Efforts to reduce, recycle and reuse water at TPPs
Water plays a significant role in power generation and is required for process cooling in condensers, ash disposal, removal of heat generated in plant auxiliaries and various other plant consumptive uses. The thermal generation industry is one of the largest consumers of water with a share of around 87.8 per cent in industrial water consumption. The diminishing water supply and pressure from competing water users are areas of concern for power plant owners. In 2015, the Ministry of Environment, Forest and Climate Change (MoEFCC) had notified water consumption norms for thermal power plants (TPPs) in order to minimise the consumption of water and ensure its judicious use in power plants. Besides, the government mandated the use of treated wastewater for all coal-based power plants located within a 50 km radius of a sewage treatment plant (STP).
Water consumption in a TPP
The estimated water use in TPPs is 35,157.4 million cubic metres (mcm) annually. Depending on the quality of water and end-use considerations it can be broadly classified as cooling tower water, and make-up water for compensating the loss due to water evaporation in cooling towers; demineralised water used in boilers for generating steam; ash handling water used during the combustion process; service water, which is used for coal dust suppression, firefighting measures, plantation, toilets and other civic amenities, and potable water. Demineralised water costs more than cooling tower water and its use is limited.
Traditionally, power plants used a once-through cooling (OTC) system, which draws a large amount of fresh water for cooling and other processes, and then discharges it to the source (with around 1 per cent lost in evaporation) though at a higher temperature than at the time of withdrawal. With a cooling tower system, the water consumption at a power plant reduces significantly. As per the Centre for Science and Environment, while the water withdrawal of an OTC plant is in the range of 70-200 cubic metres (cum) per MWh, the plants with cooling towers consume an average of 4 cum per MWh. OTC systems have been disallowed in India since 1999.
In a power plant, various factors influence its water consumption. The amount of water consumed in a plant depends on power plant technology as well as water characteristics such as the quality of raw water, wastewater programme, type of ash disposal, type of condenser cooling, quality of coal and ash utilisation at a power plant. In view of this, dry cooling systems are being used in power plants instead of wet cooling systems in order to optimise their water consumption. Further, high concentration slurry disposal and dry fly ash disposal systems are being deployed to reduce the water requirements of ash handling plants.
Water consumption norms and compliance so far
The emission control norms were announced by the MoEFCC in 2015. As per these, all TPPs with OTC were required to install cooling towers. The existing plants with cooling towers were required to achieve a maximum specific water consumption (SWC) of 3.5 cum per MWh. Also, plants installed after January 1, 2017 were required to meet an SWC limit of 2.5 cum per MWh as well as achieve zero wastewater discharge. The water consumption norms were further amended in June 2018. Through the amendment, the SWC limit of 2.5 cum per MWh for new plants installed after January 1, 2017 has been revised to 3 cum per MWh. Furthermore, seawater-based once-through plants have been exempted from the conversion to cooling tower-based systems.
As per the Central Pollution Control Board (CPCB) data on compliance for the April-June 2019 period, of the 183 power stations, only 93 stations have achieved compliance with SWC limits. Directions have been issued to plants with OTC systems to switch to cooling towers by 2022. The state pollution control boards have been asked to take the necessary action against TPPs that do not comply with the SWC limit.
Treated sewage as raw water to TPPs
As an alternative to fresh water, the government has mandated that all coal-based power plants located within a 50 km radius of an STP must use treated wastewater in their operations. In order to utilise treated sewage at TPPs, a tertiary treatment plant is required for treating the incoming secondary treated sewage to the design inlet water quality of the existing facility. A tertiary treatment plant must be capable of monitoring the incoming secondary treated sewage water. It should be equipped with a large storage facility and raw water backup. Besides this, in new plants, the pretreatment should be designed to handle secondary treated sewage with its inherent characteristics of biochemical oxygen demand, chemical oxygen demand, total oxygen demand, heavy metals, etc.
With regard to the use of treated sewage water in a plant, while a few state and central gencos have been complying with this requirement, there are concerns regarding the quality of water that hinder its uptake. In a recent development, in March 2020, the Central Electricity Authority (CEA) notified a draft MoU between urban local bodies (ULBs) and TPPs to facilitate the use of treated sewage water by power plants. The CEA also stated that the treated sewage water transportation system may be constructed by the power plants, which will also bear the cost of the transportation up to the power plant. Meanwhile, the ULBs will facilitate right of way for laying the water transportation pipeline by the power plant. The tertiary treatment plant may be constructed by the power plants.
Issues and concerns
Water shortage is one of the key concerns facing the thermal power generation segment. Excess water consumption leads to excess auxiliary power consumption because water and energy are dependent on each other. Further, an increase in the consumption of auxiliary power deteriorates the power plant efficiency. Thus, efficient and optimal use of water should be made mandatory for the benefit of both the environment and power plants.
While various old plants meet the 3.5 cum per MWh norm, others face layout constraints and retrofitting issues in installing additional treatment systems. Besides, 20-25-year-old plants equipped with OTC water systems often do not have adequate space for the installation of cooling towers. Some of the other challenges faced by developers in reducing water consumption at power plants are the high capital cost for the adoption of dry cooling systems, the high amount of chemical treatment required to increase the concentration cycles in order to lower the make-up water used in cooling towers, additional water required to operate FGDs, and handling of wastewater from FGD units.
The way forward
Although power production by burning coal is a water-intensive process, it can be optimised by conducting water audits. The domestic water requirement of power plants is unavoidable. Hence, sewage (domestic wastewater) should be treated in the STP itself and repurposed within the plant for other needs. This will help reduce domestic water consumption. Also, the wastewater generated in one system could be reused for other systems/applications. For new plants, pretreatment should be designed to handle secondary treated sewage water.
Water consumption can be reduced by modifying the existing facility and selecting an alternative wastewater treatment technology. Also, a customised tertiary treatment plant should be used as feed for TPPs can have a significant impact on the social water cycle. Besides, changing the water consumption pattern in a TPP by modifying the existing facility to suit the recycled water, and choosing water efficient technologies that reduce water consumption will be useful in optimising water consumption in a power plant. Some of the other ways to reduce water consumption in a power plant are optimising the ash-water ratio, recycling ash water from the ash dyke, reducing leakages and overflows, and wastewater recycling to achieve zero liquid discharge.
Net, net, reducing, recycling and reusing water in a power plant would go a long way in optimising water consumption. To this end, new and effective wastewater treatment technologies should be adopted.