Mechanism Of Combined Clogging Of Porous Media In Managed Aquifer Recharge

Managed aquifer recharge has become an effective measure to alleviate the negative effects of groundwater over-exploitation and its environment.However,clogging during the recharge process is one of the bottlenecks factors restricting its widespread application.Therefore,the research on the clogging formation and its mechanism during the process of managed aquifer recharge is of great theoretical and practical significance.Currently,research on the mechanisms of single-type clogging is relatively mature,but the research on composite clogging mechanism under actual field conditions is still very weak.This study completed a series of systematic laboratory study(a total of 20 groups,and each group repeated 3 times),to perform tasks such as media selection,typical clogging substances selection and the experimental concentration,and investigation of the formation and evolution of composite clogging.and the simulation of composite clogging process were completed.The study also utilized theories and techniques from groundwater dynamics,hydrogeochemistry,and biogeochemistry to examine the order,causes,distribution,and dynamic changes of different types of clogging under specific dominant factor conditions.The study analyzed the synergistic competition relationship between different clogging factors during the composite clogging process,identified the dominant factors,stages,and intensities of composite clogging under different conditions,and developed a simulation model for the formation and evolution of composite clogging based on causal mechanisms.These findings provide a scientific basis for the prevention and control of clogging in managed aquifer recharge.The main work conducted and the major achievements obtained in this study are as follows:(1)The evolution law of composite clogging was analyzed through laboratory experiments.In terms of the occurrence time of clogging,the bio-chemical clogging occurred earliest,and the physical clogging occurred latest.In terms of clogging rate,bioclogging had the fastest rate,next is the two types of composite clogging(biophysical > physical-chemical > bio-chemical),followed by bio-physical-chemical clogging,chemical clogging and physical clogging.In terms of the overall degree of clogging(on the third day of recharge)was most severe for bio-chemical clogging,and the degree of physical clogging was weakest.In terms of clogging location,biophysical-chemical clogging,chemical clogging,and bio-clogging occurred only in the surface layer of the sand column,while physical clogging and physical-chemical clogging occurred slightly deeper,bio-physical clogging and bio-chemical clogging occurred within the experimental sand column,but the degree of clogging gradually decreased from the surface layer downwards.Overall,compared with single clogging,complex clogging occurred earlier,had a faster clogging rate,a higher degree of clogging,and a greater depth of clogging.The formation and evolution process of complex clogging was not a simple superposition of single clogging.(2)The formation and evolution mechanisms of complex clogging were revealed.The mechanism of formation and evolution of combined clogging under the interaction of typical clogging-causing factors(iron,suspended solids,microorganisms)was revealed by various characterization means,such as Fourier Transform Infrared spectroscopy,X-ray photoelectron spectroscopy,Scanning electron microscopy,Energy spectrum analysis,Zeta potential,and particle size:(1)Iron and suspended solids promote the growth of microorganisms and their extracellular polymers,which can accelerate the occurrence and development of clogging in porous media;(2)The solubilization of suspended solids by microorganisms and their hydrolysis of minerals can reduce the degree of clogging;(3)Microorganisms affect the p H of the recharge environment through solubilisation and mineral hydrolysis,contributing to iron precipitation and flocculation,increasing the risk of clogging;(4)Iron,suspended solids,and microorganisms formed larger aggregates through bridging and polymerization.Iron,suspended solids,microorganisms and its EPS formed larger aggregates through bridging and polymerization,which were infiltrated by the surface layer of porous medium,leading to a decrease in medium permeability.(3)The contribution rate of typical clogging substances to complex clogging was quantitatively evaluated.Statistical analysis methods such as correlation analysis,and path analysis were used to quantitatively evaluate the contribution rate of typical clogging-causing substances(iron,suspended solids,microorganisms)to the composite clogging.The results showed that different clogging factors interacted and jointly influenced changes in medium permeability.Under the selected conditions of typical substances and concentrations in this study,iron adsorption and medium filtration were the main factors causing the evolution of physical-chemical and bio-chemical complex clogging.During the formation and evolution of bio-physical combined clogging.In the process of bio-physical composite clogging,microorganisms had a greater direct impact on the formation and evolution of clogging than suspended solids.In the entire process of bio-physical-chemical clogging,iron adsorption and medium filtration were the dominant factors causing the formation and evolution of clogging,followed by the effects of microorganisms,and finally the deposition of suspended solids.(4)A clogging process simulation model based on the formation mechanism was constructed.Based on the bacterial growth-decay model,the adsorption-desorption kinetic model of suspended solids,iron and bacteria in the porous medium,the convectiondispersion equations of microorganisms and nutrients,coupled with the KozenyCarmen equation.A numerical simulation model of combined clogging in the process of managed aquifer recharge was constructed,which realized the simulation of the formation and evolution of bio-physical-chemical combined clogging with suspended solids,iron and microorganisms as typical clogging materials,and predicted the time of clogging,the location of clogging,the clogging degree and the clogging rate.