Effects of Artificial Destratification and Induced-natural Mixing on Water Quality Improvement in a Drinking Water Reservoir

Abstract

To explore the effects of water quality improvement by artificial destratification and artificially-induced mixing, as well as realize the conditions of artificial mixing and natural mixing, the Lijiahe Reservoir was selected to monitor the indexes of water quality and hydrometeorology from June 2017 to April 2019 and to analyze the characteristics of variations in water temperature, dissolved oxygen, and pollutants during the natural and induced-mixing processes. The results demonstrated that:① The natural process had the features of a long period of thermal stratification and only a short period of mixing of about 2.5 months. Through the operation of water-lifting aerators (WLAs) during the induced-mixing process, the water body was completely mixed and entered the cooling period at the end of September, achieving the conditions of induced-natural mixing. The surface water temperature and average air temperature were 20.17℃ and 16.5℃, respectively, and the water body continued to be naturally mixed after the WLA system was shut down, which led to a natural mixing cycle of 5.5 months. ② During the natural-mixing process, the concentration of pollutants was relatively high in the whole period, and the concentration of surface pollutants in the mixing stage presented a trend of increasing first and then decreasing. The oxycline appeared with thermal stratification, and the anaerobic cycle in the bottom of water column reached 6 months. ③ Compared with the natural-mixing process, the hypolimnetic anaerobic condition was eliminated and the control effects of pollutants dominated during the induced-mixing process. Simultaneously, the concentrations of NH4+-N, TP, Fe, and Mn in the bottom of the water column were cut by 76.2%, 75.5%, 82.2%, and 82.1%, respectively, during the same period from October to March of the following year, and met the "Environmental Quality Standards for Surface Water". This study shows that the artificially-induced mixing process contributes to water quality improvement and mixing-period prolongation.