Insights into the roles of EDTA in enhancing the bioavailability of Fe(Ⅲ) and stabilizing the anammox granular sludge system

The adsorption and utilization of Fe(Ⅲ) can be impeded by precipitation in anaerobic reactors. In this study, we investigated whether the bioavailability of Fe(Ⅲ) can be enhanced by EDTA-Fe(Ⅲ), thereby improving the stability of anammox system. Fe(Ⅲ) and EDTA-Fe(Ⅲ) were added to two parallel up-flow anaerobic sludge blanket (UASB) reactors, respectively. After 186 days of operation, when the nitrogen loading rate (NLR) was 0.8 g-N/L/d, the nitrogen removal rate (NRR) of R2 was stable at 0.675 g-N/L/d, which was higher than that of R1 at 0.557 g-N/L/d. The anammox activity of R2 reached its peak in phase Ⅲ (158.55 ± 4.53 mg-N/g-VSS/d), while the anammox activity of R1 was only 28.37 ± 5.96 mg-N/g-VSS/d. Moreover, in phase Ⅱ-Ⅴ, the bioavailability of Fe(Ⅲ) in R2 was increased by 15.63%, 38.37%, 65.90%, and 39.63%, respectively, compared with R1. The results showed that EDTA-Fe(Ⅲ) can prevent precipitation encasing anammox granular sludge, which can enhance substrate transport and Fe(Ⅲ) adsorption utilization. High throughput sequencing suggested that EDTA-Fe(Ⅲ) could better increase the abundance of anammox bacteria (AnAOB). In conclusion, this research proved that EDTA could increase the bioavailability of Fe(Ⅲ) and thereby ensuring the stability of the reactors.