#01 | Let’s make Zero Liquid Discharge more efficient

By Alejandro Vilar and Carlos Echevarría (Cetaqua)

Water scarcity is one of the major global challenges of our time, representing a threat not only in the ecosystems but a direct impact on economic growth. This shortage in freshwater availability, and the incremental pollution of natural water bodies, has raised awareness in the population and regulatory bodies, which during last decades have tried to develop stricter discharge restrictions in terms of organic matter and nutrients, but also in heavy metals and recalcitrant organic compounds.

The perspective of more restrictive discharge limits, especially in the industrial sector, has pushed the development of alternative water management strategies to capture part of the economic value of wastewater effluents through the recovery of valuable by-products. Zero Liquid Discharge (ZLD) processes allow to separate the liquid fraction from solid waste, producing reclaimed water suitable to be reused in other industrial applications (e.g., cleanings, boiler feed, cooling towers, process) while recovering a solid waste, which depending on its nature can be easily managed in landfills or commercialized.

Despite the advantages of ZLD, achieving an effective minimization of wastewater volume requires the implementation of advanced separation technologies that are intensive in CAPEX and OPEX (mainly associated with high energy consumption). In this sense, the high cost of implementing ZLD processes needs to be driven by other factors such as:

  • A stricter regulatory framework.
  • Extreme freshwater scarcity that can compromise water availability for industrial production processes.
  • A relatively high cost of wastewater treatment that boost the implementation of additional treatment steps to achieve circular management.

Minimum Liquid Discharge, an energy-saving alternative to current processes

Originally, ZLD was limited to the use of thermal evaporation technologies (e.g., Mechanical Vapor Compression (MVC) or Multi-Effect Distillation (MED)) followed by brine crystallizers. This represents a robust but costly scheme, in which 100% of water can be recovered by applying a total energy consumption of 65 kWhe/m3.

The emerging trend is to use a pre-concentration step, also known as Minimum Liquid Discharge (MLD), in which advanced membrane-based technologies are applied to recover 90% of water and concentrate the stream up to 200-250 g/L before deriving the concentrate to a conventional brine crystallizer. The application of alternative MLD technologies instead of conventional thermal evaporators accounts for a reduction of 70% of the energy consumption. Additionally, the minimization of the brine volume allows a lower CAPEX of the ZLD process and a reduction of the global energy consumption of more than 20% .

“The application of alternative Minimum Liquid Discharge (MLD) technologies instead of conventional thermal evaporators accounts for a reduction of 70% of the energy consumption”

Specific energy consumption of concentration technologies used in MLD/ZLD systems (Martinez et al. 2020)

LIFE Remine Water will demonstrate an MLD scheme based on the use of Nanofiltration, Reverse Osmosis and Electrodialysis Reversal to minimize the volume of acid mine drainage (AMD) effluents and produce suitable water to be reused.