Mutual symbiosis of electroactive bacteria (EAB) and denitrifier may be the key for solving the refractory carbon and residual nitrogen in wastewater treatment plant effluent. However, its application is hampered by unclear co-metabolic model and uncertain electron transfer. Here, we achieved 3–5 times increase in refractory carbon degradation, 40 % improvement in denitrification, and 36.0 % decrease in N₂O emission by co-culturing P. aeruginosa strain GWP-1 and G. sulfurreducens. Such an enhancement is obtained by both refractory carbon co-metabolism and interspecies electron transfer (IET) between GWP-1 and G. sulfurreducens. Importantly, IET was quantified via isotopic approach, which revealed that G. sulfureducens supplies more electrons to GWP-1 when the system was fed with cellulose (0.071 mM) than glucose (0.012 mM). This study demonstrates that the residual refractory carbon and nitrogen in treated wastewater could be further converted by mutual symbiosis of EAB and denitrifiers, which paves a synergic way for pollution and carbon reduction.

This study assessed the evolution of wastewater systems during the rapid urbanization of Beijing, with special focuses on the carbon footprints and growing underground WWTPs (u-WWTPs). Specifically, the Bishui plant (in situ constructed u-WWTP) was assessed in detail regarding eco-environmental benefits. Our results showed that, the direct emission intensity of 65 WWTPs decreased from 0.47 to 0.24 kg CO_2eq/m³, when the electricity intensity increased from 0.22 to 0.39 kWh/m³ from 2010 to 2020. Bishui u-WWTP emitted 36.6 kt CO_2eq/year (0.09 kg CO_2eq/m³), with electricity intensity of 0.43 kg CO_2eq/m³. Additionally, compare to the hypothetical relocating scenario, it saved 6.67 × 10⁴ m² land and 33.0 kt CO_2eq/year, and the created urban river carries 6.5 × 10¹³ J/year heat outside town. The evaluation and balance of choice for conventional or underground WWTP should be made case by case. However, this study demonstrated that u-WWTP is not only a construction manner, but a sustainable management model with positive eco-environment effects, algin with future city expansion, and circular economy visions.