Membrane Filtration

The second phase is the membrane filtration. Sea water at high pressure is pumped to the filters. Each of the filter consists of a special membrane wrapped around an inner tube. The pressure forces the water molecules through the membranes to the inner tube. A 60 % yield of fresh water is possible in RO systems. The remaining sea water carries away the collected salts and is returned back into the sea. Increasing the number of filter modules increases the capacity of the plant.

A third stage consisting of another set of membrane filters may be required if the quality of the product has to be further improved.

The main energy requirement in an RO system is for the pumps required to pressurise water to the membranes.

Technological improvements over the last decade have seen the development of membranes that require less pressure, longer life and reduced cost. This has made RO a more economical process than other desalination methods.

This post is part of the series: Desaliantion

Depleting fresh water resources is pushing the implementation of more and more Desalination plants. A brief description of the common desalination process and their merits and demerits are discussed in the series.

  1. How Does Reverse Osmosis Plant Work?
  2. How Desalination by Multi-stage Flash Distillation Works
  3. Comparison Between the Reverse Osmosis and Multi-stage Flash Distillation Methods