Study On The Effect Of Residual Disinfectant On Pathogens In Porous Media

During the Coronavirus pandemic,in order to control the spread of the virus,disinfectants were used in large quantities and large doses,resulting in a large amount of disinfectant remaining in the environment.The effective ingredients of the residual disinfectants would infiltrate into groundwater with rainfall-runoff,which may cause potential risk to the groundwater environment,and groundwater as an important habitat for pathogens,the long-term residual disinfectants in groundwater may lead to the enhanced tolerance of pathogens to disinfectants.The influence of residual disinfectants on pathogens during infiltration into porous media is still unclear.This study intends to use static experiments and sand column experiments,particle size measurements,zeta potential tests,Raman confocal microscopy,and other means to investigate the effects of residual disinfectants on pathogens during infiltration into porous media.The research is carried out from two aspects: influence effect,mechanism,and process,the simulated rainfall-runoff process to study the effect of residual disinfectants on the survival and migration of pathogens in porous media.It provides scientific support for the use,management,and knowledge of disinfectants in public health emergencies,as well as a relevant understanding for the risk of pathogen invasion into groundwater and prevention and control measures.The main conclusions of this paper are as follows:1.Effect of residual disinfectants on the survival of pathogensDifferent residual disinfectant doses and exposure time will have different inactivation effects on pathogens.When the residual disinfectant dose is 0-6.5 mg / L,the higher the dose,the higher the inactivation rate and the lower the survival rate of pathogens.Prolonging the exposure time under the same residual disinfectant dose will also reduce the survival rate of pathogens,but with the increase of residual disinfectant dose,the impact of exposure time on pathogen inactivation will gradually decrease.Under the same residual dose（0-6.5 mg / L）,the inactivation rate and inactivation rate of sodium hypochlorite disinfectant are significantly higher than those of peracetic acid disinfectant.Both disinfectants inactivate bacteria by their strong oxidation ability,but peracetic acid disinfectant is easy to decompose and unstable,so at the low dose,the inactivation effect is more likely to be weakened by external factors.The inactivation rates and survival rates of C-3000 E.coli and P.aeruginosa were basically the same during the disinfection process under the same conditions,but sometimes the survival rate of P.aeruginosa was higher than that of C-3000 E.coli.It is speculated that the reason was that the two pathogens have different ways to resist disinfectants due to the low inactivation efficiency of disinfectants at low doses（≤ 0.5mg/L）,which lead to different survival rates.2.The effect of residual disinfectants on the migration ability of pathogens in porous mediaThe process of residual disinfectant infiltration into porous media will lead to pathogen inactivation.The higher the residual dose of disinfectant,the lower the survival rate of pathogen efflux.However,combined with the static experimental results,it was found that the inactivation effect of disinfectant was greatly weakened in the porous media environment,and the inactivation effect was weakened more with the lower the dose of disinfectant.The residual disinfectant can inhibit the migration ability of pathogens,which will prolong the peak time of the pathogen penetration curve.Under the stress of disinfectant,it was conducive to promote the attachment behavior of pathogens,and it was mainly reversible adsorption so that the pathogens that survive in the sand column could desorb and flow out in the leaching process.The influence of residual disinfectant on the migration ability of pathogens is also related to the nature of pathogens.As Pseudomonas aeruginosa has stronger migration ability and less deposition than C-3000 E.coli.In dynamic experiments,P.aeruginosa survived the first stage of effluent at a higher rate than C-3000 E.coli,which is due to the fact that the strong migration ability of Pseudomonas aeruginosa reduces the reaction time with disinfectants,resulting in a large concentration of effluent living bacteria.3.Reducing the effect of natural organic matter on disinfection processThe organic matter in the groundwater environment is the key factor affecting the interaction between residual disinfectant and pathogen.In this study,humic acid was used as a typical organic component representative,and the results showed that under disinfectant stress,the mechanism of the reduction effect of humic acid on residual disinfectant-influenced pathogens was divided into three main aspects:（1）humic acid and disinfectant can react directly,thus consuming the disinfectant and leading to a reduction or depletion of the disinfectant’s bactericidal performance;（2）during the dynamic experimental process,humic acid has physical restrictions on the diffusion of disinfectant.Due to the negative charge on the surface of the pathogen and the negative charge on hypochlorite root,it is difficult to reach the surface of the pathogen and enter the interior of bacteria,and the existence of humic acid hinders the contact between them and limits the contact of disinfectant with pathogen;（3）In the aqueous phase,the presence of humic acid can act as a carrier for the pathogen,providing attachment sites and enhancing the pathogen’s ability to aggregate and thus increase its resistance to disinfection.When migrating in porous media,pathogens and pathogens adsorbed on the surface of humic acid tend to aggregate and deposit due to the influence of disinfectants,and the rough surface embedded in porous media can maintain a shield against disinfectants,making them less susceptible to the inactivating effect of disinfectants.When the dosage of sodium hypochlorite disinfectant is less than 2mg/L,humic acid has a significant protective effect on C-3000 Escherichia coli.However,at higher disinfectant doses,the effect of influencing factors in the environment became weaker,and the protective effect of humic acid on pathogens is not obvious.