Abstract
An ordinary aerobic rotating biological contactor (NBC) has disks which are half submerged in the bulk liquid. In order to estimate the oxygen transfer rate into the biofilm fixed to a rotating disk quantatively, it should be noted that the mechanism be essentially described as unsteady-state diffusion and reaction process, because the biofilm experiences cyclic changes between the air and water phases. In this study, we gave the analytical solutions of differential equations of unsteady-state mass transfer for RBC nitrification process. We calculated the removal rate of ammonium nitrogen from the solutions and compared them with experimental results. The experiments were carried out using a small laboratory-scale RBC unit whose disk diametre was 26 cm. From these results, we could examine effects of rotational speed and disk diametre on the ammonium nitrogen removal rate. Main results were as follows:(1) Reaction mechanism changed from ammonium limitation (substrate limitation) to oxygen limitation at about 5 rpm in this study.(2) At higher rotational speed than 5 rpm, nitrification was limited by oxygen supply and the increase of rotational speed had no effect on the ammonium nitrogen removal rate. However, the dissolved oxygen concentration in the bulk liquid increased with the increase of rotational speed, because the bulk liquid was more aerated as rotational speed increased.(3) It was estimated from the unsteady-state analytical solutions that more than 95% of the amount of oxygen used in the biofilm was supplied from the air under the oxygen limitation and that the maximum ammonium nitrogen removal rate decreased as disk diametre increased.
