Abstract: A reactor apparatus including a fluid bed reactor wherein liquid to be processed is conducted upwardly therein through a bed to fluidize the bed, the bed including particulate solids having biological material supported thereon, and the liquid forming an effluent head above the bed. The reactor apparatus also includes a control apparatus for removing excess biological material from the reactor. The control apparatus includes a tubular separator column which is emersed in the liquid effluent head, and a telescoping sludge valve that has a draw-off port communicating openly with the effluent head surface and that is adjustable to raise and lower the draw-off port relative to the effluent head surface to control the sludge withdrawal rate. The control apparatus also includes an agitator arrangement that withdraws bed particles from the separator column, shears excess biological material from those particles, and then returns the partially sheared particles and sheared biological material to the separator column.