Frequently asked questions about zero liquid discharge (ZLD) in power plants

Zero liquid discharge (ZLD) is a wastewater treatment approach that recovers water for reuse within the plant rather than discharging it to the environment. The goal is simple: no regulated wastewater leaves the facility as a liquid stream.

In coal-fired power plants, ZLD is most often evaluated for challenging wastewater streams generated by processes such as flue gas desulfurization (FGD) systems. Through a series of treatment steps, water is separated from dissolved solids and returned for beneficial reuse within plant operations.

Most ZLD systems combine multiple treatment technologies, including membrane filtration, evaporation, and crystallization. These processes concentrate dissolved salts and other contaminants into solid residuals that can be managed separately, while recovering a significant portion of the water for reuse.

ZLD shifts wastewater management from discharge-based treatment to water recovery-focused operations.

Instead of treating wastewater solely for environmental release, ZLD systems recover water for reuse within the plant. This reduces freshwater demand and decreases wastewater discharge volumes.

However, as water is recovered, dissolved contaminants become concentrated into smaller residual streams such as brine, salt cake, or crystallized solids. Wastewater management therefore extends beyond treatment to include residual handling, stabilization, transport, and disposal.

These downstream requirements can significantly influence system design, operating costs, and long-term facility planning.

Yes. One of the primary goals of zero liquid discharge (ZLD) is to recover water that can be reused in plant operations rather than discharged as wastewater. Depending on water quality requirements and site-specific conditions, recovered water may be

used in applications such as flue gas desulfurization systems, cooling processes, ash handling, or other plant operations.

Reusing recovered water can help reduce freshwater demand while improving overall water management. This can be especially valuable in regions where water availability is limited or where facilities are looking to reduce their dependence on external water sources.

The amount of water that can be recovered and reused depends on the treatment approach and the quality requirements of the intended application.

Water recovery percentage is a key performance metric for zero liquid discharge (ZLD) systems because it determines how much wastewater is converted into reusable water and how much remains as residual waste. Even small increases in recovery rates can significantly reduce the volume of residuals that require handling, transportation, and disposal.

For example, increasing water recovery from 98% to 99% reduces the remaining wastewater by roughly half. If a plant starts with 100 gallons of wastewater, 98% recovery leaves 2 gallons of concentrated residuals. At 99% recovery, only 1 gallon remains.

For coal-fired power plants, this reduction can have a meaningful impact on residuals management costs, landfill requirements, transportation needs, and fly ash consumption for encapsulation. Because disposal costs are often a significant component of long-term operating expenses, improving water recovery can provide benefits beyond water reuse alone.