Multiscale Modelling of De Novo Anaerobic Granulation

F Russo,A Tenore,G Collins,L Frunzo,M R Mattei,B D’Acunto

doi:10.1007/s11538-021-00951-y

Abstract

A multiscale mathematical model is presented to describe de novo granulation, and the evolution of multispecies granular biofilms, in a continuously fed bioreactor. The granule is modelled as a spherical free boundary domain with radial symmetry. The equation governing the free boundary is derived from global mass balance considerations and takes into account the growth of sessile biomass as well as exchange fluxes with the bulk liquid. Starting from a vanishing initial value, the expansion of the free boundary is initiated by the attachment process, which depends on the microbial species concentrations within the bulk liquid and their specific attachment velocity. Nonlinear hyperbolic PDEs model the growth of the sessile microbial species, while quasi-linear parabolic PDEs govern the dynamics of substrates and invading species within the granular biofilm. Nonlinear ODEs govern the evolution of soluble substrates and planktonic biomass within the bulk liquid. The model is applied to an anaerobic, granular-based bioreactor system, and solved numerically to test its qualitative behaviour and explore the main aspects of de novo anaerobic granulation: ecology, biomass distribution, relative abundance, dimensional evolution of the granules and soluble substrates, and planktonic biomass dynamics within the bioreactor. The numerical results confirm that the model accurately describes the ecology and the concentrically layered structure of anaerobic granules observed experimentally, and that it can predict the effects on the process of significant factors, such as influent wastewater composition; granulation properties of planktonic biomass; biomass density; detachment intensity; and number of granules.

Highlights

Biofilms are complex, dense and compact aggregates comprising microbial cells immobilized in a self-produced matrix of extracellular polymeric substances (EPS) (Flemming and Wingender 2010)
The granule is affected by complex phenomena, which radically influence its structure and suddenly change its properties: granulation processes of planktonic biomass; metabolic growth and decay of sessile biomass; particle-particle interactions; EPS secretion; gas production; invasion processes by planktonic cells; and detachment processes induced by intense hydrodynamic conditions and shear stress
A mathematical model able to reproduce the de novo granulation process involved in a generic, granular biofilm system has been introduced

Summary

Introduction

Dense and compact aggregates comprising microbial cells immobilized in a self-produced matrix of extracellular polymeric substances (EPS) (Flemming and Wingender 2010). Natural biofilms typically develop as planar layers attached to suitable surfaces, under specific conditions the aggregation occurs due to the selfimmobilization of cells into approximately spherical-shaped granules (Trego et al 2020a). The movement and shape mitigate boundary layer resistances and enhance the mass transfer of substrates across the biofilm granule (Baeten et al 2019). For these reasons, granular biofilms have been successfully developed in different bioreactor configurations, for various processes, such as aerobic, anaerobic and partial nitritation-anammox treatments (Trego et al 2020b)

Results

Discussion

Conclusion