this invention relates to a process and reactor system suitable for biological treatment of wastewaters or for other applications wherein effective and intimate contact between biological populations attached to a fixed surface and liquid born chemicals and gaseous atmospheres is required . the reactor includes a sealed housing generally having a plurality of independent - vertical chambers , each open at the top and bottom and each filled with high surface - area packing and disposed annularly around a central - vertical chamber . the central vertical chamber is also open at the top and bottom , but free of packing . the bottom of the central chamber is in communication with the bottom of each of the independent - annular columns by means of a bottom - freeboard chamber . the top of the central - vertical chamber is also in communication with the top of each independent - vertical chamber by means of a top - freeboard chamber created by setting the liquid level in the reactor at a position above the top of all vertical chambers . agitation within the reactor maintained by effecting downward flow of fluid and gases in said central - vertical chamber . a central - vertical chamber contains a pump which circulates fluid from the top freeboard chamber and / or the fluid and gases from the gaseous freeboard of the reactor , downward through the central - chamber , radially across the floor of the reactor and vertically up through the vertical chambers . the fluid then flows radially inward in the top - freeboard chamber and returns to the central chamber thus providing a continuous toroidal recirculation .