Photo-anammox by vacuum ultraviolet tandem chlorine

Excessive ammonia (NH₄⁺) discharge can lead to algal blooms and disrupt water sustainability, so its control is imperative. Although microbiology-triggered anammox process is promising, its application is limited due to time-consuming cultivation of specific microorganisms and need for skilled operation. To bypass these barriers, this study proposed and verified a photo-induced anammox technology that removes NH₄⁺ and total nitrogen (TN) from water by ultraviolet (UV)/vacuum UV (VUV)/chlorine under anoxic conditions. Under the Cl/N mass ratio of 5:1, the anoxic VUV/UV/chlorine process achieved 66.8% removal of 10 mg-N/L NH₄⁺ within 10 min along with 57.8% reduction in TN. Besides the evidence from TN loss, this study confirmed nitrogen gas (N₂) as the primary degradation product at low dissolved oxygen (DO) concentration of 2.0 mg/L. The selective conversion of NH₄⁺ into N₂ was mainly attributed to reactive nitrogen species (RNS, 42.5%) and reactive chlorine species (RCS, 57.5%). The TN removal efficiency was insensitive to certain variations of pH (7.0-9.0), NH₄⁺ concentration (1-30 mg-N/L), chloride (50-125 mg/L), and sulfate (25-100 mg/L), but sensitive to DO and bicarbonate (25-100 mg/L). Given its robustness and high efficiency, the anoxic VUV/UV/chlorine technology may serve as a potentially promising alternative for NH₄⁺ and TN alleviation in wastewater.