As a large metropolitan city in south India, Chennai has undergone rapid urbanisation, alongside population growth. Many regions in the city are facing water shortage, while the outskirt areas have started using bore wells to fulfil their water requirements. The Chennai Metropolitan Water Supply and Sewerage Board (CMWSSB) has been responsible for providing adequate and good quality water to the old Chennai city and its suburbs. With the lack of perennial water sources such as rivers and major water bodies, the water board relies on alternative water sources such as rainwater and seawater. The unpredictable nature of rainfall has pushed the utility to construct efficient desalination plants to meet the growing demand for treated water. One such plant implemented by CMWSSB is the Nemmeli desalination plant, a seawater reverse osmosis (SWRO) plant with a capacity of 110 million litres per day (mld). It was constructed by VA Tech Wabag and commissioned in South Chennai in 2013. As an extension to this plant, another SWRO plant of 150 mld capacity has also been constructed on the same campus. This plant, inaugurated in February 2024, is built on an area of only 10 acres of land. The two plants together serve the South Chennai region with 142 litres of water per capita per day to a population of around 1.76 million.
The construction of the 110 mld Nemmeli desalination plant has incorporated advanced construction technologies and design methods. It is built as a fully automated plant, enabled with supervisory control and data acquisition and internet of things, and fulfils energy-efficiency and environmental requirements. The treatment techniques integrated into the plant ensure 47 per cent recovery of bacteria and virus-free potable water from seawater.
Key design mechanism
The construction techniques adopted in implementing the 110 mld Nemmeli desalination plant encompassed various design considerations. The plant has been built with different systems for multiple stages of advanced seawater treatment. These include main components such as an intake system, pretreatment system, SWRO system, post-treatment system and outfall system. These parts help in the treatment of total suspended solids (TSSs) and total dissolved salts (TDSs) in the raw water before they become fit to be used for drinking water purposes. In the initial stages of pretreatment, the TSS is removed while the TDS is removed in the reverse osmosis (RO) system.
The RO system is built with specialised membranes that help in eliminating various dissolved salts. After this, the pH imbalance of the water is treated through the post-treatment system and upon demineralising, the water is supplied to the city. Further, the outfall system helps in the careful discharge of the rejected water, also called brine. In this system, the outfall pipe allows the transfer of the rejected water up to 1.5 km back to the sea. This pipe is constituted of diffusers (multiple openings in different parts of the pipe) that dissipate the brine slowly into the seawater without impacting the aquatic life.
Main plant components for effective treatment
Design elements
The detailed construction of the plant has incorporated various aspects of advanced treatment of seawater. Many important design components have been integrated to ensure that quality water is received by the end of the treatment. For instance, there are around six huge seawater intake pumps in the plant that allow raw water to be pumped for treatment at the beginning. These pumps are connected to 1,000 metre long, high density polyethylene (HDPE) intake pipes of 1.6 metre diameter. This is followed by the travelling band screen, which removes particles above 4 mm such as shells and fish that escape from the intake pumps. A conveyor-like belt connected with these filters helps in throwing these particles outside the treatment unit. Following this, the seawater is collected through HDPE pipes in a concrete intake chamber, around 10 metres in the sea by gravity. As the initial treatment process is completed, other advanced plant components have been built to allow further filtering. Lamella Clarifiers, such as clarifiers in sewage treatment plants and water treatment plants, aid in flocculation. They are designed to remove TSS particles up to 300 parts per million (ppm) by adding chemicals that help form large clusters of solids called flocs. The water then settles down along with lowering of TSS as it comes to the sedimentation tank. This collected water from the tank is further sent to disc filters, which are built to remove the suspended particles above 100 microns. The plant consists of a total of 120 disc filters, including 30 skids of automatic disc filters.
Ultra filtration
Further, the plant design incorporates the ultra filtration of water to eliminate almost all TSSs, viruses and bacteria. With a pore size of 0.05 microns, the filters ensure that none of these particles escape and hit or impact the costly and imported membranes. Upon completion of this process, the RO system of the plant serves a critical function. This system is constructed with 14 blocks called trains with a capacity of 8.3 millilitres each, along with two stand-by trains. There are also 72 vessels in each train with seven elements in each vessel that help in the treatment of around 41,000 ppm TDS in water. The RO membranes allow the trapping of main dissolved solids. The entire system has a recovery rate of 45.4 per cent and meets the standard for the resultant treated water to less than 500 ppm. Likewise, the construction of the filtration units in the 150 mld Nemmeli desalination plant has been further upgraded to achieve a TDS of around 150-200 ppm.
Other special features
The construction of the 110 mld plant has a few outlying features for energy-efficient water treatment as well. For instance, the integration of high-pressure pumps in the plant allows an effective pressure exchange. They allow proper pumping of water into the membranes as well as recover energy on the discharge of rejected water. The recovered energy is used to power inflow pumps and RO membranes with the help of energy recovery devices. The carbon dioxide storage tanks have also been built in the plant campus to help in the final phases of post-treatment. Moreover, the dosing of limestone and sodium hypochlorite helps add some necessary salts to the water to attain the correct pH balance.
In sum
As the city continues to expand with the increasing economic opportunities, its population will grow exponentially. The population is expected to grow at a rate of 6.8 per cent. This would have a direct impact on the water demand in the city and reliance on desalination plants would increase further. The design scheme and advanced construction practices adopted in the Nemmeli desalination plant present a fine example for the upcoming desalination projects in Chennai such as the Perur desalination plant. The uptake of such a detailed construction plan with modern treatment units and membranes will help utilise the abundant seawater along the vast coastline of the city. It would also be catalytic in efficiently achieving the desired quality of tertiary-treated seawater.
Based on a presentation by G.B. Vydahi, Superintending Engineer, Desalination Wing, CMWSSB at a recent India
Infrastructure conference.