[Control of Sludge Bulking Caused by Unknown Reason Through FeCl₃ Coupled with Biochemical Methods]

Filamentous bulking could commonly influence effluent water quality in sewage treatment plants. Existing technologies are slowly effective, time consuming, and poorly adaptable. For now, enhancing organics substrates (COD) and dissolved oxygen (DO) concentration, adding FeCl₃ into the reactor, and maintaining alternant aeration conditions are common methods to control filamentous bulking, but the effects of coupling techniques on the control of bulking are rarely reported. In this study, the filamentous bulking resulting from unintentionally erupted Candidatus Saccharibacteria was controlled by FeCl₃ coupled with biochemical methods, which transforming step-feed A/O processes to the SBR process in the emergency by increasing DO to (7.45±0.49) mg·L^-1 during aeration, enhancing COD to (332.73±106.06) mg·L^-1, and adding FeCl₃ into the reactor to set the starting concentration to 120 mg·L^-1. As the results showed, FeCl₃ coupled with the biochemical method quickly counteracted the bulking sludge mainly composed of Candidatus Saccharibacteria caused by unknown reason, while the Sludge Volume Index dropped from 274 mL·g^-1 to 56 mL·g^-1 within 14 days. The relative abundance of Candidatus Saccharibacteria decreased from 97.64% to 32.67% at the genus level because FeCl₃ coupled with the biochemical method inhibited growth of Candidatus Saccharibacteria. Meanwhile, effluents of both COD and PO₄^3--P met the effluent requirements of the I-A discharge standard in China and removal efficiency of NH₄⁺-N increased from 65.33% to 74.65%. The results showed that FeCl₃ coupled with the biochemical method exhibited good performance in the control of bulking caused by unknown reasons.