Formation And Evolution Of Microbial Clogging During Artificial Groundwater Recharge Of Urban Rain-Flood Water

The problem of groundwater overexploitation caused by the increase of water demand has formed a sharp contrast with the problem of urban waterlogging caused by extreme rainfall,and the contradiction between the two has become increasingly prominent.As an effective way to solve this contradiction,using surplus rainwater to artificially recharge groundwater has attracted more and more attention.However,the recharge efficiency of the operating processes in recharge facilities has been greatly reduced or even scrapped in many cases due to clogging.Biological clogging is one of the main types of media clogging,which is mainly dominated by microbial growth and extracellular polymers(EPS)secreted by microorganisms.Existing studies on the law of biological clogging are carried out mainly with a background of saturated seepage and the development process of single microbial clogging with sufficient nutrients.The accumulation and migration characteristics of microorganisms and EPS in the medium are still unclear,and the formation and evolution of bioclogging need to be further studied in the case of urban rainfall-flood with low nutrient content and usually under variable saturation infiltration recharge condition.Therefore,the research carried out a series of seepage simulation tests in view of the unique hydrodynamic and water environment conditions of artificial recharge of rain and flood.The research systematically analyzed the influence of the seepage conditions of rain and flood on the growth,morphology,migration and other characteristics of microorganisms and EPS,and then clarified the main control mechanism and influencing factors of the formation and development of microbial clogging in porous media,and constructed the evolution process model of biological clogging in infiltration profile.The main conclusions reached are as follows:1.In the process of rain-flood infiltration and recharge,the hydrodynamic conditions,hydrochemical environment and microbial composition of the vertical profile of the medium show a significant covariation evolution law.The surface seepage state of the medium gradually changes from unsaturated to saturated with the infiltration and the interior is always in a non-saturated seepage state due to the growth of surface microorganisms.The overall permeability change trend of the medium profile is basically consistent with the microbial growth process.The p H distribution of the infiltration profile decreases gradually under the influence of microbial growth,and becomes stable after the microbial growth reached stability and the permeability of the medium is no longer reduced.The medium is always in an oxidizing environment during the whole recharge process.The microbial community richness and diversity of microbial communities in the near surface and underground water surface are significantly reduced after recharge,and the dominant bacteria genera are significantly different from those in the lower layer2.The nutrient content and temperature of recharge water will affect the development of clogging.In oligotrophic environment,the amount of biofilm formed by microorganisms was low.The biofilm is mainly bacterial body and the EPS content is extremely low.Low temperature can delay the growth of microorganisms to a certain extent and reduce the biomass of a single bacteria,but it has no effect on the total biomass of mixed bacteria,and the EPS secretion is more in the same biomass under low temperature condition.Therefore,the growth process of the mixed bacterial system should be taken as the evaluation object of bioclogging.Depending on the development of a single model of microorganisms,the degree of blockage under microbial action may be significantly underestimated,and the prediction error of low temperature is greater.3.Unsaturated seepage conditions will significantly affect the EPS morphology and component content of microorganisms.The unsaturated seepage condition will lead to the curling of EPS,which will promote the uneven distribution of the biofilm on the surface of the medium.And the lower the relative moisture content,the more obvious the EPS curling and biofilm agglomeration.In terms of composition and content,the unsaturated seepage condition will promote the microbial secretion of more EPS.The LB-EPS and polysaccharide content in EPS are significantly increased and the degree of biological blockage was more serious.4.The migration and deposition of microbial blockages in the porous medium under unsaturated seepage conditions is controlled by the moisture content of the medium,acid base environment,the strength of ions in recharge water and the infiltration rate of recharge.When the water content of the medium decreases,the p H in the medium decreases,the strength of ions in the infiltration water increases or the flow rate decreases,the microbial migration will slow down,the deposition on the medium increases and the degree of interlayer deposition non-uniformity decreases.5.The accumulation of microorganisms and their EPS in the medium will affect the permeability,moisture absorption capacity and solute dispersion capacity of the medium at a macroscopic level.When the bacteria proliferate,the permeability of the medium is reduced and the solute transport in the medium is dominated by convection.After that,at the stage of EPS secretion by microorganisms,the medium permeability decreases faster,the water absorption capacity will gradually increase and the solute transport will gradually be dominated by diffusion.6.The formation and evolution model of microbial clogging on the infiltration profile was constructed,and the main control mechanism of microbial clogging at different locations was identified.In the near surface layer,the biological blockage is mainly caused by the accumulation of bacteria,which is mainly affected by nutrients.And the evolution mainly includes four stages: the rapid development stage under the action of bacteria,the accelerated development stage under the action of bacteria and EPS,the slow development stage under the action of EPS and the blockage stability stage.The biological blockage of the variable saturation seepage area and near the groundwater surface area is mainly EPS,which is mainly affected by unsaturated seepage.In addition,the control mechanism of microbial blockage will also evolve with the recharge time and the development of upper blockage.And the evolution of biological blockage mainly includes two stages: EPS action stage and EPS and bacteria interaction stage.