The productivity and sustainability of forward osmosis (FO) - reverse osmosis (RO) system could be hindered by contaminant accumulation in the draw solution. A significant, progressive contaminant accumulation was observed when digested sludge was processed by the FO-RO system. The FO-RO system achieved stable water production and high rejections of dissolved organic matter; however, progressive contaminant accumulation was evident in the draw solution. Mechanism for contaminant accumulation was elucidated by examining the passage and accumulation of dissolved organic matter using fluorescence excitation emission matrix (EEM) spectroscopy and size exclusion (SEC) chromatography. Contaminant that accumulated in the draw solution exhibited a distinct signature in the fluorescence EEM spectra at peak T-1, suggesting protein-like substance. The molecular weight of the protein-like substance was resolved by SEC chromatography, identifying a molecular weight of 200 g/mol. The molecular weight of the protein-like substance was between the estimated molecular weight cut-offs of RO and FO membranes. As a result, such low molecular weight protein-like substance diffused through the FO membrane, and was largely rejected by the RO membrane by the virtue of steric hindrance mechanism, thereby accumulating in the draw solution of the closed-loop FO-RO system. (C) 2016 Elsevier B.V. All rights reserved.