Effect of different molecular weight organic components on the increase of microbial growth potential of secondary effluent by ozonation

Ozonation has been widely applied in advanced wastewater treatment. In this study, the effect of ozonation on assimilable organic carbon (AOC) levels in secondary effluents was investigated, and AOC variation of different molecular weight (MW) organic components was analyzed. Although the removal efficiencies were 47%–76% and 94%–100% for UV254 and color at ozone dosage of 10 mg/L, dissolved organic carbon (DOC) in secondary effluents was hardly removed by ozonation. The AOC levels increased by 70%–780% at an ozone dosage range of 1–10 mg/L. AOC increased significantly in the instantaneous ozone demand phase, and the increase in AOC was correlated to the decrease in UV254 during ozonation. The results of MW distribution showed that, ozonation led to the transformation of larger molecules into smaller ones, but the increase in low MW (< 1 kDa) fraction did not contribute much to AOC production. The change of high MW (> 100 kDa and 10–100 kDa) fractions itself during ozonation was the main reason for the increase of AOC levels. Furthermore, the oxidation of organic matters with high MWs (> 100 kDa and 10–100 kDa) resulted in more AOC production than those with low MWs (1–10 kDa and < 1 kDa). The results indicated that removing large molecules in secondary effluents could limit the increase of AOC during ozonation.