Evidence on the causes of the rising levels of COD along the middle route of the South-to-North Diversion Project in China: The role of algal dissolved organic matter.

As the biggest inter-basin water transfer scheme in the world, the South-to-North Water Diversion Project (SNWD) was designed to alleviate the water crisis in North China. The main channel of the middle route of the SNWD is of great concern in terms of the drinking water quality. In this study, we tested the hypothesis that the dissolved organic matter (DOM) derived from the planktonic algae causes the rising levels of COD along the middle route by monitoring data on water quality (2015-2019, monthly resolution). The results showed that algal density in the main channel increased along the channel and was significantly correlated with COD (p < 0.01). Five fluorescent components of DOM, including tyrosine-like (14.85%), tryptophan-like (22.48%), microbial byproduct-like (26.34%), fulvic acid-like (11.41%), and humic acid-like (24.92%) components, were detected. The level of tyrosine-like components increased along the channel and was significantly correlated with algal density (p<0.01), indicating that algae significantly changed the level of DOM in the channel. Algal decomposition and metabolism were found to be the main mechanisms that drive the changes in COD. Therefore, controlling algal density would be an important measure to prevent further increase in COD and for the guarantee of excellent water quality.