Rearrangement of existing treatment facilities for optimum bioreactor configuration offers a cost-effective upgrading possibility. Model experiments were performed to determine how the configuration of bioreactors of a given total volume can influence system performance, with the goal of influencing planned modifications of activated sludge plants in Hungary. In the first experiment, two bioreactors arranged in series were compared to two in parallel. In the second experiment, a two-stage system with sludge recycle around each stage was compared to a single-sludge selector system. In the last experiment, three types of selectors (two anoxic and one aerobic) were evaluated for addition to a conventional, flow-through activated sludge system. The experiments were performed with dairy-type synthetic wastewater and with effluent from the primary clarifier of an existing municipal treatment plant.

Reactor arrangement had a great influence on the effectiveness of the treatment, primarily by inducing significantly different sludge structures. The in-series reactor arrangement proved to be, in every respect (effluent COD, residual detergent, and sludge characteristics), more efficient than the parallel arrangement. The two-stage system produced more sludge than the selector system, and the SVI values of the second stage of the two-stage system were extremely unfavorable. However, the effluent from the second stage was significantly clearer. Separating the selector from the flow-through reactor improved the sludge settling considerably. This facilitated an increased hydraulic load in the secondary clarifier and thereby in the whole treatment system.