From Filamentous Bulking to Utilization: Formation Mechanisms of Filamentous Biofilms and Construction of Stabilized Systems

Sludge bulking in wastewater treatment is often caused by massive filamentous bacteria. This study aimed to turn such bacteria into a stable system dominated by filamentous biofilms (FBs) by using a continuous flow reactor (CFR) fed with simulated domestic wastewater; to address FBs’ poor solid–liquid separation and uncontrollable sludge retention time (SRT), string carriers were added, SRT was controlled at 30 days, and parameters like mixed liquid suspended solids (MLSS) and sludge volume index (SVI) were monitored. Results showed filamentous Sphaerotilus (68–93% of FBs) self-aggregated as FBs’ reticular skeleton (loose, porous, stable, max 8 cm) with non-filamentous bacteria anchoring; FBs achieved >80% COD/NH4+-N removal despite low MLSS (<1000 mg/L) and SVI > 350 mL/g. The application of carriers increased the proportion of non-filamentous microorganisms to over 80%, reduced SVI to 150–400 mL/g, and increased MLSS to over 2700 mg/L, enabling stable operation. This study challenges the traditional negative perception of filamentous bacteria and opens new prospects for wastewater treatment technology.