High dissolved oxygen regulation by side-stream supersaturated oxygenation impedes black-odor removal from rivers affected by combined sewer overflows

Abstract

In rivers affected by combined sewer overflows (CSOs), achieving fine-grained management of dissolved oxygen (DO) regulation is critical for effective suppression of black odor after rainfall and reduction of river management costs. However, systematic investigations of the DO regulation mechanisms underlying the removal of key pollutants that cause black, odorous waters after CSOs are lacking. In this study, we investigated the effects of different DO regulation levels associated with short- and long-term side-stream supersaturated (SSS) oxygenation on the formation of carbon, nitrogen, sulfur, and other black odorous pollutants after CSOs. In addition, the effects of different DO levels regulated by SSS oxygenation were determined using the modified entropy weight method. The results showed that the number of CSOs, the rate of the black-odor phenomenon (12 h), and its extent increased. After different SSS oxygenation steps, the odorous degree of CSOs was reduced to varying extents. Additionally, for SSS oxygenation-mediated DO regulation to yield a DO ≥ 3.0 mg L−1, the oxygen-deficient reduction state of the interface between the overlying water and CSO-induced turbid water was effectively reversed. The removal rates of sulfide, Fe2+, and other black odorous substances varied; high levels of SSS oxygenation-mediated DO regulation were more conducive to the emergency improvement of redox conditions (182.67%) and removal of NH3–N (0.616–0.638 mg L−1·d−1). However, regulation of such high DO levels was not as effective for the long-term removal of black malodorous substances, leading to a rapid decline in pH (lowest decrease to 5.68) and denitrification efficiency in later stages. Additionally, evaluating the effectiveness of different SSS oxygenations provided further evidence that medium to high DO levels (4.5 mg L−1) achieved the highest comprehensive scores of 23.81 and had the optimal cost-effectiveness for long-term regulation. These results provide a scientific basis for improving the management of black odorous water in rivers after CSOs.