Study On Disinfection And Degradation Of Organic Matter From Cyanobacteria And Its Influence On Water Quality Of Water Supply

In recent years,the occurrence of cyanobacteria in natural water bodies is frequent,which causes a series of water quality deterioration and drinking water safety problems.Cyanobacteria can produce a large amount of algal organic matter(AOM),which is divided into EOM and IOM.The former is called extracellular organic matter,which is secreted and released into the surrounding water by living algal cells through their own activities,while the latter is called intracellular organic matter,which is secreted into the surrounding water environment mainly by the natural rupture of cells in the decline period.The composition of AOM material is complex,conventional water treatment process is difficult to remove all of it,so there is residual cyanobacteria AOM in the water purification plant,into the water supply network,will provide conditions for the growth and propagation of bacteria and other microorganisms in the pipeline,and then cause a series of pipe network water quality deterioration problems.Therefore,in the process of algal water treatment,cyanobacteria AOM is an important factor affecting the water quality of water purification plant and water safety in the water supply pipeline.In this study,the disinfection and degradation mechanism of microcystis aeruginosa,the dominant algae species during the bloom period of Taihu Lake,and its algae-source organic matter as well as the influence mechanism on the water supply network quality was studied.The degradation efficiency and order of cyanobacteria EOM and IOM in the traditional disinfection and oxidation process were compared and investigated,so as to explore the organic degradation mechanism of cyanobacteria EOM and IOM in different species and different compositions.And the effects of the residual state after sodium hypochlorite chlorination into the water supply pipe network on the water quality and bacteria growth in different materials of water supply pipe.At the same time,the impact of cyanobacteria AOM on the water quality safety of continuous water supply pipe network was investigated.The findings include the following:(1)The two disinfection methods,sodium hypochlorite and ozone,had certain effect on the removal of cyanobacteria AOM,but the degradation efficiency was different,and the disinfection mechanism of the two also had great differences.The removal rates of TOC and UV254 indexes before and after disinfection and the changes of the three-dimensional fluorescence spectra before and after disinfection of cyanobacteria AOM showed that sodium hypochlorite disinfection had a better removal effect on cyanobacteria EOM,ozone disinfection had a higher removal rate on cyanobacteria IOM,but the overall removal rate of ozone on AOM was slightly lower than that of sodium hypochlorite.In addition,the overall removal rate of AOM by these two disinfection methods is not high,and it is difficult to achieve a satisfactory removal effect only through the degradation of these two disinfection processes.(2)In the static water supply pipeline,the presence of cyanobacteria AOM in the form of residual organic matter and its derivatives has no obvious effect on the fluctuation of pH value of water samples,but increases the decay rate of residual chlorine in the pipeline network,and is also the main factor leading to the rise of turbidity in the pipeline,the accumulation of TOC pollution and the decline of biological stability of the pipeline network.These indexes of cyanobacteria AOM change with time in static pipe network also verify that cyanobacteria EOM is more easily removed by sodium hypochlorite oxidation than IOM.In different pipes,the pH decline,decay of residual chlorine,rise of turbidity,TOC,BDOC content and total bacterial growth in cast iron pipes are all higher than those in other three pipes.It may be because that the water environment in the cast iron pipe is more complex than that in other pipes,the growth of biofilm is more vigorous,bacteria breeding or scaling is more obvious,so that the biological stability of the water in the pipe is poor.(3)Compared with static pipe network,the decline rate of residual chlorine value in dynamic continuous pipe network is faster and the decline range is larger.Turbidity value continues to decrease from 0 to 36h,but the overall turbidity and total number of bacteria are higher than that in static pipe network,and the total number of bacteria in the system with flow rate of 0.5m/s is higher than that in the system with flow rate of 1.0m/s,mainly because of the higher flow rate of 1.0m/s.Strong shear force will be generated,which is unfavorable to the adhesion of bacteria on the pipe wall,resulting in a low biomass in the pipe network.The ultra-low flow rate under static condition reduces the transport of nutrients,which is not conducive to the growth of microorganisms in the pipe network.The presence of high concentration of cyanobacteria EOM in dynamic pipe network is more likely to lead to TOC accumulation.When chlorine injection is 3mg/L and pipe network flow rate is 0.5m/s and 1.0m/s,TOC content in pipe network water of cyanobacteria EOM increases by 1.93 times and 1.88 times,respectively,compared with the initial moment.The TOC content in the network water of cyanobacteria IOM increased by 1.11 times and 1.81 times respectively compared with the initial time.