What is Zero liquid discharge (ZLD)?
Zero liquid discharge (ZLD) is a wastewater approach in which a facility treats and recovers all of its effluent for reuse, leaving only solid or brine residue and no liquid discharge off site.
Where ZLD is required
In India, ZLD is imposed through consent conditions set by State Pollution Control Boards and through directions from the Central Pollution Control Board, most often for highly polluting sectors. In December 2016, CPCB issued directions under Section 18(1)(b) of the Water (Prevention and Control of Pollution) Act, 1974 to the Uttar Pradesh and Uttarakhand boards to ensure zero liquid discharge from distilleries. Other companies adopt ZLD voluntarily where water is scarce or groundwater permission is tight.
How a ZLD train works
A typical train runs conventional ETP treatment, then ultrafiltration and reverse osmosis to recover most of the water, then an evaporator and a crystallizer or dryer for the RO reject. Each stage concentrates the brine further and uses more energy per cubic meter than the one before, so the thermal stages tend to dominate operating cost.
Proving the zero on Aqueduct's water twin
A ZLD claim rests on a closed water balance. On the water digital twin, metered flows into the ETP and RO, permeate returned to process, reject sent to the evaporator and condensate recovered all sit in the same equation, with RO recovery and treatment efficiency shown live. Any unaccounted flow appears as a loss term that has to be explained before the site can call itself zero discharge.