Wastewater re-use in agriculture: modelling contaminant transport and impact on soil structure

Resúmen

Conventional freshwater resources are currently under pressure, while the freshwater demand is expected to increase further due to the growing world population, urbanization, and climate change. Re-using wastewater for irrigation in agriculture may alleviate the pressure on freshwater resources.  However, the contaminants present in wastewater may pose a risk to the public health and the environment. Several aspects regarding contaminant transport in the environment are considered in this thesis. In current sustainability frameworks regarding wastewater irrigation, only one or several contaminant pathways or transport processes are considered. However, all potential pathways should be considered to accurately predict the associated public health and environmental risks. Therefore, we developed such an integrated framework for the sustainability assessment of wastewater irrigation. The framework highlights the need for accurate parametrization of contaminant behaviour in the environment, which is lacking especially for contaminants of emerging concern. The effect of degradation concepts for adsorbing contaminants under unsteady water flow conditions was also investigated. When contaminants do not degrade while adsorbed, adsorption does not affect the steady state fluxes and concentration, only the timing. When contaminants do degrade while adsorbed, the steady state fluxes and concentration are affected for nonlinear adsorption, but not for linear adsorption. Thus, it is necessary to experimentally determine the appropriate degradation concept to assess the role of adsorption on the long-term concentration and fluxes in the subsurface. Wastewater often contains high sodium concentrations, which may lead to soil structure degradation of clayey and loamy soils. The step from changes of the microscopic soil structure to changes of the macroscopic soil hydraulic properties is challenging. A methodology for upscaling flow and transport effects in such charged porous media is presented, which allows for the computation of effective electrokinetic transport parameters for heterogeneous media. The effect of sodicity on the saturated and unsaturated soil hydraulic properties was investigated by upscaling models for the microscopic scale. It was found that including ion-ion correlation forces is necessary to accurately model the swelling of sodic soils on the microscopic soil, for example with density functional theory. These results aid in developing and refining irrigation water quality standards.