Maximizing eco-environmental gains: Exploring underground wastewater treatment plants in Beijing for sustainable urban water management

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 CO2eq/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 CO2eq/year (0.09 kg CO2eq/m3), with electricity intensity of 0.43 kg CO2eq/m3. Additionally, compare to the hypothetical relocating scenario, it saved 6.67 × 104 m2 land and 33.0 kt CO2eq/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.