Pollutant exchange between sewage and sediment in urban sewer systems

In this study, the apparent phenomenon of pollutant exchange between sewage and sediment that affected the influent quality of wastewater treatment plants was evaluated by a long-term monitoring in a pilot sewer system. In order to understand the in-depth effect of this phenomenon on water quality in sewer, the pollutant exchange pathways of physical pollutant deposition (PPD), biological transformation adsorption (BTA) and biological transformation release (BTR) were first proposed in this study. And then, the controlled experiments (prohibiting the microbial activity in either biofilms or sediment) were established to quantify the physical and biological pollutant exchange capacity. The results showed that the PPD values of organic, nitrogen and phosphorus pollutants were 144.0 mg/L, 5.15 mg/L and 3.79 mg/L, respectively, and this pathway accounted for over 70% of the pollutant decrease in sewage. It indicated that the PPD was the dominant pollutant exchange pathway in sewers. Meanwhile, the pollutant exchange values in BTR were more than two times of those in BTA (39.2 mg/L > 14.7 mg/L for organic matters; 1.79 mg/L > 0.85 mg/L for nitrogen pollutants; 2.12 mg/L > 1.01 mg/L for phosphorus pollutants). The result indicated that the BTR was the dominant biological reaction which affected the pollutant structures in sewage (especially resulting in the accumulation of volatile fatty acids), and consequently, changed the biodegradability of sewage. Therefore, this study revealed the characteristics of pollutant exchange pathways in sewers and could provide a theoretical basis for the future development of urban sewer systems.