Treatment of flue gas desulfurization wastewater by integrated membrane-based process for approaching zero liquid discharge

An integrated membrane process for the treatment of wastewaters from a flue gas desulfurization (FGD) plant was implemented on laboratory scale in order to reduce their salt content and to produce a water stream to be recycled in power industry. The process is based on a preliminary pretreatment of FGD wastewaters, which includes chemical softening and ultrafiltration (UF), in order to remove Ca2+ and Mg2+ ions as well as organic compounds. The pretreated wastewaters were submitted to a reverse osmosis (RO) step to separate salts from water. The RO retentate was finally submitted to an MD step in order to extract more water, thus increasing the total water recovery factor while producing a high-purity permeate stream. The performance of RO and MD membranes was evaluated by calculating salts rejection, permeate flux, fouling index and water recovery. The investigated integrated system allowed to reach a total recovery factor of about 94%, with a consequent reduction of the volume of FGD wastewater to be disposed, and an MD permeate stream with an electrical conductivity of 80 ?S/cm, able to be reused in the power plant, with saving in fresh water demand.