Evaluating membrane cleaning for organic fouling in direct contact membrane distillation

To find an effective cleaning method for organic fouling in membrane distillation (MD), this work focused on the fouling characteristics of humic acid (HA), sodium alginate (SA), and bovine serum albumin (BSA), and the cleaning efficiency of deionized (DI) water flushing, NaCl flushing, NaOH flushing and DI water backwashing. The results showed that when Ca2+ was absent from the feed solution, severe fouling was observed for HA and BSA, while SA led to slight fouling due to its hydrophilicity. The addition of Ca2+ alleviated the fouling for HA, but intensified the fouling for SA and BSA. There was a transition from cake filtration to complete blocking in membrane fouling mechanism for most MD tests. NaOH flushing resulted in irreversible fouling ratio of 48–60% after fouling by HA and BSA, while DI water flushing can recover 70% of flux decline caused by SA without Ca2+ present in the feed. For the three organics, only 8–15% of fouling was removed after DI water backwashing. The differences in the cleaning efficiency of the four cleaning methods were further verified by the positive correlation between irreversible fouling ratio and surface energy parameter, as well as Scanning electron microscopy and Fourier transform Infrared analysis of the membranes used in this study.