S V S Phani Kumar, M V Ramana Murthy and M A Atmanand

The mismatch between increasing demand and decline in water availability due to overexploitation and climate change is a critical issue for policy makers. Desalination, which is the conversion of seawater to potable water through various physical and chemical methods, has emerged as a potential solution to India's looming water crisis. The most important concern however is cost effectivity of the methodology, type of energy used for desalination and sustainability of the plant.

Rapid urbanisation, population explosion and over exploitation of ground water resources are some of the reasons for the increasing freshwater demand in regions of the country. Coastal and island communities of the nation however, can eke potable water with the use of desalination technologies. Some of the common conventional desalination technologies are reverse osmosis (RO), multistage flashing and multi effect distillation. In RO water from a highly concentrated region is transferred to a region of low pressure. A semi-permeable membrane having pore diameter from 0.5 nm to 1.5 nm separates the two sections. The technology has the limitations - pretreatment of water to protect membranes, higher pumping power proportional to feed pressure, biofouling of membranes, frequent change of expensive membranes, etc., apart from causing ecosystem imbalances through discharged water. Similarly, multistage flashing and multi effect distillation plants are economical for higher capacity desalination plants especially when warm water above 60oC can be produced using waste heat from power plants. However, in the Indian context there are very few plants that work with these technologies - since the ultimate cost of desalination would include the cost of the waste heat that is used to take the water to the required inlet temperature. The generation and maintenance of vacuum and the problems of scaling are two technical challenges associated with such technologies.
On the other hand, low temperature thermal desalination (LTTD) process uses naturally available temperature difference in the ocean layers, and provides an option that is completely environment friendly with the added advantage of minimum maintenance. The process entails evaporation of warmer surface sea water at low pressures and condensation of the resultant pure vapour using deep sea cold water available at about 400m below sea level. Simple and easy to maintain, the desalination plant requires just a few components - a flash chamber for evaporation, a condenser for liquefying the vapour, sea water pumps, vacuum system, a long pipe to draw cold water from 400m below sea level, marine structures such as sump, plant building and bridge. The LTTD with ocean thermal gradient is an environment friendly technology as it uses naturally available heat. Fig. 1 shows a schematic diagram of LTTD process.

Desalination Plant for Islanders
Realising the misery of the islanders, Lakshadweep was identified as the most suitable for setting up of the LTTD technology on experimental basis to produce freshwater. Also, for most islands here, a 400m depth is available within 600 to 800 m from the shore. The Ministry of Earth Sciences through the National Institute of Ocean Technology (NIOT) decided to set up a desalination plant at Kavaratti, with a capacity of 1 lakh litres/day in 2005 to provide potable water from the sea and alleviate the scarcity of drinking water faced by the communities. The plant is being operated by local islanders since 2006 and meeting the drinking water requirements of the 10,000 strong local communities for over six years. Studies conducted by a team of doctors have shown an improvement in public health among the beneficiaries with a drastic drop in the water borne diseases.

Minicoy Desalination Plant
Satisfied with the ease of operation, utility and performance of the successful Kavaratti desalination plant, Lakshadweep administration approached NIOT to put up similar plants in other islands of the region. Works in Agatti and Minicoy were taken up in the first phase. The plant at Minicoy, was commissioned on 22 April 2011 to mark the Earth Day celebrations.

Challenges
Construction of marine structures that can withstand all weather conditions is the most challenging part of the project. The 400 tonne sump that houses the sea water pumps are initially cast inside the island lagoon, pushed into water, floated and towed about 10 km to the eastern side of the island, for its final installation at the site. The construction of piers of the bridge that connects the sump to the shore was a daunting task considering the fact that the piers in the breaker area experience constant wave action at every 10-15 second period.
The 700 m long HDPE pipe that draws the cold water from 400 m below the sea level is connected to the island through 12 m pieces, welded in the lagoon and towed to reach the site and then deployed to connect one end to the sump and leave other end at about 400 m depth. The pipe is designed to withstand all weather conditions in an oceanic environment. The design of process equipment is optimised to facilitate the ease in construction, transportation and erection in remote islands, while meeting the project requirements of minimal power usage to cut down running costs, compatibility for sea water use and containment of total project cost.

Concluding Remarks
LTTD is a fairly new development with significant scope for cost reduction as the technology matures. The process involves about 1 per cent conversion resulting in zero brine disposal problems, and hence does not interfere with the fragile ecosystem of the area. The discharged cold water that is let out at about 17oC is rich in nutrients and attracts a variety of fish. This results in spin offs like air conditioning for land based plants and mariculture. Considering the projected demand for drinking water in the near future, it is important to promote LTTD for parched coastal and island communities.

The author are Scientist E, Scientist F and Director, respectively at National Institute of Ocean Technology, Chennai. mvr@niot.res.in