Enhancing anammox reactor performance: the role of biotic and abiotic particle addition

The slow growth rate of anaerobic ammonium oxidation (anammox) bacteria and susceptibility of anammox sludge to washout pose significant challenges for the successful start-up and stable operation of the anammox process. Granulation may resolve this issue. This study investigated the effects of biotic and abiotic particle addition on the start-up and operation of anammox reactor by inoculating seed sludge with suspended, granular, magnesium ammonium phosphate (MAP) coupled anammox sludge, and pure MAP precipitates, aiming to promote granulation and preserve anammox functionality. The results showed that the start-up period of the reactor was consistent approximately 55 days, irrespective of inoculation type. Notably, the addition of anammox sludge and MAP precipitates did not notably expedite the start-up process. However, incorporating of biotic and abiotic particle additions significantly enhanced the nitrogen removal rate per unit volume of sludge (p < 0.05), achieving 2.20-2.62 kg N m⁻³ d⁻¹. In contrast, the control group and the group inoculated with suspended anammox sludge achieved only 1.32 and 1.38 kg N m⁻³ d⁻¹, respectively. Furthermore, particle addition stimulated the formation of high-density, larger-sized granular sludge, particularly when anammox-MAP and pure MAP particles were introduced. MAP may offer adsorption sites for bacterial retention and accelerate granulation, but it was ineffective for reactor start-up, which mainly involved the initial enrichment and activity manifestation of anammox bacteria. Although the experimental group with suspended anammox sludge exhibited comparable anammox activity, its stability deteriorated over time due to the washout of low-density sludge. Anammox bacteria was enriched in both biotic and abiotic particle addition groups. MAP contributed to a higher abundance of anammox bacteria and a shift in the dominant genus from Candidatus Brocadia to Candidatus Kuenenia, likely attributable to Candidatus Kuenenia's superior substrate affinity. Collectively, these findings provide a scalable approach to improving anammox reactor efficiency in wastewater treatment plants.