An aerated biological filtration system (10) for treating waste water includes a treatment vessel (11) of elongated configuration which incorporates a circulating filter bed (23) and a static filter bed (24). The circulating filter bed incorporates a particulate material such as granulated activated carbon having a specific gravity in the range of 1.2 to 1.4 and effective size of 0.6 mm to 1.5 mm. The circulating filter bed is disposed above the static filter bed and an aeration system (27) is disposed between the filter beds. The aeration system is arranged to diffuse air through the circulating bed either continuously or intermittently so as to provide oxygen to micro-organisms attaching to the particulate material and to cause slow circulation of that particulate material. The system has particular application for domestic recycling of grey water.