Mechanism by which micro-nano bubbles impact biofilm growth in drinking water distribution systems

Abstract

Biofilm growth in drinking water distribution systems (DWDS) has become a concern due to the various water quality issues it causes, and thus suitable disinfection methods are required to ensure drinking water safety. Micro-nano bubbles (MNBs) technology provides a possible breakthrough in dealing with the above issues. This paper simulates the hydraulic conditions of the terminal pipeline in a DWDS to explore biofilm formation under the influence of MNBs from different gas sources. To further understand the changes in water quality, this study evaluated the biofilm morphology, composition, microbial communities, and water quality at different experiment stages. Therefore, we divide the biofilm formation into three phases: the slow growth phase (0–27 days) (SP), rapid growth phase (27–42 days) (RP), and dynamic stability phase (42–66 days) (DP). Biofilm formation was significantly inhibited in the slow growth and rapid growth phases, especially after combining the MNBs with oxygen, causing a reduction in biofilm dry weight of 77.87%. The mechanism by which the MNBs regulate biofilm growth is different at each stage. During the SP stage, physical obstruction and chemical oxidation occurs, at the RP stage oxidative inactivation takes place, whilst at the DP stage adsorption and scour predominate. Notably, the MNBs first attach to the tank's inner surface, forming a hydrophobic layer to increase the difficulty of microbe adherence. Then, an extensive amount of hydroxyl radicals (˙OH) were generated by the MNBs collapsing, reducing the number of bacteria present while increasing the competitive advantage of oxidation-resistant bacteria. This disinfection method narrows the differences in number between the dominant bacterial populations in the biofilm, which changes the key strains and reduces microbial community diversity. As a result, the inactivation rate of Planctomycetes reached 54.22–61.66%, and a significant reduction of the organic matter in water was achieved (87.9% removal of TOC). These results proved that the MNBs have great potential in treating biofilms in DWDS and improving drinking water quality.