The Global Rise of Zero Liquid Discharge for Wastewater Management

Zero Liquid Discharge (ZLD) is a strategic wastewater management system that ensures that there will be no discharge of industrial wastewater into the environment. It is achieved by treating wastewater through recycling and then recovery and reuse for industrial purposes. Hence, ZLD is a cycle of a closed loop with no discharge. Although ZLD is a costly process, it paves the way for economic benefits by recovering salts and other chemical compounds.

ZLD systems are being implemented by many industrial processes, primarily for end-of-pipe treatment, due to severe regulations put into practice for industrial wastewater discharge. A ZLD system mainly involves the use of advanced treatment methods, through which industrial brine wastewater is essentially reduced to dry solids/salts. 

Factors that motivate the ZLD system are:

1. Water scarcity
2. Water economics
3. Environmental regulations

However, implementation of ZLD requires extensive research and a pilot study because the wastewater generated is not the same for different processes. Above all, it must be financially viable. The type of industry that can afford it should have an effluent treatment plant to achieve ZLD, and those that cannot afford it may become a member of a common effluent treatment plant (CETP).

So Why is Zero Liquid Discharge Important?

In a world where freshwater is an increasingly valuable resource, industrial processes threaten its availability on two fronts, unless the water is treated. Many industrial processes require water, and then reduce the availability of water for the environment or other processes, or alternately contaminate and release water that damages the local environment.

In recent years, greater recognition of the dual challenges of water scarcity and pollution of aquatic environments has revived global interest in ZLD. More stringent regulations, rising expenses for wastewater disposal, and the increasing value of freshwater are driving ZLD to become a beneficial or even a necessary option for wastewater management. The global market for ZLD is estimated to reach an annual investment of at least $100–200 million, spreading rapidly from developed countries in North America and Europe to emerging economies such as China and India.

Another important reason to consider ZLD is the potential for recovering resources that are present in wastewater. Some organizations target ZLD for their waste because they can sell the solids that are produced or reuse them as a part of their industrial process.

Regardless of an organization’s motivations to target ZLD, achieving it demonstrates good economics, corporate responsibility and environmental stewardship. By operating an in-house ZLD plant, disposal costs can be reduced, more water is reused, and fewer greenhouse gases are produced by off-site trucking, which minimizes the impact on local ecosystems and the climate.

While Early ZLD systems were based on stand-alone thermal processes, where wastewater was typically evaporated in a brine concentrator followed by a brine crystallizer or an evaporation pond. Such systems, which have been in successful operation for 40 years and are still being built, require considerable energy and capital.

Meanwhile, since the last decade, Reverse osmosis (RO), a membrane-based technology widely applied in desalination, has been incorporated into ZLD systems to improve energy and cost efficiencies. However, RO, although much more energy efficient than thermal evaporation, can be applied only to feedwaters with a limited salinity range. Accordingly, other salt-concentrating technologies that can treat higher salinity feedwaters, such as electrodialysis (ED), forward osmosis (FO), and membrane distillation (MD), have emerged recently as alternative ZLD technologies to further concentrate wastewater beyond RO.

Although ZLD holds great promise to reduce water pollution and augment water supply, its viability is determined by a balance among the benefits associated with ZLD, energy consumption, and capital/operation costs. Therefore, it is imperative to understand the drivers and benefits that make ZLD a realistic option. Incorporating new technologies, such as emerging membrane-based processes, provides opportunities to reduce the associated energy consumption and costs and to expand the applicability of ZLD.

The benefits of ZLD are:

1.  Wastewater discharge is avoided by recycling.
2.  Recovery of water and salt aids in reducing the cost of operation of ZLD.
3.  It promotes the sustainability of the industry and the environment at large.
4.  There is less use of water by the textile industry, which means water is available for other purposes such as irrigation (agriculture) and domestic utilities.
5.  ZLD helps recover the environment.

 Sludge can be effectively used by the cement industry.

Environmental Impacts

As discussed earlier, ZLD consumes large amounts of energy, leading to significant emissions of greenhouse gases (GHG). Some pre-treatment methods, such as acidification followed by degasification, release CO₂ from the feedwater into the atmosphere. Incorporating technologies with higher energy efficiency, such as RO, will significantly reduce greenhouse gas emissions. In addition, emerging ZLD technologies that can utilize low-grade or renewable energy enable further reduction of greenhouse ga footprint of ZLD systems.

Viewpoint

ZLD implementation is growing globally as an important wastewater management strategy to reduce water pollution and augment water supply. Future growth of the ZLD market will heavily rely on regulatory incentives that outweigh its economic disadvantages. As the severe consequences of water pollution are increasingly recognized and attract more public attention, stricter environmental regulations on wastewater discharge are expected, which will push more high-polluting industries toward ZLD. Intensified freshwater scarcity, caused by both climate change and freshwater overexploitation, will likely facilitate ZLD implementation too.