The aim of this study was to determine the impact of the temperature of wastewater in a biological reactor with activated sludge and the BOD₅/N-NH₄ ratio in the influent to the treatment plant on nitrification efficiency and the concentration of ammonium nitrogen in treated wastewater. Tests were carried out in a household wastewater treatment plant which collects and treats sewage from a school building and a teacher’s house. During the 3-year study, large fluctuations in the sewage temperature in bioreactor were noted which was closely related to the ambient temperature. There were also large fluctuations in the concentration of organic matter and the concentration of ammonium nitrogen in inflowing sewage. The influence of wastewater temperature in the bioreactor and the BOD₅/N-NH₄ ratio on the concentration of ammonium nitrogen in treated wastewater was determined using Pearson’s linear correlation. A statistical analysis showed that a 1°C decrease in the temperature of wastewater in the bioreactor increased the concentration of ammonium nitrogen in treated wastewater by 2.64 mgN-NH₄·L^-1. Moreover, it was found that nitrification depended on the ratio of BOD₅ to the concentration of ammonium nitrogen in wastewater flowing into the bioreactor. An increase in the BOD₅/N-NH₄ ratio by 1 value led to a 5.41 mgN-NH₄·L^-1 decrease in the concentration of ammonium nitrogen.