Desalination: A hard route to solve crisis

Reeling under one of its worst water crises, Chennai continues to look for that permanent solution that can resolve all its problems forever. Desalination, though expensive, seems to be the only and last solution unless the authorities manage to store enough water in the reservoirs and lakes.

The Chennai Metropolitan Water Supply and Sewerage Board (CMWSSB) has now called for Expression of Interest (EoI) for three 10 mld (million litres per day) desalination plants in the city, shelving the original plan for five 1 mld plants, in a bid to augment water supply to the city in the coming years. Although these will not solve the current crisis, the plan seems to be to set up many such small installations at several points in the city gradually.

Desalination companies are yet to come forward with their bids for the 10 mld plants, the last date for submitting their EoI being July 21. The cost of putting up a 10 mld desalination plant is roughly Rs 70 crore.

Once the bids are chosen, it would take about 15 months to build the plant and commission it.

The city already has two 100 mld plants operating at Nemmeli on the East Coast Road and Minjur in North Chennai. Two more plants with a total capacity of 550 mld are coming up at Nemmeli. Water from these will be available in two or three years, taking the total supply of desalinated water to 750 mld. Chennai city’s water requirement is about 850-900 mld currently. Also, the city is expanding with an ever growing population. 

Desalination was first thought of as an alternative source of potable water for the city which had for years been experiencing water scarcity. The government simultaneously promoted rainwater harvesting and conservation in an effort to see Chennai to sufficiency. And yet, here we are in the midst of the crisis.

Can more desalination plants solve the problem in the coming years? “Absolutely not,” say experts in the water industry and water conservationists. For Chennai, the most economical option is rainwater harvesting and must be the first choice. The next option should be the judicious use of ground water. The third option should be the use of recycled and treated water from domestic and industrial use and finally, as a last resort, desalinated water.

Chennai receives in excess of 800 mm of rainfall from the North-East monsoon and in excess of 400 mm from the South-West monsoon annually and collecting and storing that water is clearly the right choice. In addition, desalination is an expensive option.

Setting up a desalination plant takes time. The laying of the intake pipes for sea water, one km into the sea and the pipes for releasing the brine (salt concentrate after desalination) 500-600 metres into the sea takes a long time, says one industry expert, because the outlet pipe has to be buried under the seabed which requires skill and time. 

Almost 50 per cent of the cost of the generation goes into power. The membrane reverse osmosis process, although cheaper than other technologies, is power intensive. The cost of generating 1,000 litres of water from the Nemmeli plant is about Rs 40–Rs 50, (including the energy cost of Rs 24) depending upon the salinity of the water being fed in. Higher salinity requires greater pressure which draws more electricity. “Sometimes, there are slight variations to the daily supply due to power failures”, says K Syed Amir Basha, Chief Technology Officer (Desalination), VA Tech Wabag.  “When power failures happen, it takes around two hours to bring the plant back to operation and this causes a production loss of around 6 to 10 mld.” The cost per 1,000 litre generation at the Minjur plant is Rs 56. 

Another cost incurred is on account of the membranes that filter out pollutants. Each membrane is expected to have a lifecycle of five years, but typically, after the third year, about 15 per cent of the membranes need replacement. Each membrane costs upwards of  $ 400 (Rs 28,000). Every unit of 8.33 mld needs 504 membranes.

According to a senior CMWSSB official, “We continue to supply this water to the consumer at normal rate fixed for water, even though it costs us more.” Environmentalists question the supply of such expensive water for use in flushing and washing clothes.

Industry sources say that setting up 1 mld plants is tougher and more expensive than putting up a big plant because the overheads would be much higher and the water, too costly. While 10 mld plants may be more viable, the better option would be to put up installations of 10 mld plants in combinations of  four 2.5 mld plants or five 2 mld plants and keep them ready to be deployed in times of crisis. That way, we only tap the sea water when it is an absolute must.

The 100 mld Minjur desalination plant has been in operation since 2010 and also uses the Sea Water Reverse Osmosis technology. The project is being implemented by Spanish company Abengoa and Indian company IVRCL. 

The Nemmeli plant was awarded to VA Tech Wabag for a construction, maintenance and operation contract for seven years, in consortium with IDE Technologies of Israel. It began operations in 2013. Together, the existing plants supply Chennai one-third its daily requirement.

There are many technologies that have been tried and tested around the world, the two most popular ones being thermal distillation and membrane reverse osmosis. Both the existing and proposed plants will use the membrane reverse osmosis technology because it has been found to be the most cost-effective. (See graphics)

While desalination can be a solution for arid areas such as the Middle East, a state like Tamil Nadu receives enough rainfall to meet water requirements, but has not bothered to develop its storage facilities, says professor S Janakarajan, president of the South Asia Consortium for Interdisciplinary Water Resources Studies. Maintaining and protecting the almost 3,600 waterbodies in Tiruvallur, Chennai and Kancheepuram districts would be a more cost-effective option, he feels. 

If we start working on our water bodies today and converting any open space into a water storage facility, we will see a difference in the water situation within a year and this will be long-term. This is far cheaper than putting up desalination plants whose impact we will have to deal with later, says Nityanand Jayaraman, environmentalist.

Says Dr Indumathi Nambi of the Civil and Environmental Engineering Department of IIT, Madras, “We have a tradition of nurturing small water bodies such as temple tanks and ponds. That should actually be enough to meet our water requirement.” Countries like Germany have started working on building local facilities for water storage. We already have the facilities, we just need to maintain them, she says.

The process of converting sea water into potable water is expensive. The desalinated water loses some of its vital nutrients such as calcium, magnesium, sodium chloride, potassium and fluoride. Hence, the desalinated water needs treatment to ensure these nutrients are added back and made healthy for drinking.

The Nemmeli expansion of 150 mld will be constructed under an Engineering Procurement and Construction (EPC contract) by a joint venture between Spanish company Cobra and UAE based Tecton Engineering. The total cost of this project is Rs 1,689.34 crore. 

The second plant at Perur, coming up on 87 acres is one km north towards Chennai and downstream of the existing Nemmeli plant. The project will also use SWRO (Sea Water Reverse Osmosis). The total cost of the project is Rs 3,912.16 crore and is being financed by an Official Development Assistance (ODA) loan of approximately Rs 1,800 crore from Japan International Cooperation Agency (JICA).

It will begin operations  by 2023.

Even as desalination is emerging as an alternative source of water for many countries globally, there has been vociferous opposition to the use of this method of harnessing drinking water. 

Desalination reduces the presence of nutrients. In general, drinking water is supposed to provide vital nutrients such as calcium, magnesium, sodium chloride, potassium and fluoride. Seawater is rich in ions such as calcium, magnesium, sodium chloride and iodine, but low in zinc, copper, chromium and manganese. The water will require tertiary treatment to add the nutrients that are essential. The Minjur as well as the proposed Perur plant will use limestone to treat the water, according to a preparatory study on the project prepared by Japan International Cooperation Agency (JICA), which is funding the project. In general, chlorine and lime are used to treat the water before supply.

A CMWSSB official said that the water from the desalination plants is subject to the requirement of the IS 10500 standards for potable water before it is supplied. Desalinated water is supplied directly to areas such as Tiruvanmiyur, Adyar, Neelankarai and Sholinganallur, while at other places; it is mixed with ground water. 

The more serious objection has come from environmental activists and experts who have expressed concern over the likely damage to people living around the area and marine life in the sea near the plants. Desalination plants need about 240 mld of seawater to generate 100 mld of potable water. The rest is concentrated brine that is released into the sea, affecting the salt concentration in the region.

“Desalination plants which are being talked about as the big solution are highly unsustainable and ecologically disastrous in the long run” says Prof S Janakarajan, “The Total Dissolved Salts (TDS) levels of sea water is around 20,000 mg/litre and the fresh water output should have a TDS level of around 50-60 mg/litre. Hence the desalination process is a very energy intensive one.” 

The plants also generate a lot of brine - for every 1 litre of freshwater output, almost 30 litres of brine is produced. The brine is hyper salty with TDS levels of around 40,000-50,000 mg/litre. This brine is released back into the sea 1-2 km away from the shoreline. Small fish and turtles which live near the shoreline around this area cannot survive in such hyper salty waters and they die. This in turn will affect the big fish which depend on the small fish for food. 

“The entire food chain of the seas is affected. In the long run, rising TDS levels will result in a dead sea where there would be no aquatic life within 3-4 km from the shoreline,” he says. Needless to say, this will in turn affect the livelihoods of the fishermen. 

While it is possible to extract salt from the brine using salt pans, transporting the brine to the pans can be expensive. According to an industry source, using the brine for salt production can bring down production time by 50 per cent for salt manufacturers. In countries such as Indonesia, salt pans are located close to the desalination plants and attempts are on to extract salt from the brine.