Assimilable organic carbon (AOC) was measured combined with molecular weight (MW) distribution during a long time operation of a pilot scale ozonation/BAC (biological activated carbon) process in a water treatment plant in China. The AOC measurement was done on different samples: raw water, water samples after sedimentation, sand filtration, ozonation and BAC filtration. MW distribution variation of AOC during ozonation/BAC process was evaluated. Results showed that AOC was mainly related to the NOM (natural organic matter) with MW < 1 kDa. The fraction of the NOM with MW < 1 kDa corresponds to 53–67% of the dissolved organic carbon (DOC) to all the samples while AOC corresponds to 2.7–5.9% of DOC. So just a small part of the fraction with MW < 1 kDa was AOC. This indicates that ozonation/BAC process removing a large part of DOC will not necessarily result in a great reduction in AOC.

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