Coagulation pretreatment could deteriorate reverse osmosis membrane fouling

Coagulation is widely regarded as an indispensable pretreatment process in reverse osmosis (RO) systems of zero liquid discharge applications. Yet in practical applications, coagulation pretreatment often causes perplexing impact on membrane fouling and even deteriorates the RO performance with ambiguous mechanisms, thereby seriously disrupting the progress of RO-based applications. This study systematically reveals the RO performance devolution caused by Fe- or Al-based coagulation pretreatment, and elucidates the fundamental mechanism of membrane fouling deterioration due to residual coagulants. The Al-based coagulation predominantly triggers inorganic fouling, with the disruption of microbial ecological interaction networks within the biofilm exacerbated by copper-induced oxidative stresses. The Fe residues dramatically enhance the production of extracellular polymeric substances and facilitate robust fouling layer development, exacerbating membrane fouling and diminishing RO performance. These findings not only provide essential engineering guidance for optimizing practical operations but also deepen the understanding of the coagulation-RO interactions, establishing a refined framework for enhancing the efficiency and sustainability of advanced water treatment systems.