Modeling of Phosphorus Reactions during Wastewater Infiltration using Analytical and Numerical Modeling Techniques

Abstract

A modeling exercise was completed using an experimental phosphorus breakthrough curve from a one-dimensional sand column to test the validity of various conceptual models of phosphorus removal in porous media. Analytical and numerical modeling techniques were applied for two conceptual models of phosphorus sorption to solid surfaces, two conceptual models of precipitation of phosphorus minerals, and a conceptual model of sorption in conjunction with precipitation. An analytical solution to the advection-dispersion equation for one-dimensional conservative transport was used first with data from a conservative tracer test conducted in the sand column to determine optimized values for average linear velocity and dispersivity for the sand column. These optimized values were then used as input parameters in a linear sorption model, surface-complexation model, equilibrium precipitation model, precipitation as 1st-order decay model, and linear sorption with 1st-order removal by precipitation model. The linear sorption conceptual model provided the best fit to the experimental data. The surface-complexation model displayed sensitivity to simulated changes in pH and was unable to be calibrated to provide a reasonable fit. Precipitation modeled as an instantaneous, or equilibrium, reaction resulted in a breakthrough curve that over-estimated phosphorus removal which lead to the conclusion that equilibrium precipitation was not a process occurring in the real system. Precipitation modeled as 1st-order decay also yielded a poor fit to the data, but did not lend insight into actual removal mechanisms in the column. With a combination of linear sorption and precipitation as 1st-order decay, a better fit to the front of the breakthrough curve was obtained, but the model could not simulate the continued breakthrough of phosphorus measured from the column. This modeling project demonstrates a method of comparing the results of various conceptual models of phosphorus removal in porous media to experimental data in order to better understand actual removal mechanisms that may be occurring within the real system.