Abstract:
Moving bed biofilm reactor (MBBR) technology was introduced relatively recently to the wastewater industry to help overcome the difficulties faced by conventional treatment processes. The major advantage of this technology is the high treatment capacity due to the majority of the microbial biomass being retained for extended periods within the reactor on suspended polyethylene carriers. However, much remains unknown about the microbial ecology of these biofilm-based systems. This research examined the bacterial and archaeal community structure in full-scale MBBR systems treating municipal wastewater at Moa Point and Karori in Wellington, New Zealand. Molecular characterisation of these communities was based on 16S rRNA gene sequencing and fluorescence in situ hybridisation, together with automated ribosomal intergenic spacer analysis. Bacterial communities within the biofilm were dominated by putative anaerobes such as Deltaproteobacteria and Clostridia, with minor variations between treatment facilities. In contrast, the suspended fraction of the MBBRs was dominated by fast-growing, putatively aerobic members of Gamma-, Beta- and Alphaproteobacteria. The dominant archaea across all biofilm samples were members of the Methanosarcinaceae, which represented <5% biovolume. A subsequent study investigated biofilm succession on K1 carriers (AnoxKaldnesTM) from initial seeding to maturation using 16S rRNA gene amplicon pyrosequencing and dsrAB gene-based analyses. Comparisons of successional development of bacterial communities were also made between two commonly used carrier types (K1 and K3). The 16S rRNA gene results provided in-depth knowledge of the development of bacterial and archaeal communities within biofilm of MBBR from initial attachment to maturation. The archaeal community structure changed over time from a diverse system, in the younger biofilm, to one that was dominated by Methanosarcinales, in the older biofilm. In contrast, the bacterial community increased in diversity over time with the appearance of potential sulfate-reducing bacteria (SRB) in the later stages of biofilm growth. A major motivation of this study was to identify the SRB at Moa Point WWTP, as there are large amounts of hydrogen sulfide emitted at this site. Karori WWTP, with no reported sulfide-related issue, served as a useful comparison. SRB diversity results based on dsrAB genes indicated that Desulfovibrio and Desulfomicrobium were almost exclusively found at Moa Point WWTP. The total SRB community accounted for <1% of the bacterial community at each treatment plant. Interestingly, the larger carriers (K3) had reduced growth of SRB within the developing biofilm compared with K1 carriers. The final part of this thesis involved investigations into the microbial communities and nutrient removal efficiency of lab-scale MBBRs that were subjected to changes. Functionally important organisms including nitrifiers, sulfate reducers and phosphate accumulators, were identified by 16S rRNA gene pyrosequencing. The number of sequences detected for each of these functional groups correlated positively with measurements for nutrient removal. However, the biggest effect on nutrient removal rates was from manipulating seeding material, which reiterates the notion that seeding material is an important factor for optimal plant performance as it can also have long term effects on nutrient removal rates. This is the first report on the microbial ecology of MBBR systems treating municipal wastewater and should provide a basis for optimising MBBR plants in the future.