A critical review of the appearance of black-odorous waterbodies in China and treatment methods.

Black-odorous rivers and lakes are a serious environmental problem and are frequently reported in China. Despite this, there have been no comprehensive in-depth reviews of black-odorous water formation mechanisms, contributing factors and potential treatment technologies. Elements such as S, C and N play an important role in the biogeochemical cycle of black-odorous waterbodies, with water blackening caused by metal sulfides such as iron sulfide (FeS) and manganese sulfide (MnS). Volatile substances such as volatile organic sulfur compounds (VOSCs) are the main contributors of odor. Microorganisms such as sulfate reducing bacteria (SRB), Bacteroidetes and Proteobacteria play important roles in blackening and odor formation processes. Effectiveness of the commonly used treatments methods for black-odorous waterbodies, such as artificial aeration, sediment dredging, microbial enhanced technologies and constructed wetlands, varies significantly under different conditions. In contrast, bio-ecological engineering technologies exhibit comprehensive, long-lasting and economical treatment effects. The causes and mechanisms of black-odorous water formation require further investigation, as well as the optimal application conditions and mechanisms of treatment technologies. This study comprehensively reviews 1) the characteristics and current distribution of black-odorous waterbodies; 2) the compounds contributing to black-odorous phenomenon; 3) black-odorous waterbody production mechanisms; 4) treatment technologies for black-odorous waterbodies. Further studies on the mechanisms of blackening and odor formation are required, with treatment application conditions and mechanisms also requiring further clarification. In addition, the long-term ecological restoration of black-odorous rivers immediately after remediation is key issue that is easily overlooked but merits further investigation and development.