In this study the effect of different particle sizes of inorganic suspended solids (ISS) on the ISS accumulation in an activated sludge system was investigated. The volume mean particle diameters (Dv) of ISS were 26, 73, 106, 165, and 210 gm. There are four fates of ISS in an activated sludge system: (1) suspending in the activated sludge, (2) depositing at the bottom of the reactors, (3) discharged from the reactors via excess sludge, and lastly (4) discharged from the reactors via effluent. The accumulated ISS in the bioreactor was unevenly distributed. Based on the accumulation proportion of ISS in bioreactor, an ISS accumulation model was established, from which the ISS accumulation concentration and the MLVSS/MLSS could be predicted. The proportion of ISS suspending in activated sludge was 0.22, 0.21, 0.042 and 0.031. The proportion of ISS depositing at the bottom of bioreactors was 0.31, 0.47, 0.75, 0.76 and 0.92. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.