Enhanced mainstream nitrogen removal from synthetic wastewater using gel-immobilized anammox in fluidized bed...

Anammox retention, which is crucial for successful nitrogen removal because of slow growth, is still a major challenge. Fixed film processes or gel-immobilization techniques can minimize biomass washout. However, the detachment mechanisms from gel-immobilized beads are still unclear. Despite the widely known advantages of fluidized bed reactor (FBR) with respect to biomass retention, the technology has not been investigated for anammox processes, and thus, the current study evaluated the feasibility of using immobilized anammox gel beads as a carrier media in anammox fluidized bed reactor (AFBR), with a particular focus on understanding detachment mechanisms. The study optimized the packing ratio in AFBR and compared holed and non-holed beads. The optimum packing ratio (on a volumetric basis) was 30% (v/v) with a nitrogen removal rate (NRR) of 0.40 kg N/m³-d at a volumetric nitrogen loading rate (NLR) of 0.51 kg N/m³-d. Biomass detachment rates increased linearly with specific anammox activity (SAA). The fluidized bed reactor employing holed (more porous) anammox gel beads (HFBR) exhibited 20% lower biomass detachment rates than the non-holed fluidized bed reactor (NHFBR). Moreover, the HFBR achieved a maximum NRR of 0.81 kg N/m³-d at NLR of 1.01 kg N/m³-d after 35 days without operational problems, whereas the NHFBR with non-holed anammox gel beads failed after 30 days. The hindrance to diffusion of the generated nitrogen gas was the main mechanism of beads breakup and biomass washout, and thus, the sustainability of the beads hinges on increased external porosity. Therefore, developing microporous gel beads is critical for achieving a high rate stable anammox process that overcomes the limitations of the current technologies.