Investigation On Breeding Mechanism And Control Methods Of Mosquitoes In Sewage And Wastewater Ecological Treatment Facilities

Ecological treatment of sewage and wastewater is an important development trend in the field of water pollution control in China.However,its hidden mosquito breeding effect is likely to increase the mosquito population size and the risk of mosquito-borne diseases transmission,which may become an urgent problem to be solved in the"post-treatment"period of water environment.At present,the number and scale of sewage and wastewater ecological treatment facilities are increasing in China,but the understanding of the current situation,mechanism analysis and control methods of mosquito breeding are lagging behind,which may adversely affect the development of the industry and public health.Based on the lack of research in this field,this paper evaluated and analyzed the breeding status and mechanism of mosquitoes in common sewage and wastewater ecological treatment facilities through background investigation,on-site monitoring and simulation experiments,and preliminarily proposed specific ideas and implementation methods of mosquito-sewage co-treatment.It helps to clarify the relationship between water pollution treatment and mosquito breeding and eliminate the adverse risks to public health and the ecological environment caused by sewage ecological treatment facilities.Following four aspects are studied through the thesis:(1)Monitoring the changes in hydrology,water quality,biology,climate and other environmental conditions of mosquito breeding sites,and analyzing the formation mechanism of breeding sites and the regulation mechanism of environmental factors on mosquito breeding density;(2)To establish a suitability assessment method for small and medium-sized mosquito larvae microhabitat to provide demonstration and reference for identification of new breeding sites and prediction of mosquito larvae in related facilities;(3)To propose targeted mosquito larvae control methods and summarize mosquito larvae control strategies applicable to related breeding sites;(4)To explore the effects and mechanisms of typical water treatment technologies on mosquito breeding,so as to provide a basis for reaching the goal of mosquito-pollution co-treatment through coupling and coordination of multiple technologies.Through the research,the following main conclusions are drawn:(1)The highest larvae density of 22.8 per spoon was found in the tailwater wetland of the sewage treatment plant from 2019 to 2020,and the timing pattern of mosquito larvae density was bimodal.The highest mosquito density was found at the turn of spring and summer(April to May),and the main mosquito species was Culex pipiens pallens.When water temperature was above 20?,vegetation coverage was 30%-60%and the density of natural enemies was low,the number of mosquito larvae in the breeding site was higher.The breeding sites of tailwater wetlands are generally small semi-enclosed structures which are independent from the main part of wetlands,which are highly correlated with the blood-source area in spatial distribution.Most of the breeding sites are long-term stagnant water,and the water volume is less than the maximum capacity of the breeding sites.Among water quality factors,COD_Mn had a high correlation with the larvae of 1^st instar mosquitoes,indicating that COD_Mn is significantly oviposition attractive for adult mosquitoes.There was a significant negative correlation between the number of mosquito larvae and nitrogen/phosphorus nutrients(Dongyang Wetland)and Chla(Changshu Wetland),indicating that in small water in tailwater wetland,when there were more nutrients so that more aquatic creatures lives,the amount of larval mosuqitoes was lower.(2)The highest larvae density of 1162.3 per spoon was found from the sewage ecological treatment facilities in Chongming rural areas from 2020 to 2021,and the main mosquito species were Culex pipiens pallens,Culex tritaeniorhynchus,Armigeres subarmigeres and Anopheles Sinensis.The highest intensity and longest breeding period of mosquito larvae were found at the monitoring site of the gully of wastewater returning to field,which were mainly related to continuous heavy pollution,slow or even stagnant water flow and high density blood source in surrounding areas.Acidulous or alkaline p H(6.60-8.60),low concentration of DO(<3.50mg/L),high concentration of organic matter and nitrogen and phosphorus nutrients(mainly composed of COD_Mn,NH₄⁺-N and PP)were the most suitable water quality conditions for the survival and development of mosquito larvae.On this basis,the deeper the water,the higher the density of mosquito larvae.(3)The mosquito larvae suitability evaluation system established by fuzzy comprehensive evaluation method can be used to identify new breeding sites in related facilities and predict the breeding situation of mosquito larvae.Fuzzy comprehensive evaluation system of larval mosquitoes'suitability consists of four kinds of environment characteristics(criteria layer)and 13 evaluation factors(factor layer),AHP is used to determine the weight of criterion layer,entropy weight method and coefficient of variation method to determine the weight of factor layer.In this way,subjective analysis and objective data complement and improve each other,and the results can not only qualitatively evaluate the classification of larval mosquitoes'suitability,but also can make a horizontal comparison within the same grade through quantitative comprehensive evaluation value B^T.(4)Bacillus sphaericus(Bs)preparation could significantly kill Culex pipiens pallens larvae,and its killing effect was negatively correlated with the degree of organic pollution in water.Fermentation treatment could reduce the effective time and equivalent dose of Bs preparation in livestock and poultry feces.After Bs field spraying,the mortality rate of 1^st-2^nd instars mosquitoes was 83.83%.However,after 24 hours,the larval mosquitoes might develop resistance to pesticide stress.Artificial aeration significantly reduced mosquito egg hatching rate(2.27%-10.22%)and larval mosquito pupation rate(4.44%-6.11%),but there was no significant difference in the effect of water surface disturbance on pupation rate between different aeration modes.PACl coagulation-flocculation had a strong lethal effect on the larvae of Culex pipiens pallens,with LT₅₀ of 2d and LT₉₀ of 8-14d.The medium and low coverage(0-60%)of Lemna minor L.had no significant effect on mosquito breeding,while Azolla pinnata promoted mosquito breeding.While the coverage was high(>60%),both floating plants were not conducive to mosquito breeding.(5)In this study,physical mosquito control methods should be considered firstly in ecological wastewater treatment facilities,and the main measures including the optimization of hydrodynamic conditions,sealing of small and medium-sized standing water structures,and reduce the pollution of breeding sites.Biological control methods can be applied to ecological treatment units with large water area and rich vegetation.Ecological friendly,low quantity,high efficiency and high targeting insecticides should be selected for chemical control as emergency treatment measures.Through optimizing technological process,architectural configuration and ecological design,strengthening operation and maintenance management and mosquito suitability monitoring and evaluation,and actively exploring multi-technology coupling mode with mosquito control and pollution control effects,it is helpful to realize the goal of mosquito and pollution co-treatment in sewage ecological facilities.