Zero Liquid Discharge: recovering the water and leaving no liquid effluent to discharge.

That no liquid effluent leaves the premises. Wastewater is treated and concentrated until the water fraction has been recovered for reuse and the dissolved load remains as solid salts and residues, which are then disposed of through an authorised route. The discharge that remains is solid waste and vapour, not liquid.

ZLD is not a universal requirement. It is imposed on specified highly polluting categories and on units in particular locations through CPCB and State Pollution Control Board directions and through consent conditions, with textile dyeing, distilleries and tanneries among the categories most commonly covered. Whether it applies to a given unit is determined by the directions in force for its category and state and by what its own consent says.

Primary and secondary treatment to remove solids and organic load, softening and fine filtration to protect the membranes, reverse osmosis in one or more stages to recover permeate and concentrate the reject, then a multiple effect evaporator to concentrate further, and an agitated thin film dryer or crystalliser to produce dry salt. The exact configuration follows the effluent's TDS, its organic content and the recovery achievable.

Near-ZLD recovers most of the water and reduces discharge substantially but still leaves a small liquid stream (often the hardest, most saline fraction) to be disposed of. True ZLD carries that fraction through to solids. The last increment of recovery is disproportionately expensive in energy, which is why the distinction is a real engineering and commercial decision rather than semantics.

The dominant costs are the thermal stage's steam or fuel demand and the electrical load of the RO and pumping, followed by membrane replacement, chemicals for pre-treatment and cleaning, and disposal of the recovered salts. Because thermal energy scales with the volume reaching the evaporator, the operating cost is decided upstream, by how much effluent is generated and how well streams are segregated, more than by the evaporator itself.

It is characterised and disposed of through the route its character requires. Mixed salts from many industrial effluents are managed under the hazardous and other wastes rules and go to an authorised treatment, storage and disposal facility under manifest. Where selective crystallisation produces a saleable, sufficiently pure salt, recovery is possible, but that depends on the effluent and should not be assumed at design stage.

Permeate and evaporator condensate are usually of good quality and are commonly reused as process or boiler feed water after appropriate polishing. Reuse points have to be chosen against the quality each use demands, and the water balance has to close. A ZLD plant recovering more water than the factory can absorb simply moves the problem.

Most often because the effluent reaching them is not what the design assumed: inadequate pre-treatment fouls the membranes, unsegregated streams push TDS beyond the RO's recovery, and organics carry through to the evaporator and cause scaling and foaming. Add to that deferred membrane replacement and interrupted steam supply, and a plant that was compliant on paper becomes intermittent in operation.