Continuum modeling of bioclogging of soil aquifer treatment systems segregating active and inactive biomass

Abstract. Soil aquifer treatment (SAT) systems are used to remove pollutants from treated wastewater and store freshwater for reclamation and reuse. However, the accumulation of microbial biomass in the soil pore space, bioclogging, reduces water infiltration and hinders SAT efficiency. Since SAT systems play a crucial role in maintaining water resilience by providing an alternative to freshwater supply, optimizing their operation is essential to ensure their effectiveness. However, SAT systems are complex and dynamic systems that involve coupled interactions between microbial activity, water infiltration, and bioclogging in unsaturated media. This work proposes a continuum model that accounts for all these processes while distinguishing between active and inactive biomass, with the latter split into labile and recalcitrant fractions. The model is used to replicate a laboratory column experiment of bioclogging under unsaturated conditions and to explore how to optimize the operation of SAT systems. Specifically, we determined optimal wetting and drying periods that maximize water input to the SAT system while maintaining nutrient transformation rates. Our simulations show that the dry/wet time ratio controls biomass spatial distribution over depth. In contrast, the dry time extent dictates the degree of recovery of the soil relative to its initial (clean) infiltration capacity. We discuss the potential of this model to be extended to larger-scale experiments and to inform daily SAT operations in the field.