The RetroFAST plant for treatment of domestic sewage was monitored for BOD, COD, suspensions, bed surface and outlet intensity in 2005-2007. The measurement results were analyzed to est. the operation reliability level (85.4% for BOD, 89.2% for COD and 92% for suspensions). The aerobic conversion of org. matter was limited by the O-2 access (biofilm penetration depth 0.07-0.20 mm).