A method of biologically purifying waste water, said method including the following steps in the following sequence a) an aeration step for stripping off H.sub.2S and other gases and for removing bactericides from the water, b) a step of decomposing organic matter, c) a sludge precipitation step, d) a clarification step and e) a nitrification step followed by a completing secondary settling step. This method is carried out in a plant, preferably a mini purification plant of the type "immersed aerated bio filter plant" for purifying waste water from households without connection to a sewage system. The plant includes an aerated settling tank (1), a unit for decomposing organic matter (2), a first sludge precipitation unit (5), a nitrification unit (8) and a completing sludge settling unit (5'). The primary settling unit includes means for providing aeration.In this manner, it is rendered possible to obtain such an improved nitrification that it is possible to keep well below the limits stipulated by the authorities for the ammonia/ammonium content in waste water.