Stability and performance of two GSBR operated in alternating anoxic/aerobic or anaerobic/aerobic conditions for nutrient removal

Two granular sludge sequencing batch reactors (GSBR) with alternating anoxic/aerobic (R1) and anaerobic/aerobic (R2) conditions were operated with a 4-carbon-source synthetic influent. The physical properties of the granular sludge were very good (SVI ≈ 20 mL g−1) and high solid concentrations (up to 35 g L−1) were obtained in the bioreactor operated with a pre-anoxic phase with additional nitrate (R1). In contrast, performance and granule settleability were lower in R2 due to the development of filamentous heterotrophic bacteria on the surface of granules. These disturbances were linked to the fact that a fraction of COD remained during the aerobic phase, which was not stored during the anaerobic period. To stabilize a GSBR with a mixture of organic carbon sources, it is thus necessary to maximize the amount of substrate used during the non-aerated, anaerobic or anoxic, phase. Comparable phosphate removal efficiency was observed in both systems; enhanced biological P removal being greater in anaerobic/aerobic conditions, while the contribution of precipitation (Ca–P) was more significant in anoxic/aerobic conditions.

► Good physical properties for granules were obtained with anoxic/aerobic cycles.
► Pre anoxic phase diminishes the microbial competition for oxygen but limits the SND.
► Instability of GSBR with anaerobic/aerobic phases was due to the filamentous growth.
► Maximizing COD fraction stored anaerobically is a key parameter for process stability.
► Comparable P removal efficiency was observed in both systems.
► P removal is mainly due to precipitation in anoxic/oxic and EBPR in anaerobic/oxic process.