Deciphering the spatial fouling characteristics of reverse osmosis membranes for coal chemical wastewater treatment

•Spatial distribution of foulant deposit on 12 regions of each membrane was mapped.•Contribution of organic/inorganic/biological fouling to flux drop was quantified.•Biofouling was the dominant factor for the flux decline throughout the two stages.•Inorganic fouling was prominent at the second stage due to foulant interactions.•Customized feed spacer and membrane for specific fouling control were proposed.