Differentiating fouling on the membrane and on the spacer in low-pressure reverse-osmosis under high organic load using optical coherence tomography

•Optical coherence tomography for on-line monitoring of fouling in a feed channel under high organic load.•Fouling of the membrane and spacer both contribute to pressure drop.•Stable flux is still possible at high surface coverage.•Despite the high organic load, long term operability is possible.