The definition of ZLD differs from industry to industry depending upon consumption pattern and the contaminants.
The treatment depends on the intended usage of the recycled water. If the water is to be used for gardening purpose, BOD, COD and suspended solids removal with filtration will suffice. On the other hand if the water is to be used for high pressure boiler or for industrial production processes it's including pre treatment, Membrane filtration (UF & RO).
The rejection of Membrane filtration will require Mechanical evaporation or mist evaporation is used to further concentrate the brine prior to crystallization. If any organic are present, condensate polishing may be required for final cleanup to reuse.