Optimizing the placement of medical wastewater outlets in sewer systems to reduce chemical consumption at wastewater treatment plants

The severely low influent chemical oxygen demand (COD) concentration at wastewater treatment plants (WWTPs) has become a critical issue. A key factor is the excessive biodegradation of organic matter by microbial communities within sewer systems. Intense disinfection commonly adopted for medical wastewater leads to abundant residual chlorine entering sewers, likely causing significant changes in microbial communities and sewage quality in sewers, yet our understanding is limited. Through long-term sewer simulation batch tests, this study revealed the response mechanism of microbial communities to residual chlorine and its impact on organic matter concentration in sewage. Under residual chlorine stress, microbial community structure rapidly changed, and more complex microbial interactions were observed. Besides, pathways related to stress response such as two-component system were significantly enriched; pathways related to energy metabolism (such as carbon fixation in prokaryotes and citrate cycle) in microbial communities were inhibited, and carbon metabolism shifted from the Embden-Meyerhof pathway to the pentose phosphate pathway to enhance cellular reducing power, reduce oxidative stress, and consequently decrease organic matter degradation. Therefore, compared to sewers with normal disinfection, concentrations of COD and dissolved organic carbon in sewage under chlorine stress increased by 12.6 % and 7.4 %, respectively. Besides, the decay and transformation of residual chlorine in sewers were explored. These findings suggest a new approach to medical wastewater discharge management: placing the medical wastewater outlet at the upstream in sewer systems, which ensures that residual chlorine consumption reaches maximum during long-distance transportation, mitigating its harmful effects on WWTPs, and increases the influent organic matter concentration, thereby reducing the need for additional carbon sources.