Enhancing nutrient removal in continuous-flow anaerobic/aerobic/anoxic processes treating low C/N ratios wastewater via side-stream light irradiation: NOB suppression and PHA metabolism stimulation.

Partial nitrification (PN) and efficient internal carbon source metabolism are critical for optimal nitrogen (N) and phosphorus (P) removal in continuous-flow anaerobic/aerobic/anoxic (AOA) processes. However, achieving and maintaining these processes in engineering application is challenging. This study proposes that side-stream (SST) light irradiation is a novel strategy to enhance the nutrient removal performance of continuous-flow AOA processes for domestic wastewater with low carbon to nitrogen ratios (C/N of 3.6-4.2). Batch experiments found that light irradiation inhibited nitrite-oxidizing bacteria (NOB) activity and promoted the production of internal carbon sources, specifically polyhydroxyalkanoates (PHA). Long-term exposure to various SST light conditions showed that SST light irradiation with external light (energy density (Es) of 135-270 J/gVSS) and natural light (Es of 35-360 J/gVSS) sources achieved stable PN by inhibiting NOB activity and growth, while promoting the dominant growth of Accumulibacter clades IIC-ii and IIC-iii, which exhibited high PHA metabolic activity. Consequently, the AOA process under steady-state conditions achieved the removal of 85 % N and 95 % P. Flow cytometry sorting results revealed that SST light irradiation shifted the N removal pathway from Competibacter-dominated to Accumulibacter-driven. The dominant clades IIC-ii and IIC-iii, characterized by high intracellular free Mg²⁺ content compared to other endogenous denitrifying bacteria, enhanced glycogen accumulation metabolism (GAM)-related enzyme activity, thereby promoting PHA production and improving nutrient removal. These findings offer a novel approach for enhancing nutrient removal in AOA processes by utilizing natural light irradiation for the treatment of domestic wastewater with low C/N ratios.