An advanced online monitoring approach to study the scaling behavior in direct contact membrane distillation

Abstract One of the major challenges in membrane distillation (MD) desalination is scaling, mainly CaSO 4 and CaCO 3 . In this study, in order to achieve a better understanding and establish a strategy for controlling scaling, a detailed investigation on the MD scaling was performed by using various analytical methods, especially an in-situ monitoring technique using an optical coherence tomography (OCT) to observe the cross-sectional view on the membrane surface during operation. Different concentrations of CaSO 4 , CaCO 3 , as well as NaCl were tested separately and in different mixed feed solutions. Results showed that when CaSO 4 alone was employed in the feed solution, the mean permeate flux (MPF) has significantly dropped at lower volume concentration factor (VCF) compared to other feed solutions and this critical point was observed to be influenced by the solubility changes of CaSO 4 resulting from the various inlet feed temperatures. Although the inlet feed and permeate flow rates could contribute to the initial MPF value, the VCF, which showed a sharp MPF decline, was not affected. It was clearly observed that the scaling on the membrane surface due to crystal growth in the bulk and the deposition of aggregated crystals on the membrane surface abruptly appeared close to the critical point of VCF by using OCT observation in a real time. On the other hand, NaCl + CaSO 4 mixed feed solution resulted in a linear MPF decline as VCF increases and delayed the critical point to higher VCF values. In addition, CaCO 3 alone in feed solution did not affect the scaling, however, when CaSO 4 was added to CaCO 3 , the initial MPF decline and VCF met the critical point earlier. In summary, calcium scaling crystal formed at different conditions influenced the filtration dynamics and MD performances.