The adoption of zero liquid discharge (ZLD) is gaining momentum. ZLD is a cutting-edge engineering approach wherein the entire industrial and domestic wastewater can be reused after treatment/recycling without discharging even a drop of water outside the project boundary in natural waterbodies. It uses a method of concentration and thermal evaporation to either reuse treated wastewater on site or reduce it to solids. Notably, the Ministry of Environment, Forest and Climate Change (MoEFCC) has mandated plants installed after January 1, 2017 to meet a limit of 2.5 m3 per MWh along with ZLD. By using ZLD, the discharged water can be returned to the facility.
Reverse osmosis (RO), electrodialysis, evaporation, lime-soda ash softening, and other cutting-edge treatment methods are frequently used in ZLD systems. It typically comprises three components – pretreatment (for chemical and biological processes), RO (for the membrane process), and evaporator and crystalliser (for the thermal process). Water from the thermal power plant (TPP) is sent to the wastewater treatment facility during the ZLD process, where it is filtered using membrane technologies such as ultrafiltration. Reusing the separated water produces a concentrate made of the polluted stream, which is then transported into a brine concentrator (a mechanical evaporator) using a combination of heat and vapour compression. Water that has evaporated is retrieved and reused.
Major systems of ZLD
Ash water recirculation system (AWRS) and toe drain recirculation system (TDRS): The AWRS and TDRS are installed to optimise water consumption in a closed cycle and achieve ZLD from ash ponds. The effluent from an ash pond is recirculated to the plant for further ash slurry makeup and again sluiced to the ash pond. With this, 70 per cent of the ash handling water is recirculated to the plant from the ash pond resulting in marginal makeup of 200-300 m3 per hour for a typical 2×660 MW power station.
Liquid waste treatment plant (LWTP): The effluent generated from various sources in the plant, such as coal handling, is collected in the central monitoring basin of the LWTP. The collected effluent is analysed for quality in keeping with the prescribed norms and reused in suitable applications.
Separate drainage system for storm water and process water: Separating storm water from process water is useful for segregating pure and contaminated water, thus, saving fresh water and reducing purification costs. To this end, collecting and carrying the pristine storm water through separate storm water drains and ensuring that it does not get mixed with sewage water or industrial waste water is useful. In the modified system being implemented by NTPC, two independent drainage systems are constructed and these run concurrently for the collection and transportation of plant effluent and storm water runoff separately. With this system, uncontaminated storm water can be used as a top-up water source, to reduce load on sewerage/effluent treatment plants during periods of wet weather and optimise performance of the wastewater treatment plant.
Treated sewage water: The treated water can be used in various processes in the plant. These include horticulture (wherein most of the treated water is consuming thereby saving a huge quantum of fresh water), AWRS and flushing water for residential and non-residential units. Besides this, by making use of the treated sewage water from the municipality in the vicinity of power plants (within 50 km), a great amount of fresh water can be saved.
Conclusion
The main benefit of ZLD is that it reduces the negative environmental effects of energy production. The factors driving the uptake of ZLD are water scarcity and the rising concerns over the effects of toxic discharges on the environment. Several utilities are taking steps for reducing water discharge through recycling, reuse as well as ZLD. It is essential to ensure that the toxic solid waste generated as a by-product of the ZLD process is disposed of in an environmentally safe manner. Notably, one of the biggest power producers in the country, NTPC has adopted ZLD at its 16 power stations. Additionally, it intends to make 20 more stations ZLD compatible.
Net, net, using the ZLD procedure ensures water conservation and adherence to the most recent statutory requirements for TPP water use as announced by the MoEFCC. It also offers a number of advantages for TPP operators and is likely to gain traction, going ahead.