About four decades ago, scientists blew the dust off a 250-year-old technology and began using it to produce potable water. Called reverse osmosis (RO), it has become the leading method of generating pure water from impure water or brine.
Osmosis is a process by which a liquid moves through a semipermeable membrane from a solution of lower concentration to one of higher concentration due to natural pressure. Reverse osmosis is when the flow is reversed. If you squeeze sea water through a membrane, for instance, pure water will get to the other side. Today, RO is the leading desalination technology. And interesting technologies are further being explored in India and elsewhere to make potable water production cheaper.
Here are some technologies that have the potential of being mainstreamed in the next few years.
This is really osmosis — the word forward is merely to mark the difference between FO and RO. In this, they put the feedwater, say, brine, on one side of a membrane and a higher concentration ‘draw solution’ on the other side. Osmosis causes water from the brine to move to the draw solution. The water is then extracted from the solution. This calls for energy. More than seawater desalination, FO seems better suited for mining water from industrial waste water, such as from pharmaceutical or textile units. It helps if there is waste heat also. Today you can make this work with solar thermal.
The trick here is to design a low-cost draw solution, from which it is easy to recover water. Scientists are trying out chlorides of metals as well as some organic compounds (di-methyl ether and trimethylamine carbon dioxide). Whatever the draw solution, the energy needed to mine water from it is a lot lower than that in RO. RO needs 4 kWhr per cubic metre (1,000 litres); a US company called Trevi Systems says FO needs 1.3 kWhr and is targeting 1 kWhr.
A few FO plants are coming up in West Asia largely because there are not enough freshwater resources in the region. UK company Modern Water has built a 100 cubic-metre-per- day FO plant in Oman.
Humidification-dehumidification (HDH):
This system envisages evaporating sea or brackish water and condensing the vapours to produce potable water. It requires 650 Whr per cubic metre of water. Even if you harvest some of the heat from the de-humidifier, you need about 120 kWhr. So HDH is useful only in special cases. The US-headquartered Gradiant Corporation, a company founded by two Indians, Anurag Bajpayee and Prakash Govindan, is selling equipment with this technology, including to customers in India. Gradiant has also pioneered a new technology called ‘counter flow reverse osmosis’, in which brine, instead of being squeezed into water, is squeezed into a less salty solution — a process that requires less energy. A series of such steps results in pure water. Another company, Bahrain-based Hyrec, has the same technology, though it calls it ‘osmotically-assisted reverse osmosis’.
Membrane distillation (MD) and capacitive de-ionisation (CDI):
Two technologies are knocking at the doors of the markets. MD uses a hydrophobic (water-repelling) membrane that lets only water vapours pass through it, due to a temperature and pressure difference on either side. This technology is promising because of its ability to function on low-grade heat (70-90°C), which solar collectors can supply. One big advantage is that it doesn’t care what kind of feedwater you put in. There is also no ‘fouling’ (deposition of contaminants on the surface) of the membranes — therefore, easy maintenance.
The flipside is that it requires special membranes — those that are available are not cheap. However, scientists are trying to resolve this problem.
Capacitive de-ionisation (CDI):
This uses electrostatic forces to separate ions in the feedwater. Saline water is streamed through pairs of electrodes, held at a potential difference of 1.2V. Because the electrodes are usually made of carbon-based material with a high surface area and high electrical conductivity, ions are adsorbed into the pores of the electrodes, leaving pure water to flow through. When the current polarity is reversed, the ions are released and flushed away.
In a 2019 paper in Science Direct , Sujit Sengupta and other scientists said CDI was “an emerging new technology for the removal of ionic species” from water. The promise here is the development of better electrodes. With porous activated carbon electrodes, scientists have reported water production at a cost of 11 US cents per 1,000 litres, using low salinity (<2,000 ppm) feedwater; in India, running costs of producing RO water costs 52 cents (₹38), though mainly due to the cost of electricity (water company VA Tech Wabag pays ₹6.35 a kWhr for its Chennai plant). But better electrodes are being made — carbon aerogel, carbon nanotubes and graphene are hot candidates. CDI could, therefore, gain traction in the coming years.
Graphene membranes:
Perhaps the most significant development in water technologies is the development of graphene membranes. Graphene is a one-atom-thick sheet of carbon, peeled off graphite; when rolled, it becomes a carbon nanotube. Researchers from IIT Bombay recently reported they had developed a graphene oxide membrane that was “extremely effective in removing chlorides, residual chlorine, phosphorous and fluorides” from seawater. They said the membranes were easy to make, recyclable and “highly economical”. Even if that is an exaggeration, it is clear from scientific reports that graphene membranes are the next big thing in water technology. Dr Suryasarathi Bose, Associate Professor, Indian Institute of Science, Bangalore, is working on graphene oxide liquid crystal membranes.
Scientists are also onto developing ceramic membranes. Prof Anup Keshri of IIT, Patna, is working on one, under a government-funded project.
So, when you marry cutting-edge technology and solar power, what do you get? Drinking water.