Membrane distillation for wastewater treatment: Current trends, challenges and prospects of dense membrane distillation

Membrane distillation (MD) has excellent potential for wastewater treatment, and studies on this subject have increased recently. The unique features of MD to pass only volatile component through the pores has led to the exploration of hybrid MD, particularly membrane distillation bioreactor (MDBR) and osmotic membrane bioreactor-membrane distillation (OMBR-MD). Fouling and wetting are recognised as drawbacks in particular applications despite the excellent performance. The application of classical porous MD is highlighted as the main concern that leads to poor separation performances. This review highlights the possibility of applying dense membrane to push forward MD applications for wastewater treatment. The development of self-standing and composite dense MD and their performances in applications of wastewater treatment are summarized and discussed. The operational parameters affecting MD performance in the treatment of various types of feed are presented to analyze the robustness and the critical shortcomings of dense MD for wastewater treatment. Challenges, such as the trade-off between mechanical integrity and permeate flux, lack of studies using real wastewater for long-term operation, stability of the dense membrane against rigorous operation conditions, complicated fabrication methods, and the use of harsh chemicals during the fabrication of dense MD membranes are elaborated for further improvement in the development of dense MD membranes for wastewater treatment.