Advancing ZLD for Efficient and Sustainable Wastewater Management

In an interaction with Asia Business Outlook, Kamal Verma, Chief Executive Officer - Water Business Group at Triveni Engineering & Industries Ltd, shares his views on the rise of Zero Liquid Discharge (ZLD) processes, improving energy efficiency in ZLD processes, enhancing the efficiency and effectiveness of wastewater treatment processes, and more. He has over 30 years of experience in EPC, which is Engineering, Procurement, and Construction of Water and Wastewater Treatment Plants and Infrastructure development, including Highways, Expressways, Metro Rail, Urban Development, Water, Hydro Power, and SEZs.

Why are ZLD processes on the rise? 

In June 2019, 11 million residents of Chennai, India, endured a day without drinking water and experienced water depletion. This incident is not isolated and will escalate as climate change effects intensify. Niti Aayog released a report in 2019 that predicted day zero for 21 Indian cities in the next few years. There is a severe risk to the availability of freshwater. It has been noticed that many industries extract groundwater to meet their process requirements. Due to regulation as well as a shift in dynamics in the water sector, gradually more people are opting for recycled reuse, including zero-liquid discharge projects.

Energy consumption is a critical factor in the operational costs of ZLD systems. What innovations are being implemented to improve energy efficiency in ZLD processes, and how do they impact overall cost savings?

Early ZLD systems were based on standalone thermal processes, where wastewater was typically evaporated in brine concentrators, followed by brine crystallizers or an evaporation pond. Additionally, ZLD processes are based on water evaporation, including multistage evaporation, MEG, or multistage flash distillation, MSF, crystallizers, and evaporation ponds. However, technologies like MED, MSF, and crystallizers suffer from high energy consumption due to the use of electricity or fossil fuels. In addition, constructing the original resistant container in these brine concentrators and crystallizers is costly and brings high capital costs.

On the other hand, evaporation ponds demand extensive land space, and their evaporation efficiency is generally low. So, reverse osmosis and membrane-based technology, widely applied in distillation, have been incorporated into ZLD systems to improve energy and cost efficiency. As a result, RO is much more energy efficient than thermal evaporation. Hence, it is being preferred in the ZLD process. Incorporating new technologies, such as emerging membrane-based processes, provides an opportunity to reduce the associated energy consumption and costs and to expand the applicability of ZLD. This is how people improve energy efficiency by introducing the RO system.

With the increasing emphasis on sustainability, how are recent technological advancements in recycling & reuse with ZLD systems improving the efficiency and effectiveness of wastewater treatment processes?

ZLD systems efficiently process industrial brine wastewater to dry solids, recovering approximately 95% of the liquid waste for reuse. This reduces operational costs and allows for the recovery of valuable byproducts like salt and brine. Through advanced processes and techniques, wastewater is now converted into useful resources such as clean water, renewable energy, and nutrient-rich fertilizers. These advancements not only help to alleviate the strain on wastewater resources but also contribute to reducing greenhouse gas emissions and protecting ecosystems. Moreover, wastewater plants are becoming central hubs for green initiatives, enabling communities to move towards a circular economy and achieve long-term environmental goals, thus improving the efficiency and effectiveness of the wastewater treatment process.

How can the PPP Model drive ZLD technology adoption in industrial wastewater treatment?

Highly polluted industries like textile, dyeing, printing units, and other units require vast amounts of freshwater for their processes, and they discharge highly contaminated wastewater. Due to strict discharge norms and scarcity of freshwater, industries are turning towards ZLD with recycling and reuse of the wastewater. Due to underfunding issues and the availability of funds for industries and the government, PPP concessions are being discussed. The PPP concession model is suitable wherein grants are provided by central and state governments, and probably a quarter, which is 25%, is funded by the industries. The balance is funded through a private investor. PPP concessions will install and operate the ZLD facility for 15 years during the operation and maintenance phase.

Tertiary treated wastewater is then sold to industries for their process requirements with a guaranteed quantity and quality of water, and these industries will be paying for it. Therefore, when private capital is infused into the ZLD model through the PPP model, it can drive ZLD technology as it is an emerging trend. It provides the stability of the quality and quantity of the water to industries for their process requirements, so the PPP model will pave the way for adopting ZLD technologies in industrial wastewater treatment.

What are the emerging trends and future innovations expected in ZLD technology, and how might these advancements shape the future of wastewater treatment in industries?

The emerging trends and future innovations expected in ZLD technology are three membrane-based processes: electrode dialysis, also known as ED, forward osmosis, also known as FO, and membrane distillation, also known as MD. These are provided as an alternative to ZLD technologies for brine concentration and further concentration of the brine for the wastewater after the RO stage. The brine produced from these processes serves as feed for the crystallizers for evaporation. ED, FO, and MD are at the piloting stage worldwide and are expected to reach commercial operation application in a few years.

Electrode dialysis, or ED, applies an electric potential as the driving force to remove dissolved ions through an ion exchange membrane. In contrast to RO membranes that reject all ions, ion exchange membranes selectively permit continuous transport but prevent the passage of co-ions. ED can concentrate feed water to as high as more than one lakh milligrams per liter. Similarly, forward osmosis, unlike hydraulic pressure-driven RO, utilizes an osmotic pressure difference to drive water permeation across the semipermeable membranes. In FO, water flows from the feed water to a concentrated draw solution with a higher osmotic pressure. Since the driving force in FO is osmotic pressure, FO can treat water with much higher salinity than reverse osmosis.

FO can treat average salinity up to one lakh eighty thousand milligrams per liter, which is higher than electrode dialysis. The third is membrane distillation. This is a thermal membrane-based desalination process in which a partial vapor pressure difference drives water vapor across a hydrophobic microporous membrane. In membrane distillation, the feed water is heated, and the resultant temperature difference between the hot feed water and the cold permeate site creates a vapor pressure difference that drives the water vapor flux. The MD technology is more energy-intensive than RO and ED, as water separation by MD requires a liquid-vapor phase transition. These are futuristic technologies like ZLD and are at the piloting stage. I'm sure that in a few years, some more new technologies will be introduced.