Effect of Influent Carbon-to-Nitrogen Ratios on the Production and Bioavailability of Microorganism-Derived Dissolved Organic Nitrogen (mDON) in Activated Sludge Systems

The impact of the carbon-to-nitrogen (COD/N) ratio is typically a crucial factor in the operation of activated sludge systems; however, knowledge of its impact on effluent microorganism-derived dissolved organic nitrogen (mDON) is limited. This study investigated the effect of influent COD/N ratios (8, 5, and 3) on the production and bioavailability of mDON in effluents from sequencing batch reactors (SBRs). Significant differences were not observed between the mDON concentrations in effluents from SBRs operated at different COD/N ratios (p = 0.273-0.568, t-test). However, the molecular composition analysis showed that the mDON produced at low COD/N ratios (3 and 5) exhibits more lability than that produced at a COD/N ratio of 8. The results of an algal bioassay also showed that the mDON produced at low COD/N ratios is highly bioavailable to algae and microorganisms. Gray relational analysis revealed that Bacteroidetes and Actinobacteria (at the phylum level) and Microbacterium (genus level) had the most significant influence on mDON bioavailability. Above all, this study highlights the importance of controlling the mDON in effluents from SBRs operated at low influent COD/N ratios to reduce the potential impact of wastewater-derived nitrogen on receiving waters.