Reverse osmosis (RO) plays a critical role in sewer mining plants for the reclamation of high-quality water from wastewater. However, membrane fouling remains a major operational challenge that limits the long-term performance and economic viability of RO systems in such applications. In this study, a comprehensive autopsy of fouled RO membrane elements from a full-scale sewer mining plant was performed to characterise foulant composition and elucidate fouling mechanisms. The results revealed a pronounced spatial distribution of foulants along the RO train, with the first element in the lead stage exhibiting more severe fouling than the tail elements due to higher hydraulic loading and contaminant exposure. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analyses indicated that the foulant layers consisted of heterogeneous organic and inorganic deposits and biofilms. Additionally, total organic carbon (TOC) measurements and elemental analysis of foulant extracts confirmed the variation in foulant composition across elements. Cover densities of TOC, Si and Al foulants on the first membrane element were 1.6, 1.7, and 7.6-fold, respectively, higher than those on the last element. This spatial distribution of various foulant composition emphasised the necessity for combined acid and alkaline chemical cleaning strategies to achieve effective fouling control.
