Desalination is a natural process that recycles water. Precipitation in the form of rain or snowfall finally flows into the sea or returns backs to the atmosphere through evaporation or else, percolates into the sub-soil. This is a process of natural desalination.
However, with population growth the world over, the demand for fresh water has increased substantially. Existing sources of fresh water are today woefully inadequate to meet the growing needs of populations. This has necessitated the move towards engineered desalination processes.
Logically, desalination activities are concentrated in regions where availability of water is scarce. This is precisely the reason why more than 80 per cent of the world’s desalination plants are located in the water scarce Middle East. In India, too, fresh water desalination technology is concentrated around water-stressed parts of Gujarat, Tamil Nadu and Rajasthan.
Besides producing desalted water for human consumption and industrial requirements, desalination technologies are advantageous in the recovery of water from waste streams. However, reliable statistics are lacking on the number of desalination plants, their capacities, technologies adopted and status in India. As per rough indications, there are more than 1000 membrane based desalination plants of capacities ranging from 20 to 10,000 m3 per day in India. There are a few thermal based desalination plants as well.
Conventional Desalination Processes
Conventional processes for seawater desalination include reverse osmosis and distillation. In the reverse osmosis process, sea water is pressurised through a semi-permeable membrane, with a pore size of 0.5-1.5 nm, to separate the salts. Pre-treatment of the inlet water, multistage filtration, post -treatment and the optional pressure recovery systems associated with the reverse osmosis plants need regular maintenance, supply of spares and trained personnel. Owing to logistical and maintenance issues, only one out of 10 reverse osmosis plants, installed in the Lakshadweep islands during the 1990s, survived.
In the distillation process, which includes multi-stage flashing and multi-effect distillation, the seawater is evaporated using either oil or excess steam from a power plant. The lack of viable options and cost-effectivity are some of the deciding factors in choosing such technology. The prohibitive cost of fuel, however makes the conventional desalination processes difficult to opt for in India.
Low temperature thermal desalination
The Low Temperature Thermal Desalination (LTTD) process, which converts sea water into potable water using natural ocean thermal gradient, was initially demonstrated at Kavaratti in Lakshadweep way back in 2005. This indigenised technology, was implemented in two other Lakshadweep islands namely Agatti and Minicoy in 2011 by the National Institute for Ocean Technology (NIOT), an autonomous body under the Ministry of Earth Sciences (MoES), Government of India.
NIOT has been working on Low Temperature Thermal Desalination Technology (LTTD) since 2000. The three plants established in Lakshadweep each have fresh water generation capacity of 100 m3 per day.
The difference in the surface and deep sea water temperatures, termed as ocean thermal gradient, is used in LTTD to evaporate the surface water at about 22oC and condensing its vapour with the 12oC sea water drawn from depths of about 350m (Fig. 1). The availability of the required water depth within 1 km from the shore of Lakshadweep islands, makes it ideal for this technology. The LTTD plant includes the—flash evaporator, condenser, sea water pumps, vacuum system, a long submarine pipeline made of high density polyethylene (HDPE) to draw the cold water from required depth; and, marine structures such as sump, approach bridge and plant building.
MoES-NIOT have currently installed three plants, which cater to the drinking water needs of the population in these islands. Based on the success of the plants in these islands, the Lakshadweep Administration is pursuing proposals to establish six similar plants in the region, one each in the rest of the inhabited islands.
These LTTD-based desalination plants are catering to the water requirements of 28,000 island people, and are being successfully maintained by the local authorities since its handover after the initial year of maintenance.
The execution of the marine structures and deep sea cold water pipeline were adapted to suit the complex island conditions. For the Agatti desalination plant, a concrete caisson was floated with a cast weight of about 250 tonnes and constructed in stages. Towed through the narrow channel of the island lagoon, it was installed at a depth of about 5 m. The deep water piers of the 230 m long approach bridge were cast and floated as encased caissons, while the shallow water piers were cast in stages. A flexible, HDPE pipeline 0.63 m in diameter and 850 m in length was assembled in the lagoon and towed about 10 km, to be installed with the front end fixed to the sump, through a 100 m long steel pipe, and, the back end weighed down with a 10 tonne dead weight to lower to the required depth. All the structures and the pipelines were designed for the prevalent environmental conditions to withstand the complex loads arising out of breaking waves, seasonal currents, and, freak wave conditions. The condenser with a weight of 12 tonnes; the flash chamber with a diameter of 3 m and 5.2 m height; and other heavy plant equipment were erected with the minimal use of the heavy lifting equipment, as per the island conditions.
The equipment were designed to avail the existing low temperature gradients to yield potable drinking water with a total dissolved solids (TDS) level below 200 ppm. A team of doctors from Sri Ramachandra Medical University, Chennai, have concluded from the local hospital records of Kavaratti that the incidence of water borne diseases in the island dropped from 200 before the installation to 12 within six months of the installation of the plant.
A National Council for Applied Economic Research (NCAER) study conducted in 2012 too confirmed a drop in the topmost prevailing water-borne diseases like typhoid (13 per cent), dysentery (88 per cent), amoebiasis (6 per cent) and cholera (6 per cent) to the almost negligible 2 per cent following the introduction of the LTTD desalination plant.
MoES-NIOT has undertaken detailed studies to assess the wave loads on the structures and pipelines and also fine tuned the process equipment as part of its activities to standardise the design of these components. The standardised designs are proposed to be used in newer plants.
A barge mounted desalination plant of 1 million litres per day capacity was also set up 40 km off the Chennai coast for demonstration. The concept was further extended to generate fresh water using waste heat, from coastal thermal power plants. A pilot project has been successfully demonstrated at the North Chennai Thermal Power Station using condenser reject water. Designs of scaled up thermal power plant based and offshore platform mounted desalination are now ready.
Since India is blessed with abundant solar radiation, the NIOT is currently looking into the feasibility of solar desalination techniques as well.
The temperature difference available due to the ocean thermal gradient has been used to establish simple, easily maintained desalination plants in the Lakshadweep islands. The innovative designs used by MoES-NIOT for these plants in keeping with the prevalent local conditions can be easily replicated in other coastal regions in the near future.