The paper presents a methodology of estimating various reliability indices for analyzing a wastewater treatment plant. The continuous operation of the plant is of utmost importance and therefore its reliability analysis under different stipulated conditions helps in understanding the overall plant performance. Relevant data for a specific waste water treatment plant have been used to validate the methodology. Here, the plant failure is categorized into minor or major reasons of failures and the type of failure is detected by inspection only. Semi-Markov process and regenerative point techniques are used in the entire analysis.