Over the years, modelling frameworks have been proposed to describe and predict the degradation of municipal solid waste in landfills. These frameworks, however, fail to couple the kinetic solutions to the real-time equilibrium state of the system. Because of this, fundamental processes such as pH changes, NH4+ adsorption to the waste surface and changes to the fluid composition are often not (or inadequate) taken into account. In this study, we propose a degradation model that couples a kinetic solver to the chemical equilibrium software, ORCHESTRA. For this, a tool is developed that allows seamless operability between ORCHESTRA and the Python environment. The modelling framework is used to simulate the degradation of solid organic matter in a batch reactor following a simplified, but comprehensive, reaction network. Reaction rates are based on Monod kinetics that includes a wide variety of inhibition functions. The Freundlich equation is used to capture NH4+ adsorption to the waste surface. Results from various case studies show that the modelling framework is suitable for coupling real-time landfill chemistry to degradation kinetics. We found that NH₄⁺ concentrations play a dominant role in the presented model. While low NH₄⁺ concentrations limit biomass growth by substrate inhibition, high concentrations (>0.002 4
mol/L) affect the hydrolysis rate by toxicity inhibition. This translates to a model that is sensitive to a variety of input parameters such as the initial C/N ratio, the Kd (degree of adsorption) value and the initial organic fraction. A comparison with literature studies, however, implies that the model lacks fundamental processes such as gas and fluid transport or changes in volume and temperature. In addition to this, we discuss that the Freundlich isotherms may be inappropriate to capture adsorption in a coupled framework as no ion exchange is taken into account. Models that do so, such as the NICA-Donnan model, could improve the model results significantly.

In this work, a novel globally implicit framework for reactive multiphase flow and transport has been implemented using the simulator DARTS and the geochemical package Reaktoro. This framework applies the Operator-Based Linearization (OBL) approach together with a Gibbs Energy Minimization (GEM) scheme to model complex nonlinear reactive transport and flow. Using component- to element-based transport allows for numerically stable equations. This formulation is used to model the dry-out effect and subsequent precipitation of minerals in the near-wellbore region of subsurface aquifers when injecting CO$_2$. A kinetic rate in combination with Reaktoro equilibrium calculations is implemented to achieve more accurate precipitation results. The kinetic products are not considered inside of Reaktoro and also not as an element. The consistency at the phase transition boundary has been corrected by using a projection of a plane within the hypercube of zero charge balance. Highly sensitive reactions such as pH, require a high resolution to produce physical results. The parameterization grid can be implemented such that some species have a high resolution within a small variation of concentration. Reaktoro is highly accurate for the complex multiphase thermodynamic and chemical equilibrium calculation, but at phase transition boundaries, the equilibration of the state can be nonphysical. The results have been benchmarked against the existing geochemical simulator from Shell, showing that more physics needs to be implemented, but gives an overall accurate result.