Rapid and successful start-up of anammox process by immobilizing the minimal quantity of biomass in PVA-SA gel beads

• PVA-SA gel immobilization is an efficient way to initiate anammox process with less biomass.
• Only 13% of anammox biomass was good enough for rapid start-up by gel immobilization.
• Nitrogen removal performance of immobilized biomass was far better than granular biomass.
• Higher diffusivity of immobilized biomass resulted in higher activity throughout the gel beads.
• Only outer layer of granular biomass was metabolically active due to severe pH increase.

Rapid start-up of anaerobic ammonium oxidation (anammox) process in up-flow column reactors was successfully achieved by immobilizing minimal quantity of biomass in polyvinyl alcohol (PVA)-sodium alginate (SA) gel beads. The changes in the reactor performance (i.e., nitrogen removal rate; NRR) were monitored with time. The results demonstrate that the reactor containing the immobilized biomass concentration of 0.33 g-VSS L−1 achieved NRR of 10.8 kg-N m−3 d−1 after 35-day operation, whereas the reactor containing the granular biomass of 2.5 g-VSS L−1 could achieve only NRR of 3.5 kg-N m−3 d−1. This indicates that the gel immobilization method requires much lower seeding biomass for start-up of anammox reactor. To explain the better performance of the immobilized biomass, the biological and physicochemical properties of the immobilized biomass were characterized and compared with the naturally aggregated granular biomass. Effective diffusion coefficient (De) in the immobilized biomass was directly determined by microelectrodes and found to be three times higher than one in the granular biomass. High anammox activity (i.e., NH4+ and NO2− consumption rates) was evenly detected throughout the gel beads by microelectrodes due to faster and deeper substrate transport. In contrast, anammox activity was localized in the outer layers of the granular biomass, indicating that the inner biomass could not contribute to the nitrogen removal. This difference was in good agreement with the spatial distribution of microbes analysed by fluorescence in situ hybridization (FISH). Based on these results, PVA-SA gel immobilization is an efficient strategy to initiate anammox reactors with minimal quantity of anammox biomass.

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