Granular biomass selection in a double-stage biogas collection UASB reactor: effects on SMA, abundance and diversity of the methanogenic population.

The present work aimed at investigating biomass selection in a pilot-scale double-stage biogas collection (DSBC) upflow anaerobic sludge bed (USAB) reactor treating domestic wastewater. Specific methanogenic activity (SMA) measurements and FISH countings were applied to sludge samples collected during 102 days of operation of the DSBC–UASB and of a control reactor. Results showed that both reactors presented similar SMA values in early stages of operation however the UASB–DSBC reactor showed much higher SMA after day 45, when the biomass was in granular stage. In terms of archaeal abundance, no statistical difference was observed between the reactors. Polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE) revealed a similar composition of the archaeal communities in the two reactors and during the operational period, mainly constituted by Methanosaeta concilii . The results suggest that cell activity rather than archaeal abundance or diversity drive the methane production in the UASB reactors.