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

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 0.33g-VSSL^-1 achieved NRR of 10.8kg-Nm^-3d^-1 after 35-day operation, whereas thereactor containing the granular biomass of 2.5g-VSSL^-1 could achieve only NRR of 3.5kg-Nm^-3d^-1. This indicates that the gel immobilization method requires much lowerseeding biomass for start-up of anammox reactor. To explain the better performance of the immobilized biomass, the biological and physicochemical properties of theimmobilized biomass were characterized and compared with the naturally aggregated granular biomass. Effective diffusion coefficient (D_e) 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.