Evaluation of phytoplankton-stimulating potency of effluent nitrogen depending on its chemical forms: A comparison between inorganic and organic nitrogen

Abstract

Wastewater-originated nitrogen (N) is considered a primary N source in urban waters. In order to mitigate eutrophication in such waters, decreasing N discharges from wastewater treatment plants (WWTPs) is necessary. Upgrading WWTPs from conventional activated sludge (CAS) to biological nutrient removal (BNR) is the most common measure to lower levels of effluent N. However, in spite of successful N reduction through such upgrades, eutrophication persists in numerous urban waters. In this study, we investigated why decreased N discharge resulting from upgrading CAS to BNR, particularly predenitrification BNR, cannot necessarily alleviate eutrophication. Our laboratory reactor study demonstrated that compared to CAS effluent N, predenitrification BNR effluent N contains less dissolved inorganic N (DIN) but more dissolved organic N (DON), especially low molecular weight DON (LMW-DON). Bioassay-based experimental and numerical analyses found that effluent N has dissimilar phytoplankton-stimulating potency depending on its chemical forms. In particular, effluent LMW-DON showed significantly greater potency than effluent DIN. This difference in potency makes predenitrification BNR effluent N more productive for causing primary production than CAS effluent N. These results indicate that the impact of effluent N on eutrophication should be evaluated based on not only the total quantity but also the qualitative aspect of N.