Dissolved organic nitrogen (DON) is currently considered as one of the most important parameters in drinking water treatment due to its potential to form toxic nitrogenous disinfection by-products (N-DBPs). A comprehensive investigation was made in this study on the variation of DON in a full-scale drinking water treatment plant with a treatment train of coagulation/sedimentation, biofiltration and disinfection. The results showed that DON could be removed effectively by coagulation/sedimentation and disinfection, while biofiltration increased the DON concentration significantly. To determine the mechanism of DON increase, DON and other related parameters at different media depths of the biofilter were studied. The results showed that the DON concentration in the biofilter presented a rapid decrease from 0.73 to 0.44 mg l−1 in the top media (0–10 cm), and a slow increase from 0.44 to 1.08 mg l−1 in the bottom media (10–100 cm). Soluble microbial products (SMPs) released by bacterial metabolism might be a main source of the DON in the biofilter. These SMPs contained aromatic protein-like fractions, which were confirmed by EEM analysis.

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