Tofu wastewater as a carbon source flowing into municipal wastewater treatment plants for reductions of costs and greenhouse gas emissions

Wastewater treatment processes significantly contribute to greenhouse gas (GHG) emissions. Municipal wastewater treatment also faces challenges related to low strength and a low carbon-to-nitrogen (C/N) ratio. This study investigates the high-carbon tofu wastewater flowing into municipal sewers for co-treatment at a wastewater treatment plant (WWTP) directly, with the goal of enhancing nitrogen removal and reduce GHG emissions. Within the framework of a circular economy for wastewater treatment, tofu wastewater serves as an external carbon source for sustainable solutions. The concentrated tofu wastewater had an average chemical oxygen demand (COD_Cr) of 21,894 ± 11,485 mg/L, total nitrogen (TN) of 591.8 ± 238.2 mg/L, and a C/N ratio of 36.9 ± 7.4. The denitrification rate reached 3.05 mg NO₃^--N/(g MLVSS·h). Therefore, tofu wastewater is a suitable alternative carbon source. A full-scale WWTP with a capacity of 20,000 m³/day was monitored from 2017 to 2022 to evaluate the co-treatment effects of municipal wastewater and tofu wastewater. The results showed an increase in 53.3% in the average COD_Cr concentration of the influent wastewater, while the total nitrogen and total phosphorus removal efficiencies were enhanced to 75.8% and 95.2%, respectively. In addition, the study quantified GHG emissions from tofu wastewater and municipal wastewater treatment. Compared to separate treatment processes, the co-treatment reduced GHG emissions by 337.9 t CO₂-eq., approximately 15.8% of the total emissions of WWTP, and achieved a cost saving of 7-10% of the total operational costs. These findings demonstrate the environmental and economic advantages of integrating high-carbon industrial wastewater treatment directly into wastewater treatment plants.