The paper determines the reliability of the wastewater treatment plant (WWTP) at the Czernichów Municipality. The study was carried out from February 2010 to March 2011. The following physic-chemical sewage quality parameters were analyzed: BOD5, COD and total suspended solids (TSS). While assessing the plant reliability, two indicators were considered: a reliability indicator and an indicator of technical plant efficiency. The number of violations of the effluent limit values, specified in the Regulation of the Minister of the Environment of 24 July 2006  was established.