New Dimensions of Moving Bed Biofilm Carriers : Influence of biofilm thickness and control possibilities

Sammanfattning: The moving bed biofilm reactor (MBBR) is a biological wastewater treatment process in which microorganisms grow as biofilms on suspended carriers. Conventionally, MBBRs are mainly designed and optimized based on the carrier surface area, neglecting the dynamic relationship between carrier design, reactor operation and biofilm characteristics, such as biofilm thickness and the composition of the microbial community. The purpose of this research project was to learn more about the roles of the biofilm carriers in the MBBR process, with the intention to improve process performance and develop new MBBR applications. In doing so, the MBBR performance was evaluated in several lab studies, considering different aspects such as carrier design and operational strategies. A new carrier type, the Z-carrier, was developed, with which it was possible to control the biofilm thickness in the MBBR. Hence, the Z-carrier enabled the evaluation of having different, pre-defined biofilm thicknesses in the MBBR process, something that has not previously been achievable. This thesis shows that biofilm thickness control can be used to ensure a more stable process performance as well as to avoid carrier clogging and minimize issues with biofilm scaling that may have detrimental effects on the MBBR performance. It was also shown that the microbiology in biofilms can be altered by biofilm thickness control. Based on these findings, a novel process configuration was developed, showing that successful nitritation of mainstream municipal wastewater could be achieved when combining thin, controlled biofilms with a periodic exposure of the biofilm to reject water from sludge dewatering. Finally, the role of suspended biomass in the MBBR was evaluated in relation to carrier surface area, HRT and loading rate, showing that the suspended biomass can have a considerable effect on the overall process performance, and that the design of MBBRs should not always be solely based on biofilm surface area. For future studies, the potential of using biofilm thickness as a control parameter for the MBBR should be investigated further, especially for specific microbial applications such as nitritation and anammox, and the importance of suspended biomass in the MBBR should be studied in relation to the settling characteristics of the excess sludge.