A comparison of the characteristics of soluble organic nitrogen in untreated and activated sludge treated wastewaters

Soluble organic materials containing nitrogen (SON) are present in effluents from activated sludge treatment of domestic wastewater, but little is known about the sources and characteristics of these materials. The objective of this research was to evaluate the characteristics of SON in untreated wastewaters and activated sludge effluents. Characterization techniques used included low microbial seed biodegradability, molecular weight distribution using gel filtration chromatography, removal by activated carbon and ion exchange, and analysis for free and combined amino acids. Activated sludge effluent SON was more refractory (40–50%; first-order decay rates for the remainder were about 0.014 day⁻¹ than SON in untreated wastewater (18–38%; decay rates for the remainder were 0.08–0.16 day⁻¹). SON produced biologically during treatment had decay rates (about 0.028 day⁻¹) similar to SON in municipal activated sludge effluents, and was from 20 to 100% refractory. Less than 10% of the SON in municipal activated sludge effluent consisted of free or combined amino acids. Approximately 15–30% appeared to nucleic acid degradation products. Fifty to 60% of the SON and SCOD had apparent molecular weights of less than 1800. Apparent molecular weight distributions of treated and untreated wastewaters were similar; however, the excess SON produced during activated sludge start-up contained considerably more SON with molecular weights greater than 1800. The 165–340 molecular weight fraction had a significantly higher SON to SCOD ratio than any other fraction for all wastewaters examined. Activated carbon adsorption efficiently removed SON (72 ± 9%) and SCOD (78 ± 6%) from treated and untreated wastewaters, and from biologically produced organics. Significantly more SON was removed by cationic exchange at pH 2.0 (33–56%) than by anionic exchange at pH 9.5 (10–24%) for all wastewaters tested. Cationic exchange at pH 2.0 selectively removed more biologically produced SON relative to SCOD.