Novel Nitrogen-Doped Zinc Oxide (ND-ZnO) Algal-Bioreactor System for Methylene Blue Wastewater Treatment

Herein we present the production of a two-stage wastewater treatment system for industrial azo-dye using an immobilized nitrogen-doped zinc oxide (ND-ZnO) nanoparticles and a microalgae bioreactor. The treatment system was found capable of reducing up to 95% of methylene blue wastewater in 48 h under a seminatural condition. In the first stage, the immobilized ND-ZnO chamber was found to have photocatalyzed up to 80% using natural sunlight in 4 h. In the second stage, the algae (Chlorella vulgaris) were found to be capable of repeatedly handling the semitreated wastewater and found remained active for at least 7 cycles (hydraulic retention time = 44 h, solid reaction time = 14 days). The algae not only take part in purifying the wastewater but also absorb and metabolize the toxic cleaved by-products therein. The presented system demonstrated a low-cost alternative, both in terms of production and operation, for treatment of recalcitrant azo-dye. Furthermore, unlike other pilot systems using suspended nanoparticles, expensive recovery of nanoparticles is not required and accidental discharge of nanoparticles is avoided. Toxicity assay using fish gill cell line demonstrated that the acute toxicity of the effluent was negligible. To conclude, this novel treatment system opened a new and safe way for effective yet simple treatment of azo-dye wastewater.