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Partial nitrification/anammox (PN/A) is an energy-efficient biological nitrogen removal process, that is used for treating reject water in wastewater treatment plants (WWTPs). Although PN/A processes have lower aeration demand and chemical use compared to conventional nitrificationdenitrification, stable full-scale operation remains challenging due to the sensitivity of the process to oxygen supply, temperature, pH, hydraulic load, microbial balance, and control strategies. The purpose of the thesis is to investigate the relationship between operational stability and energy performance in a full-scale sidestream PN/A process. Thus, the study addresses two research questions: What characterizes operational stability in the PN/A process, and how operational stability relates to variations in aeration demand and energy consumption. The study was conducted as a single-case study of a full-scale PN/A process at a wastewater treatment plant. An abductive research approach was applied, combing theoretical literature, semi-structured interviews, and historical operational data. Regarding the interviews, there were five in total that were conducted with personnel involved in the operation and monitoring of the PN/A process. In addition, operational data from 2022 to 2026 was analysed using descriptive statistics, Pearson correlation analysis, lagged correlation analysis and one-way analysis of variance (ANOVA). The variables that got included in the study were reactor temperature, dissolved oxygen, pH, reject water flow ammonium, nitrate, aeration flow, and heating energy consumption. The results show that operational stability in the PN/A process is characterized by maintaining key process variables within suitable operating ranges, supporting the microbial balance between ammonium-oxidizing bacteria and anammox bacteria, limiting unwanted nitrate formation and enabling the process to withstand and recover from disturbances. The findings also show that stability is influenced by both technical, as well as human factors. This includes measurement systems, control strategies, operational routines, and operator experience. In relation to energy performance, stable operation supports more efficient use of aeration and heating by allowing the process to be controlled close to actual process demand. Instability can instead increase energy use through over-aeration, delayed operational responses, longer treatment times, or additional heating requirements. The thesis concludes that operational stability is a central condition for achieving the desired energy-efficiency that a full-scale operation of a PN/A sidestream can offer. The study further shows that stability should be understood as a socio-technical outcome, where biological process conditions, automation, monitoring, and operator knowledge interact to influence both treatment performance and energy consumption.