Water production technologies can be a good alternative to large waterworks in countries with access to the sea.
In late 2015, India began construction of a vast hydraulic network joining 37 rivers from around the country. A precedent occurred years earlier, when the Chinese diverted the Yangtze River, the longest river in China, and the third longest in the world after the Amazon and the Nile.
This monumental project initiated by the Indian authorities to alleviate the major problem of water shortage in India requires the construction of over 15,000km of river links and will cost at least $168 billion.
As well as the financial cost, there are the resulting environmental consequences of completely redesigning the geography and the river flow in the country. The works will require the deforestation of large areas, changing the biodiversity of rivers and displacing more than half a million people.
This large project may solve the problem of water shortages in large parts of the country, but it will definitely not solve the other major existing water-related problem: its poor quality. The river Ganges, the holy river of Hinduism, which feeds hundreds of millions of people, is one of the most polluted in the world; not surprisingly, as it has to deal with the waste of 10% of the world’s population. It is estimated that the poor quality of the river water in India is responsible for the deaths of 600,000 people a year.
Countries with access to the sea or the ocean – and India has more than 7,500km of coastline – have a much more economical, sustainable and viable alternative to such ambitious hydraulic projects. They can obtain water fit for human consumption by desalination of sea water using membrane technologies. Reverse osmosis is the most developed of these for desalination of seawater, but recently a new technique has emerged, representing a step forward from reverse osmosis, and is called forward osmosis.
High quality water production from forward osmosis is possible and technically feasible. It is based on the principle that two solutions with different concentrations of a solute, separated by a semipermeable membrane, experience a net flow of solvent to equalise the two concentrations. The solvent flows from the lower concentration solution (of lower osmotic pressure) to the higher solute concentration (with higher osmotic pressure).
Moreover, this technique does not require any special environmental conditions, as an ambient temperature with a pressure of 2-3 bar, to overcome the frictional resistance of the membrane, is sufficient. These mild operating conditions mean very little energy is consumed in the process, which is a key factor for producing a high quantity water of without suffering escalating costs.
The process is based on the use of a solution of high osmotic pressure, called the draw solution (or osmotic agent), which receives the solvent crossing the membrane. This draw solution must be one that allows quick, easy and affordable separation of the recovered solvent.
Forward osmosis is more competitive than reverse osmosis, as it has key advantages. One of these is the very low operating pressure, resulting in controlled energy costs. Also, forward osmosis membranes are more resistant to fouling and attack by chlorides than reverse osmosis ones, so cleaning is less frequent and less aggressive, resulting in membranes that last longer.
However, a second step is required after forward osmosis. The first step extracts pure water from the contaminated water and mixes it with the draw solution; and the second step removes this extracted water from the draw solution, to produce high quality water (Figure 1).
The two processes, forward osmosis and draw solution regeneration, are linked by recirculating the draw solution, which has a higher osmotic pressure than the feed solution. Pure water flows from the feed solution through the membrane into the concentrated draw solution, thus diluting it. The diluted draw solution can be concentrated again and pure water extracted in the regeneration system. The combination of the two systems is a key parameter in the system design so that the assembly operation is simple, robust and reliable.
The most important advantages of forward osmosis for water production compared to conventional reverse osmosis are:
- Reduced energy consumption; especially in solutions of high osmotic pressure.
- Less fouling of the membrane.
- Easier and more effective cleaning of the membrane.
- Longer membrane life.
- Lower operating costs.
Figure 1
Thus, forward osmosis is a fully viable and reliable technology emerging as a clear competitor to conventional reverse osmosis and other separation technologies, and is an excellent choice for producing high quality water, especially when the feed solution has a high salt concentration. To summarise, forward osmosis is:
- A process producing high quality water at low operating costs.
- An emerging alternative technology to conventional processes.
- A viable, reliable and efficient process.
- A process with quickly recoverable investment costs due to its low operating costs.
