Effluent treatment plant comprises: (a) a first elongate vessel (5) for anoxic metabolisation of effluent contained in the vessel. The first elongate vessel has a vertical first downcomer zone (4) for transport of effluent towards the bottom of this first vessel, and a vertical first riser zone (7) for transport of effluent towards the top of the first vessel. The bottom region of the first downcomer zone connects with a bottom region of the first riser zone; (b) feed system (2) for supplying the effluent to be treated to a top region of the first downcomer zone;(c) first recovery means for recovery of effluent that has passed through the first downcomer zone and then through to the top of the first riser zone; (d) a second elongate vessel (12) connected downstream from the first elongate vessel for aerobic metabolisation of effluent that has passed through the first elongate vessel. The second elongate vessel has a vertical second downcomer zone (11) for transport of effluent towards the bottom region of this second elongate vessel and a vertical second riser zone (14) for transport of effluent towards the top of this second vessel. The bottom region of the second downcomer zone connects with a bottom region of the second riser zone; (e) feed system for feeding effluent from the first elongate vessel to the top of the second downcomer zone; (f) gas injection system for injecting an oxygen-containing gas into the second elongate vessel so as to facilitate and maintain circulation of effluent through the second elongate vessel; and (g) second recovery system for recovering effluent from the top region of the second riser zone.    ADVANTAGE -   Effluent circulating in the plant is highly aerated in the second elongate vessel and can therefore handle a high concentration of biomass, typically 6000-20000 mg biomass/litre of liquid. The plant runs at a high food/microorganism (f/m) ratio, typically 1.0/day or higher, in comparison with conventional plants. This has consequent capital cost advantages since the use of a higher f/m ratio enables a smaller plant to cope with the same quantity of effluent as a larger plant operating at a lower f/m ratio. Due to high liquid velocities in the downcomers and risers, there is a reduced risk of solid materials settling out of the effluent and clogging the reactor. The provision of the first elongate vessel operating under anoxic conditions avoids the attendant disadvantages of prior art denitrification tanks. The aerobic bacteria are not destroyed but merely subdued, the bacteria once again thriving when entering the aerated second vessel.