Conceptual Design and Evaluation of Zero Liquid Discharge Systems for Management of Industrial Wastewater

Zero liquid discharge (ZLD) is becoming a more viable option for discharge of complex industrial wastewater. Drivers for ZLD include lowering of water quality standards for metals, implementation of chloride, sulfate, and total dissolved solids (TDS) water quality standards, as well as scarcity of water resources. ZLD of a gasification wastewater and two flue-gas desulfurization (FGD) scrubber blowdown wastewaters were evaluated. In all cases, complex water chemistry required use of a software model to evaluate boiling point rise, changes in pH, and solids formed. Formation of hygroscopic solids occurred with full crystallization of both wastewaters requiring careful handling. For the FGD blowdown, the chlorine concentration in the coal and the FGD scrubber operating chloride level drives the quality of the FGD blowdown and the extent of concentration using evaporation. The magnesium content of the limestone also impacts ZLD due to increases in the boiling point. Disposal of the concentrated brine with the fly ash is a convenient method for disposal of the brine and avoids full crystallization. This requires dry fly ash handling and a dependence on ash production for brine disposal.