- Coagulation and adsorption are two widely recommended pretreatments to ultrafiltration (UF) membranes. Optimal concentrations of ferric chloride and powdered activated carbon (PAC) are reducing membrane fouling and increasing the retention of the organic matter. Intuitive pretreatment may result in either increased fouling or in low retention of organic matter. In addition, the inside-out capillaries might become partially plugged by the formed flocs. The understanding of the fouling mechanisms in inside-out capillaries is therefore a challenging and much practically needed task.We investigated the fouling using an unique setup with separate measurements of flux and retention of organic matter in five segments of a 1.5-m long multibore capillary. The high coagulant doses resulted in average 90% humic acid retention and in severe fouling. The use of powdered activated carbon at higher pH also contributed to severe increases in membrane resistance. As the filtration progressed, the dead-end part of the capillary (far from the feed) gradually became clogged with flocs.Graphical abstractHighlights► The hollow fiber UF capillary was divided into five segments with separate drainage. ► The flux and retention of organic matter were sensitive to coagulant dose. ► The dead-end (far from the feed) part of the capillary gradually became clogged with flocs. ► They can be subdivided into active and “backyard” sectors (used to store formed flocs).