The decrease in effluent inorganic nitrogen concentration in water resource recovery facilities (WRRFs) has been effectively achieved by including or finetuning the biological nutrient removal process. However, soluble organic nitrogen (sON) remains relatively unchanged, resulting in no reduction in the proportion of sON in effluent nitrogen. Therefore, for some WRRFs subjected to stringent nitrogen discharge limits, the removal of sON is crucial. Furthermore, the interest in sON is growing due to its impact on eutrophication, close relationship with the membrane fouling in wastewater treatment and the formation of nitrogenous disinfection byproducts in drinking water treatment. The overall aim of this research work was to determine the limits and capabilities pertaining to the removal and production of organic N in two different biological wastewater treatment processes, activated sludge process and moving bed biofilm reactors (MBBRs). Three research tasks were performed. In the first task, the effect of sludge retention time (SRT) on the production of organic nitrogen fractions (particulate, colloidal and soluble) and the biodegradability of produced sON in an activated sludge process was investigated. SRT influenced each fraction (particulate, colloidal and soluble) of organic nitrogen along with the biodegradability of effluent sON. The second task investigated sON activity in batch reactors mimicking nitrifying MBBRs. Although net production of sON was observed, both production and ammonification coexisted in the reactors which regulated the sON concentration. Organic carbon bioavailability and/or ammonia concentration were found to influence the production and ammonification of sON. The task also identified the variation in the bacterial activity in the biofilm during nitrification when exposed to different C/N ratios in the influent. The third task examined sON activity under heterotrophic and nitrifying sludge to identify how one sludge type is better than the other in removing sON under a simple aerobic reactor. Higher degradation of sON was identified in the presence of heterotrophic sludge than nitrifying sludge. Results from this dissertation research provide a better understanding of sON activity for two different biological wastewater treatment processes, which is critical for optimizing the removal of sON in WRRFs.