An unconventional solution to water woes of Chennai

April 28, 2024 01:00 am | Updated 04:45 pm IST

It is that time of the year again when sweltering heat and water shortage take centre stage and contingency plans are chalked out. So far this summer, Chennai has averted water scarcity, despite the storage dipping in reservoirs, as desalination plants have come in handy. In a few years, Chennai will emerge as the desalination capital of India, with three operational plants in Minjur and Nemmeli and Southeast Asia’s largest plant, with a capacity to treat 400 million litres of seawater a day (mld), coming up at Perur along the East Coast Road. Once the Perur facility is up and running, these plants will together meet nearly half of the daily needs of the growing metropolis.

With the effect of climate change threatening to increase the vicious cycle of floods and droughts and cause an imbalance between demand and supply with limited sources of freshwater, desalination has a larger role to play, as one of the sustainable solutions, in addressing water scarcity in the city.

Parts of south Chennai have already been receiving desalinated water through pipelines. Work is in full swing on creating drinking water supply infrastructure on the southern fringes, including Neelankarai, Uthandi, and Semmencheri. The plants are expected to quench the thirst of residents living in areas along the Information Technology Corridor. They have been relying heavily on private water suppliers and the groundwater for more than a decade.

Harsha Koda, co-founder, FOMRRA (Federation of OMR Residents Association), says Metrowater must complete the pending infrastructure projects to link all households. Only four or five residents’ welfare associations on Old Mahabalipuram Road get piped supply and others depend on private tankers. Though 99% of the pipelines have been laid, connections are waiting for the work to be completed in a few metres.

Metrowater plans to cover more areas with desalinated water supply in three-four years, officials say.

Environmental concerns

While desalination plants have helped in assured supply in parts of the city, they have their critics. Metrowater has been conscious of the environmental concerns over the establishment of new desalination plants. The 150-mld Nemmeli plant, which was recently inaugurated by Chief Minister M.K. Stalin, focused more on the pre-treatment process to extend the life of reverse osmosis membranes. The dissolved air flotation (DAF) process has been introduced to remove suspended solids and impurities like algae. Ultrafiltration, coupled with RO membranes, will remove micro-organisms and bring down the total dissolved solids (TDS) from 39,000 parts per million (ppm) to the desired standards of 500 ppm.

The new plant at Perur, spread over 85.5 acres, will have components such as lamella clarifier units and the DAF to remove lighter materials and gravity dual media filters to separate total suspended solids in the seawater. The process will be more efficient in removing the finer particle impurities from raw water.

Learning from the past experience, Metrowater has incorporated the recommendations of the National Institute of Ocean Technology on environmental impact, including an erosion management plan. Diffusers in the reject water outfall system will slow the release of brine and disperse the high salt concentration over a wider space to protect the marine life.

“We have not received complaints about risks to the marine ecology since the 110-mld Nemmeli plant started in 2013. We have provided a caisson structure and appropriate screens in the intake pipe to prevent entrapment of marine species. Marine organisms will not be affected, thanks to the low velocity intake head and no suction involved,” says an official.

The Perur plant will have two separate 200-mld units to ensure that it produces 200 mld of treated water at any given point of time. With the plant expected to be an energy-guzzler, there are plans to install energy recovery devices and booster pumps. Units will be added for post-mineralisation of treated water before supply.

Besides compensation to the fishermen families during offshore work on the Nemmeli plant, jobs were given to Sulerikattukuppam residents. Once the Perur facility and the ring main system are in place, other places, including those up to Choolaimedu, can be supplied from the plants on the East Coast Road, officials say.

2019 drought

Officials recall that the two plants saved the city from slipping into a ‘Day Zero’ crisis, along with tankers and water trains during the 2019 drought. The Nemmeli facility has been a game changer this year as it has filled the gap left by the Veeranam tank that has gone dry and stabilised the daily supply. Only about 40 mld is being drawn from the Minjur plant, which has conventional technology, to meet the daily requirements of two north Chennai zones.

Residents like R. Sureshkumar of Sholinganallur said they received saline water with a high TDS level for the past few days. Officials respond that water supplied now has a permissible TDS level of 700 ppm. The RO membranes will be replaced at the Nemmeli plant by next week and residents will get better quality water by the first week of May. Metrowater is also retrofitting the components in the old facility. Work on enhancing production to the full capacity will begin in two or three months.

More plants

Tamil Nadu has been a pioneer in conceiving technology-aided solutions to water shortage in the coastal region. The Tamil Nadu Water Supply and Drainage Board had established 15 small-scale desalination plants in Ramanathapuram three decades ago. The facilities, including the one at Narippaiyur, were closed because the technology was outdated and the maintenance cost was high.

Drawing inspiration from the success of desalination technology in Chennai, the government is considering the possibility of establishing desalination plants in Ramanathapuram, Villupuram, and Thoothukudi, with 60-mld capacity each. The TWAD Board is looking for financial support from the government as the production cost of desalinated water, including conveyance, is put at ₹60 per kilolitre.

Sources say that unlike Metrowater that collects charges directly from domestic and industrial consumers, the TWAD Board depends on local bodies, and the gap between charges and production cost is filled with government subsidy. Moreover, Metrowater can recover the cost by charging industries more. With a limited number of industries in rural areas and lesser population density, the TWAD Board’s desalinated water supply must cover an entire district with pipelines running hundreds of kilometres.

While desalination plants do address the issue of water scarcity, experts note that a multi-pronged approach, including the use of reclaimed water and abandoned quarries, will enhance water resilience.

A study conducted by Anna University and IIT-Madras, using the water evaluation and planning model, has revealed that increasing desalination capacity can reduce the unmet demand in times of normal and deficit rainfall. In 2040, the unmet demand will be nearly 717.4 mld in Chennai, with normal rainfall, and this can be reduced to 227.7 mld with an expanded desalination capacity.

L. Elango, the author of the study and visiting faculty member at IIT-M, says desalination must be one of the water management strategies and expansion must not be done without water pricing and incentives for frugal use. The need for more supply through desalination must be balanced with the preservation of ecosystems to sustain water management in Chennai. Rigorous environmental impact assessments and mitigation measures are essential to minimising the ecological impact, he says.

Safer and cheaper

Desalination is a reliable, drought-proof source of water security and economy, says Rajiv Mittal, chairman and managing director, VA Tech Wabag Limited, which is operating the 110-mld Nemmeli plant. It has bagged an order to design, build, and operate the 400-mld desalination plant at Perur.

Noting that desalination is a viable technological solution, he says desalinated water is safer and cheaper than water supplied by tankers. Consumers will pay only seven or eight paise for each litre of desalinated water as against 15 or 20 paise for a litre of tanker water. The cost of establishing a desalination plant has come down over the past decade, thanks to advances in technology. “At the Perur plant, reverse osmosis membranes will be sturdier and need not be replaced often,” he says.

Traditional waterbodies

The Tamil Nadu Public Works Department Seniors Engineers’ Association has come up with a plan to increase the storage of the tanks in the Chennai Metropolitan Area. The Association says it will be a cost-effective alternative to desalination plants. The State government has incorporated the proposal in its ₹40,000-crore project to rejuvenate the waterbodies in the State. The project will be executed with the support of the government of the Netherlands.

The Association, a forum of retired engineers, has prepared a list of 507 tanks in 322 villages in the neighbouring districts to augment sources of surface water and supply drinking water to Chennai. According to the Association, an additional storage of 3.75 tmcft can be created by desilting the tanks at a cost of ₹500 crore.

Listing the demerits of desalination, the Association notes that steps to reduce environmental impact will entail additional cost. The reverse osmosis membranes need to be replaced every seven years.

K. Nagarajan, former Special Chief Engineer, Water Resources Department, who made the study, says the production cost of desalinated water will be 10 paise a litre, whereas the cost of treating one litre of surface water will be around one paisa. Desilting and augmenting the storage of the tanks will help to conserve surplus water draining into the sea. Chennai will get another 3 mcft, if major reservoirs are deepened by one metre. The additional storage in all these waterbodies will amount to nearly 526 mld, he says.

Desalination has given a reprieve to the city grappling with water deficit. Innovations in desalination technology, coupled with strong monitoring to assess environmental impact, will help to work out a sustainable solution, according to experts.