Application of computational fluid dynamics to the biopile treatment of hydrocarbon contaminated soil

Abstract

Biopiles are a common treatment for the ex-situ remediation of contaminated soil. Much research has been carried out on understanding and modelling of bioremediation techniques related to biopiles, but hitherto no study has attempted to model the effect on a biopile by its ambient surroundings. A hydraulics-based approach to simulating a biopile in the context of its ambient surroundings is presented in this study, taking into account physical, chemical and biological processes within the pile, external conditions of wind and temperature, the location of aeration pipes and venting pressure, and considering the spatial distribution of treatment as well as contaminant within the pile. The simulation approach was based upon a fluid flow model which couples Eulerian multiphase flow model and Darcy’s Law for immiscible fluid flow through porous media, a species transport model integrating advection, diffusion/dispersion and biodegradation, and a heat transfer model considering the interphase temperature equilibrium. A Computational Fluid Dynamics (CFD) system has been developed to solve this set of mathematical models by applying the commercial CFD package FLUENT, and various trial simulations have been carried out to examine the potential of the hydraulics approach for practical applications. The simulation produces reasonable results: the biodegradation process relates to the temperature within the pile, and the temperature in turn relates to wind speed and aeration details; due to the various fluid flow patterns, the contribution of each remediation mechanism (contaminant loss to atmosphere via pile surface, contaminant loss to aeration pipe and biodegradation) varies according to the aeration method; contaminant interphase transfer between different pairs of phases have greatly different impacts on contaminant removal. A number of counter-intuitive results are presented, indicating that simulations of this type will give valuable insight into the practical design of biopiling systems. The simulation system also allows the total environmental footprint of biopiling to be considered, examining not just degradation of contaminant but also its removal via volatilization and the energy used in heating air for venting. Further, the application of the approach formulated in this study is not limited to biopiles, but can also be expanded to related in situ bioremediation techniques.