CHENNAI : Water-starved Chennai could earn the distinction of being the first in the world to set up an offshore desalination plant, for which a deep sea site that is 40 km from the Ennore coast has been finalised. The National Institute of Ocean Technology (NIOT) has already submitted a detailed project report (DPR), which is likely to be considered by the Union Government under the `10,000-crore Deep Sea Mission to be launched in March 2018.
Unlike the existing desalination plants in Nemmeli and Minjur that uses Reverse Osmosis (RO) technology to separate fresh water from seawater, this plant will use indigenously developed low temperature thermal desalination (LTTD), which is an environment-friendly technology.
The LTTD utilises temperature difference between surface water and deep sea water. In this methodology, the warmer surface sea water is made to evaporate at low pressures and the vapour obtained is condensed using the colder deep sea water to get potable water.
The current RO-based desalination plants use lots of chemicals and produce highly concentrated brine that is usually sent back into the ocean. This brine is toxic to bottom-dwelling marine life when it settles on the ocean floor, and can also cause hypoxia or oxygen deficiency in the ocean floor area.Also, ocean scientists are exploring the possibility of powering the proposed desalination plant with electricity generated through ocean thermal energy conversion (OTEC), instead of diesel, which will drastically reduce the operating cost of the plant. This makes the project the first of its kind in the world. The OTEC uses the ocean thermal gradient to produce electricity.
M Rajeevan Nair, secretary, Ministry of Earth Sciences, said that NIOT had finalised the design and submitted the DPR. It would be a 10-MLD (million litres per day) capacity plant costing an estimated `2,000 crore. “Since Chennai has shallow waters, we have to travel 40 km into the sea to get enough depth to take in deep sea cold water needed for the process. It is a great challenge for ocean engineers,” Nair said.
Satheesh C Shenoi, director, NIOT, said the off-shore desalination plant by itself is a first-of-its-kind. “In the DPR submitted, the use of OTEC to power the plant is not included. But, the NIOT is optimising the technology at a laboratory scale for which an OTEC test lab has been commissioned on the NIOT campus. Next, we will be setting up India’s first OTEC powered desalination plant at Kavaratti in Lakshadweep Islands for which tendering process is on. Then, we will bring the best of technology to the proposed Chennai plant that will go a long way in solving the water crisis of the city,” he said.
Purnima Jalihal, head, Energy and Freshwater Programme, NIOT, said the initial high capital cost was due to building of an offshore platform. Otherwise, the technology is very economical and environment-friendly. “We are operating three 1 MLD desalination plants using LTTD technology in Kavaratti, Minicoy, and Agatti islands of Lakshadweep. Six more are planned at Amini, Chetlet, Kadamath, Kalpeni, Kiltan and the Andrott islands. The use of OTEC for Chennai plant depends on the budget sanctioned.”
With diminishing fresh water resources, desalination plants will be future sources of drinking water. The Tamil Nadu government is investing heavily in the desalination plants and plans to meet 65 per cent of Chennai’s water requirement through these plants. The government is setting up two more desalination plants with a combined capacity of 550 MLD. This would reduce stress on natural resources like lakes as well as ground water.