Fig. 1: Zero liquid discharge versus minimal liquid discharge.

Zero liquid discharge (ZLD) and minimal liquid discharge (MLD) treatment trains typically involve a pretreatment step and a brine concentration step. The brine concentration step usually includes a reverse osmosis process and a brine concentrator. The concentrated brine can be sent to either an engineered process (such as a thermal brine crystallizer) or a natural process (such as an evaporation pond) for crystallization. The brine concentration step and the crystallization step combined comprise the ZLD treatment train. Alternatively, the concentrated brine can also be disposed via deep-well injection or discharge (to ocean or other allowable receiving water bodies). The brine concentration step and the disposal step combined comprise the MLD process.