This article evaluates and compares the actual behavior of 18 UASB reactor systems operating without (10 plants) and with (8 plants) post treatment, considering their performance and their reliability in terms of compliance with specified discharge standards. The following types of post-treatment processes were analyzed: aerated filter; anaerobic filter; trickling filter; dissolved air flotation unit; facultative pond and maturation pond. The effluent quality and the removal efficiencies were compared with typical values reported in the technical literature. A methodology developed by Niku et al. (1979) was used for the determination of the coefficients of reliability, in terms of the compliance of effluent BOD and TSS with discharge standards. The results showed that, in general, the inclusion of a post-treatment step, be it either aerobic, anaerobic or physical-chemical, can provide a substantial improvement of the effluent quality from UASB reactors in terms of BOD and TSS. In terms of the systems reliability, the actual effluent concentrations from UASB reactor followed by some post-treatment processes would meet more restrictive discharge standards than those considered in this study (60 mg L−1 for both constituents—values prevailing in most states in Brazil).