Thermal power plants (TPPs) are among the largest industrial consumers of water. With India’s rapid economic expansion in recent years, the demand for both energy and water has increased, putting the resources under increasing pressure. It is estimated that TPPs account for nearly 88 per cent of industrial water consumption in the country. As per the International Energy Agency (IEA), India’s energy sector currently withdraws (the volume of water removed from a source) roughly 30 billion cubic metres (bcm) of water and consumes (the volume withdrawn but not returned to a source) almost 6 bcm. A continuation of current trends in water usage would put the projected demand for water far ahead of the available supply and threaten the country’s energy output. Besides, water scarcity will hamper the productivity of TPPs. Hence, efficient and optimal use of water by power plants is beneficial not only for the environment but also for TPPs.
The Ministry of Environment, Forest and Climate Change (MoEFCC) notified emission control norms along with water consumption norms on December 7, 2015. The three essential norms introduced for water consumption were:
- All TPPs with once-through cooling (OTC) systems are required to install cooling towers (CTs), implying that they need to shift to recirculating cooling.
- All these plants, as well as the existing plants with CTs, need to achieve a maximum specific water consumption of 3.5 m3 per MWh.
- TPPs installed after January 1, 2017 will have to achieve a specific water consumption of 3 m3 per MWh and zero wastewater discharge.
All seawater-based plants were exempted from meeting the norms. In October 2017, these norms were eased for all operating TPPs, and they are now allowed to utilise up to 20 per cent more water than permitted earlier. Further, the Tariff Policy of 2016 mandates that treated water from sewage treatment plants (STPs) of local bodies and municipalities should be used directly by TPPs located within a 50 km radius. To set up TPPs in water-scarce regions, the state and central agencies that are responsible for water allocation from rivers/reservoirs need to ensure water availability before granting environmental clearance to a power plant. TPPs are required to meet the water consumption norms prescribed by the MoEFCC.
Best practices for controlling water use in TPPs
As per the CEA’s Report on Minimisation of Water Requirement in Coal-Based TPPs, the consumptive water requirement for coal-based plants with CTs used to be about 7 m3 per MWh without ash water recirculation, and 5 m3 per MWh with ash water recirculation. In the recent past, plants have been designed with consumptive water requirement in the range of 3.5-4 m3 per MWh. In TPPs, water is typically used for cooling tower make-up, ash disposal, demineralising water make-up, wet flue gas desulphurisation (FGD) systems, etc. More than 80 per cent of input water is required for cooling tower make-up.
To reduce the usage of water, innovative technologies should be adopted for wastewater recycling, water flow and quality monitoring. To ensure water conservation in the cooling system, the cycles of concentration (CoC) of cooling water need to be increased, thus reducing the blowdown and make-up water quantity. Many systems operate at two to four CoC, but six cycles or more may be possible. Increasing the number of cycles from three to six reduces cooling tower make-up water by 20 per cent and cooling tower blowdown by 50 per cent.
Ash is a major by-product of coal-based TPPs and must be safely disposed of. The traditional method of ash disposal is to make a slurry by mixing ash with water and pumping it to ash ponds through long distance pipes. This method requires a considerable amount of water. Utilities can install a high concentration slurry disposal (HCSD) system for wet ash disposal. The water requirement in an HCSD is about one-tenth that of low concentration slurry disposal. In case the plant has a lean slurry ash disposal system, then an ash water recirculation system (AWRS) from an ash dyke should be in place. Another method to reduce water consumption is to adopt dry ash handling systems, in which ash is extracted and disposed of in dry form using pneumatic vacuum systems. Besides this, dry bottom ash handling systems are used.
Zero liquid discharge (ZLD) is a practice whereby the all the industrial and domestic wastewater can be reused after treatment/recycling. Changes in the original design of systems, such as the installation of CTs in the cooling water system, modifications in wastewater disposal systems and use of independent storm water drains can help achieve ZLD. Wastewater from FGD systems, which contains high total dissolved solids, high chlorides and heavy metals, should be used in the ash water system to conserve water. Further, installation of STPs can facilitate the provision of treated wastewater, which can be used for low-grade applications such as dust suppression. Some plants that have achieved ZLD are CLP’s Jhajjar plant and CESC Limited’s Budge Budge plant.
The desalination of seawater and brackish water is another feasible and attractive option for water conservation. Further, an HDPE lining should be used for reservoirs, as it helps in preventing seepage losses. The substitution of water-cooled condensers by air-cooled condensers is another innovative method.
As utilities are adopting digitalisation amidst the Covid-19 pandemic, a real-time data monitoring system could also be put in place for monitoring distributed water usage in a plant through a water supervisory control and data acquisition (SCADA) system. For this, according to the Confederation of Indian Industry, utilities need to install electromagnetic flow meters at prominent water supply and consumption points and connect them to real-time monitoring or SCADA-based water monitoring system to compute water balance.
NTPC has pioneered the adoption of several new technologies, including AWRS, liquid waste management systems and STPs. To reduce water usage, air-cooled condenser technology is being installed at the North Karanpura super TPP and Patratu STPP. NTPC has also taken the initiative to conduct water balance audits at its stations. The company has completed it in four stations as of December 2020, with two more stations set to be audited in the fourth quarter of 2020-21. NTPC has taken a proactive approach in progressively making all its power stations ZLD capable. Further, the company has released a Water Policy, 2021 to minimise its water footprint to the extent possible and become one of the most water efficient power companies globally, by generating more power per drop.
Challenges and the way forward
Coal power plants were supposed to meet the water norms by December 2017. However, even after six years of introduction of water consumption norms for TPPs, there has been no progress on compliance. Although timelines for emission norms have been revised twice, in 2017 and more recently in 2021, the implementation of and compliance with water norms have been completely overlooked in the absence of any firm action. Freshwater-based OTC plants withdraw enormous amounts of water, and many of these plants continue to operate, with no plans to upgrade or install CTs. Further, there is no uniform format for reporting specific water consumption. The lack of on-ground monitoring and inspection by regulatory authorities to check compliance with zero discharge norms is also a major concern. The use of wastewater for cooling is also limited, with just 5-8 per cent of all coal-based TPPs in India getting access to treated wastewater, as per the IEA.
Going ahead, there is a need to review the implementation of water consumption norms and issue clear deadlines to non-complying plants, besides ensuring ZLD implementation, notes Down To Earth. The implementation of norms should be prioritised for TPPs in water-scarce regions; and old and inefficient freshwater-based OTC plants should be considered for decommissioning. Cross-verification and monitoring of self-reported water consumption data should be done by a third party agency through annual water audits. Further, a uniform format should be adopted by all plants to report water consumption; and raw water meters should be periodically calibrated, and calibration reports submitted to the regulatory agency. The best practices of water efficient, norms-compliant plants with low specific water consumption and those that have achieved ZLD should be highlighted.
In line with the recent revision to the emission norms, a penalty mechanism may be introduced for effective deterrence from flouting water norms. Operators should increase the share of non-freshwater cooling in new TPPs and retrofit old ones to make better use of water recycling. The prospect of shifting to dry cooling, or air-cooled condenser technology, which eliminates the need for water, should be explored. Utilities should also constitute focused working groups at the corporate, regional and station levels, responsible for entire water systems, besides investing in research and adopting cutting-edge technologies to improve water efficiency.
Net, net, there is scope for reducing water consumption in the power sector by implementing norms that not only help curb the water stress situation and ensure availability of water for other needs, but also make TPPs more efficient.