Dissolved free amino acids (DFAA) and dissolved combined amino acids (DCAA, e.g., discrete dissolved polypeptides and proteins as well as amino acids, peptides, and proteins adsorbed to humic material) were measured in unconcentrated domestic wastewater at a detection limit of 10 to
100 nM (coefficient of variation ≤0.10 at 100 nM) using precolumn ortho-phthaldialdehyde derivatization, reverse-phase high performance liquid chromatography separation, and fluorometric detection. Wastewaters were sampled at three types of wastewater treatment plants: trickling
filter, conventional activated sludge, and pure-oxygen activated sludge. Raw wastewaters contained the largest concentrations (58 μM) of dissolved total amino acids (DTAA) with 7% as DFAA. The DTAA were between 28 and 40 μM in primary clarifier effluents with 7% to 29%
as DFAA. Removal percentages during biological treatment were >70% for DFAA and 23% to 79% for DCAA. Molecular weight separations indicated that most dissolved organic carbon (DOC) (>68%) in the trickling filter influent had small molecular weights (<1 000 atomic mass unit [AMU]),
whereas the majority of DTAA had large molecular weights (>10 000 AMU). The DTAA were a small fraction (8% to 13%) of DOC in both influent and effluent samples.
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