Innovative reverse-irradiation UV photocatalytic system for algae inactivation from eutrophic water: Limitation breakthrough of algae blocking and light decay

[Display omitted] •Reverse-irradiation photocatalysis overcame limitations of light blocking and decay.•Reverse-irradiation photocatalysis greatly inactivated algae and inhibited regrowth.•Photocatalysis majorly contributed to algae cell lysis and cytoplasm degradation.•Intracellular and extracellular degradations were achieved in two stage oxidation.•Sustainable catalytic performance and high UV availability saved energy by 160%. The particle blocking and light energy decay were major limitations in algae-rich water treatment by photo-catalysis. This study developed an effective photocatalytic system in innovative reverse-irradiation mode to overcome these limitations towards improving algae inactivation. Suitable light transmittance and reverse photocatalytic activity of TiO2 thin film were obtained by deposition and calcination optimization, with the optimal UV wavelength at 365 nm. Such reverse-irradiation photocatalytic system presented sustainable and durable algae degradation performance in long-term operation (7 cycles), which generated reactive oxygen species and increased superoxide dismutase activity in two-stage catalytic oxidation process. The algae cells were lysed for intracellular substance solubilization in 2-h treatment, followed by numerous degradation and removal of the dissolved cytoplasm in 2–4 h. Reverse-irradiation photocatalytic oxidation dominantly contributed 62.66–85.60 % of overall algae inactivation, while environment and photolysis provided feeble contributions of 0–4.80 % and 14.40–32.54 %, respectively. As such, 19.79 % of algae cells were lyzed, with the degradations of intracellular chlorophyll a (26.42 %) and proteins (58.14 %). Particularly, the innovative photo-catalysis in reverse-irradiation mode presented much greater performances in algae removal than those in traditional forward-irradiation mode at the long light transmission path (>1.75 cm) and high algae density (>2.8 × 107 cells/L). The reverse-irradiation photo-catalysis not only overcame limitations of algae blocking and light decay, but also remarkably inhibited residual algae regrowth, contributing to the continuous algae inactivation. Moreover, the energy-saving efficiency of 160 % was achievable due to photocatalytic promotion, contributing to economic benefit of 87.59 CNY/(m3 order) and carbon-emission reduction of 0.091 ton CO2/(m3 order). The findings provided novel insights into photo-catalysis promotion and limitation breakthrough from system configu