Treating Seawater: Desalination trends, developments and outlook

Urban areas in India are experiencing burgeoning growth in population, creating an urgent need for clean and po­t­able water. Seawater desalination has been serving as a promising technique for sourcing potable water. The unique advantage of the In­dian coastal states with 7,800 km of water shoreline offers a large potential for desalination. Various factors govern the use of desalination for meeting water supply requirements, some of the critical ones being geographical proximity to seawater, environmental impact, co­st of treatment and consumption of energy in the process of treatment. Desalination pla­nts in India use membrane, thermal ba­sed and hybrid technologies to process water. In recent years, trends such as brine disposal through the use of zero-liquid discharge (ZLD), counter-flow reverse osmosis (CFRO) and low temperature thermal distillation (LLTD) technologies have also gained traction.

Uptake of technologies

Membrane technology is the most prevalent ac­ross plants in India. It involves separating the salt from seawater by passing it through membranes with high pressure, through rever­se os­mosis (RO), electrodialysis and forward os­mosis. The industry has also witnessed the up­take of thermal-based technologies such as multi-stage flash distillation (MSFD), multieffect distillation (MED), LLTD and thermal/me­chanical vapour compression. Hybrid technologies, which make use of two or more of these technologies for tr­eatment are also being explored.

As mentioned above, RO technology is mo­st widely used in desalination plants in the country. The Minjur desalination plant in Chennai is one such example, which uses 8,600 seawater RO membranes along with 248 pressure vessels, 23 pressure exchangers, five highpressu­re pumps, 16 pressure filter vessels, etc. The treated water from the plant is used for industrial purposes, including at Ennore port and the North Chennai Thermal Power Station as well as for public use during water-scarce seasons. Recen­tly, in April 2023, the Chennai Met­ropolitan Wa­ter Supply and Sewerage Board has awarded a project for the development of a 400 million litre per day (mld) RO-based desalination plant to Va Tech Wabag. Water treated through the plant is expected to serve the water requirement of arou­nd 750 mld in the city.

In another instance, MED has been taken up in the Reliance desalination plant in Jam­nagar by IDE Technologies India Private Limi-ted. It has been used for reducing energy consumption and costs with a low temperature process and minimal pretreatment. The plant is one of the largest MED sites worldwide, with a total capacity of 400 mld.

Integration with renewable energy

The industry is increasingly turning towards renewable energy to power desalination plants. For instance, renewable energy is planned to be integrated into the Manori desalination pl­ant in Mumbai. The plant will be operated com­pletely on solar energy by the Brihanmum­bai Muni­cipal Corporation. It would treat 200 mld of seawater, which would be augmented to a capacity of 400 mld for future needs. The project is likely to be commissioned in the second half of 2023.

Meanwhile, government programmes such as the National Water Mission by the Ministry of Jal Shakti have laid down targets for renewable energy adoption. Goal 3 of the mission emphasises the adoption of RO and MSFD-based desalination with the use of renewable energy such as solar. Meanwhile, NITI Aayog, the Council of Scientific and Industrial Re­search- Central Salt and Marine Chemicals Research Institute and the Department of Science and Technology (DST), Ministry of Sci­ence and Technology, are some of the bodies that are also working with various stakeholders to enable desalination projects to operate on renewable energy. To this end, NITI Aayog is promoting the establishment of plants with a capacity of about 1 mld or less.

An impetus to such initiatives is also witnessed under the National Green Hydrogen Mission. VOC Port in Thoothukudi is planning the installation of a 5 mld desalination plant to be operated through renewable energy generated at the port. It would be developed on RO te­chnology through public-private partnership (PPP) mode and operators are mandated to de­velop a solar power plant for the electricity requ­ired on the site. Such integration would also help in reducing the carbon footprint of de­sa­li­na­tion plants with reduced dependence on coal/carbon-intensive power generation. As per industry estimates, desalination plants ha­ve a high carbon footprint with an estimated re­lease of 7 tonnes of carbon dioxide for the desalination of 1,000 cubic metres of seawater.

Focus on appropriate brine disposal

One of the biggest challenges of operations of desalination projects is the disposal of brine in seawater. Brine is generated as a by-product in desalination plants. Given its high salinity and temperature, the brine disposed of in the sea deteriorates the aquatic ecosystem and the quality of water resources. Brine management, therefore, has caught the attention of the government and solutions such as ZLD, CFRO and LTTD are being explored.

ZLD-based desalination creates a closed loop of water flow, which lowers the impact of brine discharged, while CFRO is a membrane-based technique that desalinates brines. In contrast, in LTTD, the surface seawater is evaporated at low pressures and the resultant fresh vapour is condensed with deep-sea cold water.

Desalination plants in the country are de­ploying these technologies to reduce brine disposal. For instance, desalination plants coming up at Amini, Androth, Chetlet, Kadmat, Kalpeni and Kiltan in Lakshadweep, having 0.15 mld capacity each, are deploying LTTD technology. These plants are being established by the National Institute of Ocean Technology, under the Ministry of Earth Sciences. The technology is appropriate to be used at these islands since the coastline offers a temperature difference of about 15 °C  between sea surface water and de­ep-sea water, suitable for the treatment process.

Future outlook and focus areas

The desalination market in India is projected to grow at a rate of over 3 per cent per annum. According to India Infrastructure Research, around 31 municipal and industrial desalination plants with a total capacity of around 1,800 mld are expected to come up in the country by 2026. States such as Tamil Nadu and Gujarat are expected to have the largest share of the upcoming projects. The upcoming refineries, power plants and metal industries in the coastal areas will drive demand as the­se are likely to adopt new desalination tech­no­logies. This, in turn, will open up opportunities for plant equipment and membrane providers in the future.

Moreover, government programmes such as the National Solar Mission, Swachh Bharat Abhi­yan, and Make in India promote the use of rene­w­able energy in desalination, conscious disposal and reuse of brine, and the indigenous development of modern membranes for plants respectively. At present, the National Water Mis­sion encourages these projects to be undertaken in PPP mode in order to increase the quality of work, while lowering capex and op­erating co­sts. The uptake of private financing models such as build-own-operate is also foreseen in the sector. Some of the upcoming big projects under PPP mode include the Perur desalination plant in Tamil Nadu and a plant each in Mandvi, Dw­arka, Mundra and Ghogha in Gujarat.

Moreover, the adoption of automation and digital tools would reduce the production costs of these projects in the coming years. Going forward, the use of high resolution satellite im­agery of sea and internet of things platforms are also expected to help in the accurate ass­essment of seawater temperature, salinity and detection of red tide.

Water scarcity is a huge threat to urban ar­eas and scaling up the desalination technology would offer plausible support for a stable supply of water. The alternative use of brackish wa­ter, which is more saline than fresh water and lesser than seawater, is also being researched by the DST. The source of this water is commonly found in estuaries, aquifers and coastal areas and its use could serve the arid areas.