Environmentally friendly approach for advanced treatment of municipal secondary effluent by integration of micro-nano bubbles and photocatalysis.

Numerous organic contaminants existing in the municipal secondary effluent (SE) have gained considerable attention because of their persistent nature and adverse environmental effects. In this study, micro-nano bubbles (MNB) and UV/Titanium dioxide (TiO 2) system, both individual and in combination, were investigated for advanced treatment of the SE. The integrated MNB and photocatalysis system significantly improved the removal of chemical oxygen demand (COD) and persistent organic pollutants by 27.52 and 14.38%, respectively. The hybrid system efficiently decreased all five categories of organic contaminants according to excitation-emission matrix (EEM) spectrum, while the molecular weight (MW) fractions were decreased and the UV-absorbance at wavelength 254 nm (UV 254) of MW (<3 kDa) increased by 16%. The enhanced removal was induced by the elevated production of reactive oxygen species (ROSs) in a two-stage model. Further experiments were conducted to identify the mechanism involved, and the results showed that the MNB treatment mainly generated ·OH species and the UV/TiO 2 process produced most of the other ROSs (H 2 O 2 , HO 2 − and O 2 −). The results showed that the photocatalytic degradation could be readily enhanced by the green MNB technique for SE treatment without any modifying, micrifying or doping of the photocatalyst. • An integrated micro-nano bubbles and photocatalysis system was developed. • The hybrid system significantly improved the treatment of organic contaminants. • Enhanced removal was induced by the elevated ROSs production in a two-stage model. [ABSTRACT FROM AUTHOR]