Desalination - A Solution to Water Needs

Desalination is fast emerging as a viable solution to the drinking water shortage the world over. India has taken a few good strides towards this end.

Commissioning of desalination plants to meet the drinking water needs of people is a concept that is fast emerging in India in recent years, after the West had succeeded in converting seawater into drinking water. A Rs 1,000 crore- desalination plant that is to be set up in Minjur near North Chennai by the IVRCL Infrastructure and Projects, to generate 100 million litres of water per day, is bound to bring relief to this water-starved metro.

Former President APJ Abdul Kalam too is a votary of desalination plants. A few years ago, he said 'There are many technologies in desalination. A cost-effective methodology has to be selected to suit the need of our country. The most important aspect is the type of energy we use in the desalination method. Every country has to use the energy that is easily available in the country. For example, in the UAE, they use gas and oil for desalination process. From the type of conditions that prevail in our country, solar power energy can be one of the options for this purpose. Before, we proceed with the project, it would be better to go in for a project report reconciling the technology and energy sources that suit our needs.'

According to a study, there are over 7500 desalination plants across the globe, of which at least 60 per cent are located in the Middle East. The world’s largest plant in Saudi Arabia produces 128 MGD of desalted water. In contrast, 12% of the world’s capacity is produced in the Americas, with most of the plants located in the Caribbean and Florida.

'Investigations have shown that desalination is among the most cost effective options for boosting water supply in major Australian state capitals. The city of Perth, for example, in Western Australia, has been successfully operating a reverse osmosis seawater desalination plant since 2006, and the West Australian Government has announced that a second plant will be built to service the city's needs,' says an online article.

However, desalination is best suited and cost-effective for regions that are in proximity to oceans. Supplying desalinated water to a place like Delhi, which is away from the sea, will be an expensive affair. The cost of transporting water to a city like Delhi from a desalination point must also be factored for. However, for a Metro like Chennai bordering the coastline, desalination is an ideally suited technology that remains untapped.

An expert points out that Israel was desalinizing water for just 53 cents per cubic meter and Singapore for 49 cents per cubic meter. Other large coastal urban cities in developed countries are increasingly looking at the feasibility of seawater desalination, due to its cost effectiveness when compared to other water supply augmentation options.

He further indicated that the proposed Minjur plant is not Chennai’s first experiment with desalination. 'The Chennai Port Trust had planned for a desalination plant to meet the water requirement inside the harbor and its residential complex at Tondiarpet'. 'The technology had been successfully experimented at Kalpakkam, where 45,000 people are getting uninterrupted drinking water thanks to a technology developed by the Bhaba Atomic Research Center (BARC), which could convert seawater into drinking water at a cost of just Rs. 2.50 a litre,' he said.

Earlier, in 2004, the Chennai-based National Institute of Ocean Technology (NIOT) had developed a cost-effective technology to desalinate one-lakh litres of water daily at Lakshwadeep for 25 paise a litre.

Likewise in May 2005, the world’s first ever-Low Temperature Thermal Desalination plant (LTTD) was opened in Kavaratti, an island in Lakshadweep. The plant cost about Rs 50 million and produces 1,00,000 litres of potable water daily from seawater.

Desalination-The process

Seawater desalination has been considered as a long -term water source. Seawater also contributes to reduce the supply demand gap, as it is the most dependable source during drought. Desalination is a process that removes dissolved minerals including salt from seawater, brackish water, or treated wastewater. The desalinated product water is usually purer than the standard drinking water. It is highly acidic and therefore corrodes the pipes. Hence it has to be mixed with other sources of water or else adjusted for hardness and alkalinity before being used, experts say.

A desalting device essentially separates saline water into two streams: one with a low concentration of dissolved salts (the fresh water stream) and the other containing the remaining dissolved salts (the concentrate or brine stream).

The device requires energy to operate and can use a number of different technologies for the separation.

The various major desalting processes include:

• Thermal, Multi -Stage Flash Distillation (MSF),
• Multiple Effect Distillation (MED),
• Vapor Compression Distillation (VC)
• Reverse Osmosis (RO)
• Electro dialysis (ED).

Minor desalination processes include freezing, membrane distillation and solar humidification.

There is no 'best' method of desalination. There are several desalination technologies, the most common being the Reverse Osmosis (RO) process and distillation.

Generally, distillation and RO are used for seawater desalting, while RO and electro dialysis are used to desalt brackish water. However, the selection of a method should be dependent on a careful study of site conditions and the application at hand. Local circumstances may play a significant role in determining the most suitable method for an area. The 'best' desalination system should work when it is installed and continue to work and deliver suitable amounts of fresh water at the expected quantity, quality, and cost, experts note.

According to the Indian Desalination Association, which was formed in Chennai in 1991, desalination facilities exist in about 120 countries. The capital and operating cost for desalination have decreased over the years although energy prices have increased. However, the cost of obtaining and treating water from conventional sources has increased because of the increased levels of treatment being required in various countries to meet more stringent water quality standards. This rise in cost is also the result of an increased demand for water, leading to the need to develop more conventional supplies as the readily- obtainable water sources have already been exhausted.

Many factors enter into the operating costs for desalination. They are:

• Capacity and type of plants,
• Plant location,
• Feed water quality,
• Labor cost,
• Energy cost,
• Financing cost,
• Ease of concentrate disposal,
• Level of instrumentation / automation
• Plant reliability.

However, as a guideline, the reader can take the production cost of a brackish- water desalination plant to be Rs. 10 to 15 per m3. The production cost for a sea- water desalination plant varies between Rs. 40 to 50 per m3. Whereas the production cost of desalted water from effluent varies from Rs. 15 to 50 per m3 depending upon the TDS load in the effluent stream.

Desalination ensures safe, hygienic and adequate supply of drinking water and provides an ideal future plan to sustain life, particularly in coastal, drought-driven metros.

Source-Medindia