Water management is a key priority for power generation companies across the world, owing to stricter environmental regulations related to wastewater discharge and usage of treated sewage water for thermal power plants (TPPs), regional water shortages and the negative public view about TPPs. The Indian government has also tightened water consumption norms in recent years. As per the Ministry of Environment, Forest and Climate Change’s norms, plants installed after January 1, 2017 are required to meet a water consumption limit of 3 cubic metres per kWh and achieve zero liquid discharge (ZLD). Power plant owners, therefore, 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.
A host of water and wastewater treatment solutions are available to meet the varying needs of TPP operators. TPPs require water for several processes such as steam cycle, ash handling and FGD systems, among others, but about 90 per cent of the total is for cooling purposes. Since TPPs need a large volume of water for various processes, systems for water pretreatment, boiler feed water treatment, condensate polishing and seawater desalination systems are required to purify and filter water. The use of pure or filtered water is crucial to prevent corrosion, scaling and fouling of equipment such as boilers and turbines and associated tubes and pumps. After water is used by individual TPP systems, it contains a significant amount of impurities and needs to be treated before it is discharged. These impurities are removed in stages through wastewater treatment systems such as cooling tower blowdown treatment plants and FGD water treatment plants to improve water quality in line with statutory limits.
Water treatment solutions
TPPs typically store water for seven to eight days in a plant’s reservoir to reduce bacteria, algae and other suspended solids. Pre-chlorination or injection of chlorine in water is carried out to further reduce biological impurities. Subsequently, this water is led to the water pretreatment system to further remove suspended solids, colloids, organic matter, iron and manganese. Typically, sedimentation, flocculation and coagulation processes are carried out using a variety of chemicals (sodium hypochlorite, lime, iron trichloride and polyelectrolyte) to remove undissolved impurities at this stage. Filtration, ultrafiltration, microfiltration and nanofiltration processes along with reverse osmosis are also used to filter out particles of varying sizes. Before the water is fed into the boiler, the boiler feed water treatment system further purifies water to ensure an efficient process and generate quality steam. A basic boiler feed water treatment system is based on processes such as filtration and ultrafiltration, ion exchange/softening, membrane processes such as reverse osmosis and nanofiltration, de-aeration/degasification, and coagulation/ chemical precipitation. Separate processes might be required to remove dissolved solids and organic material.
Another important water treatment system is the condensate polishing plant (CPP), which prevents contaminants and corrosion products in return steam condensate from entering into the turbine or boiler. During the process of steam generation in power plants, the steam cools and condensate forms, which is collected and then used as boiler feed water. Prior to re-use, the condensate must be polished/purified to remove impurities (such as silica oxides and sodium) and suspended matter (iron oxide particulates from corrosion) as well as other contaminants, which can cause corrosion and maintenance issues that can effectively be mitigated by condensate polishing treatment. The CPP reduces the operating cost of power plant by improving its efficiency and is typically deployed in large-sized power plants to increase the boiler efficiency and to make the condensate appropriate for recycling. CPPs are equipped with ion exchange technology to remove dissolved trace minerals and suspended matter.
Coastal power plants often use seawater desalination systems, which treat water with high salt content to produce water of acceptable or potable standards for plant operations. Some key desalination techniques include thermal distillation and membrane filtration. The latter is a dominant technology and includes reverse osmosis and electro-dialysis processes. A seawater reverse osmosis plant (SWRO) system consists of pretreatment plant equipped with clarifiers as well as dual media and pressure sand filters. An example of power plant deploying seawater desalination system is Tata Power’s 4,000 MW Mundra ultra mega power project in Gujarat. Commissioned in 20212-13, the project comprises 25,200 cubic metres per day (permeate) SWRO. The plant produces 25.2 million litres per day of the desalinated water for various end user applications. A part of desalinated water is treated further through a brackish water reverse osmosis system to produce permeate for use in the upstream mixed beds demineralisation plant for boiler feed application.
Wastewater treatment solutions
Wastewater produced by cooling towers and wet FGD systems contains salts such as sodium sulphate, sodium chloride, calcium, magnesium and bicarbonates. A significant quantity of blowdown water is generated when the cooling towers are operated at different cycles of concentration. The cooling tower blowdown treatment plant reduces corrosion in the blowdown system and discharges cleaner water into the environment. In the plant, the blowdown water is generally treated by softening and the use of reverse osmosis membranes to remove suspended and dissolved contaminants. New technologies such as microfiltration are also increasingly being employed in cooling tower blowdown treatment plants to meet the stringent environmental specifications. The treated blowdown water (permeate or filtrate) is then reused as boiler feed or process water. A portion of the blowdown, the reverse osmosis reject stream, is directed to brine concentrators or evaporators to enable ZLD.
The wastewater produced by FGDs based on wet-scrubbing is also required to be treated before being discharged as it contains contaminants from coal, limestone and make-up water. The wastewater is acidic in nature and highly saturated with gypsum, contains high concentrations of heavy metals, chlorides as well as certain dissolved organic compounds. However, wastewater composition can vary significantly from plant to plant depending upon scrubber chloride concentrations, efficiency of the fly ash removal, the type of gypsum dewatering system and the type of FGD process used. The treatment process consists of several steps including calcium sulphate (gypsum) desaturation, primary clarification, equalisation, trace metal precipitation and suspended solids removal, secondary clarification and filtration. For pretreatment, FGD wastewater is directed into reactor tanks, where chemicals such as caustic soda/ lime and sodium sulphide/organosulphide are added to precipitate heavy metal ions as insoluble hydroxide and sulphide salts. Subsequently, ferric chloride/alum and certain polymers are added to coagulate the precipitates and form large flocs, which settle at the bottom of a clarifier and are filtered later. Though this method is well suited to reduce the suspended solids, metals and acidity in the wastewater, it is ineffective in filtering the soluble salts, including calcium, magnesium, sodium and ammonium combined with chloride and nitrate, as well as any organic compounds. These are later removed using biological treatment.
Conclusion
There is a pressing need for gencos to reduce their water footprint and recycle as much water as possible. New power plants need to be designed from the ground up with ZLD systems in place. Utilities also need to step up to utilise sewage treated water wherever available to reduce the dependence on freshwater sources. At present, there are just a handful of utilities using or planning to use such water. For instance, NTPC Limited signed an MoU with Noida Authority for the supply of 80 million litres per day of treated sewage water for its Dadri TPP in June 2018. Tata Power has also installed an STP at its Jojobera TPP in Jharkhand, which reduces its water consumption. Going forward, water scarcity and the increasing focus on the environmental impact of TPPs place an onus on gencos to recycle and reuse water, which is a valuable resource.
Neha Bhatnagar