Understanding the influence of dissolved organic nitrogen characteristics on enhanced coagulation performance for water reuse.

With the recent focus on using advanced water treatment processes for water reuse, interest is growing for utilizing enhanced coagulation to remove dissolved chemical species. Up to 85% of the nitrogen in wastewater effluent is made up of dissolved organic nitrogen (DON), but there is a knowledge gap regarding its removal during coagulation, which can be influenced by DON characteristics. To address this issue, tertiary-treated wastewater samples were analyzed before and after coagulation with polyaluminum chloride and ferric chloride. Samples were size-fractionated into four molecular weight fractions (0.45 μm, 0.1 μm, 10 kDa, and 3 kDa) using vacuum filtration and ultrafiltration. Each fraction was further evaluated by coagulating it separately to assess DON removal during enhanced coagulation. The size fractionated samples were also separated into hydrophilic and hydrophobic fractions using C18 solid phase extraction disks. Fluorescence excitation-emission matrices were used to investigate the characteristics of dissolved organic matter contributing to DON during the coagulation process. The results showed that DON compounds of size <3 kDa constituted a majority of the total DON. Coagulation removed more than 80% DON from size fractions 0.45 μm-0.1 μm and 0.1 μm-10 kDa, but less than 20% was removed from 10 kDa to 3 kDa and <3 kDa fractions. Coagulation on pre-filtered samples removed 19% and 25% of the <3 kDa DON fraction using polyaluminum chloride and ferric chloride, respectively. In all molecular weight fractions, hydrophilic DON compounds were found to be dominant (>90%), and enhanced coagulation was not effective in removing hydrophilic DON compounds. LMW fractions respond poorly to enhanced coagulation due to their hydrophilic nature. Enhanced coagulation effectively removes humic acid-like substances, but poorly removes proteinaceous compounds such as tyrosine and tryptophan. This study's findings provide insights into DON behavior during coagulation and factors affecting its removal, potentially improving wastewater treatment strategies.