Alternative Supply

Desalination proves to be a commercially viable drinking water model

By Rajneesh Chopra, Global Head, Business Development, VA Tech WABAG Limited

Overview and challenges: Bridging the demand supply gap in Chennai

Chennai city, located on the coast of Tamil Nadu, currently has a 426 square km area, and has always had a turbulent relationship with rainfall. The city has been heavily dependent on the Northeast monsoons for its drinking water and agricultural needs. The rainwater, transferred to the lakes of Veeranam located 220 km from Chennai, Chembarakkam, Red Hills, Cholavaram and Porur, is prioritised to sustain the city’s 7 million people. But given the troubled alliance with wind-bearing rains, the city is often left bereft of its fair share of water, making the possibility of occurence of a water crisis all the more possible. In recent years, another factor that has aggravated the water crisis is the reckless depletion of groundwater sources to satisfy the demand arising as a result of rainfall failure. Chennai city has also faced several droughts in the past. Water tankers carrying groundwater and bubble cans vendors supplying drinking water from door to door are a common sight.

It is apparent that providing a solution to the issue of water scarcity was becoming a pressing concern. In addition, the solution would have to ensure sustainability in the long run instead of being a temporary measure.

Fortunately for Chennai the solution lay in its coastal location. Sitting on the Coromandel Coast, it is blessed with a 25 mile coastline making it the second longest in the world after Miami. The marine waterfront has provided a solution in the form of seawater desalination. Currently, about 25 per cent of the city’s water needs are sourced through the two desalination plants situated in Nemmeli and Minjur.

Nemmeli desalination plant: Background and key features

The 110 million litre per day (mld) Nemmeli desalination plant is the largest in the country on a design-build-operate basis supplying 100 per cent potable water for domestic consumption. Situated about 35 km south of the city centre along the East Coast Road, the project is a membrane reverse osmosis (RO)-based seawater desalination plant. It aims at bolstering piped water supply to the southern parts of Chennai which includes the areas of Guindy, Velachery, Alandur, Sholinganallur, Neelangarai, Perungudi, Besant Nagar, Nandanam, MRC Nagar, Raja Annamalaipuram and Mylapore.

The project was awarded to the Chennai-based water solutions provider VA Tech WABAG on an engineering, procurement and construction turnkey basis at an approximate cost of Rs 10.33 billion. The project scope included design, supply, construction and commissioning of the facility along with seven years of operation and maintenance. Funding for the project was facilitated through a grant by the central government.

Integrating state-of-the-art technologies

WABAG equips the plant with the most advanced technology in the field of desalination. The entire process starts with the inflow of seawater, drawn at a depth of 10 metres, through gravity flow via a 1 km pipeline embedded in the seabed. The intake caisson and velocity head are designed, with generous area, to make sure no marine life is sucked into the pipeline. The laying of this 1 km long high density polyethylene pipeline required massive dredging works of about 175,000 cubic metres in the seabed.

The success of a desalination plant depends on the effectiveness of the pretreatment system to make the raw water suitable for desalination. At the Nemmeli desalination plant, the inlet water is pretreated with a state-of-the-art pretreatment technology – the lamella clarifier – followed by ultrafiltration (UF). The lamella clarifier not only removes the high amount of suspended solids in the incoming water but its compact size also reduces the area footprint of the desalination facility. After removal of suspended solids, the water flows to the disc filter. These Arkal disc filters are designed to screen 260 mld of water every day, removing impurities as small as 100 microns in size. The water is then passed through a second stage of filtration which is the UF. At this stage, over 3,600 membranes are deployed in over 30 UF skids. The highly advanced pretreatment at the Nemmeli desalination plant ensured that the plant could operate at full capacity during the recent cyclone when the seawater total suspended solids (TSS) reached around 1,400 parts per million (ppm) as against the design value of 300 ppm. Even six years after commissioning, the Nemmeli desalination plant’s state-of-the-art technology is at par with the latest and most advanced seawater reverse osmosis (SWRO) plants being built elsewhere in the world.

From here, water devoid of suspended solids and organic particulates is stored in two massive steel fabricated filtered water storage tanks each with a capacity of 15 million litres to be pumped to the RO section. The heart of the desalination process is a single-pass SWRO system removing the salinity of the seawater. High pressure pumps make the water flow into the RO membranes through a guard 5 micron cartridge filter mechanism ensuring removal of any residual contaminants. In this stage, salinity level as high as 39,000 ppm is reduced to less than 400 ppm. The reject water which is produced from the RO process comes out at a very high pressure of around 58 to 60 bars. This pressure is not allowed to go untapped by the use of energy efficient pressure exchangers which transfer 97 per cent of the reject water pressure to boost the pressure of the incoming feedwater to the RO membranes. Energy savings are ensured at every step with the use of variable frequency drives which feed only the required amount of power for all processes. The plant is also fully automated with a highly sophisticated distributed control system.

The treatment cycle is concluded with a remineralisation process using carbon dioxide and lime to make the desalinated water fit for human consumption. Chlorine is added to keep the water disinfected during storage, transportation and distribution. In 2016, the plant’s capacity was further enhanced by 10 mld taking it to 110 mld, with an innovative process retrofit and optimisation by the water technology company.

Over the years, with advancements in technology, the cost of desalinated water has come down from $3.2 per cubic metre of water in 1980 to $0.55 per cubic metre in 2018. This reduction in price has been possible due to improvements in membrane treatment efficiency, mass production of these RO membranes, and owing to energy savings through highly efficient pressure exchanger devices. It is noteworthy that the life cycle cost to produce desalinated water at Nemmeli is as low as 4.4 paise per litre.

Future of desalination in India

In a country like India, experiencing acute water crisis due to limited freshwater sources and intermittent rainfall, desalination has emerged as an alternative to ensure water security. The Nemmeli desalination plant supplies water to over 1 million residents of Chennai daily. meeting almost 25 per cent of its daily municipal water requirement. In light of such examples, it is safe to say that seawater desalination is an extremely successful potable water model which can be applied to other coastal cities in the country and across the globe. Constant optimisation of the desalination membranes coupled with the lower power consumption footprint will definitely encourage more desalination projects being envisaged. The recent announcements of two new desalination plants with capacities of 150 mld and 400 mld at Chennai emphasise the same conviction.

Besides, states such as Gujarat and Karnataka are already in the process of setting up mid- to large-scale (30-100 mld) desalination plants to serve domestic as well as industrial consumers. The desalination segment is evolving with projects coming up on a build-own-operate-transfer basis and hybrid annuity model to further enhance the viability and timely execution of such projects with the belief that water is a finite resource and must be priced appropriately.

Given the country’s rapidly growing economy coupled with a burgeoning population and stricter regulations against groundwater extraction, it is evident that desalination is the answer to water needs not only for drinking water consumption but for the sustenance of the industrial sector as well.

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