A safe operation of a wastewater collection and transport system must consider the probability of overflow events in and off the industry limits. In this paper the reliability of the collection and wastewater transport system of a refinery is assessed by using Cox reliability models. These models were adjusted to take into account the complex nature of the system. Rainfall is considered a model covariable since we are dealing with an open system. The probabilities of overflow from one retention tank either to another or to the water body were evaluated. The results indicate that through the creation of two scenarios, with different rainfall indices, in which it was possible to verify that the higher the index, the lower the reliability of the system. In conditions of rain, non-pumping, and in a one-day retention time, the reliability of the system did not exceed 76 % with a probability of overflow of 24 %. Nevertheless, after 4-day retention, the estimated reliability of the system dropped to 11.6 % and the probability of overflow increased to 88.4 %. And the system gets even more chaotic, if in the presence of rain, the activation of the pumps does not occur. The presence of rain is a factor that accelerates the occurrence of overflow. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.