The effect of polymer addition on granulation in an anaerobic baffled reactor (ABR). Part II: compartmentalization of bacterial populations

The microbial ecology of wastewater treatment plants remains one of the least understood aspects in both aerobic and anaerobic systems, despite the fact that both processes are ultimately dependent on an active biomass for operational efficiency. Ultimately, future developments in anaerobic treatment processes will require a much greater understanding of the fundamental relationships between bacterial populations within the biomass if optimum process efficiency is to be fully realised. This study assesses the influence of polymer addition on granule formation within an ABR and compares the ecology of the biomass in each compartment of two ABRs treating ice-cream wastewater. To our knowledge, this is the first reported characterisation of the microbiology of acidogenic and methanogenic bacteria in the individual compartments of an ABR. The polymer-amended reactor contained sludge that had a greater density of anaerobic bacteria and larger and denser granules than the control reactor, indicating that polymer addition possibly contributed to the retention of active biomass within the ABR. The average fraction of autofluorescent methanogens was lower, with 1.5% being in the initial compartments of the ABRs, compared to the last compartment which had 15%, showing that each compartment of an ABR had a unique microbial composition. Partial spatial separation of anaerobic bacteria appeared to have taken place with acidogenic bacteria predominating in the initial compartments and methanogenic bacteria predominating in the final compartments. Scanning electron micrographs have revealed that the dominant bacteria in the initial compartments of the ABR (Compartments 1 and 2) were those which could consume H2/CO2 and formate as substrate, i.e. Methanobrevibacter, Methanococcus, with populations shifting to acetate utilisers, i.e. Methanosaeta, Methanosarcina, in the final compartments (Compartments 3 and 4). In addition, there appeared to be a stratified structure to the bacterial genera present within the granules.