Context of the topic:
- With reports of major cities in India struggling to stave off a water crisis, there’s talk about exploring technologies to harness fresh water.
- The one idea that’s been around for a while is desalination, or obtaining freshwater from salt water.
- The city of Chennai already uses desalinated water. However, it only has a limited application, given the operation costs.
What is desalination technology?
- Desalination refers to the removal of salts and minerals from water.
- Today there are two main types of desalination technologies:
- Reverse Osmosis (RO) and
- Thermal (MED, MVC and MSF) desalination
- Reverse Osmosis (RO) desalination uses the principle of osmosis to remove salt and other impurities, by transferring water through a series of semi-permeable membranes.
- RO desalination came about in the late 1950s.
- Osmosis involves ‘a solvent (such as water) naturally moving from an area of low solute concentration, through a membrane, to an area of high solute concentration.
- A reverse osmosis system applies an external pressure to reverse the natural flow of solvent and so seawater or brackish water is pressurised against one surface of the membrane, causing salt-depleted water to move across the membrane, releasing clean water from the low-pressure side’.
- Seawater has Total Dissolved Solids (TDS) i.e. a measure of salinity, close to 35,000 parts per million (ppm), or equivalent to 35 g of salt per one litre/kg of water.
- An effective network of RO plants reduce this down to about 200-500 ppm.
- There are about 18,000 desalination plants in the world across 150 countries and nearly half of Israel’s water is sourced through desalination.
RO Desalination Plants in India
- Years of water crises in Chennai saw the government set up two desalination plants between 2010 and 2013.
- Together, they meet little under a fourth of the city’s water requirement of 830 MLD.
- Buoyed by the success of these plants, the city’s water authorities are planning to install two more plants with capacities of 150 MLD (to be operational by 2021) and 400 MLD, at a cost of around Rs 1,260 crore (funded by the German agency, KfW) and Rs 4,000 crore (funded by the Japan International Cooperation Agency), respectively.
- In November 2018, Gujarat government also announced plans of setting up a 100 MLD RO plant at the Jodiya coast in Jamnagar district.
- This would go a long way in ‘solving’ the water availability problems in the drought-prone Saurashtra region.
- Other plants of a similar size are expected to come up in Dwarka, Kutch, Dahej, Somnath, Bhavnagar and Pipavav, which are all coastal places in Gujarat.
- There are also a slew of desalination plants that cater to industrial purposes.
- For now, India’s real-world experience with desalination plants is restricted to Chennai.
Environmental issues with RO Plants
- The major environmental challenge they pose is the deposition of brine (highly concentrated salt water) along the shores.
- The brine being deposited along the seashore is triggering changes along the coastline and reducing the availability of prawn, sardine and mackerel.
- Hyper salinity along the shore affects plankton, which is the main food for several of these fish species.
- Moreover, the high pressure motors needed to draw in the seawater end up sucking in small fish and life forms, thereby crushing and killing them.
Is RO water healthy?
- Earlier, there were concerns that desalinated water was shorn of vital minerals such as calcium, magnesium, zinc, sodium, potassium and carbonates, referred to as TDS.
- Higher quantities of these salts in desalination plants tend to corrode the membranes and filtration system in these plants.
- Highly desalinated water has a TDS of less than 50 milligrams per litre, is pure, but does not taste like water. However, anything from 100 mg/l to 600 mg/l is considered as good quality potable water.
- Most RO plants, put the water through a ‘post-treatment’ process whereby salts are added to make TDS around 300 mg/l.
- Several of the home-RO systems that are common in affluent Indian homes, too employ post-treatment and add salts to water.
Low- Temperature Thermal Desalination
- This is a low-temperature thermal desalination (LTTD) technique for instance which works on the principle that water in the ocean 1,000 or 2,000 feet below is about 4º C to 8º C colder than surface water.
- Salty surface water is collected in a tank and subject to high pressure (via an external power source).
- This pressured water vapourises and this is trapped in tubes or a chamber.
- Cold water plumbed from the ocean depths is passed over these tubes and the vapour condenses into fresh water and the resulting salt diverted away
Thermal Desalination Plants in India
- In 2005, National Institute of Ocean Technology (NIOT), set up a 100,000 litre-a-day plant in Kavaratti, Lakshwadeep islands and this has been providing water to about 10,000 residents.
- There are also 1.5 lakh litres a day plants proposed at Amini, Androth, Chetlat, Kadamat, Kalpeni and Kiltan islands.
- There are plants of similar capacity proposed at Minicoy and Agatti islands.
Ocean Thermal Energy Conversion:
- The most ambitious research project is a 10 million litre a day plant that is proposed to be built in the deep ocean, 50 kilometres away from the Chennai coast.
- Ocean Thermal Energy Conversion plant will draw power from the vapour generated as a part of the desalination process.
- This vapour will run a turbine and thereby will be independent of an external power source.
- While great in theory, there is no guarantee it will work commercially.
- For one, this ocean-based plant requires a pipe that needs to travel 50 kilometres underground in the sea before it reaches the mainland.
- The NIOT has in the past had significant problems in managing such a pipe.
- Therefore it could be hard to convince private players to invest in such a technology.