Comparison of calcium scaling in direct contact membrane distillation (DCMD) and nanofiltration (NF)

Abstract

Calcium scaling is commonly encountered in membrane separation processes with a profound impact on permeate flux and energy efficiency of these processes. Direct contact membrane distillation (DCMD) and nanofiltration (NF) were selected to study calcium scaling impact on thermal and pressure driven membrane processes. Calcium sulfate (CaSO4) and calcium carbonate (CaCO3) solutions with salinity in the range from 3000 to 30,000 mg/L were used as the feed. All DCMD and NF tests were performed at identical feed temperature (i.e., 40 °C) and shear conditions (i.e., Re = 771 ± 28). The average size of CaSO4 and CaCO3 crystals formed in the DCMD system was larger and the scale was more porous than in the NF system. The permeate flux in NF system was impacted by both calcium scales regardless of the feed salinity, while only CaSO4 scale formed at low salinity affected the performance of DCMD system. Surface crystallization in NF systems is likely extended into the membrane pores, which lead to permanent reduction in membrane permeability. This study demonstrated that the impact of calcium scaling is affected by both feed salinity and separation driving force, and is much less severe in thermal than pressure driven membrane processes.