Ultraviolet (UV) disinfection is widely used in municipal wastewater and reuse systems, yet its full-scale outcomes depend strongly on how fluence is delivered under real-water conditions rather than on nominal lamp output. This review consolidates the photochemical basis and plant-relevant limitations of UV disinfection, with emphasis on key factors such as matrix optics (e.g., UVT254, color, and turbidity/suspended solids) and particle shielding. Building from these constraints, UV-enhanced disinfection is examined as an engineering strategy that couples UV with oxidants or catalytic/physical processes to expand inactivation pathways and improve robustness in challenging effluents. Representative configurations have been reported (e.g., UV/free chlorine, UV/monochloramine) and are compared in terms of dominant reactive species and reaction networks, matrix dependence and scavenging effects, disinfection performance trends across microbial targets, and process-specific trade-offs including transformation products/disinfection byproducts, energy and chemical demands, and materials durability. Finally, practical considerations for implementation are summarized, including monitoring and control variables, validation approaches (e.g., biodosimetry, challenge testing), and operating windows that balance inactivation with risk and resource inputs, to support more reliable selection and operation of UV and UV-hybrid disinfection for water reuse.

Reducing nitrous oxide (N₂O) emissions from biological wastewater treatment is critical for achieving low-carbon environmental goals. In this study, a Chlamydomonas reinhardtii -driven algal–bacterial granular sludge system was successfully established in a photo-sequencing batch reactor to enhance nitrogen removal while suppressing N₂O generation. Compact granules formed within 48 days, exhibiting good settling ability (SVI₅/SVI₃₀ = 1.0), an average diameter of 0.5 mm, and a mixed-liquor suspended solid concentration of 2.1 g/L. Algal enrichment was confirmed by an increase in chlorophyll-a to 6.6 mg/g-VSS and substantial accumulation of protein-rich extracellular polymeric substances, which improved granule stability and mass transfer. The system achieved efficient pollutant removal when treating synthetic municipal wastewater, maintaining a chemical oxygen demand removal efficiency of approximately 90% and total nitrogen removal of up to 69.4%, with effluent NH₄⁺-N consistently below 1.6 mg/L. Notably, the N₂O emission factor decreased from 4.2 to 0.4 g N₂O-N/kg N-removed, which is lower than that of conventional activated sludge processes. These results demonstrate the potential of microalgae-driven granulation as a promising low-carbon biotechnology for sustainable wastewater treatment.