Research On The Law Of Water Quality Change And The Improvement Effect Of Water-lifting Aerator In Lijiahe Reservoir

In recent years,with the deteriorating water quality of reservoirs and the periodic outbreak of harmful Cyanobacteria,the process of eutrophication of water bodies is accelerating,seriously damaging the ecological balance of water bodies and affecting the safety of water supply.As a new in-situ control algae technology,it not only improves the water quality,but also inhibits the reproduction of algae and provides a guarantee for the water quality safety of the water source.This study taked Lijiahe Reservoir,an important water source in Xi'an,as the research object.Through continuous in-situ monitoring and laboratory analysis,it analyzed the evolution law of water quality and the characteristics of planktonic algae under natural conditions,and researched the improvement effect of water-lifting aerator.This article main includes the following three aspects:1)the characteristics of seasonal thermal stratification in Lijiahe Reservoir and its impact on water quality evolution;2)growth and reproduction law of phytoplankton in Lijiahe Reservoir and its influencing factors;3)the improvement effect of water quality and control effect of planktonic algae in water-lifting aerator.The main results are as follows:(1)The seasonal thermal stratification of the Lijiahe Reservoir was very significant.From the beginning of June,the temperature difference of the vertical water has gradually increased,resulting in the density difference between the upper and lower water,so the material and energy exchange between the vertical water obstructed,dissolved oxygen(DO)appeared obvious stratification,DO of the middle water layer(5~25m)and the bottom water were close to 0mg/L,showing an anaerobic state,aggravating the release of reducing substances in the bottom sediment,after the stratification period,the vertical water began to gradually mix,and the reducing substances enriched at the bottom began to migrate and diffuse into the upper layer of water,causing secondary pollution,which seriously affected the water quality of the reservoir.(2)The peak density of algae in the Lijiahe Reservoir occurred during the stable stratification of water(August 2017),which was as high as 2.76×10~8/L.During this period,the main dominant algae in the Lijiahe Reservoir was the Microcystis aeruginosa and the Aphanizomenon flos-aquae.The algae accounted for more than80%of the total algae cells,mainly distributed in the 0~5 m water layer,while the Chlorella was the second.Then it entered the mixing period,the water structure was weakened,and the algae began to decay gradually.By the end of October,the density of algal cells decreased to 9.0×10~6 cells/L.At the same time,the Cyclotella and Synedra of the diatoms became the main dominant algae species.(3)The water-lifting aerator can not only improve anaerobic environment on the bottom and inhibit the release of pollutants in the sediment,but also destroy the water layer structure and improve the water quality of the reservoir,finally inhibit the reproduction of algae.When the equipment was not installed at the same time last year,the stable thermal stratification of the water was maintained for 3 months,and the thickness of the thermocline was 18m.After the installation of the water-lifting aerator in the Lijiahe Reservoir in 2018,the thermal stratification of the water was obviously weakened.After 8 days of continuous operation,the thermocline sneaked into the water depth by 45m from 25m in the same period of last year.The thickness of the thermocline was reduced to 10m.After one month of operation,the vertical water was completely mixed,the water temperature remained consistent,and the anaerobic environment of the bottom water was improved,the internal source release has been effectively controlled.The density of algae cells also decreased from 2.76×10~8 cells/L in the same period of last year to 4.0×10~6 cells/L,and the reduction rate was as high as98%,and the corresponding community structure also changed.The ratio of Microcystis aeruginosa was greatly reduced,from 80%in the same period last year to40%,and the main dominant algae became Chlorella and Cyclotella.