Experiment And Numerical Simulation For Bioclogging In Aquifer Media During Artificial Recharge

Artificial groundwater recharge can effectively solve problems such asgroundwater recession, saltwater intrusion, and ground subsidence. However,recharge water quality is often not so ideal, providing a suitable condition formicroorganism growth, and leading to bioclogging of aquifer, that can serousilyrestrict the injection efficiency.In this article, suitable conditions was satisfied in a one-dimension seepage sandcolumn, aiming at simulate the bioclogging process of typical aquifer medium duringartificial recharge. The change of permeability coefficient, biomass, and water qualitywithin the sand column were monitored and then analyzed during the experiment.Based on the microorganism growth kinetics, nutrient degradation, and porosity-permeability coefficient theories, and specified to the hydrodynamic, boundary, andinitial conditions of column experiment, the mathematical model of biologicalclogging during recharge process under one-dimensional constant flow condition wasestablished, and then examined by experiment data. By the method of numerical simulation,the temporal and spatial variation of water quality, biomass, and permeabilitycoefficient were analysized comprehensively, and the influences of recharge waterquality and hydrodynamic condition to bioclogging effect were discussed, in order toexplore the general rule of bioclogging development primarily. In addition, themathematical model of bioclogging during single well recharge was established basedon the Dupuit theory, and the bioclogging development rule and characteristics ofspace and time were discussed. Conclusions were reached as follows:(1) Microorganism growth and aggregation were the key reason of thepermeability coefficient decreasing. Development of bioclogging can be divided tothree phases as unclogged stage, clogging stage, and gently clogging/stable stage. Distance from the inlet within6cm is the most seriously clogged area, where therelative permeability coefficient reduced to lower than0.0017, yet the cloggingdevelopment toward deeper area was confined, because the nutrition was nearlyexhausted in the upstream.(2) The numerical simulation curve fitted well with measurement curve,indicating that this mathematical model is reliable and be could be applied to forecastthe occurrence and development of biological clogging in aquifer medium duringrecharge progress. Local analysis method was applied to access the parametersensitivity, and result showed that the sensitive parameters include μmaxand Y, themedium sensitive parameters include kaand kd, which should be selected cautiously inthe practical applications.(3) Numerical analysis results indicated that, the permeability coefficient declineoccurs only in the area of distance from the inlet within6cm, which agreed with theexperiment data. Consumed for organism growth and propagation, the CODconcentration decreased along the column length faster and faster, and the nutritiondepleted sooner in the area near the inlet. Both the suspended and attached biomassalong the column increased first and then slightly decrease, and the further away fromthe inlet, the more significant the decreasing trend was. The biomass concentrated inthe area within5cm away from the inlet, and the gap between upstream anddownstream grew with time, the attached biomass difference even reached five ordersof magnitude. The discharge flow decreased with clog developing, and theparticularly acute decline happened in the27thto35thday, with the decreasingamplitude up to64%. The permeability decline rate accelerated in the early stage, butin the later stage there the detachment of microbial occupied the dominant position,leading to a clog relieving phenomenon.(4) The worse recharge water quality resulted in faster bio-clogging process, andthe sever clog stage occurred earlier, and the cumulative injection amount whenreached the same clog degree was smaller. The better water quality led to smallerclogged scope, but stronger spatial heterogeneity, i.e. bioclogging in the superficialpart stood out. Hydrodynamic condition had little impact on the process of bioclogging development, but the clog degree and extent increased with enhancedhydrodynamic condition.(5) During the well recharge, the permeability coefficient of media around thewell decreased due to bioclogging effect, present that the water level in the well riseup gradually. Bioclogging occurred only in a very small scope around the well,indicating that the microorganism deposited among the media particles could bewashed out through back flush approach, and reach certain clog removal effect.Analysizing the influence of water quality to the bioclogging during well recharge, thebioclogging effect could be ignored with COD concentration of5mg/L, and the clogdegree and extent increased with COD concentration increasing.