Nutrient removal from digested piggery wastewater (DPW) using microalgae is of increasing interest as a secondary treatment prior to discharge to avoid environmental contamination. In this study, we investigated the main mechanisms involved in ammonium removal from DPW by Oedogonium sp. using the¹⁵N mass balance approach with a focus on the relationship between algal growth and ammonium removal and the dominant ammonium removal pathway. We noted 96.2% ammonium removal and 0.04~0.15 specific growth rate of Oedogonium sp. in the diluted autoclaved DPW during the incubation period and 94.1 % ammonium removal and -0.14~0.13 specific growth rate in the diluted raw DPW. Aeration provided a significant benefit to ammonium removal via the stripping effect, which was favored by the high pH in the experimental conditions. Isotope mass balance analysis indicated that bacteria present in the initial DPW had little effect on ammonium removal in the experiment. Algal uptake and gaseous loss were the dominant pathways for NH₄—N removal from the diluted DPW using Oedogonium sp. cultures and accounted for 40.97% and 32.59% of the total ¹⁵N amount, respectively. Regression and path analyses between NH₄—N removal and its main influencing factors indicated that to improve NH₄—N removal efficiency, the levels of Oedogonium sp. and dissolved oxygen (DO) should be increased under weakly alkaline conditions.