Hydrodynamic and biochemical computational fluid dynamic modelling of full-scale anaerobic digesters for wastewater treatment
- Autores: Maria Rosario Arnau Notari
- Directores de la Tesis: Sergio Chiva Vicent (dir. tes.), Raúl Martínez Cuenca (codir. tes.)
- Lectura: En la Universitat Jaume I ( España ) en 2022
- Idioma: español
- Tribunal Calificador de la Tesis: Leonor Hernández López (presid.), Ángel Robles Martínez (secret.), Borja Valverde Pérez (voc.)
- Programa de doctorado: Programa de Doctorado en Tecnologías Industriales y Materiales por la Universidad Jaume I de Castellón
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Anaerobic digestion is one of the most widely used biological treatments for the stabilisation of the sludge from wastewater treatment plants, livestock manure or organic waste from agriculture or the processed food industry. Indeed, its capability to produce biogas or biomethane is gaining more and more attention from the public and administrations due to the implementation of the circular economy and the rising climate crisis. In fact, most of the efforts made on anaerobic digesters are focused on increasing biogas production, leaving sludge stabilisation as a secondary objective. In this context where priority is given to biogas production, it is advisable to ensure homogeneous mixing of the sludge. This mixing is achieved by means of agitation systems that generate an intense mixing inside the anaerobic digesters. However, the efficiency of this mixing becomes evident when dead volumes or low biogas production occur. The Computational Fluid Dynamics (CFD) technique allows the mixing and hydrodynamics of these process units to be analysed, which is why it is now a widespread technique.
Firstly, in this PhD thesis, the hydrodynamics of a full-scale anaerobic digester has been studied by means of non-Newtonian single-phase CFD models, evaluating the mixing systems by means of different CFD scenarios. The CDF model was comprehensively and globally validated using an experimental 76-day RTD curve, and different mixing parameters were evaluated in terms of generality and usability. The local and global mixing parameters were sensitive and helped to define the differences of the mixing scenarios, but the design parameters failed in this task. B100 scenario, with a mechanical propeller installed in the centre proved to be the best and most efficient mixing scenario. Indeed, homogenisation times were obtained by means of local mixing parameters and were less than 30 minutes in the case of intensive mixing scenarios and less than 1 hour in the base scenario. This useful information was used to define an efficient mixing regime with the addition of co-substrates.
These CFD scenarios were then used to assess the robustness and sensitivity of the dead volume criteria from the literature. However, they did not fulfil this goal so new dead volume criteria were proposed and calibrated with experimental dead volume data. The new criteria considered the buoyancy force and low turbulence dispersion and are generally applicable in CFD models of full-scale anaerobic digesters.
In a second part, a new solver was developed in an open-source CFD code to couple a biological model, i.e., the Anaerobic Digestion Model 1, with hydrodynamics. The solver included the pH calculation, simplified liquid-gas transfer and the equations of the anaerobic digestion process and was called ADM1Foam. It was tested at lab-scale and at the full-scale studied previously: On the one hand, the lab-scale allowed to confirm the correct implementation of the model by means of experimental data and 0D-CSTR models. On the other hand, different mixing regimes were simulated with this solver in both configurations in order to gain some knowledge about the link between anaerobic digestion performance and mixing. In summary, the solver reproduced transient simulations and showed that lower mixing would be detrimental to anaerobic digestion process. This solver is the basis for the development of new advanced models considering two-phase CFD simulations.
The last part is devoted to evaluating the hydrodynamic performance of rising biogas bubbles inside an anaerobic sludge matrix by means of two-phase Euler-Euler Volume of Fluid models. Different bubble sizes and two types of anaerobic sludge were tested, so that apparent viscosity, terminal velocity and shape were evaluated. From the bubble size analysis, the large bubbles showed large terminal velocity, so they would coalesce with the small bubbles on their trajectory to the biogas chamber. As for the sludge types, the anaerobic digestate revealed a lower apparent viscosity, thus a higher terminal velocity and wider bubbles shapes were observed in contrast to waste anaerobic sludge. Moreover, different drag coefficients from literature that can be used on two-phase CFD models were tested. However, a new drag correlation was proposed according to the obtained results. In the end, the biogas bubbles enhanced the local mixing in the anaerobic sludge matrix, so that their contribution to global mixing should be considered in future CFD models.
Accordingly, CFD modelling is a powerful and alternative tool for modelling anaerobic digesters, reproducing their local and global mixing with single-phase and two-phase models. On the other hand, experimental validation of CFD models of both their rheological properties and their hydraulic behaviour is crucial to ensure accurate and reliable models. In the near future, CFD will be able to reproduce the biogas generation and mass transfer from the liquid to the gas phase resulting from the anaerobic digestion process.
