Effect of Fe3+ on the sludge properties and microbial community structure in a lab-scale A2O process

During biological waste water treatment, ferric salt (Fe3+) usually serves as an inorganic flocculant to improve the agglomeration and sedimentation of suspended solids, and thus the removal efficiency of pollutants to meet the increasing strictly regulated waste water discharge standards. In this study, we investigated the effects of Fe3+ on the removal efficiencies of pollutants, sludge properties, dominant flora and metabolic pathways of bacterial community in a classical anaerobic-anoxic-oxic (A(2)O) process. The results showed that a Fe3+ concentration lower than 10 mg.L-1 could improve the removal efficiencies of chemical oxygen demand (COD) and total nitrogen (TN), while an inhibition effect was exerted at concentration higher than 10 mg.L-1. The maximum removal efficiencies of COD and TN were 97% and 89%, respectively, under the critical Fe3+ concentration of 10 mg.L-1. Total phosphorous (TP) removal was constantly positively correlated with Fe3+ concentration, due to the enhanced adsorption of phosphorus on activated sludge with the increase of surface roughness. Thauera displayed the highest relative abundance, and certain bacteria in Proteobacteria, Dehloromonas and Candidatus Competibacter exhibited good adaptability to high concentration of Fe3+. In the context of metabolic collaterals, the most abundant functional gene families were identified to be Carbohydrate Metabolism, Amino Acid Metabolism, Cell Motility, Membrane Transport, and Replication and Repair. This study provides an extensive mechanistic insight into the impact of Fe3+ on the A(2)O process, which is of fundamental significance to exploit the contributions of inorganic salts to biological waste water treatment. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study demonstrated that an appropriate concentration of Fe3+ can enhance the removal efficiencies of COD and TN in wastewater treatment, while excessive concentration may lead to inhibition. Furthermore, the positive correlation between Fe3+ concentration and TP removal was attributed to the increased surface roughness for enhanced phosphorus adsorption onto activated sludge. Thauera was identified as the dominant bacterial genus, and certain bacteria showed good adaptability to high concentration of Fe3+ within the bacterial community. Gene families related to Carbohydrate Metabolism, Amino Acid Metabolism, Cell Motility, Membrane Transport, and Replication and Repair were found to be the most abundant functional genes in the metabolic pathways.