Reverse Osmosis...A water solution for a city ?
RO water treatment idea.
Chennai has been facing acute water shortage problems for quite some time now. The only primary source of water for citizens are the water tankers. Each water tanker can satisfy the need of only 50 households and that too for one day. This is not a long term answer to the problem, obviously.
Then what is ?
One solution can be 'RO(Reverse Osmosis) Desalination plants'. Chennai has already been using this technology. However, not very popular because of the high operation costs of each plant. City has two plants running each with a capacity of 100 million litres a day. Government is planning to build two more plants till 2021 with capacity of 150 MLD and 400 MLD respectively.
Basically, what RO desalination plant does is to desalinate sea water using a process called Reverse Osmosis. Let me explain. Reverse Osmosis is a process derived from a natural phenomena called 'Osmosis'. To understand the concept of reverse osmosis well enough, we must understand osmosis first.
OSMOSIS
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| Diagram showing Osmosis |
Osmosis is a process where a solution of low salt concentration migrates towards a solution of higher salt concentration when the two solutions are separated by a semi permeable membrane. We have many real life examples of osmosis like in living cells: Cells contain dilute solution ions, sugars and amino acids. So, have a high water potential. The cell membrane is partially permeable. Water will move into and out of the cell by osmosis.
The diagram above shows how the process works. the clean water moves towards saline water through a 'semi permeable membrane'. The minimum pressure required for the clean water to move towards the saline water through the semi permeable membrane is called as Osmotic Pressure, P in this case.
What is Reverse Osmosis?
Reverse Osmosis is the process of osmosis conducted in reverse order. That means, here the more concentrated solution will migrate towards less concentrated solution through a semi permeable membrane with the help of an external pressure that needs to be greater than the 'Osmotic pressure'. For the opposite flow of water to take place the external pressure applied *on the high concentrated water side* needs to be greater than natural osmotic pressure.
Read more about Reverse Osmosis
City's one fourth need for a day is about 830 Million litres a day, which is in hand of RO desalination plants in Chennai. They are a success overall. But also has some downsides which are:-
"I am personally a big fan of The Hindu. The quality of each article is very nice and easy to understand. The process was wisely mentioned by Jacob Kushy, Science editor at The Hindu. He mentions the process briefly in his article. Here's what he says."
Osmosis involves ‘a solvent (such as water) naturally moving from an area of low solute concentration, through a membrane, to an area of high solute concentration. A reverse osmosis system applies an external pressure to reverse the natural flow of solvent and so seawater or brackish water is pressurised against one surface of the membrane, causing salt-depleted water to move across the membrane, releasing clean water from the low-pressure side’. Seawater has Total Dissolved Solids (TDS) — a measure of salinity — close to 35,000 parts per million (ppm), or equivalent to 35 g of salt per one litre/kg of water. An effective network of RO plants reduce this down to about 200-500 ppm. There are about 18,000 desalination plants in the world across 150 countries and nearly half of Israel’s water is sourced through desalination. Read the full article here.Is it being done ?
City's one fourth need for a day is about 830 Million litres a day, which is in hand of RO desalination plants in Chennai. They are a success overall. But also has some downsides which are:-
- Deposition of brine, brine is the left out highly concentrated salt water. After the water is treated. Waste is generated. The waste is in the form of brine, which then poses as an environmental problem.
- Motor sucking waters also end up taking in lots of small fishes and other aquatic life into the containers. They end up getting crushed by a large external pressure. So, another environmental problem.
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