Research On The Effects Of Artificially-Induced Mixing On Organic Matter And Algae Control In Lijiahe Reservoir

To explore the effects of artificially-induced mixing by the water-lifting aerators（WLAs）on organic matter and algae control in Lijiahe Reservoir,this study divided the year into natural mixing period,stratification period（including stratification formation period）and artificial destratification period three times according to the reservoir mixing state,the stratified sampling of Lijiahe Reservoir was carried out from July 2019 to September 2020.By detecting the physical and chemical indexes,water quality indexes and algae population structure of the water,combining with the excitation emission matrices coupled with parallel factor analysis method（EEMs-PARAFAC）and ultraviolet-visible（UV-Vis）spectrum,the damage of artificially-induced mixing to thermal stratification structure and the influence on spectral characteristics of colored dissolved organic matter（CDOM）and algae population structure in Lijiahe reservoir were analyzed,so as to provide scientific basis and theoretical support for drinking water reservoir water quality improvement,the main research contents include:1)The effect of artificially-induced mixing on prolonging natural mixing period of reservoir;2)The characteristics of reservoir organic matter reduction by artificially-induced mixing;3)The characteristics of artificially-induced mixing on reservoir algae control.The main conclusions of this paper are as follows:（1）The operation of WLAs in the reservoir stratification period can effectively weaken the thermal stratification state.The artificial destratification by WLAs increased the mixing water temperature of the reservoir,and induced the natural mixing period of the reservoir to last from the beginning of October to the middle of March of the following year,which increased the natural mixing period of the reservoir to 5.5 months.The combined action of artificial destratification and hypolimnion oxygenation of WLAs effectively eliminated the phenomenon of metalimnetic oxygen minimum（MOM）and hypolimnetic anoxia during the stratification period,and increased the DO content in the water,so that the average DO concentration in the natural mixing period was within the range of 7.26-10.85 mg/L and maintained at a high level.Temperature and WLAs are the two main factors of reservoir stratification and mixing,surface water temperature higher than air temperature is the connecting condition of artificial destratification and natural mixing,the operation of WLAs in high temperature season destroys the natural stratification of the reservoir,and induces the reservoir to be complete mixing until the surface water temperature is higher than air temperature,then the WLAs can be shut down,and the continuous natural mixing of the reservoir can be realized through air temperature conditions,thus saving WLAs operating costs.（2）Input of storm runoff is the main source of organic matter pollution of Lijiahe Reservoir,the contents of organic matter and nutrients in Lijiahe Reservoir were significantly increased by storm runoff in a short period of time,causing serious damage to the water quality,UV-Vis spectrum and EEMs analysis results showed that the storm runoff significantly increased concentrations of CDOM,increased the proportion of humus components,and decreased the proportion of protein components.（3）The artificially-induced mixing by WLAs effectively controlled the organic matter pollution of the reservoir,and improved the self-purification ability of the reservoir.The EEMs analysis results showed that Lijiahe Reservoir contains four CDOM components,namely microbial humus（C1）,terrestrial humus（C2 and C3）and tryptophan（C4）.During the natural mixing period,C1-C4 components were significantly reduced.The operation of WLAs in the artificial destratification period can degrade the C4component by destratification and oxygenation.The characteristic parameters of UV-Vis spectrum indicated that the molecular weight of CDOM decreased gradually during the natural mixing period,and the operation of WLAs during the artificial destratification period increased the molecular weight of CDOM.The results of principal component analysis indicated that the water quality of Lijiahe Reservoir is characterized by endogenous organic matter in stratification period and exogenous organic matter in natural mixing period,the operation of WLAs in artificial destratification period makes the reservoir organic matter change from endogenous to exogenous.（4）During the artificial destratification period,the operation of the WLAs had a significant reduction effect on algae.After 20 days of WLAs continuous operation,the reduction rates of algae density in the surface,middle and bottom water were 96.1%,94.9%and 81.9%,respectively,within the radial range of 80 m of the WLAs.A total of 5 phyla,43 genera and 64 species of algae were detected in Lijiahe Reservoir.The operation of WLAs will affect the population structure of algae in the reservoir.Chlorophyta is the dominant population in the operation period of WLAs.Correlation analysis results showed that algal density was positively correlated with COD_Mn（r=0.654,P<0.01）and DOC（r=0.706,P<0.01）,and controlling algal propagation through WLAs in summer was an important way to reduce organic matter in water.The algae density in the surface water was significantly positively correlated with the surface water temperature（r=0.517,P<0.01）,and negatively correlated with the depth of mixing zone(Z_mix)（r=-0.492,P<0.05）.The decrease of water temperature and increase of mixing zone depth caused by WLAs were the reasons for controlling algae propagation.