Invasion moving boundary problem for a biofilm reactor model

TL;DR

This paper analyzes a free boundary problem modeling species invasion in biofilm reactors, combining nonlinear PDEs with boundary evolution, and provides qualitative analysis, existence results, and numerical simulations of bacterial invasion.

Contribution

It introduces a novel free boundary model for biofilm invasion, including existence and uniqueness analysis, and applies it to simulate bacterial colonization in wastewater treatment.

Findings

Numerical simulations show invasion impacts biofilm structure.

Model captures switching between planktonic and sessile states.

Invasion influences biofilm activity and growth patterns.

Abstract

The work presents the analysis of the free boundary value problem related to the invasion model of new species in biofilm reactors. In the framework of continuum approach to mathematical modelling of biofilm growth, the problem consists of a system of nonlinear hyperbolic partial differential equations governing the microbial species growth and a system of semi-linear elliptic partial differential equations describing the substrate trends. The model is completed with a system of elliptic partial differential equations governing the diffusion and reaction of planktonic cells, which are able to switch their mode of growth from planktonic to sessile when specific environmental conditions are found. Two systems of nonlinear differential equations for the substrate and planktonic cells mass balance within the bulk liquid are also considered. The free boundary evolution is governed by a differential equation that accounts for detachment. The qualitative analysis is performed and a uniqueness and existence result is discussed. Furthermore, two special models of biological and engineering interest are discussed numerically. The invasion of Anammox bacteria in a constituted biofilm inhabiting the deammonification units of the wastewater treatment plants is simulated. Numerical simulations are run to evaluate the influence of the colonization process on biofilm structure and activity.