Thermal power plants (TPPs), especially coal-based power plants, are among the largest industrial water consumers. According to a study by The Energy and Resources Institute (TERI), TPPs are responsible for nearly 88 per cent of the total water consumption by the industrial sector.
As per Ministry of Environment, Forest and Climate Change norms, plants installed after January 1, 2017 are required to meet a water consumption limit of 3 cubic metres (cum) per kWh and achieve zero liquid discharge (ZLD). Therefore, power plant owners need to take firm steps to recycle wastewater. Further, as per the revised Tariff Policy, 2016, power plants located within a radius of 50 km from a sewage treatment plant (STP) have been mandated to use treated sewage water. The associated costs on this account will be treated as pass-through. In addition, with the Ministry of Power mandating the installation of flue gas desulphurisation (FGD) systems for about 166 GW of coal-based capacity, TPP operators opting for wet FGD technology will be required to install FGD wastewater treatment systems as well.
Wastewater treatment techniques
Coal-based power generation is a water-intensive process, and conducting water audits can enable efficient usage. Water used in various plant processes can be treated and recycled. Typically, effluent treatment plants use evaporators, crystallisers and dewatering units installed at power plants to remove dissolved salts such as sodium sulphate, sodium chloride, calcium, magnesium and bicarbonates from wastewater to produce clean water to be recycled into the plant.
The cooling process in power plants requires the highest amount of water. The once-through cooling systems used in TPPs are inefficient as they do not recycle the cooling water. This causes high volumes of water withdrawal. They can be replaced by closed-cycle cooling systems, where the cooling water is recycled between a cooling tower and a heat exchanger, thus minimising the amount of water withdrawn from the river. Another suitable method that can be adopted is dry cooling. It requires little or no water as it relies on air as a medium of transfer. It is even better from the cost perspective as it requires less maintenance than cooling towers that require water.
The cooling tower blowdown is mostly saturated with calcium sulphate and silica, which are tough to separate through evaporation. So, to treat this water, calcium sulphate seeds are added to the
saturated wastewater to suspend these precipitating salts. A similar principle is used in the seed slurry brine concentrator, which allows concentration without scaling surfaces and reduces the volume of waste. It is used for waste streams with high levels of dissolved solids.
Meanwhile, the wastewater from wet FGD systems is difficult to crystallise by evaporation due to the presence of soluble salts like ammonium and calcium chlorides and heavy metal salts. Wastewater from this source requires pretreatment with chemicals to produce a crystalline solid. For pretreatment, wastewater is directed into reactor tanks, where chemicals are added to precipitate heavy metal ions as insoluble hydroxide and sulphide salts. The chemicals used in the process are caustic soda, lime and sodium sulphide. Then, ferric chloride/alum and specific polymers are added for the coagulation of the precipitates to form large flocs, which settle at the bottom of a clarifier and are filtered later. This method is suited to reduce the suspended solids, metals and acidity in the wastewater. For removing any organic compounds, biological treatment is needed.
TPPs use a large amount of water in their ash handling system, wherein ash is mixed in water to form an ash-water slurry, which is pumped into the ash pond. An approach for reducing the water footprint in ash disposal is to recover this ash water. Water from the ash pond could be recycled and used in cooling towers. In addition, the water overflow from the pond could be directed to the wastewater treatment plant, and also be reused for coal heap wetting, roadways wetting and other processes.
Zero liquid discharge systems
ZLD is an advanced wastewater treatment technology used to purify and recycle all the wastewater produced in industries including TPPs. A ZLD system includes a range of technologies for the recovery, recycling and reuse of treated wastewater. The implementation of ZLD would ensure that the discharged water is recycled back to the plant. It is a system wherein all wastewater is either retained on site or reduced to solids by adopting a method of concentration and thermal evaporation.
A ZLD system typically includes one or more of the advanced treatment technologies like lime-soda ash softening, reverse osmosis (RO), electrodialysis and evaporation. It typically comprises three components – pretreatment (for chemical and biological processes), RO (for the membrane process), and evaporator and crystalliser (for the thermal process). During the ZLD process, the wastewater from the TPP is directed to the wastewater treatment plant and subsequently into the ZLD system, where it is filtered using membrane technologies like ultrafiltration. The separated water is reused, and a concentrate, which is the polluted stream, is obtained which is sent inside a brine concentrator (a mechanical evaporator using a combination of heat and vapour compression). The evaporated water is recovered and recycled.
The brine is further concentrated to a higher solids concentration, resulting in the formation of wet sludge. The crystalliser converts the sludge to solid waste using highpressure steam. Crystallised solids, in most cases, are categorised as hazardous waste and disposed of periodically. Any remaining water is clean enough for reuse. After the solid salts are separated for disposal, there is 98 per cent water recovery for reuse.
A few challenges of the ZLD system are that it generates solid waste, which is hazardous, creating challenges in its disposal. It has a high operating cost. Further, the economic viability of the system depends on the cost and availability of water. The biggest advantage of ZLD is that helps to decrease the environmental impact of power generation.
This system is used in the 750 MW Budge Budge power plant in Kolkata, West Bengal, and the JSWEL Torangallu power plant in Bellary, Karnataka. The power plants have an average specific water consumption of 2.25 cum per MWh and 2.23 cum per MWh respectively, and have been identified by the Centre for Science and Environment among the most water-efficient plants in its 2015 rating study. The initiatives adopted by the Budge Budge power plant for zero discharge handling include bottom ash handling, complete dry fly ash evacuation and handling, 100 per cent fly ash use, emergency fly ash disposal via high concentration slurry disposal, volatile treatment for boiler water, and cooling tower with concentration cycles of six. The plant uses cooling-tower blowdown water instead of fresh water for road washing, bottom ash handling, high concentration ash handling and sprinkling in the coal plant. In the Torangallu power plant, an RO plant has been installed for unused cooling tower blowdown water treatment. The treated RO permeate is recycled for cooling tower make-up, and the RO plant rejects are used in the JSW Steel plant for its ore beneficiation unit.
Conclusion
According to a report by the International Renewable Energy Agency, in a business-as-usual scenario, power generation is expected to account for nearly 9 per cent of national water consumption by 2050, growing from 1.4 per cent in 2025. A World Resources Institute report claims that by 2030, more than 70 per cent of TPPs in India are likely to experience an increased level of water competition from the agricultural, urban and industrial sectors. The 2018 Composite Water Management Index estimates that water demand will exceed water supply by the year 2030.
In view of the looming water crisis, it is crucial to regulate water use. Continuous policy and regulatory support and technological changes can divert water usage in a more efficient direction. However, the process is not simple. Cost-benefit analysis, technical limitations and implementation time are some of the many aspects that need to be looked into before applying any of the changes.