18 Jan : A Flagship Drinking Water Programme of the Ministry of Earth Sciences, desalination refers to any of several processes that remove excess salt and other minerals from water. Water is desalinated in order to be converted to freshwater suitable for human consumption or irrigation. It is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on developing cost-effective ways of providing freshwater for human use in regions where the availability of freshwater is limited. Large-scale desalination typically uses extremely large amounts of energy as well as specialised, expensive infrastructure, making it very costly compared to the use of freshwater from rivers or groundwater. The Ministry of Earth Sciences (MoES) through its National Institute of Ocean Technology (NIOT) has worked extensively inquest for techno-economically viable solutions for desalination. Low Temperature Thermal Desalination (LTTD) is one process that uses the availability of a temperature gradient between two water bodies or flows to evaporate the warmer water at low pressure and condense the resultant vapour with the colder water to obtain freshwater. While ocean, with its temperature variation across its depth, presents one such scenario of two water bodies, a coast based thermal power plant discharging huge amounts of condenser reject water into the nearby ocean represents an alternate scenario. The simplicity of LTTD process also enables to control the quality of product water in order to provide either good quality drinking water or boiler grade water as the situation warrants.
Low Temperature Thermal Desalination
The main components that are required for LTTD plant are the evaporation chamber, the condenser, pumps and pipelines to draw warm and cold water, and a vacuum pump to maintain the plant at sub-atmospheric pressures. One of the advantages of the process is that it can be implemented even with a low temperature gradient of about 8°-10°C between the two water bodies. Even though flash distillation is a commonly used desalination process worldwide and especially in Middle East, none of the established plants work with the temperature gradient as low as 8°C that exists in the North Chennai Thermal Power Station (NCTPS).
Conventional Desalination Processes
Some of the other commonly used desalination processes such as: Reverse Osmosis is the most commonly used membrane process in the industry. Water at high pressure is made to pass through a porous membrane. As the water passes from the high pressure area to the low pressure area through pore of the size of 0.5-1.5 nm, the dissolved solids are left behind. Over the past 20 years, the process has been fine tuned. However, high power consumption and the disposal of the concentrated brine are two of the major drawbacks of the process. Multi Stage Flash Desalination (MSF) is a flash distillation process that is similar to LTTD process, but works on a higher temperature difference. The flashing is done in multiple stages. Most MSF plants use inlet feed water in the temperature range of 60°- 80°C. In Multi Effect Desalination (MED), the energy available from the steam in the power plants is used to distill the sea water in multiple stages in this method. The use of steam as the primary source makes the process expensive in the Indian context.
Low Temperature Thermal Desalination
National Institute of Ocean Technology started working with the LTTD applications in 2004 and established various plants, namely : Laboratory scale model with a capacity of 5 m3/day (2004); 100 m3/day capacity land based plant in the Kavaratti Island of Lakshadweep (2005); 1000 m3/day capacity Barge Mounted Experimental Plant off Chennai coast (2007) – pilot/experimental plant.
LTTD on the Lakshadweep Islands
Land based demonstration plant in Kavaratti with a capacity of producing one lakh litres per day of freshwater was set up by NIOT in May 2005. The sea bed bathymetric (depth) near the island was such that 350m water depth was available at about 600m from the shore. Temperature gradient of 15°C was utilized. High Density Polyethylene (HDPE) pipes of 630mm diameter and 600m long were deployed to draw cold water from a depth of about 350m. The plant has been running continuously ever since, fulfilling the needs of the 10000 strong local community for over three years. The salinity of the freshwater produced was reduced from 35000 ppm of the seawater to 280 ppm whereas the permissible limit for drinking water is 2000 ppm. Subsequent to the commencement of the plant water supply for drinking water needs, there have been significant drop in the incidence of water-borne diseases among the consumers NIOT is currently in the process of establishing similar plants in three more islands in Lakshadweep, namely Agati, Minicoy and Androth.
Barge-Mounted LTTD Plant
For an LTTD plant meant for the mainland needs, NIOT has demonstrated an experimental 1000 m3/day (one million litres per day) barge mounted desalination plant 40 km off Chennai coast meant for mainland usage. Temperature gradient of about 18°C was utilised with surface water at 28°C and the water at 550m depth at 10°C. The plant was commissioned in April 2007 and the sea trials were successfully conducted for a few weeks. Thereafter, the plant was dismantled.
Application of LTTD in Power Plants
It can be seen from these LTTD plants that temperature difference and adequate vacuum levels should be sufficient for the generation of freshwater. Thermal power plants discharge warm water from their condensers. The process that involves transfer of tremendous levels of energy usually includes heat recovery systems like cooling towers or heat dissipating open channels before the condenser reject water at acceptable temperatures is discharged back into the surrounding environment. Consequently, the resultant thermal pollution by the power plants is a serious issue today. An efficient way to utilise the heat available in the condenser reject water would reduce the load on the cooling towers and in turn the resultant thermal pollution. One of the aspects of LTTD is that it transfers the available heat from warmer water to the colder water while generating freshwater from the warm water. This aspect could therefore be aptly used in thermal power plants resulting in the double benefits of cooling the condenser reject water and generating the freshwater. A small temperature gradient of about 8°-10°C, as is the case with most power plants, would be sufficient to utilise the concept. With the idea of demonstrating the concept in a coast based thermal power plant, where the co-existence of warm power plant condenser reject water and the nearby surface sea water with a gradient of about 8°-10°C presents an ideal case for an LTTD application. NIOT took up the task of setting up the LTTD plant in Northern Chennai Thermal Power Station (NCTPS).
The Road Ahead
Drinking Water in Lakshadweep : With LTTD, technology has stabilised over last four years. The Ministry of Earth Sciences targets to cover by December 2010 all 10 major habited islands of Lakshadweep with plants of capacity of one lakh litres each per day. Freshwater in Thermal Power Station on Seacoast : The successful demonstration of an LTTD plant with the existing temperature gradient of mere 8°C at the NCTPS provides another new area for its application. Since most power plants discharge the condenser reject water at 8°-10°C above the ambient sea water temperature, any increase in the available temperature difference or the provision of additional process steam from power plant will increase the efficiency of the LTTD process, resulting in the generation of huge quantity of freshwater. If implemented in the design stages, further optimisation of the power consumption is also possible. Newer power plant projects are also coming up all over the country, most along the coast. Thus, the LTTD technology, if applied to power plants, would be useful in providing high quality freshwater for boilers, while reducing the thermal pollution.