Direct contact membrane distillation with softening Pre-treatment for effective reclaiming flue gas desulfurization wastewater

Abstract

Comparisons of the formation, distribution, and composition of the membrane foulants caused by raw and pre-treated flue gas desulfurization wastewaters during membrane distillation process. • Softening-treated DCMD for concentration of flue gas desulfurization wastewater. • Mineral ion is effectively removed by NaCO 3 , NaOH and polyacrylamide pre-treatments. • Pretreatment inhibits membrane fouling and changes composition of membrane fouling. • CaSO 4 is the main foulant. • Mg-Si, Al colloid and organic matter weld scaling layer. • Colloidal silica/organic matter is the main contaminant in DCMD to pre-treat FGDW. The flue gas desulfurization wastewater (FGDW) contains extremely high concentration of heavy ions (e.g., Ca 2+ , Mg 2+ ) and unbiodegradable organic pollutants such as humic substances. In this paper, the effects of softening pre-treatment on water reclamation performance and membrane fouling phenomenon of the direct contact membrane distillation of FGDW were investigated systematically. Results showed that fouling on the membrane caused by crystallization of CaSO 4 , Mg-Si complex, Al colloid, and organic matter in the FGDW decreased the permeate flux. Pre-treatment with NaCO 3 , NaOH and polyacrylamide in combination with coagulation and chemical precipitation processes resulted in nearly 100% removal of Ca 2+ and Mg 2+ and prevented membrane from fouling. This led to 11 times-concentrated FGDW with excellent salt and organic matter rejections. Overall, membrane distillation combined with softening pre-treatment can effectively reclaim FGDW water with high concentration and salt rejection rates, without causing significant membrane fouling.