Stepping up from its ongoing initiative of providing potable water on six islands of Lakshadweep using low temperature thermal desalination (LTTD) technology, the Chennai-based National Institute of Ocean Technology (NIOT) is working at making this process free of emissions.
- Currently, the desalination plants are powered by diesel generator sets — there being no other source of power on the islands.
- The LTTD exploits the difference in temperature (nearly 15 degrees Celsius) in ocean water at the surface and at depths of about 600 feet.
- This cold water condenses water at the surface, which is warmer but whose pressure has been lowered using vacuum pumps.
- Such depressurised water can evaporate even at ambient temperatures, this resulting vapour when condensed is free of salts and contaminants and fit to consume.
- Need for an emission free process:
- The need for diesel power to reduce the water pressure means that the process is not fossil-fuel free and also consumes diesel.
- The National Institute of Ocean Technology (NIOT) was established in 1993 as an autonomous society under the Ministry of Earth Sciences.
to develop reliable indigenous technologies to solve various engineering problems associated with harvesting of non-living and living resources in India’s exclusive economic zone.
What is desalination?
- Desalination is the process by which the dissolved mineral salts in water are removed to obtain fresh water for human consumption or agricultural purposes.
- Desalination occurs naturally during the water cycle:
- The evaporation of seawater leaves salt behind and forms clouds that give rise to rain.
- The first country to adopt this process at a large scale was Australia.
- It has plants in the main cities that operate through reverse osmosis.
- Saudi Arabia is the leading desalination country by volume, followed by the United Arab Emirates, both of which are desert countries and highly dependent on this process.
Distillation, consisting of boiling seawater in a still, collecting steam and condensing it to obtain fresh water, is the most obvious method for removing salt, but not the most effective one since it consumes large amounts of energy.
- It is the most used process and consumes less energy than the rest, as it is based on the use of semipermeable membranes that allow the water to pass, but not the salt.
- These membranes are made of ultra-thin polyamide, which can become contaminated with bacteria so the water must be treated.
- Imitating the water cycle, it consists of evaporating seawater in large facilities with roofs where it is condensed and collected as fresh water.
- Although the energy used is the sun’s heat, large areas of land are required.
It consists of moving the salt water through electrically charged membranes that trap the salt ions dissolved in the water, allowing fresh water to be extracted.
- It is a process that uses nanotube membranes with higher permeability than reverse osmosis ones, which allows more water to be processed in less space using less energy.
- These membranes are manufactured with sulfonated compounds which, in addition to salt, eliminate traces of pollutants.
Gas hydrate formation
- Gas hydrates are solid crystals that are formed by combining water with a gas, such as propane, at high pressure and at low temperature.
- During the process, all the salts and impurities present in the water disappear and as the temperature increases the gas can be recovered leaving fresh water
Key issues with desalination:
- It may be key to solving water scarcity in the future.
- The water desalination process is not impact-free since the residue resulting from the process is brine, wastewater with a high concentration of salt and pollutants, which in many cases is discharged into the sea and affects ecosystems.
- There is also a risk of seepage that can contaminate coastal aquifers.
- Many desalination processes require heating water, pressurising it, or both, entailing a high energy cost.
- Use renewable energy, such as solar, to reduce the consumption of the desalination plants.
- To use biotechnology, for example, by cultivating cyanobacteria that are capable of processing seawater, forming a low-salinity deposit around it.
|Desalination in India
· In India, Tamil Nadu has been the pioneer in using this technology, setting up two desalination plants near Chennai in 2010 and then 2013.
· The other states that have proposed these plants are Gujarat, which has announced to set up a 100 MLD RO plant at the Jodiya coast in Jamnagar district.
· Andhra Pradesh, too, has plans of setting up a plant.
SOURCE: THE HINDU, THE ECONOMIC TIMES, PIB