Sustainable Tech: Advanced solutions for desalination

Advanced solutions for desalination

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.

Overarching challenges

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.

Emerging technologies

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