Three small sewage treatment plants with activated sludge, sprinkled biol. deposit and a hybrid reactor (activated sludge and immersed trickling filter) were 5 yr long tested for reliability of N and P removal. The anal. of reliability was performed by using the Weibull method. The technoL reliability was 24% for total N and 5% for total P for the active sludge treatment plant, 15% and 10%, resp., for the plant with bioL deposit, and 21% and 18%, resp., for the hybrid reactor. The plants did not meet the required limits of the wastewater quality.