Aerobic granulation in a sequencing batch airlift reactor.

Aerobic granular sludge was cultivated in an intensely mixed sequencing batch airlift reactor (SBAR). A COD loading of 2.5 kg Acetate-COD/(m3 d) was applied. Granules developed in the reactor within one week after inoculation with suspended activated sludge from a conventional wastewater treatment plant. Selection of the dense granules from the biomass mixture occurs because of the differences in settling velocities between granules (fast settling biomass), and filaments and flocs (slow settling biomass). At 'steady state' the granules had an average diameter of 2.5 mm, a biomass density of 60g VSS/I of granules, and a settling rate of > 10 m/h. The biomass consisted of both heterotrophic and nitrifying bacteria. The reactor was operated over a long period during which the granular sludge proved to remain stable. The performance of the intermittently fed SBAR was compared to that of the continuously fed biofilm airlift suspension reactor (BASR). The most importance difference was that the density of the granules in the SBAR was much higher than the density of the biofilms in the BASR. It is discussed that this could be due to the fact that the SBAR is intermittently fed, while the BASR is continuously fed.

Fundamental bottlenecks in the application of continuous bioprocesses.

Continuous culture is applied mainly as a research tool and much less as a production process. Fundamental bottlenecks in continuous culture are discussed to help shed light on this apparent contradiction. Based on a discussion of technical, process related, and economic/market bottlenecks it is concluded that the often mentioned productivity argument in favor of continuous processing is much too simple. The optimal choice of a process mode is determined by a full understanding of the equipment and production plant factors and of the economic/market factors. Very often the resulting choice will be the fed batch and/or the cell retention process mode which is characterized by low growth rates. Therefore more research towards product formation at low growth rates (less than 0.05 h-1) is needed.