Thiosulfate-mediated PAO-GAO competition via enhanced anoxic glycogen consumption in AOA-mode constructed wetland

The competition between phosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs) in carbon-limited constructed wetland (CW) always results in low phosphorus removal efficiency. This study investigated the long-term effect of thiosulfate on PAO-GAO competition in CW operated in alternating anaerobic-aerobic-anoxic (AOA) mode for treating carbon-limited (120 mgCOD/L) wastewater. Four reactors were fed with 0 (R1), 20 (R2), 40 (R3) and 60 (R4) mgS/L thiosulfate, respectively. The results revealed that the relative abundance ratios of PAO/GAO in R2, R3 and R4 increased to 1.13, 1.22 and 1.01, respectively, compared with 0.6 in R1. Correspondingly, the TN removal rates in R2-R4 (80.40%, 83.67% and 87.84%) and the TP removal efficiencies (78.78%, 88.15% and 72.05%) were all higher than those of in R1 (67.74% and 50.43%, respectively). Electron flow analysis revealed that the enhanced TP removal in thiosulfate addition systems was attributed to the extra energy supplied by intracellular polysulfur (poly-S) accumulation. Candidatus Accumulibacter and Dechloromonas remained the dominant phosphorus-metabolizing microorganisms in R2-R4, rather than sulfur-metabolizing bacteria. Moreover, higher thiosulfate concentration strengthened the anoxic glycogen oxidization in GAOs, impairing their subsequent anaerobic carbon uptake. This Gly consumption was greater in R2-R4 than in R1, likely due to lower redox potential from sulfate accumulation and poly-S storage.