A process for the biological treatment of wastewater in which the performance of a conventional activated sludge system is improved by adding an aerobic granular biomass system in a hybrid parallel process configuration. Waste biomass and suspended material from the aerobic granular biomass system is introduced into the conventional activated sludge system for this purpose. In the hybrid process configuration the advantages of both systems are combined to produce new advantages, while drawbacks of the individual systems are reduced to great extent.