With the growing focus on optimising water use at thermal power plants (TPPs), wastewater treatment and zero liquid discharge (ZLD) technologies are fast gaining traction. The water usage norms by the Ministry of Environment, Forest, and Climate Change (MoEFCC) mandate TPPs installed after January 1, 2017 to meet the water consumption limit of 2.5 cubic metres per kWh and achieve ZLD. The ZLD process requires effective wastewater treatment, recycling and reuse, and contributes to water conservation by reducing the intake of freshwater.
Plant operators are increasingly adopting technologies to treat the wastewater streams generated from cooling tower blowdown, ash pond water and flue gas desulphurisation (FGD) systems, making it suitable for further use in other power plant processes. Advanced wastewater treatment facilities such as liquid waste treatment plants (LWTPs) and ash water recirculation systems (AWRSs) are increasingly being used at TPPs for the treatment and reuse of treated effluents.
The wastewater generated by a TPP includes clarifier sludge, filter backwash, cooling tower blowdown, regeneration waste of the demineralisation plant and boiler blowdown. The cooling process in power plants requires the highest amount of water. A major amount of cooling tower blowdown is utilised in ash handling and unused blowdown is collected in the central monitoring basin (CMB). Apart from unused cooling tower blowdown, unused wastewater at different locations within the plant is collected through a network of drains at the CMB. The wastewater in the CMB has high total dissolved solids on account of regeneration waste and CT blowdown water. It is made suitable for further consumption through various wastewater treatment processes including reverse osmosis (RO) and water softening. In ash management, the water requirement can be drastically reduced with the adoption of efficient AWRS. In these systems, the decanted ash water from ash ponds is recirculated back to the plant for reuse in ash handling. Industry estimates suggest that with an effective operational AWRS in place, more than 70 per cent of water can be recirculated back to the plant from the ash pond. The wastewater streams from FGD systems contain a number of soluble salts and heavy metal salts, which can be eliminated using chemicals such as lime and soda ash. Wastewater can be further processed through water softening and RO processes.
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 advanced treatment technologies like lime-soda ash softening, RO, electrodialysis and evaporation. It typically comprises three components – pretreatment (for chemical and biological processes), RO (for the membrane process), and evaporators and crystallisers (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, that is, the polluted stream, is obtained. It is sent inside a brine concentrator (a mechanical evaporator) using a combination of heat and vapour compression. The evaporated water is recovered and recycled.
Notably, the power gencos are adopting a proactive approach towards wastewater management. NTPC Limited has equipped all its stations with advanced wastewater treatment facilities such as state-of-the-art technology sewage treatment plants, LWTPs, coal slurry settlement pits and AWRS for treatment and reuse of treated effluents. It is also in the process of implementing ZLD at all its stations. As per the company’s annual report of 2020-21, 15 stations have already achieved ZLD, five have completed the ZLD work as per schemes and technical specifications during 2020-21, and another 20 stations are likely to have completed ZLD works during 2021-22. Apart from this, ZLD systems have been implemented at various other TPPs in the country such as CESC’s Budge Budge generating station and JSW Energy’s Torangallu power plant.
To conclude, besides environmental benefits, wastewater treatment and ZLD systems provide a wide range of benefits for TPP operators. Power plants with ZLD from the design stage are likely to gain public acceptance and obtain statutory approvals easily. Meanwhile, wastewater recycling provides more options for site selection as the concerns regarding adequate water supply are mitigated substantially. Wastewater recycling significantly reduces the amount of makeup water that the generation company may need to purchase from the state water boards.
Overall, while there are several reasons to implement ZLD technologies across power plants, the most significant are the regulatory considerations for the discharge. Several concerns related to the treatment of FGD wastewater, freshwater shortages as a result of accelerated growth of water-intensive industries, government discharge permits, and climate change are set to be the drivers for prioritisation of ZLD.