Direct contact membrane distillation: Capability to treat hyper-saline solution

• Study the influence of operating conditions on mass transfer
• Investigate the temperature and the concentration polarization phenomena
• Discuss the permeate flux decline due to salt crystallization on membrane surface
• Supervise the permeability and the scaling of the membrane

In this paper, we focused our work on the direct contact membrane distillation (DCMD) capability to treat hyper-saline solution. The governing operative model for mass transfer was investigated. The measured flux has been well predicted by the Knudsen-molecular mechanism model. The effects on the DCMD flux of polarization phenomena TP and CP were underlined. The optimum operating parameters were defined: the hot and cold stream temperatures were set, respectively at 59 and 20 °C, and the feed and permeate velocities were fixed both to 0.046 m s− 1. With regard to membrane performance to treat the reverse osmosis brine, a long-term experiment was carried out under the optimal experimental conditions. The increase in feed RO brine concentration provoked a noticeably decrease in flux from 8.43 to 4.06 kg m− 2 h− 1. The RO brine experiments proved that the DCMD process was capable to concentrate the solution tell to reach concentration factor (CF) further than four times, which corresponded to the super-saturation of saline solution. Based on the characterization methods, the occurrence of the membrane wetting and scaling was shown and interpreted. These extreme phenomena promote the salt crystallization on the feed side of the membrane. The onset crystallization phenomenon starts when the permeate decreases so fast. Their sudden decline was about 90% for a working period of 20 h.