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Coupled biomass growth and flow in unsaturated soil

Munoz Criollo, Jose, Cleall, Peter and Harbottle, Michael 2017. Coupled biomass growth and flow in unsaturated soil. Presented at: International Symposium on Coupled Phenomena in Environmental Geotechnics, University of Leeds, 6 - 7 September 2017.

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Abstract

A numerical model based on the finite element method is presented in this paper to describe transient changes in the hydraulic properties of porous media due to bioclogging in the pore space. Physical processes like ground water flow and delivery of nutrients in the domain are considered through the coupling of the Richards equation and the convection-diffusion equation. Biomass growth dynamics is described by the mathematical model defined by the Monod equation. Four theoretical relationships available in the literature are used to estimate the change in hydraulic conductivity due to biomass accumulation in the domain. Variations in the model’s predictions due to uncertainty in the Monod coefficients (yield factor, half velocity constant, maximum substrate use rate) are assessed through a sensitivity analysis. The model is validated against results from sand column experiments performed using the bacterium Beijerinckia indica. The columns were prepared with a defined initial concentration of bacteria and are subjected to periods of nutrient supply and periods of rest without flow. The columns’ effective hydraulic conductivity was determined using a constant head test across the column. Preliminary numerical results are in agreement with the literature showing that bacteria tend to reproduce more readily in nutrient injection points causing reductions in hydraulic conductivity in these regions due to biomass accumulation. The combination of increased biomass content (and corresponding nutrient consumption) and reduction of hydraulic conductivity prevents further flow downstream limiting the growth and possibly increasing the decay of bacteria in those sections of the domain. The effective hydraulic conductivity estimated using coefficient values readily available in the literature and others obtain from ongoing experiments at Cardiff University for the hydraulic conductivity models considered here show noticeable differences attributed to the underlying assumptions in their theoretical formulations. However, one model allows for a good comparison between the numerical results and the experimental measurements thus proving the validity of the mathematical model here proposed. A sensitivity analysis shows a relatively strong dependency on the yield coefficient, followed by the maximum substrate use rate and a negligible effect of the half velocity constant. Further work is ongoing to increase the accuracy of these parameters for the case of B. indica.

Item Type: Conference or Workshop Item (Paper)
Date Type: Completion
Status: Unpublished
Schools: Dentistry
Engineering
Date of First Compliant Deposit: 9 October 2017
Last Modified: 27 Jul 2019 22:16
URI: http://orca.cf.ac.uk/id/eprint/105345

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