The world's water consumption rate is doubling every 20 years, outpacing by two times the rate of population growth. It is projected that by the year 2025 water demand will exceed supply by 56%, due to persistent regional droughts, shifting of the population to urban coastal cities, and water needed for industrial growth. To cope with the demand desalinating brackish water or sea water is one of the solutions adopting desalination technology.
Desalination is the process of removing salts from seawater to provide purified water for industry, irrigation and drinking.
Primary methods of desalination, or removing salt from water, include thermal processes, such as distillation, and membrane processes, such as reverse osmosis and electro dialysis. Other methods of desalination include freezing, solar dehumidification and membrane distillation (combination of thermal and membrane methods).
Thermal methods require a large input of energy to heat the water. Membrane methods of desalination include reverse osmosis and electro dialysis. Membrane processes use selective permeability to separate salts and water.
Membrane methods of desalination require energy for mechanical processes. Reverse osmosis is the most cost effective in many applications. Other considerations include the degree of purity required, scale of production and cost of chemical pretreatments.
Reverse osmosis, uses membranes that filter out the salt, but these require strong pumps to maintain the high pressure needed to push the water through the membrane, and are subject to fouling and blockage of the pores in the membrane by salt and contaminants. The reverse osmosis process also requires tremendous amounts of energy for desalination.
Nanoparticles for cheaper desalination
The key problems that exist with the current membrane technology are biofouling and mechanical degradation of the membranes.Hence it is necessary to develop a coating for commercially available membranes, which will inhibit biofouling and/or biofoulant growth or reproduction, and synthesize an additive for membrane materials, which will significantly reduce the compaction experienced by current, state of the art polymeric membranes. By introducing a thin layer of coating and nanoparticles in the membrane system, the flow of water can be improved, the need for cleaning can be reduced and the membrane structure can be strengthened. If the potential reduction of biofouling by 75 per cent is achieved, the energy bill of a desalination plant will be reduced by up 30 per cent, downtime for cleaning could be halved and the life of the membranes extended from two to 10 years.
Nanotubes help to solve desalination problem
Current desalination methods force seawater through a filter using energies four times larger than necessary. Throughout the desalination process salt must be removed from one side of the filter to avoid the need to apply even larger energies.
A team of researchers from The Australian National University have discovered a way to remove salt from sea water using nanotubes made from boron and nitrogen atoms that will make the process up to five times faster. Boron nitride nanotubes can be thought of as a hollow cylindrical tube made up of boron and nitrogen atoms. Using boron nitride nanotubes and the same operating pressure as current desalination methods, we can achieve 100 percent salt rejection for concentrations twice that of seawater with water flowing four times faster, which means a much faster and more efficient desalination process.
A carbon nanotube technology can be used to desalinate water and can be applied to other liquid based separations also.
Scientists have used computational tools to simulate the water and salt moving through the nanotubes and found that the boron nitride nanotubes not only eliminate salt but also allow water to flow through extraordinarily fast, comparable to biological water channels naturally found in the body.
Nano magnetic particles for desalination
It is the use of a self-assembled structure of the iron-storage protein ferritin to make nanoscale magnetic particles.
The Nanomagnetic approach exploits a water purification process called Forward Osmosis. Basically, it is similar to a bag made of a nanoporous membrane filled with these magnetic nanoparticles that serves, and the bag is placed into contaminated water. The dirty water would be pulled through the membrane into the bag by osmotic pressure exerted by the difference in concentration and the nanoparticles in the bag and the bag can be used again
Compared to RO, forward (or Direct) osmosis offers lower energy process and efficiency, increased membrane lifetimes and reduced fouling and water recovery exceeding 85% for seawater (40-60% for Reverse Osmosis).