Study On Effect And Mechanism Of Nitrogen Removal And Algae Control In Shallow Water Reservoir By Water-lifting Aeration System

As an important source of drinking water in China,lakes and reservoirs are often eutrophic due to excessive input of pollutants such as nitrogen,phosphorus and organic matter by human activities,which has a great impact on the water safety of residents.The outbreak of algae caused by eutrophic water bodies will not only destroy the balance of the aquatic ecological environment system,but also increase the cost of water treatment plants.As a shallow watwer reservoir,Yuzigou Reservoir is an important water source in Xixian New Area,it is effected by exogenous input and presents eutrophication and algae outbreak.In this paper,the water quality was evaluated by studying the seasonal variation of water quality in Yuzigou Reservoir from September 2021 to August 2022.The effect and mechanism of the water quality improvement,the influence on bacterial community and the algae control were studied.The main conclusions are as follows:(1)The annual water temperature of Yuzigou Reservoir changes significantly,and thermal stratification occurs in the middle reaches of the reservoir in summer.The p H of the reservoir fluctuates less throughout the year.TN and DTN were in an increasing trend for a long time,and the average concentrations were up to 3.73 mg/L and 3.23mg/L,respectively.Nitrogen was mainly dissolved,and nitrate nitrogen was the main form of nitrogen,up to 2.79 mg/L.For a long time,the concentration of TP exceeded the Class III water quality standard of surface water,and the phosphorus in water was mainly particulate.The WQI index showed that the water quality was good most of the time,and the TLI index showed that the water quality was between mesotrophic and mild eutrophic.(2)During the summer operation of the water-lifting aeration system,the nitrogen removal effect was significant,and the decline rate reached the highest in the water area of the equipment center.The average concentration of TN continued to decrease from3.59 mg/L to 1.62 mg/L.The average concentration of DTN continued to decrease from 3.30 mg/L to 1.21 mg/L,and the average concentration of NO3~--N continued to decrease from 2.67 mg/L to 0.48 mg/L,with a decrease rate of 54.87%,63.33%,and82.02%,respectively.During the summer operation,the decrease rates of CODMn and DOC in the central water area of the equipment were 11.76%and 29.13%,respectively.(3)The ACE index,Chao1 index and Shannon index of the bacterial community at0 m in the radial direction increased by 300.00%,296.86%and 88.19%,respectively,after the summer operation of the water-lifting aeration system.The ACE index,Chao1index and Shannon index at the radial 40 m were 249%,244%and 67.16%higher than those before operation,respectively.The ACE index,Chao1 index and Shannon index at 100 m in the radial direction increased by 90.69%,124%and 51.95%respectively compared with those before operation.During the summer operation,the dominant bacteria were Proteobacteria,Firmicutes,Actinobacteriota and Cyanobacteria.(4)The annual algae density of the Yuzigou Reservoir ranged from 4.438 to 36.701million/L.A total of 4 phyla,19 genera and 34 species were detected throughout the year.The dominant algae species were Cyclotella,Chlorella,Synedra and Pseudoanabaena in spring,Synedra and Pseudoanabaena in summer,Chlorella,Merismopedia and Tetracera in autumn,Cyclotella,Synedra and Pseudoanabaena in winter.The decrease rates of algal density at 0 m in the radial direction were 58.34%and 9.53%,respectively,and the decrease rates of Fv/Fm values were 62.74%and35.55%,respectively,in winter and summer.