In this study, fouled reverse osmosis (RO) membranes from the full-scale low pressure reverse osmosis (LPRO) plant were autopsied in March (RO-M) and September (RO-S), and investigated before and after the chemical cleaning. The characteristics of residual matters on LPRO membrane surfaces were then determined via several analysis methods including excitation-emission matrix, molecular weight distribution, thermogravimetric analysis, and organic analyses. The foulants were predominantly composed of hydrophobic organic matters with dense structures on RO-S, while loose structure of CaSO4 and BaSO4 scaling dominantly covered membrane surfaces on RO-M. Interestingly, loose structure of scaling provided more significant growth of biofilm and subsequently higher organic mass per unit area on RO-M. The order of basic-acidic cleaning showed improved flux recovery (87%) than acidic-basic cleaning (79%) due to the dense structure of biofilm layer on RO-S. In case of RO-M, foulants were easily removed without regarding the order of chemical cleaning (higher than 93% of initial flux recovery) due to the loose structure of fouling layer. The results proposed that the membrane foulants might have seasonal differences, and the cleaning order of basic-acidic chemicals would provide higher flux recovery in the case of the fluctuation in influent quality.