LAUSR

Upgrading wastewater treatment plants (WWTPs) from conventional activated sludge (CAS) to predenitrification biological nutrient removal (BNR) results in improved removal of dissolved inorganic nitrogen (DIN) from wastewater. However, changes in dissolved organic nitrogen (DON) with these WWTP upgrades and their potential impacts on receiving waters have been little researched. In this study, we investigated characteristics of effluent DON derived from CAS and predenitrification BNR, paying special attention to proteins and humic substances. Through a lab-scale reactor study and analysis of full-scale WWTP effluents, we found that in predenitrification BNR effluent, proteins are much more dominant than humic substances, whereas in CAS effluent, proteins and humic substances are similarly abundant. In terms of molecular weight, the majority of proteins were present in the effluent's low molecular weight (LMW) fraction (<1 kDa), while humic substances were found mostly in the effluent's high molecular weight (HMW) fraction (0.45 μm - 1 kDa). Determination of dissolved organic carbon (DOC)/DON ratios in effluents supports that proteins (and LMW-DON) were most likely microbial-derived organic N produced during treatment processing, whereas humic substances (and HMW-DON) more likely originated outside of treatment systems. Bioassay tests demonstrated that effluent DON derived from predenitrification BNR was more bioavailable than that derived from CAS. We also found that LMW-DON and proteins were highly bioavailable DON compared to HMW-DON and humic substances. The results of this study suggest that upgrading CAS to predenitrification BNR makes effluent DON to become more conducive to phytoplankton blooms in receiving waters.