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A pilot-scale UASB reactor coupled with an external ultrafiltration (UF) membrane was operated under three different hydraulic retention times (HRT: 4, 8 and 12 h) for municipal wastewater treatment in order to assess the composition and distribution of the fouling layer, as well as to identify the predominant fouling mechanisms. For that purpose, membrane autopsies were carried out based on fouling layer density determination, thermogravimetric, SEM and EDX analysis. Results showed a  variable density of the fouling layer (average values were 13.90 ± 0.22, 13.46 ± 1.15 and 12.78 ± 0.49 mg/cm2 for HRT of 4, 8 and 12 h, respectively), indicating that this parameter had an impact on the fouling density. Organic material was predominant in the fouling layer, being around 75% of its composition for the three HRT studied. Regarding pore-blocking mechanisms, standard blocking was the predominant mechanism at the beginning of filtration, coexisting at the end of it with cake filtration. In the first filtration cycle (1 h), after standard blocking,  intermediate and complete blocking developed simultaneously during a short period of time and, finally, cake filtration prevailed. However, in the last (19th) filtration cycle, standard blocking and cake filtration occurred  simultaneously from the  beginning, suggesting the existence of an irreversible fouling layer, in spite of  chemical cleaning. Keywords: fouling layer density, pore blocking mechanisms, irreversible fouling

Fouling layer characterization and pore-blocking mechanisms in an UF membrane externally coupled to a UASB reactor