What is zero liquid discharge (ZLD) and when do you need it?

5 min read · RTW engineering team

Zero liquid discharge means exactly what it says: no wastewater leaves the site. Water is recovered and reused, and dissolved salts leave as a solid. It is the most complete form of water recycling — and the most demanding to design and operate.

How it works, in short

In outline, pre-treatment and membranes recover most of the water for reuse, and a thermal stage concentrates what is left until the salts can be handled as a solid. The exact combination depends on the chemistry of your effluent, which is why every ZLD system starts with detailed testing.

When ZLD is the right answer

• There is no permitted discharge route, or discharge limits on salinity cannot be met.
• Water is scarce or expensive, so every recovered cubic metre has value.
• The facility wants to secure its operating licence against future tightening of rules.
• Salts or products in the brine can be recovered and sold.

What to watch

• Energy: evaporation is the most energy-intensive step, so maximising membrane recovery first keeps running costs down.
• Scaling and fouling: good pre-treatment and antiscalant programmes are essential.
• Solid waste: plan how the final salt will be handled and disposed of.