Sequencing Batch Reactor (SBR) experiments have been conducted to study factors affecting composition and concentration of residual organics in the effluent of biological wastewater treatment plants. Wastewater discharged by a papermill was used as a reference. First results indicate that sludge age plays an important role when industrial wastewaters are to be treated. Increase of the sludge age led to elimination of key halogenated compounds in the effluent, although the effluent COD often remained hardly changed. Of importance is also the duration of the starvation period during each cycle. To maintain high substrate removal efficiency it is necessary to extend the duration of the aeration phase until the enzyme activity has been reduced to the background level. It was observed that the enzyme activity remains high even when the respiration rate had reached the level of endogenous respiration. High treatment efficiency can only be maintained if the bacteria are supplied with feed, and then exposed to starvation conditions on a periodic basis.