18 Feb 2020 19:49 IST

Need some drinking water? FO!

No, it’s not what you think! It means forward osmosis, a process that can be cheaper with renewables

Don’t blame me for the headline. Water technologists have abbreviated ‘forward osmosis’ to FO — a term you will frequently come across in the future when you discuss the critical subject of drinking water.

Most of us have heard of ‘reverse osmosis’, which is a method of obtaining pure water from, say, sea water, by forcing the water through a membrane. When this is done, drinking water is obtained on the other side, leaving behind sea water that is far heavier in salts and other minerals — in this context, called ‘impurities’. This dense water is disposed of back in the sea.

Pretty much the same happens in the RO system in your kitchen. Only, instead of sea water, you input brackish water, which means water from underground aquifers that is somewhat salty, or less than potable.

Back to basics

Osmosis is something we all learnt about in school. For the benefit of those who may have forgotten what it is: if you dissolve one kg of salt in a bucket of water and two kg of salt in another bucket of water, and pour the two waters into a vessel so that they are separated by a ‘semi-permeable membrane’, guess what would happen? Water in the chamber with less salt would slowly seep into the other chamber containing water with more salt.

Liquids move through a membrane from a less concentrated region to more concentrated one. This is osmosis. This is why if you soak a raisin in water, it bloats up.

Reverse osmosis is making the liquid flow the other way, by pushing it. Thus, when sea water is pushed through the membrane, you get drinking water on the other side.

Now, these RO systems consume energy, because water has to be forced through the membrane. Energy costs money.

The operating cost of a desalination plant near Chennai that produces 100 million litres of water every day works out to ₹38 to produce a 1,000 litres; of that, ₹23 is the electricity cost.

Emerging alternative

To get around the energy consumption problem, water technologists have been, for some years, toying with other methods of producing pure water. There are many of them, several in use. For example, almost all the water needs of the countries of the Arabian peninsula are met by desalinated water. Here, they obtain water by flash distillation, which calls for thermal power. This is even costlier than RO water. The Arabians can afford it because they have cheap oil and gas in abundance; others don’t.

Of the emerging alternatives is FO. Forward osmosis is just plain vanilla osmosis. It is called ‘forward osmosis’ only to distinguish it from ‘reverse osmosis’ which has become popular and ubiquitous.

But now, by all accounts, FO is gaining acceptance.

To produce drinking water through FO, you only need water that is more concentrated on the other side. For example, you could have normal sea water on one side of the membrane and the highly salty discharge water from a RO plant on the other side; or brackish water on one side and sea water on the other. Pure water will diffuse from the less concentrated side to the more concentrated one. You end up with more concentration on ‘this’ side; more dilution on the other.

Obviously, you will not get pure drinking water right at this stage. You have to take the more diluted liquid and process it further to get your product. This is the conventional stage — use either thermal or RO technologies. Only, with FO married to it, you now produce water a lot cheaper.

“This process has the ability to desalinate ocean water at one-eighth the electrical energy of the current RO systems,” says a US-based company called Trevi Systems, which is one of the very few companies in the world that runs a commercial-scale FO plant.

Many experts believe that FO has a big role in industrial water applications. Another American company, aptly called Forward Water Technologies, says that FO can be “deployed in several industries for water reuse and recycling because of its (FO’s) “low energy requirements and extraordinary capabilities.”

FO’s cousin

Now, FO has a cousin or a rival, depending on how you see it. It is another technology for desalination. It is called membrane distillation.

It is simplicity itself. In an osmosis unit — forward or reverse — you push or pull water through a membrane. In membrane distillation — let’s call it MD — you vaporise the water and let it pass through the membrane. With this technology, you’d use a lot less energy than you would if you were to just make steam out of sea water and condense the steam back into pure water.

MD is only a step behind FO in terms of technological readiness for commercial use. The beauty about both FO and MD is that they offer immense scope to be married with renewable energy, say, solar.

The upshot is this: emerging technologies are going to make desalination more ubiquitous. Given the oncoming water stress, this is just what the doctor ordered.