Impact of pollutant-laden stormwater inflow on urban river blackening and odor: Processes and mechanisms

Abstract

The inflow of pollutant-laden stormwater due to cross-connections between stormwater and sewage pipelines contributes to the occurrence of blackening and odor in urban rivers following rainfall events. To elucidate the distinct roles of particulate and dissolved pollutants contained in pollutant-laden stormwater driving water bodies blackening and odor, this study investigated the effects of individual and synergistic inputs of particulate and dissolved pollutants on blackening and odor formation, revealing the mechanisms by which pollutant-laden stormwater inflow drives river blackening and odor. Results demonstrated that particulate pollutants input alone induced only transient visual darkening during particle suspension, without causing persistent blackening and odor. In contrast, dissolved pollutants input could create anaerobic conditions, stimulating sulfide and Fe²⁺ generation, which directly triggered blackening and odor formation. Synergistic input of both fractions exhibited an accelerated effect, promoting faster blackening and odor onset compared to dissolved pollutants alone under equivalent organic loading. Dissolved organic matter influenced Fe²⁺ and chromophoric dissolved organic matter (CDOM) release from particulate pollutants, with peak impact at soluble chemical oxygen demand (SCOD) constituted 60 % of total chemical oxygen demand (TCOD). The blackening and odor process driven by pollutant-laden stormwater primarily comprises two stages: shock and induction. Shock stage: Particulate pollutant suspension directly causes visual blackness. Induction stage: The anaerobic and moderately acidic conditions created by pollutant-laden stormwater promote biological reduction of SO₄²⁻ and Fe³ ⁺ in the overlying water, as well as release of sulfur and iron from porewater. Key blackening agents were iron sulfides adsorbed onto organic flocs and CDOM in the overlying water. Gas ebullition suspended black flocs, CDOM enhanced light absorption, and fulvic-like CDOM promoted particle suspension, collectively inducing blackening. For odor, elevated H₂S concentrations in headspace gases were identified as the dominant source of post-rainfall odor. Finally, it is recommended to set the primary treatment objective for pollutant-laden stormwater as limiting the SCOD concentration of the incoming river pollutant loading to ≤ 100 mg/L, while maintaining an SCOD/TCOD ratio > 0.8.