Direct contact membrane distillation (DCMD) process for simulated brackish water treatment: An especial emphasis on impacts of antiscalants

Membrane distillation (MD) has been considered as a promising technique for brackish water desalination, while the membrane fouling issue, especially the membrane scaling, have been the main obstacle for the widespread application of MD system. Antiscalant, as an economical tool for scaling control, has been widely applied to other membrane separation processes, like RO and UF. Therefore, three types of typical antiscalants, including PBTCA, DTPMPA, and HPMA, were applied in this work to investigate their anti-scaling performances during the MD desalination of brackish water. Additionally, the impacts of antiscalants on fouling layer development as well as the desalination efficiency of MD system were also deeply researched in this study. Compared to DTPMPA, PBTCA and HPMA performed better scaling inhibition in MD system, resulting in remarkable mitigation of flux decline. With the usage of PBTCA, these mineral substances were largely distributed in feed solutions with the dissolved state, partly facilitating the biofilm formation. Differently, these salts mainly existed in the form of distorted slags with the presence of HPMA, and the biofilm development was notably retarded by the deposited slags. These loose slags could be easily removed from the MD membrane by increasing the feed flow velocity, thus it can be summarized that HPMA was more suitable for the long-period MD desalination of brackish water. The application of DTPMPA should be strictly restricted in MD desalination units considering the risk of pore wetting.