A Study on Bacterial Transport Through Subsurface Media

Abstract

Groundwater is a natural source on which everyone depends on it for fulfilling their daily needs. Increasing of the demand of clean water due to urbanisation and industrialisation force the urban people extract more from groundwater. Eventually, the recharge of groundwater occurs through precipitation, adopting various mechanism of artificial recharge of surface water and wastewater. During this process, there is a possibility of transport of existing microbes towards the aquifer and make it contaminant. Several processes in the soil environment independently or in combination works and control the fate and transport of microbes through sub-surface media. Therefore, in order to understand these processes, the present study investigates the transport of E. coli through one-dimensional vertical columns packed with three different types of porous media under various moisture content conditions. Column experiments were conducted to understand the influence of different factors such as media type, media size, media depth, minerals present in media, and presence of fertilizer in media in bacterial transport under variable moisture content conditions. E. coli suspensions were pumped through the columns, and effluent samples were collected to determine bacterial viability using the MTT protocol. Results were analyzed using Breakthrough Curves (BTC), which provided insights into peak concentrations, time to peak, and tail concentrations of E. coli. Several numbers of experiments were conducted by systematically varying the influencing factors and observed that particle size, travel distance, porosity, particle surface condition and types of mineral present in the particle are the parameters affecting the mechanism of bacteria transportation through the subsurface media. It is also observed that the sorption mechanism during the transport of E. coli through porous media is being affected by the presence of aluminium oxide. The present study also explored the effect of adding fertilizer to the porous media on the transportation of E. coli. It is observed that the peak concentration of E. coli in each case is substantially increased in the presence of fertiliser in the soil media. It is also observed that the percentage of peak concentration is a function of amount of fertiliser present in soil media. It is observed from the present study that the soil having finer particles, non-uniform shape of particle and more alumina content in the soil sample are responsible for getting an extended tail in breakthrough curve. The extended tail of the breakthrough curve is of concern as it shows the persistence of low concentration of bacteria for a longer period in the groundwater. The hydrodynamic dispersion coefficient of the bacterial transportation through saturated medium is determined by optimizing the analytical solution and observed that the hydrodynamic dispersion coefficient is also affected by the uniformity coefficient of media, porosity and sorption mechanism. The column experiments were also conducted in variable moisture content conditions and effluent samples were collected to determine bacterial viability. The results consistently indicated that as the moisture content percentage of the porous medium is decreased, the maximum concentration of E. coli in the effluent is increased, and the time to reach peak concentration decreases. The E. coli transport through sub-surface medium is observed to be a function of the soil moisture content apart from all other media parameters affecting the transport during saturated condition. The present study simulated the transportation and interception of E. coli in porous media experimentally using packed columns to address certain aspects of underexplored sorption potential and validated using several kinetic models. The breakthrough curves obtained through experiments are observed to be in good agreement with its prediction using kinetic models namely Thomas, Yoon-Nelson and Modified Dose-Response. The overall comparisons of R2 among all the three models suggest that the MDR model fits more perfectly to experimental results. It is observed that the travel distance and particle size are the statistically significant parameters that efficiently impact on sorption of E. coli during their transport whereas the alumina content also affects the sorption but is observed to be a statistically non-significant.