Desalination : Cost-Effective Ways to Provide Fresh Water
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16:34 IST
The Ministry of Earth Sciences (MoES) through its National Institute of Ocean Technology (NIOT) has worked extensively in quest 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. Te 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.

LTTD Process

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 8o – 10oC 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 8o C that exists in the North Chennai Thermal Power Station (NCTPS).

Other Conventional Desalination Processes

Some of the other commonly used desalination processes are Reverse Osmosis, Multi Stage Flash Desalination and Multi Effect Desalination. 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 60o-80oC.

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.

NIOT 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 UT Lakshadweep (2005) and 1000 m3/day capacity Barge Mounted Experimental Plant off Chennai coast (2007) – ilot/ experimental plant.

NIOT set up a land based demonstration plant in Kavaratti, Lakshadweep Islands with a capacity of producing one lakh litres per day of freshwater in May 2005. The sea bed bathymetric (depth) near the island was such that350m water depth was abailable at about 600m from the shore. Temperature gradient of 15oc was utilised (temperature at surface water at 28oc, water at 350 m depth at 12oC). High Density Polyethylene (HDPE) pipes of 630mm diameter and 600m long were deployed to draw cold water from a depth of about 350m. The sea water umps inside the partitioned sump supply, warm and cold water to the plant on the land. 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. NIOT is also the process of establishing similar plants in three more island in Lakshadweep, namely Agati, Minicoy and Androth.

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 18oC was utilised with surface water at 28oC and the water at 550m depth at 10oC. The plant was commissioned in April 2007 and the sea trials were successfully conducted for a few weeks. Thereafter, the plant was dismantled.

LTTD has been applied in Power Plants also. An efficient way to utilise the heat available in the condenser reject water of a power plant 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 8o-10oC, 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 8o-10oC 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 600 MW NCTPS plant discharges about 100,000 m3/hr of condenser reject water at about 37oC. In order to reduce the thermal pollution issues arising out of mixing this water with the nearby seawater at 29oC, NCTPS lets the water run through a long open channel where the water is brought to about 33oC.

The power plant consists of three units each with a capacity of 200 MW. For each unit, the condenser coolant water is supplied through a 2.7m diameter concrete pipe. The reject water from the condenser is led back to the open channel through the 2.7m diameter concrete pipes were provided with a manhole for servicing. These manholes were modified to accommodate the inlet and discharge piping required for the LTTD plants. A structure was designed to accommodate the plant components of flash chamber, condenser and the vacuum system. Being a demonstration plant, it was decided to utilise the existing components from the decommissioned barge-mounted desalination plant. A pump situated in a sump is used to provide water to the flash chamber while the available head in the cold water is used to pass the cold water through the condenser. The final piping connections between the LTTD plant and the main power plant were completed during their annual maintenance schedules. After letting the water through the LTTD plant, the vacuum system was operated to bring the system vacuum to the design pressure. The warm water was flash evaporated at about 33oC and freshwater was obtained. The cold water exit temperature was measured at about 31oC. A fraction of the cold water flow intended for the main power plant condenser is bypassed to the LTTD plant, thereby removing the necessity of a separate cold water pump for the LTTD plant. Thus, the warm water pump and the vacuum system are the main power consumption devices for the LTTD plant, with a power consumption of about 45 kW for the designed capacity of about 2.5 lakh litres per day. The salinity of the freshwater was reduced from 35000 ppm of the seawater to about 24 ppm, the quality well suited for drinking water as well as for use in the boilers. Water Quality Analysis for LTTD Plant at Kavaratti and NCTPS, Chennai .

The successful demonstration of an LTTD plant with the existing temperature gradient of mere 8oC at the NCTPS provides another new area for its application. Since most power plants discharge the condenser reject water at 8o-10oC 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, world be useful in providing high quality freshwater for boilers, while reducing the thermal pollution.

KP