This study evaluated the fate of chemical oxygen demand (COD) fractions of an organized industrial district effluent treated with pilot membrane bioreactor (MBR) system. A series of respirometric analyses were performed on raw wastewater together with 450 nm, 100 kDa and 1 kDa filtered samples obtained from particle size distribution (PSD) experiments. The PSD analysis revealed that more than 300 mg/L of the influent COD accumulated in the lowest size range (<2 nm), accounting for 49-52 % of the total COD. The bulk (64 %) of the influent COD consisted of soluble hydrolysable COD (S-H1). The hydrolysis rates for S-H1 (0.80-1.28 1/day) were slightly below the levels suggested for domestic wastewaters, indicating the presence of inhibitory compounds in industrial wastewaters. The COD removal efficiency of MBR pilot was found as 85 %; however, 43 % of effluent COD (106 mg/L) still included rapidly hydrolysable organics in the size bracket of 5-30 nm. In this respect, the COD fractions for activated sludge process models used for MBR applications require a new-insight to correctly reflect organic matter balances in the treatment system. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.