Effect of coagulation on dissolved organic nitrogen (DON) bioavailability in municipal wastewater effluents

Bioavailable dissolved organic nitrogen (ABDON) can support algal growth and consume oxygen in surface waters. In this study, the variations of ABDON and DON bioavailability in municipal wastewater effluents after coagulation were evaluated by algal bioassay. Surprisingly, the bioavailability of DON increased from 45 to 81% (mean 62%) to 50 to 96% (mean 79%) after coagulation and the concentration of ABDON were not significantly different before and after coagulation (p > 0.05, t-test). Correlations of carbon characteristics of DON, specific DON compounds, different fractions of DON, and DON bioavailability were assessed. Spearman's correlation analysis showed DON bioavailability correlated strongly with hydrophobicity-based DON fractions (r = 0.67, p < 0.01 and r = -0.60, p < 0.05 for hydrophilic and hydrophobic fraction, respectively). Fluorescence excitation-emission matrix (EEM) and X-ray photoelectron spectroscopy (XPS) analysis confirmed that proteinaceous materials mainly exist in the hydrophilic fraction, improving the bioavailability of hydrophilic DON. Meanwhile, coagulation preferentially removed the hydrophobic DON, 75 +/- 6% of which was non-bioavailable. It was, therefore, the removal of hydrophobic DON that led to the increase of the bioavailability of DON in effluents after coagulation. Further, coagulation pH has been shown to have little effect on the removal of ABDON. These results indicated that coagulation is ineffective at sequestering the DON forms that most readily pose algal growth stimulation problems. (C) 2016 Elsevier Ltd. All rights reserved.