Several avenues for technological interventions in the desalination sector are opening up in India. The membrane-based and thermal technologies that have been used in projects across the country have backfired due to their environmental drawbacks. Therefore, the industry is turning towards finding better alternatives. The high scope of integrating renewable energy into these desalination plants has been recognised by the government. The Department of Science and Technology, Central Salt and Marine Chemical Research Institute (CSMCRI); the Bhabha Atomic Research Centre; and the National Environmental Engineering Research Institute have been working on the research and development (R&D) of such solutions.
The state governments of Gujarat, Maharashtra, Tamil Nadu, Karnataka and Odisha have made investments in desalination plants. Several initiatives towards advanced and sustainable technology adoption are also under way. In addition, emerging solutions like zero-liquid discharge (ZLD) and low temperature thermal desalination (LTTD) have made their way into the Indian market.
A look at the solutions available in India for the conversion of seawater into potable water using desalination, their challenges and emerging technologies in the country and globally…
Membrane-based and thermal technologies
The key technologies that have been widely implemented in many of the desalination projects in India are membrane and thermal based. While membrane-based solutions include reverse osmosis (RO), electrodialysis and electrodialysis reversal, thermal technologies mainly focus on multistage flash distillation (MSF), multi-effect distillation (MED), low temperature evaporation and mechanical vapour compression solutions. Both technologies are at par in terms of their land and capex requirements.
Several completed projects have deployed one or more of these technologies. The Reliance desalination plant in Jamnagar built by IDE Technologies India Private Limited with MED application is one of the largest MED sites worldwide with a total capacity of 400 million litres per day (mld). It has reduced its energy consumption with a low temperature process and minimal pretreatment that saves costs. The Mundra desalination plant in Gujarat executed by Aquatech Systems Asia Private Limited, the Jamnagar refinery in Gujarat, and the pilot plant at the Panipat refinery in Haryana by VA Tech Wabag Limited are some of the projects that have successfully used membrane-based technology.
The application of these technologies faces some challenges. The disposal of a large amount of brine, produced as a by-product in RO desalination, into the sea deteriorates the aquatic ecosystem with high salinity and temperature. The carbon footprint of RO desalination of seawater is another challenge. An estimated 6.7 tonnes of carbon dioxide is released with the desalination of 1,000 cubic metres of seawater. Other complexities include seasonal fluctuation in seawater characteristics, the high cost of desalinated water as compared to freshwater sources, and the high cost of brine management. Moreover, the storage requirement for desalinated water before or during its distribution can corrode the storage material, which affects the water quality with the growth of pathogenic and harmful organisms.
The technologies that are gradually picking up pace and have the potential for further commercialisation in India include the ZLD solution and LTTD. These are based on the principles of sustainable use, efficient energy consumption and improved performance of desalination plants. ZLD-based desalination mitigates the impact of brine discharge by creating a closed loop of water flow. Meanwhile, LTTD utilises the temperature difference to produce potable water by evaporating surface seawater at low pressures and condensing the resultant fresh vapour with deep sea cold water.
The exploration of ZLD has been undertaken by a few companies in India such as Aquatech Systems Asia Private Limited, Hubert Enviro Care Systems Private Limited and IDE Technologies India Private Limited. They have used the technology of thermal evaporation, crystallisation and concentrators to implement this solution in a few projects. The Chemplast Mettur plant in Tamil Nadu is one such project by Aquatech Systems Asia Private Limited. It includes a ZLD system based on high efficiency RO technology, which operates in a high pH environment. The overall recovery of the system is over 97 per cent in this project. Similarly, LTTD technology has been implemented in three towns of Lakshadweep, that is, Kavaratti, Agatti and Minicoy, and further projects are under way in six other islands, each with a capacity to generate 0.15 mld of drinking water. The growth of such mature technologies that reduce the cost of energy used in plants has a positive effect on the cost of desalination. As per India Infrastructure Research, the cost of desalination has reduced sharply from Rs 120 per kilolitre (kl) in 1995 to Rs 40-50 per kl in 2019.
The use of renewable energy sources for desalination has also gained attention in recent times. Their application would further optimise costs with the integration of plants with solar, wind or tidal power. NITI Aayog has made efforts to enable such plants with a capacity of about 1 mld or less by organising various stakeholder consultations. Meanwhile, CSMCRI and the Council of Scientific and Industrial Research with the Department of Science and Technology, the Government of India and the Government of Gujarat have discussed the possibility of expanding these sustainable measures.
Several initiatives have been taken by start-up companies like Solnce Green Energy to develop solar energy-based desalination plants in coastal states such as Gujarat. The start-up has been successful in converting 1,500 litres of seawater into potable water every day through the use of solar energy-based desalination. However, the supply of water is currently limited to regular use and is not potable. Furthermore, the LTTD plants installed at Ramanathapuram and Kanyakumari in Tamil Nadu have made use of concentrated solar energy with linear fresnel-type collectors for heating and evaporating seawater.
Global best cases
Some of the advanced technologies that are being developed and implemented worldwide are nanofiltration, forward osmosis (FO), MSF and hybrid systems. Nanofiltration has membrane properties that lie between ultrafiltration and RO. It works on lower power consumption due to its lower pressure requirement. The technology of FO is guided by an osmotic pressure gradient across the membrane, whereas in MSF the seawater feed is pressurised, heated and discharged to a chamber that is maintained below the saturation vapour pressure of the water. Further, a fraction of this water is converted into steam and condensed on the exterior surface of the heat transfer tubing.
Many research organisations, government bodies and private companies globally are taking large leaps towards R&D and encouraging the application of these technologies. King Abdullah University of Science and Technology in Saudi Arabia has developed a membrane with rapid water desalination performance in FO and RO configurations. The team is also working on the improvement of its anti-fouling property, mechanical strength and long-term chemical stability for future practical applications. Similarly, the Water Authority of Israel has focused on new energy resources as part of its future technology plans. This includes the incorporation of Skoda steam turbines and GE Gas turbines into its operations. Also, a low-cost passive PV evaporation system is being developed by researchers from the Massachusetts Institute of Technology and Shanghai Jiao Tong University in China for desalination. This solution uses wicks to draw salty water to avoid salt build-up, thus improving the system’s effectiveness and maintenance.
The way forward
The potential of technological intervention can be tapped with the greater use of sustainable energy sources. Although there is immense potential for meeting the water demand through desalination given the long Indian coastline, it comes with enhanced risks associated with climate change, which would increase the vulnerability of these plants. A significant ground of research has been done in India for the use of renewable energy, but the desalination sector is still leapfrogging in the adoption of advanced solutions in its plants. Nanofiltration, FO and PV evaporation systems that are being applied in other countries have immense scope for implementation in India with higher investments, infrastructural readiness and regulatory support of the government.
With inputs from presentations and remarks at a recent India Infrastructure conference