How to measure diffusion coefficients in biofilms: A critical analysis

Abstract

Biofilm and granular sludge processes depend on diffusion of substrates. Despite their importance for the kinetic description of biofilm reactors, biofilm diffusion coefficients reported in literature vary greatly. The aim of this simulation study was to determine to what extent the methods that are used to measure diffusion coefficients contribute to the reported variability. Granular sludge was used as a case study. Six common methods were selected, based on mass balances and microelectrodes. A Monte Carlo simulation was carried out to determine the theoretical precision of each method, considering the uncertainty of various experimental parameters. A model‐based simulation of a diffusion experiment was used to determine the theoretical accuracy as a result of six sources of error: solute sorption, biomass deactivation, mass transfer boundary layer, granule roughness, granule shape, and granule size distribution. Based on the Monte Carlo analysis, the relative standard deviation of the different methods ranged from 5% to 61%. In a theoretical experiment, the six error sources led to an 37% underestimation of the diffusion coefficient. This highlights that diffusion coefficients cannot be determined accurately with existing experimental methods. At the same time, the need for measuring precise diffusion coefficients as input value for biofilm modeling can be questioned, since the output of biofilm models has a limited sensitivity toward the diffusion coefficient.

Biological processes of nitrogen removal and modeling oxygen diffusion in flocculent sludge and in granular sequencing batch reactors

“…An oxygen diffusion coefficient of 2 × 10 −9 m 2 s −1 has been used in a diffusion model for aerobic granules 38 . Diffusion coefficient in granules of 1.2 × 10 −9 m 2 s −1 has been reported by van den Berg et al 39 …”

An Overview of Wastewater Treatment and Reuse in The Gulf Cooperation Council Countries

“…These systems are differentiated based on whether the process is continuous or discontinuous [19]. The other biological wastewater treatment processes which can also be used include the rotating biological contactors (RBC) and trickling filters [19], [24]- [26]. They are beneficial since no ponding is needed; they wet the medium completely, use unlimited oxygen, produce low noise and odor, and require less space or land to set up.…”

Impacts of Biofilm Properties on the Start-Up and Performance of a Membrane Biofilm Reactor Performing Anammox and Nitrate/Nitrite-Dependent Anaerobic Methane Oxidation Integrated Processes: A Model-Based Investigation

“…However, various factors could affect the performance of an MBfR and hence hinder its large-scale applications to the anammox/n-DAMO integrated processes. Therein lie the initial biofilm properties (e.g., the initial microbial composition of the biofilm and the initial biofilm thickness) and the inherent biofilm properties (e.g., the boundary layer thickness of the biofilm and the diffusivity of solutes in the biofilm structure), which have been shown to influence the practical operation of biofilm-based reactors. − First, changes in microbial composition during the operation have been demonstrated to exert a distinct influence on the performance of the MBfR, thus highlighting the potentially critical role of the initial microbial composition of the biofilm. Second, the initial biofilm thickness has been known as a key parameter regulating the mass transfer and would therefore influence the substrate acquisition by microorganisms, the evolution of the microbial structure, and the start-up process of the MBfR.…”