Dissolved organic nitrogen (DON) can react with disinfectants to produce numerous disinfection
byproducts (DBPs), particularly nitrogen-containing DBPs (N-DBPs), and produce serious adverse effects
on public health. Widely used biological processes in drinking water treatment can increase DON in effluents,
and enhance these ill effects. This study investigated DON in a full-scale two-stage ozonation-biological
activated carbon (O3-BAC) filtration system. DON concentrations generally increased as media depth increased. There was an ebb and flow pattern between DON and NO3 -_N along the media depth in the absence of NH4 +_N and NO2 -_N. This suggests that NO3 -_N is the nitrogen source for DON. Ozonation and nutrient availability significantly impacted microbial biomass and microbial activity. Microbial biomass and microbial activity were both very important to DON formation as they affected the release of soluble microbial products (SMPs). Typical SMPs such as tyrosine/tryptophan amino acids and proteins were found to be formed during biofiltration, and this formation correlated well with DON from the same sampling ports. In order to balance the mass difference between the increased DON and disappeared NO3 -_N, a hypothesis on the generation and consumption equilibrium of DON and NO3 -_N was posited. This hypothesis involves the existence of nitrogen in the influent, effluent, and backwashing water, and the synthesis of said nitrogen by microorganisms.