The Fate Of Cyanobacteria And Odorants In Drinking Water Distribution Systems

The occurrence of unpleasant taste and odor（T&O）in drinking water influences the normal life of residents.Although the finished water treated by the water plant conforms to the"Sanitary standards for Drinking Water",there is still the problem of T&O in tap water.Therefore,the T&O problem caused by secondary pollution of the water distribution system cannot be ignored.Algae is one of the main sources of odorants,and many researches have found that a small amount of algae could not be completely removed by water plants,but the life activities of leaked algae after entering the water distribution system have not been studied.Therefore,this study selected two dominant cyanobacterial species—Microcystis aeruginosa and Pseudanabaena,and the number of algae was about 1×10⁷ cells/L.Based on the comprehensive simulation platform of drinking water distribution system of Zhejiang University,this paper explored the rule and mechanism of odorants and disinfection by-products produced by these two algae under the influence of hydraulic conditions,temperature,light,chlorine/chloramine,antibiotics such as tetracycline and norfloxacin in water distribution system,to find suitable control strategies to reduce the problem of T&O.The results showed that after 48 hours of running at the flow velocity of 0.05 m/s,0.6 m/s and 1.5 m/s,Microcystis aeruginosa was neither destroyed nor grown,and the number of algae cells in finished water and tap water changed little,indicating that algae cells arrived at the users with the water.However,the damage of shear force to Pseudanabaena was intense,and algae could not maintain the cell morphology even at the flow velocity of 0.05 m/s.The number of cells in tap water gradually decreased as time goes by,and the 2-methylisopropanol（2-MIB）produced by the cells was released into the water,and reached the end of the pipes steadily.With the increase of flow velocity,the cell damage rate and 2-MIB release rate increased.At the flow velocity of0.05 m/s,0.6 m/s and 1.5 m/s,the proportion of extracellular 2-MIB reached 93%,99%and 98%at 48 h,28 h and 24 h,respectively.However,when 16S r RNA test was used to test the microbial community of pipe biofilm before and after the long-term test,it was found that cyanobacteria could not be detected before the test,while it was found after the test.The relative abundance of cyanobacteria accounted for 0.83%,among which the relative abundance of Microcystis and Pseudanabaena accounted for 20.5%and 1.5%,respectively,indicating that Microcystis aeruginosa and Pseudanabaena could adsorb on the biofilm after long-term accumulation,and the adsorption capacity of the former was stronger than that of the latter.Pseudanabaena was damaged seriously by shear force in pipe,so the influence of light and temperature was only investigated for Microcystis aeruginosa.Temperature and light had little effect on Microcystis aeruginosa.The light permeability of the cast iron pipe was low,and the algae could not reproduce under dark conditions.However,the algae could reproduce again after the tap water being irradiated by light.Light was more effective at limiting algae reproduction than the nutrient-poor environment of tap water.The algae kept alive after 48 h even at 0℃and the concentration of extracellularβ-cyclocitral remained low.In order to reduce the influence of microbes on water quality during water supply,the finished water contains chlorine/chloramine disinfectants.This test showed that when different concentration of chlorine/chloramine coexist with algae,the decay rate of chlorine was faster than chloramine,and chlorine,chloramine work on each part:water>pipe wall>algal cells,pipe wall>water>algal cells,respectively.The degradation efficiency of chlorine onβ-cyclocitral was stronger than that of chloramine.When the concentration of chlorine was 0.05 mg/L,the degradation rate ofβ-cyclocitral was 81.52%after 4 h,and noβ-cyclocitral was detected when the concentration of chlorine was more than 0.1 mg/L.While when the initial total chlorine concentration was 0.34 mg/L,0.7 mg/L and 0.97 mg/L,the degradation rates ofβ-cyclocitral after 48h were only 14.9%,36.5%and 52.9%,respectively.Part of the chloramine in pipe worked on the biofilm,making the degradation rates ofβ-cyclocitral in pipe less than the flask test.The degradation efficacy of chlorine or chloramine on 2-MIB produced by Pseudanabaena was poor.The degradation rate of 2-MIB in pipe and flask were 19%and 22%respectively when the initial chlorine concentration was 1.0 mg/L,21%and21%respectively when the initial total chlorine concentration was 0.97 mg/L.Chlorine disinfection mainly generate trichloromethane,trichloroacetic acid,while chloramine disinfection mainly generate trichloroacetic acid,the concentration of disinfection by-products produced in pipe was greater than in the flask due to the presence of biofilm.Comparing the results with or without the existence of algae,it indicated that the small amount of algal organic matter released by Microcystis aeruginosa and Pseudanabaena did not make great contribution to the production of disinfection by-products.Except for the inorganic substances such as chlorine,antibiotics,as a typical organic matter which can affect the life activities of algae,may coexist with algae in pipe,when 40μg/L and 400μg/L tetracycline coexist with the two kinds of algae respectively,40μg/L tetracycline was able to reduce the number of Microcystis aeruginosa and damage the cells.The C_t/C₀ of Microcystis aeruginosa decreased to0.80 and 0.72 respectively after 48 h.The tolerance of Pseudanabaena on tetracycline was stronger than Microcystis aeruginosa,because until 400μg/L tetracycline increased the cell destruction rate slightly.However,for norfloxacin,even increasing the concentration of norfloxacin to 400μg/L,it had no effect on the two algae.The kinetics of 400μg/L tetracycline in pipe and flask were in line with the pseudo-first-order reaction kinetics.The attenuation coefficients of 400μg/L tetracycline co-existed with Microcystis aeruginosa and Pseudanabaena in pipe were 0.023 and 0.025,respectively,which were much higher than those in the flask test.And the kinetics of400μg/L norfloxacin in pipe was in line with the pseudo-first-order reaction kinetics,while it is unconspicuous in flask.Algae had little influence on the attenuation rate.