This article presents a study on the daily reliability analysis of a full-scale activated sludge wastewater treatment plant located in Northern Jordan. The coefficient of reliability (COR) is calculated to assess the performance of the plant in regard of wastewater concentrations of COD, BOD5, and TSS. The results indicate that the global effluent performances for COD, BOD5, and TSS comply with Jordanian standards. However, the BOD5 performance is found to be below the quality level due to issues in the ventilation basin and exceeding treatment capacity, while the TSS performance is affected by problems in the grit chamber and variations in influent quality and quantity. The study fills a research gap by focusing on the reliability of wastewater treatment plants using activated sludge technology and utilizes a probabilistic model proposed by Niku et al., (1979) to evaluate the reliability. The findings provide valuable insights into the daily variability of the treatment plant and suggest measures for improving reliability and overall efficiency.