A wastewater treatment system comprising one apparatus (1) for de-nitrification and effluent solids removal, another apparatus (2) for biological main treatment, and a further apparatus (3) for solids removal (5), each apparatus including a volume (40) in which each process is performed, a media-retaining sieve (6) at the volume (40) and arranged to retain buoyant media particles (4) in the volume (40), and a gas sparging device (8), (9) which is located in a lower end region of the volume (40) and which is controllable by way of valves (11) and (15). The three volumes (40) are substantially (10) identical to, or multiples of, each other, the three sieves (6) are substantially identical to each other, and the three sparging devices (8), (9) are substantially identical to each other. The gas is sparged into the main treatment bed at a lower flow rate for wastewater aeration but intermittently at a higher flow rate to disrupt and clean the bed. In a preferred embodiment it is possible to use relatively small media particles because of the provision of the in-situ cleaning arrangement. The biological main stage comprises up to four treatment stages either in internal partitions or in separate tanks.