The paper presents results of the study on reliability of the system: “Wastewater Treatment Plant (WWTP) – Receiver”, using different models for predicting probability that the river will stay in required purity class after receiving discharge from WWTP. Probabilistic model based on the total probability theorem and Monte Carlo (MC) simulations of the water quality below wastewater discharge were used. The results show that probabilistic model generates lower values of “WWTP–Receiver” system’s reliability than MC simulations. Results from MC model may give more realistic values, being able to reflect stochastic nature of the process, also showing that for the overall reliability of the system “WWTP-Receiver” – quality of the receiver is more sensitive factor than WWTP’s reliability.