The depleted fresh water sources & regulations on usage have compelled almost all industries consuming large quantities of water to look for recycle strategies.
The textile, paper, pulp industries and oil and gas producers are the one who are increasingly made to go for zero liquid discharge due to scarcity as well as the local & international waste water discharge regulations.
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, both manay other parameters are need to be considered.
The COD removal is normally done with combination of both chemical precipitation & bio-degradation. pH is corrected of the collected effluent & then chemical precipitation is done, which is followed by biological degradation. Chlorine dosing is done for disinfecting the treated effluent as bio fouling of membranes is major concern in waste water recycle systems. After chlorination the water is filtered in series of filters & fed to RO. The RO permeate is taken to process after conditioning or to Polisher MB if being used as boiler feed water. The reject may sometimes be passed through another RO and the concentrate is taken to either evaporators or to a salt facility To brine concentrator unit.
If reject is evaporated in brine concentrators, it is normally done in two stages. In first stage the 2 -4 % reject from RO reject is concentrated to approximately 15 -20% solids. In next step the 15 -20% solids are concentrated further to 45 -50% solids. This solids slurry is sometimes further processes in crystallizers to form salt crystals which are sellable as commercial salts.
If reject is used for purposes like gardening/flushing etc., it needs to be conditioned properly so that it does not scale the plumbing.