Effective management of pre-existing biofilms using UV-LED through inactivation, disintegration and peeling

Managing undesirable biofilms is a persistent challenge in water treatment and distribution systems. Although ultraviolet-light emitting diode (UV-LED) irradiation, an emerging disinfection method with the chemical-free and emission-adjustable merits, has been widely reported effective to inactivate planktonic bacteria, few studies have examined its effects on biofilms. This study aims to fill this gap by exploring the performance and mechanism of UV-LEDs on the prefabricated Escherichia coli (E. coli) biofilms under varying irradiation conditions. The results showed that the wavelength of 275 nm exhibited the best inactivation effect on the biofilm-bound E.coli compared to 268, 312 and 370 nm, achieving 3.2 log inactivation at a fluence of 346.5 mJ/cm² and an irradiance of 660 μW/cm². Furthermore, irradiance and irradiation time are proposed for the first time to be a pair of conjugate variables correlated to log inactivation, as a modification of the Bunsen-Roscoe reciprocity law. Additionally, the effect of UV irradiation on extracellular polymeric substances (EPS) in terms of the structure and chemical properties was investigated. The findings support that the oxidative degradation of the polysaccharides and proteins in EPS matrix should be the primary reason for destroying the biofilm framework. Finally, additional hydraulic shear was applied on the irradiated biofilms, suggesting an effective approach for enhancing biofilm removal.