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.